Silicic acid limitation drives bloom termination and potential carbon sequestration in an Arctic bloom

Antibiotics can alter the bacterial extracellular polymeric substances and surface properties affecting the cotransport of bacteria and antibiotics in porous media

Currently, studies on the environmental impact of antibiotics have focused on toxicity and resistance genes, and gaps exist in research on the effects of antibiotics entering the environment on bacterial surface properties and the synergistic transport of antibiotics and bacteria in porous media. To fill the gaps, we investigated the interactions between bacteria and antibiotics in synergistic transport in saturated porous media and the effects of media particle size, flow rate, and ionic concentration on this synergistic transport. This study revealed that although synergistic transport was complex, the mechanism of action was clear. Antibiotics could affect bacterial extracellular polymeric substances (EPS), thus altering their surface hydrophobicity and roughness, thereby affecting bacterial transport. The effects of antibiotics on bacterial transport were dominated by altering bacterial roughness. Antibiotics had a relatively high adsorption on bacteria, so bacterial transport directly affected antibiotic transport. The antibiotic concentrations below a certain threshold increased the bacterial EPS quality, and above the threshold decreased the bacterial EPS quality. This threshold was related to antibiotic toxicity and bacterial type. Bacterial surface hydrophobicity was determined by the combination of proteins and sugars in the EPS, and roughness was positively correlated with the EPS quality.

Essential omega-3 fatty acids are depleted in sea ice and pelagic algae of the Central Arctic Ocean

Microalgae are the main source of the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), essential for the healthy development of most marine and terrestrial fauna including humans. Inverse correlations of algal EPA and DHA proportions (% of total fatty acids) with temperature have led to suggestions of a warming-induced decline in the global production of these biomolecules and an enhanced importance of high latitude organisms for their provision. The cold Arctic Ocean is a potential hotspot of EPA and DHA production, but consequences of global warming are unknown. Here, we combine a full-seasonal EPA and DHA dataset from the Central Arctic Ocean (CAO), with results from 13 previous field studies and 32 cultured algal strains to examine five potential climate change effects; ice algae loss, community shifts, increase in light, nutrients, and temperature. The algal EPA and DHA proportions were lower in the ice-covered CAO than in warmer peripheral shelf seas, which indicates that the paradigm of an inverse correlation of EPA and DHA proportions with temperature may not hold in the Arctic. We found no systematic differences in the summed EPA and DHA proportions of sea ice versus pelagic algae, and in diatoms versus non-diatoms. Overall, the algal EPA and DHA proportions varied up to four-fold seasonally and 10-fold regionally, pointing to strong light and nutrient limitations in the CAO. Where these limitations ease in a warming Arctic, EPA and DHA proportions are likely to increase alongside increasing primary production, with nutritional benefits for a non-ice-associated food web.

Gene Duplication, Shifting Selection, and Dosage Balance of Silicon Transporter Proteins in Marine and Freshwater Diatoms

Numerous factors shape the evolution of protein-coding genes, including shifts in the strength or type of selection following gene duplications or changes in the environment. Diatoms and other silicifying organisms use a family of silicon transporters (SITs) to import dissolved silicon from the environment. Freshwaters contain higher silicon levels than oceans, and marine diatoms have more efficient uptake kinetics and less silicon in their cell walls, making them better competitors for a scarce resource. We compiled SITs from 37 diatom genomes to characterize shifts in selection following gene duplications and marine-freshwater transitions. A deep gene duplication, which coincided with a whole-genome duplication, gave rise to two gene lineages. One of them (SIT1-2) is present in multiple copies in most species and is known to actively import silicon. These SITs have evolved under strong purifying selection that was relaxed in freshwater taxa. Episodic diversifying selection was detected but not associated with gene duplications or habitat shifts. In contrast, genes in the second SIT lineage (SIT3) were present in just half the species, the result of multiple losses. Despite conservation of SIT3 in some lineages for the past 90-100 million years, repeated losses, relaxed selection, and low expression highlighted the dispensability of SIT3, consistent with a model of deterioration and eventual loss due to relaxed selection on SIT3 expression. The extensive but relatively balanced history of duplications and losses, together with paralog-specific expression patterns, suggest diatoms continuously balance gene dosage and expression dynamics to optimize silicon transport across major environmental gradients.

Phytoplankton community shift in response to experimental Cu addition at the elevated CO2 levels (Arabian Sea, winter monsoon)

Understanding phytoplankton community shifts under multiple stressors is becoming increasingly important. Among other combinations of stressors, the impact of trace metal toxicity on marine phytoplankton under the ocean acidification scenario is an important aspect to address. Such multiple stressor studies are rare from the Arabian Sea, one of the highest productive oceanic provinces within the North Indian Ocean. We studied the interactive impacts of copper (Cu) and CO₂ enrichment on two natural phytoplankton communities from the eastern and central Arabian Sea. Low dissolved silicate (DSi < 2 µM) favoured smaller diatoms (e.g. Nitzschia sp.) and non-diatom (Phaeocystis). CO₂ enrichment caused both positive (Nitzschia sp. and Phaeocystis sp.) and negative (Cylindrotheca closterium, Navicula sp., Pseudo-nitzschia sp., Alexandrium sp., and Gymnodinium sp.) growth impacts. The addition of Cu under the ambient CO₂ level (A-CO₂) hindered cell division in most of the species, whereas Chla contents were nearly unaffected. Interestingly, CO₂ enrichment seemed to alleviate Cu toxicity in some species (Nitzschia sp., Cylindrotheca closterium, Guinardia flaccida, and Phaeocystis) and increased their growth rates. This could be related to the cellular Cu demand and energy budget at elevated CO₂ levels. Dinoflagellates were more sensitive to Cu supply compared to diatoms and prymnesiophytes and could be related to the unavailability of prey. Such community shifts in response to the projected ocean acidification, oligotrophy, and Cu pollution may impact trophic transfer and carbon cycling in this region.

Interlinking diatom frustule diversity from the abyss of the central Arabian Sea to surface processes: physical forcing and oxygen minimum zone

This study analyzed the diversity and abundance of diatom frustules including the ancillary parameters using the core top sediments from five locations (21, 19, 15, 13, and 11°N) