The role of iron in phytoplankton photosynthesis, and the potential for iron-limitation of primary productivity in the sea

Vertical heterogeneity of dissolved ferrous and iron speciation in tropical seagrass beds

Iron is an important trace metal in regulating the ocean biogeochemical cycle. Through in situ field measurements using reverse flow dual-injection analysis (rFDIA), we demonstrate a significant spatial heterogeneity in iron distribution in the water column in three Chinese seagrass beds. Fe2+ concentrations ranged from 2.38 to 10.82 nM and dFe from 10.44 to 18.35 nM across 26 sampling stations. The concentration of Fe2+ in the surface water layer was significantly higher than the bottom within seagrass beds, while opposite trend was observed in unvegetated area, revealing that the seagrass canopy presence exerts a profound influence on iron dynamics, particularly vertical stratification. Furthermore, our characterization of suspended mineral compositions provides novel insights into possible mineral-mediated iron transformation processes. This study elucidates the potential interplay between seagrass and iron biogeochemical cycling. Nevertheless, the driving mechanism behind this still needs further study, including both biological and geochemical processes on the iron cycling.

Molecular Mechanisms for Iron Uptake and Homeostasis in Marine Eukaryotic Phytoplankton

The micronutrient iron is essential for phytoplankton growth due to its central role in a wide variety of key metabolic processes including photosynthesis and nitrate assimilation. As a result of scarce bioavailable iron in seawater, marine primary productivity is often iron-limited with future iron supplies remaining uncertain. Although evolutionary constraints resulted in high cellular iron requirements, phytoplankton evolved diverse mechanisms that enable uptake of multiple forms of iron, storage of iron over short and long timescales, and modulation of their iron requirement under stress. Genomics continues to increase our understanding of iron-related proteins that are homologous to those characterized in other model organisms, while recently, molecular and cell biology have been revealing unique genes and processes with connections to iron acquisition or use. Moreover, there are an increasing number of examples showing the interplay between iron uptake and extracellular processes such as boundary layer chemistry and microbial interactions.

Exploring the potential nutritional benefits of Arthrospira maxima and Chlorella vulgaris: A focus on vitamin B12, amino acids, and micronutrients

Arthrospira (Limnospira) maxima (A. maxima) and Chlorella vulgaris (Ch. vulgaris) are among the approved microalgae and cyanobacteria (MaC) in the food industry that are known to be safe for consumption. However, both organisms are controversial regarding their vitamin B12 content, due to the possible occurrence of pseudo-cobalamin. Concurrently, their nutrition profiles remain understudied. The main purpose of the present study was to identify their nutrition profiles, focusing mainly on vitamin B12, amino acids, and micronutrients under iron-induced hormesis (10 mg/L Fe in treated samples). Our findings indicate a higher B12 content in A. maxima compared to Ch. vulgaris (both control and treated samples). Using liquid chromatography with tandem mass spectrometry (LC-MS/MS), the cyanocobalamin content was determined as 0.42 ± 0.09 μg/g dried weight (DW) in the A. maxima control and 0.55 ± 0.02 μg/g DW in treated A. maxima, resulting in an insignificant difference. In addition, the iron-enriched medium increased the amount of iron in both tested biomasses (p < 0.01). However, a more pronounced (approximately 100×) boost was observed in Ch. vulgaris, indicating a better absorption capacity (control Ch. vulgaris 0.16 ± 0.01 mg/g Fe, treated Ch. vulgaris 15.40 ± 0.34 mg/g Fe). Additionally, Ch. vulgaris also showed a higher micronutrient content. Using both tested microalgae, meeting the sufficient recommended daily mineral allowance for an adult is possible. By combining biomass from A. maxima and Ch. vulgaris in a ratio of 6:1, we can fulfill the recommended daily allowance of vitamin B12 and iron by consuming 6 tablets/6 g. Importantly, iron hormesis stimulated amino acid composition in both organisms. The profile of amino acids may suggest these biomasses as promising potential nutrition sources.

Classification of eco-zones from the factors and processes controlling phytoplankton biomass

Phytoplankton is of utmost importance to the marine ecosystem and, subsequently, to the Blue Economy. This study aims to explain the reasons for variability of phytoplankton by estimating the dependency of Chlorophyll-a (Chl-a) on various limiting factors using statistics. The global oceans are classified into coherent units that display similar sensitivity to changing parameters and processes using the k-means algorithm. The resulting six clusters are based on the limiting factors (PAR, iron, or nitrate) that modulate Chl-a yield divisions of the oceans, similar to regions of different trophic statuses. The clusters range from the polar and equatorial regions with high nutrient values limited by light, to open oceanic regions in downwelling gyres limited by nutrients. Some clusters also show a high dependency on marine dissolved iron. Further, oceans are also divided into eight clusters based on the processes (stratification, upwelling, topography, and solar insolation) that impact ocean productivity. The study shows that considering temporal variations is crucial for segregating oceans into ecological zones by utilizing correlation of time-series data into classification. Our results provide valuable insights into the regulation of phytoplankton abundance and its variability, which can help in understanding the implications of climate change and other anthropogenic effects on marine biology.

PSKR1 balances the plant growth-defence trade-off in the rhizosphere microbiome

Microbiota benefit their hosts by improving nutrient uptake and pathogen protection. How host immunity restricts microbiota while avoiding autoimmunity is poorly understood. Here we show that the Arabidopsis phytosulfokine receptor 1 (pskr1) mutant displays autoimmunity (plant stunting, defence-gene expression and reduced rhizosphere bacterial growth) in response to growth-promoting Pseudomonas fluorescens. Microbiome profiling and microbiota colonization showed that PSKR1-mediated reduction in bacterial growth and stunting is largely specific to Pseudomonas. Transcriptional profiling demonstrated that PSKR1 regulates the growth-defence trade-off during Pseudomonas colonization: PSKR1 upregulates plant photosynthesis and root growth but suppresses salicylic-acid-mediated defences. Genetic epistasis experiments showed that pskr1 stunting and restriction of bacterial growth are salicylic acid dependent. Finally, we showed that Pseudomonas, but not other bacteria, induces PSKR1 expression in roots, suggesting that Pseudomonas might manipulate plant signalling to promote its colonization. Our data demonstrate a genetic mechanism to coordinate beneficial functions of the microbiome while preventing autoimmunity.

Symbiodiniaceae photophysiology and stress resilience is enhanced by microbial associations

Symbiodiniaceae form associations with extra- and intracellular bacterial symbionts, both in culture and in symbiosis with corals. Bacterial associates can regulate Symbiodiniaceae fitness in terms of growth, calcification and photophysiology. However, the influence of these bacteria on interactive stressors, such as temperature and light, which are known to influence Symbiodiniaceae physiology, remains unclear. Here, we examined the photophysiological response of two Symbiodiniaceae species (Symbiodinium microadriaticum and Breviolum minutum) cultured under acute temperature and light stress with specific bacterial partners from their microbiome (Labrenzia (Roseibium) alexandrii, Marinobacter adhaerens or Muricauda aquimarina). Overall, bacterial presence positively impacted Symbiodiniaceae core photosynthetic health (photosystem II [PSII] quantum yield) and photoprotective capacity (non-photochemical quenching; NPQ) compared to cultures with all extracellular bacteria removed, although specific benefits were variable across Symbiodiniaceae genera and growth phase. Symbiodiniaceae co-cultured with M. aquimarina displayed an inverse NPQ response under high temperatures and light, and those with L. alexandrii demonstrated a lowered threshold for induction of NPQ, potentially through the provision of antioxidant compounds such as zeaxanthin (produced by Muricauda spp.) and dimethylsulfoniopropionate (DMSP; produced by this strain of L. alexandrii). Our co-culture approach empirically demonstrates the benefits bacteria can deliver to Symbiodiniaceae photochemical performance, providing evidence that bacterial associates can play important functional roles for Symbiodiniaceae.

Iron and manganese availability drives primary production and carbon export in the Weddell Sea

Next to iron (Fe), recent phytoplankton-enrichment experiments identified manganese (Mn) to (co-)limit Southern Ocean phytoplankton biomass and species composition. Since taxonomic diversity affects aggregation time and sinking rate, the efficiency of the biological carbon pump is directly affected by community structure. However, the impact of FeMn co-limitation on Antarctic primary production, community composition, and the subsequent export of carbon to depth requires more investigation. In situ samplings of 6 stations in the understudied southern Weddell Sea revealed that surface Fe and Mn concentrations, primary production, and carbon export rates were all low, suggesting a FeMn co-limited phytoplankton community. An Fe and Mn addition experiment examined how changes in the species composition drive the aggregation capability of a natural phytoplankton community. Primary production rates were highest when Fe and Mn were added together, due to an increased abundance of the colonial prymnesiophyte Phaeocystis antarctica. Although the community remained diatom dominated, the increase in Phaeocystis abundance led to highly carbon-enriched aggregates and a 4-fold increase in the carbon export potential compared to the control, whereas it only doubled in the Fe treatment. Based on the outcome of the FeMn-enrichment experiment, this region may suffer from FeMn co-limitation. As the Weddell Sea represents one of the most productive Antarctic marginal ice zones, our findings highlight that in response to greater Fe and Mn supply, changes in plankton community composition and primary production can have a disproportionally larger effect on the carbon export potential.

Effect of PAR irradiance intensity on Phaeocystis antarctica (Prymnesiophyceae) growth and DMSP, DMSO, and acrylate concentrations

Phaeocystis antarctica forms extensive spring blooms in the Southern Ocean that coincide with high concentrations of dimethylsulfoniopropionate (DMSP), dimethylsulfoxide (DMSO), dimethylsulfide (DMS), and acrylate. We determined how concentrations of these compounds changed during the growth of axenic P. antarctica cultures exposed to light-limiting, sub-saturating, and saturating PAR irradiances. Cellular DMSP concentrations per liter cell volume (CV) ranged between 199 and 403 mmol · LCV -1 , with the highest concentrations observed under light-limiting PAR. Cellular acrylate concentrations did not change appreciably with a change in irradiance level or growth, ranging between 18 and 45 mmol · LCV -1 , constituting an estimated 0.2%-2.8% of cellular carbon. Both dissolved acrylate and DMSO increased substantially with irradiance during exponential growth on a per-cell basis, ranging from 0.91 to 3.15 and 0.24 to 1.39 fmol · cell-1 , respectively, indicating substantial export of these compounds into the dissolved phase. Average cellular DMSO:DMSP ratios increased 7.6-fold between exponential and stationary phases of batch growth, with a 3- to 13-fold increase in cellular DMSO likely formed from abiotic reactions of DMSP and DMS with reactive oxygen species (ROS). At mM levels, cellular DMSP and acrylate are proposed to serve as de facto antioxidants in P. antarctica not regulated by oxidative stress or changes in ROS. Instead, cellular DMSP concentrations are likely controlled by other physiological processes including an overflow mechanism to remove excess carbon via acrylate, DMS, and DMSO during times of unbalanced growth brought on by physical stress or nutrient limitation. Together, these compounds should aid P. antarctica in adapting to a range of PAR irradiances by maintaining cellular functions and reducing oxidative stress.

Subcellular proteomics for determining iron-limited remodeling of plastids in the model diatom Thalassiosira pseudonana (Bacillariophyta)

Diatoms are important primary producers in the world's oceans, yet their growth is constrained in large regions by low bioavailable iron (Fe). Low-Fe stress-induced limitation of primary production is due to requirements for Fe in components of essential metabolic pathways including photosynthesis and other chloroplast plastid functions. Studies have shown that under low-Fe stress, diatoms alter plastid-specific processes, including components of electron transport. These physiological changes suggest changes of protein content and in protein abundances within the diatom plastid. While in silico predictions provide putative information on plastid-localized proteins, knowledge of diatom plastid proteins remains limited in comparison to well-studied model photosynthetic organisms. To address this, we employed shotgun proteomics to investigate the proteome of subcellular plastid-enriched fractions from Thalassiosira pseudonana to gain a better understanding of how the plastid proteome is remodeled in response to Fe limitation. Using mass spectrometry-based peptide identification and quantification, we analyzed T. pseudonana grown under Fe-replete and -limiting conditions. Through these analyses, we inferred the relative quantities of each protein, revealing that Fe limitation regulates major metabolic pathways in the plastid, including the Calvin cycle. Additionally, we observed changes in the expression of light-harvesting proteins. In silico localization predictions of proteins identified in this plastid-enriched proteome allowed for an in-depth comparison of theoretical versus observed plastid-localization, providing evidence for the potential of additional protein import pathways into the diatom plastid.

Exploratory evaluation of iron and its speciation in surface waters of Admiralty Bay, King George Island, Antarctica

The determination of dissolved iron concentrations and speciation was conducted for the first time in surface seawater coastline samples collected during the austral summer of 2020 in Admiralty Bay, King George Island, Antarctica. The technique of competitive ligand exchange/adsorptive cathodic stripping voltammetry with 2,3-dihydroxynaphthalene as the competing ligand was evaluated, showing a sensitivity between 14.25 and 21.05 nA nmol L-1 min-1, with an LOD of 14 pmol L-1 and a mean blank contribution of 0.248 nmol L-1. Physicochemical parameters such as pH (7.85 ± 0.2), salinity (32.7 ± 0.8) and dissolved oxygen (51.3 ± 26.6%) were compatible with those of the literature; however, the average temperature (4.2 ± 0.8 °C) was higher, possibly as a reflection of global warming. The dissolved iron mean value was 18.9 ± 6.1 nmol L-1, with a total ligand concentration of 23.6 ± 12.2 nmol L-1 and a conditional stability complex constant of 12.2 ± 0.2, indicating humic substances as possible ligands. On average, the calculated free iron concentrations were 0.7 ± 0.3 pmol L-1. Relatively high concentrations of iron indicate a possible local source of Fe, likely predominantly from upwelling sediments and secondarily from ice-melting waters, which does not limit the growth of the phytoplankton.

Effect of micronutrient iron on bioactive compounds isolated from cryptophytes

Iron is one of the important micronutrients affecting algal growth due to its fundamental role in the physiological processes, including photosynthetic electron transport, respiration, and nitrogen fixation. In this study, the effect of different iron levels on growth and the production of bioactive compounds (phycoerythrin (PE), extracellular polymeric substances (EPS), and phenolic compounds (PCs)) of five cryptophyte strains were investigated. Also, the antioxidant capacity of the bioactive compounds was explored. The results showed species-specific responses to the impact of iron on growth of cryptophytes and accumulation of bioactive compounds. The growth rates of C. pyrenoidifera and Cryptomonas sp. varied significantly at different iron levels, and a reduction in the PE content was observed for several cryptophytes cultured at the highest iron level. However, no significant differences were detected in EPS content at different iron levels. Differences in PC contents of C. pyrenoidifera and Cryptomonas sp. at medium iron level were statistically significant compared with the other two treatments. The results also revealed species-specific differences in antioxidant activity at different iron levels; each studied strain followed its own pattern in response to change in iron level, and each bioactive compound had a different antioxidant activity. Overall, however, PCs demonstrated higher antioxidant activity than PE and EPS. In summary, iron has an impact on growth, bioactive compound accumulation, and antioxidant activity. However, the species-specific responses to changes in iron level should not be ignored when modifying culture conditions for optimal harvest of bioactive compounds.

Variability in Macro- and Micronutrients of 15 Rarely Researched Microalgae

Microalgae have enormous potential for human nutrition, yet the European Commission has authorized the consumption of only eleven species. Strains of fifteen rarely researched microalgae from two kingdoms were screened regarding their nutritional profile and value for human health in two cultivation phases. Contents of protein, fiber, lipids, fatty acids, minerals, trace elements and heavy metals were determined. In the growth phase, microalgae accumulated more arginine, histidine, ornithine, pure and crude protein, Mg, Mn, Fe and Zn and less Ni, Mo and I₂ compared to the stationary phase. Higher contents of total fat, C14:0, C14:1_n5, C16:1_n7, C20:4_n6, C20:5_n3 and also As were observed in microalgae from the chromista kingdom in comparison to microalgae from the plantae kingdom (p < 0.05). Conversely, the latter had higher contents of C20:0, C20:1_n9 and C18:3_n3 as well as Ca and Pb (p < 0.05). More precisely, Chrysotila carterae appeared to have great potential for human nutrition because of its high nutrient contents such as fibers, carotenoids, C20:6_n3, Mg, Ca, Mn, Fe, Se, Zn, Ni, Mo and I₂. In summary, microalgae may contribute to a large variety of nutrients, yet the contents differ between kingdoms, cultivation phases and also species.

The contribution of penguin guano to the Southern Ocean iron pool

Iron plays a crucial role in the high-nutrient, low-chlorophyll Southern Ocean regions, promoting phytoplankton growth and enhancing atmospheric carbon sequestration. In this area, iron-rich Antarctic krill (Euphausia superba) and baleen whale species, which are among their main predators, play a large role in the recycling of iron. However, penguins have received limited attention despite their representing the largest seabird biomass in the southern polar region. Here, we use breeding site guano volumes estimated from drone images, deep learning-powered penguin census, and guano chemical composition to assess the iron export to the Antarctic waters from one of the most abundant penguin species, the Chinstrap penguin (Pygoscelis antarcticus). Our results show that these seabirds are a relevant contributor to the iron remobilization pool in the Southern Ocean. With an average guano concentration of 3 mg iron g^-1, we estimate that the Chinstrap penguin population is recycling 521 tonnes iron yr^-1, representing the current iron contribution half of the amount these penguins were able to recycle four decades ago, as they have declined by more than 50% since then.

The trace metal economy of the coral holobiont: supplies, demands and exchanges

The juxtaposition of highly productive coral reef ecosystems in oligotrophic waters has spurred substantial interest and progress in our understanding of macronutrient uptake, exchange, and recycling among coral holobiont partners (host coral, dinoflagellate endosymbiont, endolithic algae, fungi, viruses, bacterial communities). By contrast, the contribution of trace metals to the physiological performance of the coral holobiont and, in turn, the functional ecology of reef-building corals remains unclear. The coral holobiont's trace metal economy is a network of supply, demand, and exchanges upheld by cross-kingdom symbiotic partnerships. Each partner has unique trace metal requirements that are central to their biochemical functions and the metabolic stability of the holobiont. Organismal homeostasis and the exchanges among partners determine the ability of the coral holobiont to adjust to fluctuating trace metal supplies in heterogeneous reef environments. This review details the requirements for trace metals in core biological processes and describes how metal exchanges among holobiont partners are key to sustaining complex nutritional symbioses in oligotrophic environments. Specifically, we discuss how trace metals contribute to partner compatibility, ability to cope with stress, and thereby to organismal fitness and distribution. Beyond holobiont trace metal cycling, we outline how the dynamic nature of the availability of environmental trace metal supplies can be influenced by a variability of abiotic factors (e.g. temperature, light, pH, etc.). Climate change will have profound consequences on the availability of trace metals and further intensify the myriad stressors that influence coral survival. Lastly, we suggest future research directions necessary for understanding the impacts of trace metals on the coral holobiont symbioses spanning subcellular to organismal levels, which will inform nutrient cycling in coral ecosystems more broadly. Collectively, this cross-scale elucidation of the role of trace metals for the coral holobiont will allow us to improve forecasts of future coral reef function.

Macronutrients, iron and humic substances summer cycling over the extended continental shelf of the South Brazil Bight

We surveyed macronutrients and dissolved iron (DFe) concentrations and speciation in a transect over the shelf of the South Brazil Bight (SBB) at Santa Marta Grande Cape (SE Brazil) during a coastal downwelling episode. Driven by dominant NE winds, coastal downwelling is a common feature during the austral summer and force after water convergence, with contribution of internal wave breaking at the shelf edge, upwelling of macronutrients into the nutrient-depleted waters of the southbound Brazil Current at ~100 km from the coastline. As a result, we found a plume of high turbidity that reached the euphotic layer, a deepening of the silicate, nitrate, and phosphate isolines over the shelf and a bulging of the nitrate and phosphate isolines over the shelf edge and the slope. Our first measurements of DFe concentration and speciation in the area revealed that against prior findings in other coastal areas, macronutrients, DFe, and iron ligand cycles were disentangled. Higher DFe concentrations were often found at the surface indicating aerial deposition. Secondary DFe maxima over the sediment-water interface and in the upwelled plume indicated DFe fluxes from the sediment and from resuspended instable colloids. Iron ligand concentrations were higher than DFe concentrations in most stations with a clear land-to-ocean gradient. Subtraction of HS iron ligands revealed that except in upwelled water, the bulk of surface ligands was the result of local biological processes. The analysis of the concentrations of Fe-HS complexes showed that the contribution of HS to DFe was dominant in upwelled waters, significant in waters close to the coast, but nearly negligible in the rest of the studied area. We hypothesize that the injection of iron-humic complexes into the euphotic layer during summer upwelling episodes is the key to understanding the persistent high chlorophyll meanders found over the shelf edge of the SBB coast.

Iron Fertilization Can Enhance the Mass Production of Copepod, Pseudodiaptomus annandalei, for Fish Aquaculture

Copepods are proven nutritious food sources for the mariculture/larviculture industry, however, unreliable methods for mass production of copepods are a major bottleneck. In this study, we modified a previously reported inorganic fertilization method (N: 700 μg L-1 and P: 100 μg L-1) by the addition of iron (Fe: 10 μg L-1, using FeSO4·7H2O) (+Fe treatment) and compared its suitability for copepod culture (Pseudodiaptomus annandalei) to the original method (control). The experiment was conducted outdoors in 1000 L tanks for 15 days. The addition of iron prolonged the growth phase of the phytoplankton and resulted in the production of significantly more small phytoplankton (0.45-20 μm, average 2.01 ± 0.52 vs. 9.03 ± 4.17 µg L-1 in control and +Fe, respectively) and adult copepods (control: 195 ± 35, +Fe: 431 ± 109 ind L-1), whereas copepodid-stage was similar between treatments (control: 511 ± 107 vs. +Fe: 502 ± 68 ind L-1). Although adding iron increased the cost of production by 23% compared to the control, the estimated net profit was 97% greater. We concluded that inorganic fertilization, with the addition of iron (Fe: 10 μg L-1), could be an effective method for the mass production of copepods for larviculture.

Short-term response to iron resupply in an iron-limited open ocean diatom reveals rapid decay of iron-responsive transcripts

In large areas of the ocean, iron concentrations are insufficient to promote phytoplankton growth. Numerous studies have been conducted to characterize the effect of iron on algae and how algae cope with fluctuating iron concentrations. Fertilization experiments in low-iron areas resulted primarily in diatom-dominated algal blooms, leading to laboratory studies on diatoms comparing low- and high-iron conditions. Here, we focus on the short-term temporal response following iron addition to an iron-starved open ocean diatom, Thalassiosira oceanica. We employed the NanoString platform and analyzed a high-resolution time series on 54 transcripts encoding proteins involved in photosynthesis, N-linked glycosylation, iron transport, as well as transcription factors. Nine transcripts were iron-responsive, with an immediate response to the addition of iron. The fastest response observed was the decrease in transcript levels of proteins involved in iron uptake, followed by an increase in transcript levels of iron-containing enzymes and a simultaneous decrease in the transcript levels of their iron-free replacement enzymes. The transcription inhibitor actinomycin D was used to understand the underlying mechanisms of the decrease of the iron-responsive transcripts and to determine their half-lives. Here, Mn-superoxide dismutase (Mn-SOD), plastocyanin (PETE), ferredoxin (PETF) and cellular repressor of EA1-stimulated genes (CREGx2) revealed longer than average half-lives. Four iron-responsive transcripts showed statistically significant differences in their decay rates between the iron-recovery samples and the actD treatment. These differences suggest regulatory mechanisms influencing gene transcription and mRNA stability. Overall, our study contributes towards a detailed understanding of diatom cell biology in the context of iron fertilization response and provides important observations to assess oceanic diatom responses following sudden changes in iron concentrations.

Nitrogen fertilization coupled with iron foliar application improves the photosynthetic characteristics, photosynthetic nitrogen use efficiency, and the related enzymes of maize crops under different planting patterns

Photosynthetic rate (Pn) and photosynthetic nitrogen use efficiency (PNUE) are the two important factors affecting the photosynthesis and nutrient utilization of plant leaves. However, the effect of N fertilization combined with foliar application of Fe on the Pn and PNUE of the maize crops under different planting patterns (i.e., monocropping and intercropping) is elusive. Therefore, this experiment was conducted to determine the effect of N fertilization combined with foliar application of Fe on the photosynthetic characteristics, PNUE, and the associated enzymes of the maize crops under different planting patterns. The results of this study showed that under intercropping, maize treated with N fertilizer combined with foliar application of Fe had not only significantly (p < 0.05) improved physio-agronomic indices but also higher chlorophyll content, better photosynthetic characteristics, and related leaf traits. In addition, the same crops under such treatments had increased photosynthetic enzyme activity (i.e., rubisco activity) and nitrogen metabolism enzymes activities, such as nitrate reductase (NR activity), nitrite reductase (NiR activity), and glutamate synthase (GOGAT activity). Consequently, intercropping enhanced the PNUE and soluble sugar content of the maize crops, thus increasing its yield compared with monocropping. Thus, these findings suggest that intercropping under optimal N fertilizer application combined with Fe foliation can improve the chlorophyll content and photosynthetic characteristics of maize crops by regulating the associated enzymatic activities. Consequently, this results in enhanced PNUE, which eventually leads to better growth and higher yield in the intercropping system. Thus, practicing intercropping under optimal nutrient management (i.e., N and Fe) could be crucial for better growth and yield, and efficient nitrogen use efficiency of maize crops.

Textile Dye Removal by Acacia dealbata Link. Pollen Adsorption Combined with UV-A/NTA/Fenton Process

The decolourization of an aqueous solution of the textile dye Acid Red 88 (AR88) and the control of the invasive plant species Acacia dealbata Link. (ADL) were addressed in this work. The aims of the study were (1) characterization of the ADL pollen, (2) application of the pollen powder in adsorption processes, (3) selection of the best operational conditions for nitriloacetic acid (NTA)-UV-A-Fenton process and (4) assess the efficiency of the combined treatment adsorption and NTA-UV-A-Fenton in AR88 decolourization. In a first step, ADL pollen was used as a AR88 bioadsorbent. Fourier-transform infrared spectroscopy (FTIR) analysis were performed and revealed the presence of proteins, fatty acids, carbohydrates and lignin in the pollen. Afterwards, trough scanning electron microscopy (SEM), it was possible to verify that ADL pollen has several empty spaces that can be used for dye adsorption. Biosorption results showed higher adsorption of AR88 with application of pH 3.0 and [pollen] = 3.0 g/L with 18.8 mg/g of dye adsorbed. The best fitting was observed with Langmuir, SIPS and Jovanovic isotherms (0.993, 0.996 and 0.994, respectively). To complement the biosorption, a UV-A-Fenton process was applied, and results showed a higher AR88 removal with (NTA) addition. Higher irradiance power favored the oxidation process with high Ф photodegradation value and low Electric Energy per Order ($$E_{\text{EO}}$$ E EO ) and Specific Applied Energy ($$E_{\text{SAE}}$$ E SAE ). The combination of biosorption with NTA-UV-A-Fenton was the most efficient system with an AR88 decolourization of 98.5% and a total organic carbon (TOC) removal of 83.5%.

Graphical Abstract

Bioavailable iron titrations reveal oceanic Synechococcus ecotypes optimized for different iron availabilities

The trace metal iron (Fe) controls the diversity and activity of phytoplankton across the surface oceans, a paradigm established through decades of in situ and mesocosm experimental studies. Despite widespread Fe-limitation within high-nutrient, low chlorophyll (HNLC) waters, significant contributions of the cyanobacterium Synechococcus to the phytoplankton stock can be found. Correlations among differing strains of Synechococcus across different Fe-regimes have suggested the existence of Fe-adapted ecotypes. However, experimental evidence of high- versus low-Fe adapted strains of Synechococcus is lacking, and so we investigated the transcriptional responses of microbial communities inhabiting the HNLC, sub-Antarctic region of the Southern Ocean during the Spring of 2018. Analysis of metatranscriptomes generated from on-deck incubation experiments reflecting a gradient of Fe-availabilities reveal transcriptomic signatures indicative of co-occurring Synechococcus ecotypes adapted to differing Fe-regimes. Functional analyses comparing low-Fe and high-Fe conditions point to various Fe-acquisition mechanisms that may allow persistence of low-Fe adapted Synechococcus under Fe-limitation. Comparison of in situ surface conditions to the Fe-titrations indicate ecological relevance of these mechanisms as well as persistence of both putative ecotypes within this region. This Fe-titration approach, combined with transcriptomics, highlights the short-term responses of the in situ phytoplankton community to Fe-availability that are often overlooked by examining genomic content or bulk physiological responses alone. These findings expand our knowledge about how phytoplankton in HNLC Southern Ocean waters adapt and respond to changing Fe supply.

Micronutrient content drives elementome variability amongst the Symbiodiniaceae

BACKGROUND

Elements are the basis of life on Earth, whereby organisms are essentially evolved chemical substances that dynamically interact with each other and their environment. Determining species elemental quotas (their elementome) is a key indicator for their success across environments with different resource availabilities. Elementomes remain undescribed for functionally diverse dinoflagellates within the family Symbiodiniaceae that includes coral endosymbionts. We used dry combustion and ICP-MS to assess whether Symbiodiniaceae (ten isolates spanning five genera Breviolum, Cladocopium, Durusdinium, Effrenium, Symbiodinium) maintained under long-term nutrient replete conditions have unique elementomes (six key macronutrients and nine micronutrients) that would reflect evolutionarily conserved preferential elemental acquisition. For three isolates we assessed how elevated temperature impacted their elementomes. Further, we tested whether Symbiodiniaceae conform to common stoichiometric hypotheses (e.g., the growth rate hypothesis) documented in other marine algae. This study considers whether Symbiodiniaceae isolates possess unique elementomes reflective of their natural ecologies, evolutionary histories, and resistance to environmental change.

RESULTS

Symbiodiniaceae isolates maintained under long-term luxury uptake conditions, all exhibited highly divergent elementomes from one another, driven primarily by differential content of micronutrients. All N:P and C:P ratios were below the Redfield ratio values, whereas C:N was close to the Redfield value. Elevated temperature resulted in a more homogenised elementome across isolates. The Family-level elementome was (C19.8N2.6 P1.0S18.8K0.7Ca0.1) · 1000 (Fe55.7Mn5.6Sr2.3Zn0.8Ni0.5Se0.3Cu0.2Mo0.1V0.04) mmol Phosphorous-1 versus (C25.4N3.1P1.0S23.1K0.9Ca0.4) · 1000 (Fe66.7Mn6.3Sr7.2Zn0.8Ni0.4Se0.2Cu0.2Mo0.2V0.05) mmol Phosphorous -1 at 27.4 ± 0.4 °C and 30.7 ± 0.01 °C, respectively. Symbiodiniaceae isolates tested here conformed to some, but not all, stoichiometric principles.

CONCLUSIONS

Elementomes for Symbiodiniaceae diverge from those reported for other marine algae, primarily via lower C:N:P and different micronutrient expressions. Long-term maintenance of Symbiodiniaceae isolates in culture under common nutrient replete conditions suggests isolates have evolutionary conserved preferential uptake for certain elements that allows these unique elementomes to be identified. Micronutrient content (normalised to phosphorous) commonly increased in the Symbiodiniaceae isolates in response to elevated temperature, potentially indicating a common elemental signature to warming.

Biodiversity and Biogeography of Abundant and Rare Microbial Assemblages in the Western Subtropical Pacific Ocean

The levels of chlorophyll a and nutrient concentrations in the surface waters of the western subtropical Pacific Ocean are among the lowest globally. In addition, our knowledge of basin-scale diversity and biogeography of microbial communities in this vast extremely oligotrophic environment is still rather limited. Here, high-throughput sequencing was used to examine the biodiversity and biogeography of abundant and rare microbial assemblages throughout the water column from the surface to a depth of 3,000 m across a horizontal distance of 1,100 km in the western Pacific Ocean. Microbial alpha diversity in the 200-m layer was higher than at other depths, with Gammaproteobacteria, Alphaproteobacteria, and Clostridia as the dominant classes in all samples. Distinctly vertical distributions within the microbial communities were revealed, with no difference horizontally. Some microbes exhibited depth stratification. For example, the relative abundances of Cyanobacteria and Alphaproteobacteria decreased with depth, while Nitrososphaeria, Actinobacteria, and Gammaproteobacteria increased with depth in the aphotic layers. Furthermore, we found that environmental (selective process) and spatial (neutral process) factors had different effects on abundant and rare taxa. Geographical distance showed little effect on the dispersal of all and abundant taxa, while statistically significant distance-decay relationships were observed among the rare taxa. Temperature and chlorophyll a were strongly associated with all, abundant, and rare taxa in the photic layers, while total inorganic nitrogen was recognized as the crucial factor in the aphotic layers. Variance partitioning analysis indicated that environmental selection played a relatively important role in shaping all and abundant taxa, while the variation in rare taxa explained by environmental and spatial processes was relatively low, as more than 70% of the variation remained unexplained. This study provides novel knowledge related to microbial community diversity in the western subtropical Pacific Ocean, and the analyzes biogeographical patterns among abundant and rare taxa.

Iron and manganese co-limit the growth of two phytoplankton groups dominant at two locations of the Drake Passage

While it has been recently demonstrated that both iron (Fe) and manganese (Mn) control Southern Ocean (SO) plankton biomass, how in particular Mn governs phytoplankton species composition remains yet unclear. This study, for the first time, highlights the importance of Mn next to Fe for growth of two key SO phytoplankton groups at two locations in the Drake Passage (West and East). Even though the bulk parameter chlorophyll a indicated Fe availability as main driver of both phytoplankton assemblages, the flow cytometric and microscopic analysis revealed FeMn co-limitation of a key phytoplankton group at each location: at West the dominant diatom Fragilariopsis and one subgroup of picoeukaryotes, which numerically dominated the East community. Hence, the limitation by both Fe and Mn and their divergent requirements among phytoplankton species and groups can be a key factor for shaping SO phytoplankton community structure.

Iron and manganese play an important role in phytoplankton biomass control, but the exact effect of these elements on species composition has remained unknown. Conducting phytoplankton incubation experiments at two Drake Passage sites, we demonstrate how iron and manganese regulate phytoplankton community structure.

Dynamics of trace metals with different size species in the Pearl River Estuary, Southern China

The Pearl River Estuary (PRE), the largest estuary in Southern China, regulates the fluxes of riverine trace metals into the South China Sea. However, the geochemical behavior of trace metals in this estuary is still ambiguous. In this study, we investigated the dynamics of trace metals in different phases (i.e., particulate, colloidal and truly dissolved) in the PRE in both wet and dry seasons characteristic of the region. Transformations of trace metals between particulate (>0.45 μm), colloidal (1 kDa to 0.45 μm) and truly dissolved (<1 kDa) phases were observed during the estuarine mixing. Colloidal metals (except for Pb) showed non-conservative 'removal' behavior in the low-salinity zone (S < 10‰), suggesting the coagulation of colloidal metals and subsequent deposition to bed sediments being an important sink of dissolved trace metals. By contrast, truly dissolved metals exhibited a mid-salinity maximum distribution (i.e., Mn, Ni, Cu and Cd) or little variation along the salinity gradient (i.e., Fe). The increase of truly dissolved metal concentration was accompanied by the decrease of particulate metal concentration, indicating that the desorption of suspended particles was an important source of dissolved Mn and Cd in the PRE. Metal released from the suspended particles increased in the dry season due to the high suspended particulate matter concentration. Dissolved Mn concentration in the bottom water in wet season was higher than that in dry season, implying that benthic Mn input increased as the bottom water became hypoxic. Abnormally high concentrations of particulate and dissolved Pb were observed at the lower PRE, implying the presence of a potential point source pollution. A flux model predicted that total dissolved Fe, Ni, Cu and Zn underwent net removal while dissolved Mn and Cd had net inputs during the estuarine mixing. This study raveled contrasting geochemical behaviors of trace metals with different size phases and the different sources and sinks of dissolved metals in the PRE.

Multi-Element Composition of Diatom Chaetoceros spp. from Natural Phytoplankton Assemblages of the Russian Arctic Seas

Simple Summary

Despite the long history of studying the elemental composition of phytoplankton and its individual ecological and systematic groups or specific algae species, the global dataset is far from completed. Our original research aims to study the elemental composition of a certain taxonomic group of marine diatoms, whose representatives make a significant contribution to primary production in the Arctic Ocean. The data on the chemical composition of diatom microalgae are discussed concerning their role in the global biogeochemical circulation of elements in the ocean. In particular, the obtained data make a prominent input to the study of the multi-element composition of marine diatom species, namely Chaetoceros spp., inhabiting the shelf seas of the Arctic Ocean. These data may be used as a basis for the cultivation of marine diatom strains for obtaining commercially promising producers of biogenic silica or valuable biological products that can be used as raw materials in the production of feed and nutrition for agriculture and aquaculture.

Abstract

Data on the elemental composition of the diatom Chaetoceros spp. from natural phytoplankton communities of Arctic marine ecosystems are presented for the first time. Samples were collected during the 69th cruise (22 August–26 September 2017) of the R/V Akademik Mstislav Keldysh in the Kara, Laptev, and East Siberian Seas. The multi-element composition of the diatom microalgae was studied by ICP-AES and ICP-MS methods. The contents of major (Na, Mg, Al, Si, P, S, K and Ca), trace (Li, Be, B, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Rb, Sr, Mo, Ag, Cd, Sn, Sb, Cs, Ba, Hg, Tl, Pb, Bi, Th and U) and rare earth (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) elements varied greatly, which was probably associated with the peculiarities of the functional state and mineral nutrition of phytoplankton in the autumn period. Biogenic silicon was the dominant component of the chemical composition of Chaetoceros spp., averaging 19.10 ± 0.58% of dry weight (DW). Other significant macronutrients were alkaline (Na and K) and alkaline earth (Ca and Mg) metals as well as biogenic (S and P) and essential (Al and Fe) elements. Their total contents varied from 1.26 to 2.72% DW, averaging 2.07 ± 0.43% DW. The Al:Si ratio for natural assemblages of Chaetoceros spp. of the shelf seas of the Arctic Ocean was 5.8 × 10⁻³. The total concentrations of trace and rare earth elements on average were 654.42 ± 120.07 and 4.14 ± 1.37 μg g⁻¹ DW, respectively. We summarize the scarce data on the average chemical composition of marine and oceanic phytoplankton and discuss the limitations and approaches of such studies. We conclude on the lack of data and the need for further targeted studies on this issue.

Dissolved iron and isotopic geochemical characteristics in a typical tropical river across the floodplain: The potential environmental implication

Iron (Fe) is an essential element for bio-physiological functioning terrestrial organisms, in particular of aquatic organisms. It is therefore crucial to understand the aquatic iron cycle and geochemical characteristics, which is also significant to obtain the key information on earth-surface evolution. The stable iron isotopic composition (δ⁵⁶Fe) of the dissolved fraction is determined in the Mun River (main tributary of Mekong River), northeast Thailand to distinguish the human and nature influenced riverine iron geochemical behavior. The results show that dissolved Fe concentration ranges from 8.04 to 135.27 μg/L, and the δ⁵⁶Fe ranges from -1.34‰ to 0.48‰, with an average of 0.23‰, 0.14‰ and -0.15‰ in the upper, middle and lower reaches, respectively. The δ⁵⁶Fe values of river water are close to that of the bulk continental crust and other tropical rivers. The correlations between δ⁵⁶Fe and Fe, Al, and physicochemical parameters show mixing processes of different Fe end-members, including the rock weathering end-member (low Fe/Al ratio and high δ⁵⁶Fe), the urban activities end-member (high Fe/Al ratio and moderate δ⁵⁶Fe), and a third end-member with probable sources from the Chi River and reservoir. For the most river water samples, the primary contribution is attributed to rock weathering, and the second is urban activities (only a few samples are from the upper and middle reaches). Thus, Fe isotopes could be employed as a proxy to identify and quantify the natural and anthropogenic contributions, respectively. These findings also provide data support for the scientific management of water resources in the Mun River catchment and other large tropical rivers.

High Iron Requirements for Growth in the Nuisance Alga Gonyostomum semen (Raphidophyceae)

The bloom-forming freshwater alga Gonyostomum semen is associated with acidic, mesotrophic brown water lakes in boreal regions. However, researchers have been unable to conclusively link G. semen abundance and bloom formation to typical brown water lake traits, that is, high water color and DOC (dissolved organic carbon) concentrations. Iron is a main driver of water color in boreal lakes, and a recent study of lake monitoring data indicated a connection between lakes with high G. semen abundance and iron concentrations >200 µg · L^-1 . Thus, iron may be the missing link in explaining G. semen abundance and growth dynamics. We experimentally assessed the effects of different iron concentrations above or below 200 µg · L^-1 on the growth of G. semen batch monocultures. Iron concentrations <200 µg · L^-1 limited G. semen growth, while iron concentrations >200 µg · L^-1 did not. Moreover, the iron concentration of the medium required for growth was higher than for other common phytoplankton (Microcystis botrys and Chlamydomonas sp.) included in the experiment. These results indicate that G. semen requires high levels of iron in the lake environment. Consequently, this and previous findings using lake monitoring data support the hypothesis that high concentrations of iron favor the formation of high-density G. semen blooms in boreal brown water lakes. As lakes get browner in a changing climate, monitoring iron levels could be a potential tool to identify lakes at risk for G. semen blooms, especially among lakes that provide ecosystem services to society.

Semi-Quantitative Analysis of Major Elements and Minerals: Clues from a Late Pleistocene Core from Campos Basin

Element and mineral associations are fundamental parameters for palaeoceanographical reconstructions but laboratory methodologies are expensive, time-consuming and need a lot of material. Here, we investigate the quality and reliability of XRF measurements of major elements (Fe, Ti and Ca) using BTX II Benchtop, by comparing them with previous ICP-OES elemental analysis for a set of Late Pleistocene marine sediments from Campos Basin. Although the numerical values of the logarithmic form of the elementary ratios were different, the lnTi/Ca and lnFe/Ca ratios measured by both techniques (XRF and ICP-OES) presented similar downcore results. To correct the XRF intensity data, a linear regression model was calculated and, based on the linear equation generated, the logarithmic values of the elementary XRF ratios were corrected. After the correction, One-Sample t-test and Bland–Altman plot show that both techniques obtained similar results. In addition, a brief paleoceanographic interpretation, during the MIS 5 and MIS 4 periods, was conducted by comparing mineralogical and elementary analysis aiming to reconstruct the variations of the terrigenous input to the studied area. As a conclusion, the results from XRF measurements (BTX II) presented to confirm the viability of such a technique, showing that analysis using BTX II is a reliable, cheap, rapid and non-destructive option for obtaining elementary ratios and mineralogical downcore results at high resolution, allowing stratigraphic and paleoceanographic interpretations.

Reciprocal Effect of Copper and Iron Regulation on the Proteome of Synechocystis sp. PCC 6803

Cyanobacteria can acclimate to changing copper and iron concentrations in the environment via metal homeostasis, but a general mechanism for interpreting their dynamic relationships is sparse. In this study, we assessed growth and chlorophyll fluorescence of Synechocystis sp. PCC 6803 and investigated proteomic responses to copper and iron deductions. Results showed that copper and iron exerted reciprocal effect on the growth and photosynthesis of Synechocystis sp. PCC 6803 at combinations of different concentrations. And some proteins involved in the uptake of copper and iron and the photosynthetic electron transport system exhibit Cu-Fe proteomic association. The protein abundance under copper and iron deduction affected the photosynthetic electronic activity of Synechocystis sp. PCC 6803 and eventually affected the growth and photosynthesis. Based on these results, we hypothesize that the Cu-Fe proteomic association of Synechocystis sp. PCC 6803 can be elucidated via the uptake system of outer membrane-periplasmic space-inner plasma membrane-thylakoid membrane, and this association is mainly required to maintain electron transfer. This study provides a broader view regarding the proteomic association between Cu and Fe in cyanobacteria, which will shed light on the role of these two metal elements in cyanobacterial energy metabolism and biomass accumulation.

Iron availability is a key factor for freshwater cyanobacterial survival against saline stress

Estuaries are among the most productive ecosystems and dynamic environments on Earth. Varying salinity is the most important challenge for phytoplankton survival in estuaries. In order to investigate the role of iron nutrition on phytoplankton survival under salinity stress, a freshwater cyanobacterial strain was cultivated in media added with different proportions of seawater (measured with siderophore activities), and supplied with gel-immobilized ferrihydrite as iron source. Results showed that the strain grew well in media with 0% seawater supplied with ferrihydrite as iron source. Surprisingly, the biomasses in media with 50% seawater, with more newly excreted siderophore, were similar to those with 0% seawater, but better than those with 6.25%, 12.5% and 25% seawater. Smaller iron isotopic discriminations between the cyanobacterial cells associated iron and dissolved iron were observed in media with 0% and 50% seawater suggested that higher fractions of iron uptake from aqueous dissolved iron reservoir by these comparatively larger biomasses. In summary, this study proved that iron availability plays a key role in cyanobacterial survival under varying salinity stress, and suggested that siderophores introduced by seawater may accelerate iron dissolution, increase iron availability, and make cyanobacterial cells overcome the adverse effects of high-salinity, and indicated that siderophore excretion is a kind of survival strategy for phytoplankton in face of salinity stress.

Photosynthetic adaptation to light availability shapes the ecological success of bloom-forming cyanobacterium Pseudanabaena to iron limitation

The poorly understood filamentous cyanobacterium Pseudanabaena is commonly epiphytic on Microcystis colonies and their abundances are often highly correlated during blooms. The response and adaptation of Microcystis to iron limitation have been extensively studied, but the strategies Pseudanabaena uses to respond to iron limitation are largely unknown. Here, physiological responses to iron limitation were compared between one Pseudanabaena and two Microcystis strains grown under different light intensities. The results showed that low-intensity light exacerbated, but high-intensity light alleviated, the negative effect of iron limitation on Pseudanabaena growth relative to two Microcystis strains. It was found that robust light-harvesting and photosynthetic efficiency allowed adaptation of Pseudanabaena to low light availability relative to two Microcystis strains only during iron sufficiency. The results also indicated that a larger investment in the photosynthetic antenna probably contributed to light/iron co-limitation of Pseudanabaena relative to two Microcystis strains under both light and iron limitation. Furthermore, the lower antenna pigments/chlorophyll a ratio and photosynthetic efficiency, and higher nonphotochemical quenching and saturation irradiance provided Pseudanabaena photoadaptation and photoprotection advantages over the two Microcystis strains under the high-light condition. The lower investment in antenna pigments of Pseudanabaena than the two Microcystis strains under high-light intensity is likely an efficient strategy for both saving iron quotas and decreasing photosensitivity. Therefore, when compared with Microcystis, the high plasticity of antenna pigments, along with the excellent photoadaptation and photoprotection ability of Pseudanabaena, probably ensures its ecological success under iron limitation when light is sufficient.

Assessment and mechanisms of microalgae growth inhibition by phosphonates: Effects of intrinsic toxicity and complexation

The effects of phosphonates, the heavily-used antiscalants in reverse osmosis systems, on microalgae are controversial, although they are harmless to most aquatic organisms. Herein, we assessed the inhibitory effects of etidronic acid (HEDP) and diethylenetriamine penta(methylene phosphonic acid) (DTPMP) on algal growth and revealed the mechanisms involved in both intrinsic toxicity and complexation. The phosphonates showed weak influences on Scenedesmus sp. LX1 in the first 4 d of cultivation. In contrast, a significant growth inhibition was observed subsequently with half maximal effective concentrations of 57.6 and 35.7 mg/L for HEDP and DTPMP, respectively, at 10 d. The phosphonates had little effect on cellular energy transfer and oxidative stress, quantified by adenosine triphosphate level and superoxide dismutase activity, respectively, demonstrating weak intrinsic toxicities to algal cells. Phosphonates blocked the algal assimilation of iron ions through complexation. Severe iron deficiency limited photosynthetic activity and caused chlorophyll decline, resulting in a functional loss of the photosystem followed by complete algal growth inhibition at the late cultivation stage. Our findings point to a potential ecological impact wherein harmful algal blooms are induced by the natural degradation of phosphonates due to the release of both iron and phosphate ions that stimulate algal regrowth after disinhibition.

Seasonal modulation of phytoplankton biomass in the Southern Ocean

Over the last ten years, satellite and geographically constrained in situ observations largely focused on the northern hemisphere have suggested that annual phytoplankton biomass cycles cannot be fully understood from environmental properties controlling phytoplankton division rates (e.g., nutrients and light), as they omit the role of ecological and environmental loss processes (e.g., grazing, viruses, sinking). Here, we use multi-year observations from a very large array of robotic drifting floats in the Southern Ocean to determine key factors governing phytoplankton biomass dynamics over the annual cycle. Our analysis reveals seasonal phytoplankton accumulation ('blooming') events occurring during periods of declining modeled division rates, an observation that highlights the importance of loss processes in dictating the evolution of the seasonal cycle in biomass. In the open Southern Ocean, the spring bloom magnitude is found to be greatest in areas with high dissolved iron concentrations, consistent with iron being a well-established primary limiting nutrient in this region. Under ice observations show that biomass starts increasing in early winter, well before sea ice begins to retreat. The average theoretical sensitivity of the Southern Ocean to potential changes in seasonal nutrient and light availability suggests that a 10% change in phytoplankton division rate may be associated with a 50% reduction in mean bloom magnitude and annual primary productivity, assuming simple changes in the seasonal magnitude of phytoplankton division rates. Overall, our results highlight the importance of quantifying and accounting for both division and loss processes when modeling future changes in phytoplankton biomass cycles.

Potential utility of reflectance spectroscopy in understanding the paleoecology and depositional history of different fossils

The potential of reflectance spectroscopy to infer the paleoecological and depositional evolution of different micro and macro invertebrate fossils has been evaluated by analyzing their reflectance spectra within the spectral domain of 350–2500 nm using the FIELDSPEC3 spectroradiometer. Mineralogical information derived from the rapid and non-destructive spectral analysis has been substantiated using concurrent mineralogical data from conventional geochemical analyses. The diagnostic Fe-crystal field effect induced spectral features are identified on the representative spectra of different benthic foraminifera. These spectral features are resulted due to the incorporation of Fe during the biomineralization process. These features are absent in planktic foraminifera. The encrustation of Fe-oxides is inferred to be responsible for imprinting the Fe-crystal field feature in the spectra of micro and macrofossils at 900–1200 nm. Vibrational spectral features of the Al–OH bond are also identified. Both of these features are an indicator of post-depositional diagenetic history. The presence of Al and Fe in macrofossil shells is also believed to be related to ecological conditions as these elements are biogenically incorporated during shell formation. This study reveals the value of reflectance spectroscopy to infer ecological behavior and post-depositional environment of different organisms.

Effects of Different Nutrient and Trace Metal Concentrations on Growth of the Toxic Dinoflagellate Gymnodinium catenatum Isolated from Korean Coastal Waters

The effects of the addition of nutrients (nitrate: N; phosphate: P; and vitamin B1) and trace metals (iron: Fe; Copper: Cu; and selenium: Se) on the growth of Gymnodinium catenatum, which was isolated from Korean coastal waters, were investigated. The Korean isolate of G. catenatum grew under a wide range of concentrations of N and P. Whilst high concentrations of N (> N: P ratio of 23.5) did not stimulate the growth rate, an enhanced growth rate and cell density were observed with the addition of P. The experimental addition of vitamin B1 revealed that G. catenatum is not dependent on vitamin B1 for growth. Moreover, the addition of Fe and Cu resulted in no significant differences in the growth patterns and rates of G. catenatum between the controls and treatments. It is thus possible that growth of the Korean isolate of G. catenatum does not require high concentrations of Fe and Cu. However, the cell densities were enhanced in the stationary phases of treatments upon addition of Se, and the maximum cell densities were higher than those in the culture experiments upon additions of other nutrient and trace metals. Our findings indicate that G. catenatum prefers P and Se for proliferation, rather than other nutritional sources.

Heme b distributions through the Atlantic Ocean: evidence for "anemic" phytoplankton populations

Heme b is an iron-containing cofactor in hemoproteins that participates in the fundamental processes of photosynthesis and respiration in phytoplankton. Heme b concentrations typically decline in waters with low iron concentrations but due to lack of field data, the distribution of heme b in particulate material in the ocean is poorly constrained. Here we report particulate heme b distributions across the Atlantic Ocean (59.9°N to 34.6°S). Heme b concentrations in surface waters ranged from 0.10 to 33.7 pmol L^-1 (median = 1.47 pmol L^-1, n = 974) and were highest in regions with a high biomass. The ratio of heme b to particulate organic carbon (POC) exhibited a mean value of 0.44 μmol heme b mol^-1 POC. We identified the ratio of 0.10 µmol heme b mol^-1 POC as the cut-off between heme b replete and heme b deficient (anemic) phytoplankton. By this definition, we observed anemic phytoplankton populations in the Subtropical South Atlantic and Irminger Basin. Comparison of observed and modelled heme b suggested that heme b could account for between 0.17-9.1% of biogenic iron. Our large scale observations of heme b relative to organic matter provide further evidence of the impact of changes in iron supply on phytoplankton iron status across the Atlantic Ocean.

Iron and manganese co-limit growth of the Southern Ocean diatom Chaetoceros debilis

In some parts of the Southern Ocean (SO), even though low surface concentrations of iron (Fe) and manganese (Mn) indicate FeMn co-limitation, we still lack an understanding on how Mn and Fe availability influences SO phytoplankton ecophysiology. Therefore, this study investigated the effects of Fe and Mn limitation alone as well as their combination on growth, photophysiology and particulate organic carbon production of the bloom-forming Antarctic diatom Chaetoceros debilis. Our results clearly show that growth, photochemical efficiency and carbon production of C. debilis were co-limited by Fe and Mn as highest values were only reached when both nutrients were provided. Even though Mn-deficient cells had higher photochemical efficiencies than Fe-limited ones, they, however, displayed similar low growth and POC production rates, indicating that Mn limitation alone drastically impeded the cell’s performance. These results demonstrate that similar to low Fe concentrations, low Mn availability inhibits growth and carbon production of C. debilis. As a result from different species-specific trace metal requirements, SO phytoplankton species distribution and productivity may therefore not solely depend on the input of Fe alone, but also critically on Mn acting together as important drivers of SO phytoplankton ecology and biogeochemistry.

Regulation of the Phytoplankton Heme b Iron Pool During the North Atlantic Spring Bloom

Heme b is an iron-containing co-factor in hemoproteins. Heme b concentrations are low (<1 pmol L^-1) in iron limited phytoplankton in cultures and in the field. Here, we determined heme b in marine particulate material (>0.7 μm) from the North Atlantic Ocean (GEOVIDE cruise - GEOTRACES section GA01), which spanned several biogeochemical regimes. We examined the relationship between heme b abundance and the microbial community composition, and its utility for mapping iron limited phytoplankton. Heme b concentrations ranged from 0.16 to 5.1 pmol L^-1 (median = 2.0 pmol L^-1, n = 62) in the surface mixed layer (SML) along the cruise track, driven mainly by variability in biomass. However, in the Irminger Basin, the lowest heme b levels (SML: median = 0.53 pmol L^-1, n = 12) were observed, whilst the biomass was highest (particulate organic carbon, median = 14.2 μmol L^-1, n = 25; chlorophyll a: median = 2.0 nmol L^-1, n = 23) pointing to regulatory mechanisms of the heme b pool for growth conservation. Dissolved iron (DFe) was not depleted (SML: median = 0.38 nmol L^-1, n = 11) in the Irminger Basin, but large diatoms (Rhizosolenia sp.) dominated. Hence, heme b depletion and regulation is likely to occur during bloom progression when phytoplankton class-dependent absolute iron requirements exceed the available ambient concentration of DFe. Furthermore, high heme b concentrations found in the Iceland Basin and Labrador Sea (median = 3.4 pmol L^-1, n = 20), despite having similar DFe concentrations to the Irminger Basin, were attributed to an earlier growth phase of the extant phytoplankton populations. Thus, heme b provides a snapshot of the cellular activity in situ and could both be used as indicator of iron limitation and contribute to understanding phytoplankton adaptation mechanisms to changing iron supplies.

Towards the development of a new generation of whole-cell bioreporters to sense iron bioavailability in oceanic systems-learning from the case of Synechococcus sp. PCC7002 iron bioreporter

Whole-cell bioreporters are genetically modified micro-organisms designed to sense bioavailable forms of nutrients or toxic compounds in aquatic systems. As they represent the most promising cost-efficient tools available for such purpose, engineering and use of bioreporters is rapidly growing in association with wide applicability. Bioreporters are urgently needed to determine phytoplankton iron (Fe) limitation, which has been reported in up to 30% of the ocean, with consequences affecting Earth's global carbon cycle and climate. This study presents a critical evaluation and optimization of the only Cyanobacteria bioreporter available to sense Fe limitation in marine systems (Synechococcus sp. PCC7002). The nonmonotonic biphasic dose-response curve between the bioreporters' signal and Fe bioavailability impairs an appropriate data interpretation, highlighting the need for new carefully designed bioreporters. Here, limitations under low Fe concentrations were related to cellular energy stress, nonlinear expression of the targeted promoter and siderophore expression. Furthermore, we provide critical standard criteria for the development of new Fe bioreporters. Finally, based on gene expression data under a range of marine Fe concentrations, we propose novel sensor genes for the development of new Cyanobacteria Fe bioreporters for distinct marine regions.

Tracing and constraining anthropogenic aerosol iron fluxes to the North Atlantic Ocean using iron isotopes

Atmospheric dust is an important source of the micronutrient Fe to the oceans. Although relatively insoluble mineral Fe is assumed to be the most important component of dust, a relatively small yet highly soluble anthropogenic component may also be significant. However, quantifying the importance of anthropogenic Fe to the global oceans requires a tracer which can be used to identify and constrain anthropogenic aerosols in situ. Here, we present Fe isotope (δ⁵⁶Fe) data from North Atlantic aerosol samples from the GEOTRACES GA03 section. While soluble aerosol samples collected near the Sahara have near-crustal δ⁵⁶Fe, soluble aerosols from near North America and Europe instead have remarkably fractionated δ⁵⁶Fe values (as light as -1.6‰). Here, we use these observations to fingerprint anthropogenic combustion sources, and to refine aerosol deposition modeling. We show that soluble anthropogenic aerosol Fe flux to the global surface oceans is highly likely to be underestimated, even in the dusty North Atlantic.

The effect of vanadium on essential element uptake of Setaria viridis' seedlings

High concentrations of vanadium, a ubiquitous element in the environment, in growing media leads to deformation of root structure and leaf chlorosis and necrosis, consequently affecting the translocations of nutrients and essential elements. However, the effects of vanadium on essential element uptake, and the interactions of essential elements in the presence of vanadium, remain incompletely understood. To elucidate the effects of different concentrations of vanadium on major and trace essential elements and plant growth, a native plant species growing in a vanadium mining area, Setaria viridis (dog tail's grass), was incubated in solutions containing 0-55.8 mg/L vanadium. The shoot accumulation of four major essential elements and five trace essential elements was detected, and the root length and stem height were measured. The results showed that vanadium in soil solution enhanced the accumulation of all major essential elements in shoot. Vanadium concentrations lower than 47.4 mg/L showed an obvious positive (p < 0.05) effect on P accumulation and translocation. In the case of trace essential elements, there were threshold values for solution vanadium stimulation of element uptake. The threshold values for Cu and Zn, Fe, and Mo uptake were 4.3, 16.3, and 40.6 mg/L, respectively. When vanadium levels surpassed these values, accumulation was suppressed and the solution vanadium concentrations attenuated the solution-to-shoot translocation of most of the essential elements. Among the trace essential elements, translocation of Fe was obviously enhanced (p < 0.05) by vanadium. Solution vanadium also enhanced plant growth at lower concentrations and inhibited it at higher levels. The threshold values for stem height and root length were 36.8 and 16.3 mg/L, respectively. Concentrations of 40 and 55.8 mg/L vanadium in soil solution caused a 50% decrease in root length and stem height, respectively, showing that root length of Setaria viridis is more susceptible to vanadium toxicity than stem growth.

Differences in the optical properties of valve and girdle band in a centric diatom

Diatoms are phototrophic single-celled microalgae encased in a cell wall (frustule) made of amorphous silicate. The frustule comprises two valves connected by a variable number of girdle bands, all exhibiting periodic micro/nanoporous structures. We studied the optical properties in water of girdle bands from the centric diatom Coscinodiscus granii, a frustule part that so far has received little attention by the scientific community. We show that valves and girdle bands exhibit different optical properties, as valves attenuate shorter wavelengths and girdle bands attenuate longer wavelengths of the visible light spectrum. Girdle bands show iridescent coloration in dependence of the light direction. Although the biological meaning of periodic nanoscale structures of frustules is still a matter of debate, the differences of valve and girdle band optical properties indicate that living diatoms are complex optical systems, where valves, girdles and pigments modulate light inside the cell.

Evidence for mitigation of coral bleaching by manganese

Unprecedented mass coral bleaching events due to global warming and overall seawater pollution have been observed worldwide over the last decades. Although metals are often considered as toxic substances for corals, some are essential at nanomolar concentrations for physiological processes such as photosynthesis and antioxidant defenses. This study was designed to elucidate, the individual and combined effects of nanomolar seawater enrichment in manganese (Mn) and iron (Fe), on the main physiological traits of Stylophora pistillata, maintained under normal growth and thermal stress conditions. We provide, for the first time, evidence that Mn is a key trace element for coral symbionts, enhancing cellular chlorophyll concentrations, photosynthetic efficiency and gross photosynthetic rates at ambient temperature. Our experiment also highlights the key role of Mn in increasing coral resistance to heat stress-induced bleaching. While Mn-enriched corals did not bleach and did not reduce their rates of photosynthesis and calcification, control corals experienced significant bleaching. On the contrary to Mn, Fe enrichment not only impaired calcification but induced significant bleaching. Such information is an important step towards a better understanding of the response of corals to seawater enrichment in metals. It can also explain, to some extent, species susceptibility to environmental stress.

Relationship between Photosynthetic Capacity and Microcystin Production in Toxic Microcystis Aeruginosa under Different Iron Regimes

Blooms of harmful cyanobacteria have been observed in various water bodies across the world and some of them can produce intracellular toxins, such as microcystins (MCs), which negatively impact aquatic organisms and human health. Iron participates significantly in cyanobacterial photosynthesis and is proposed to be linked to MC production. Here, the cyanobacteria Microcystis aeruginosa was cultivated under different iron regimes to investigate the relationship between photosynthetic capacity and MC production. The results showed that iron addition increased cell density, cellular protein concentration and the Chl-a (chlorophyll-a) content. Similarly, it can also up⁻regulate photosynthetic capacity and promote MC⁻leucine⁻arginine (MC⁻LR) production, but not in a dose⁻dependent manner. Moreover, a significant positive correlation between photosynthetic capacity and MC production was observed, and electron transport parameters were the most important parameters contributing to the variation of intracellular MC⁻LR concentration revealed by Generalized Additive Model analysis. As the electron transport chain was affected by iron variation, adenosine triphosphate production was inhibited, leading to the alteration of MC synthetase gene expression. Therefore, it is demonstrated that MC production greatly relies on redox status and energy metabolism of photosynthesis in M. aeruginosa. In consequence, more attention should be paid to the involvement of photosynthesis in the regulation of MC production by iron variation in the future.

Oceanographic boundaries constrain microbial diversity gradients in the South Pacific Ocean

Marine microbes along with microeukaryotes are key regulators of oceanic biogeochemical pathways. Here we present a high-resolution (every 0.5° of latitude) dataset describing microbial pro- and eukaryotic richness in the surface and just below the thermocline along a 7,000-km transect from 66°S at the Antarctic ice edge to the equator in the South Pacific Ocean. The transect, conducted in austral winter, covered key oceanographic features including crossing of the polar front (PF), the subtropical front (STF), and the equatorial upwelling region. Our data indicate that temperature does not determine patterns of marine microbial richness, complementing the global model data from Ladau et al. [Ladau J, et al. (2013) ISME J 7:1669-1677]. Rather, NH₄⁺, nanophytoplankton, and primary productivity were the main drivers for archaeal and bacterial richness. Eukaryote richness was highest in the least-productive ocean region, the tropical oligotrophic province. We also observed a unique diversity pattern in the South Pacific Ocean: a regional increase in archaeal and bacterial diversity between 10°S and the equator. Rapoport's rule describes the tendency for the latitudinal ranges of species to increase with latitude. Our data showed that the mean latitudinal ranges of archaea and bacteria decreased with latitude. We show that permanent oceanographic features, such as the STF and the equatorial upwelling, can have a significant influence on both alpha-diversity and beta-diversity of pro- and eukaryotes.

Ion and metabolite transport in the chloroplast of algae: lessons from land plants

Chloroplasts are endosymbiotic organelles and play crucial roles in energy supply and metabolism of eukaryotic photosynthetic organisms (algae and land plants). They harbor channels and transporters in the envelope and thylakoid membranes, mediating the exchange of ions and metabolites with the cytosol and the chloroplast stroma and between the different chloroplast subcompartments. In secondarily evolved algae, three or four envelope membranes surround the chloroplast, making more complex the exchange of ions and metabolites. Despite the importance of transport proteins for the optimal functioning of the chloroplast in algae, and that many land plant homologues have been predicted, experimental evidence and molecular characterization are missing in most cases. Here, we provide an overview of the current knowledge about ion and metabolite transport in the chloroplast from algae. The main aspects reviewed are localization and activity of the transport proteins from algae and/or of homologues from other organisms including land plants. Most chloroplast transporters were identified in the green alga Chlamydomonas reinhardtii, reside in the envelope and participate in carbon acquisition and metabolism. Only a few identified algal transporters are located in the thylakoid membrane and play role in ion transport. The presence of genes for putative transporters in green algae, red algae, diatoms, glaucophytes and cryptophytes is discussed, and roles in the chloroplast are suggested. A deep knowledge in this field is required because algae represent a potential source of biomass and valuable metabolites for industry, medicine and agriculture.

A potential biotechnological process for the sustainable production of vitamin K1

The primary objective of this review is to propose an approach for the biosynthesis of phylloquinone (vitamin K₁) based upon its known sources, its role in photosynthesis and its biosynthetic pathway. The chemistry, health benefits, market, and industrial production of vitamin K are also summarized. Vitamin K compounds (K vitamers) are required for the normal function of at least 15 proteins involved in diverse physiological processes such as coagulation, tissue mineralization, inflammation, and neuroprotection. Vitamin K is essential for the prevention of Vitamin K Deficiency Bleeding (VKDB), especially in neonates. Increased vitamin K intake may also reduce the severity and/or risk of bone fracture, arterial calcification, inflammatory diseases, and cognitive decline. Consumers are increasingly favoring natural food and therapeutic products. However, the bulk of vitamin K products employed for both human and animal use are chemically synthesized. Biosynthesis of the menaquinones (vitamin K₂) has been extensively researched. However, published research on the biotechnological production of phylloquinone is restricted to a handful of available articles and patents. We have found that microalgae are more suitable than plant cell cultures for the biosynthesis of phylloquinone. Many algae are richer in vitamin K₁ than terrestrial plants, and algal cells are easier to manipulate. Vitamin K₁ can be efficiently recovered from the biomass using supercritical carbon dioxide extraction.

An Integrated Response of Trichodesmium erythraeum IMS101 Growth and Photo-Physiology to Iron, CO2, and Light Intensity

We have assessed how varying CO₂ (180, 380, and 720 μatm) and growth light intensity (40 and 400 μmol photons m^-2 s^-1) affected Trichodesmium erythraeum IMS101 growth and photophysiology over free iron (Fe') concentrations between 20 and 9,600 pM. We found significant iron dependencies of growth rate and the initial slope and maximal relative PSII electron transport rates (rP_m). Under iron-limiting concentrations, high-light increased growth rates and rP_m; possibly indicating a lower allocation of resources to iron-containing photosynthetic proteins. Higher CO₂ increased growth rates across all iron concentrations, enabled growth to occur at lower Fe' concentrations, increased rP_m and lowered the iron half saturation constants for growth (K_m). We attribute these CO₂ responses to the operation of the CCM and the ATP spent/saved for CO₂ uptake and transport at low and high CO₂, respectively. It seems reasonable to conclude that T. erythraeum IMS101 can exhibit a high degree of phenotypic plasticity in response to CO₂, light intensity and iron-limitation. These results are important given predictions of increased dissolved CO₂ and water column stratification (i.e., higher light exposures) over the coming decades.

Bioavailability of Mineral-Bound Iron to a Snow Algal-Bacterial Coculture and Implications for Albedo-Altering Snow Algal Blooms

Snow algae can form large-scale blooms across the snowpack surface and near-surface environments. These pigmented blooms can decrease snow albedo and increase local melt rates, and they may impact the global heat budget and water cycle. Yet, the underlying causes for the geospatial occurrence of these blooms remain unconstrained. One possible factor contributing to snow algal blooms is the presence of mineral dust as a micronutrient source. We investigated the bioavailability of iron (Fe)-bearing minerals, including forsterite (Fo⁹⁰, Mg_1.8Fe_0.2SiO₄), goethite, smectite, and pyrite as Fe sources for a Chloromonas brevispina-bacterial coculture through laboratory-based experimentation. Fo⁹⁰ was capable of stimulating snow algal growth and increased the algal growth rate in otherwise Fe-depleted cocultures. Fo⁹⁰-bearing systems also exhibited a decrease in the ratio of bacteria to algae compared to those of Fe-depleted conditions, suggesting a shift in microbial community structure. The C. brevispina coculture also increased the rate of Fo⁹⁰ dissolution relative to that of an abiotic control. Analysis of 16S rRNA genes in the coculture identified Gammaproteobacteria, Betaproteobacteria, and Sphingobacteria, all of which are commonly found in snow and ice environments. Archaea were not detected. Collimonas and Pseudomonas, which are known to enhance mineral weathering rates, comprised two of the top eight (>1%) operational taxonomic units (OTUs). These data provide unequivocal evidence that mineral dust can support elevated snow algal growth under otherwise Fe-depleted growth conditions and that snow algal microbial communities can enhance mineral dissolution under these conditions.IMPORTANCE Fe, a key micronutrient for photosynthetic growth, is necessary to support the formation of high-density snow algal blooms. The laboratory experiments described herein allow for a systematic investigation of the interactions of snow algae, bacteria, and minerals and their ability to mobilize and uptake mineral-bound Fe. Results provide unequivocal and comprehensive evidence that mineral-bound Fe in Fe-bearing Fo⁹⁰ was bioavailable to Chloromonas brevispina snow algae within an algal-bacterial coculture. This evidence includes (i) an observed increase in snow algal density and growth rate, (ii) decreased ratios of bacteria to algae in Fo⁹⁰-containing cultures relative to those of cultures grown under similarly Fe-depleted conditions with no mineral-bound Fe present, and (iii) increased Fo⁹⁰ dissolution rates in the presence of algal-bacterial cocultures relative to those of abiotic mineral controls. These results have important implications for the role of mineral dust in supplying micronutrients to the snow microbiome, which may help support dense snow algal blooms capable of lowering snow albedo and increasing snow melt rates on regional, and possibly global, scales.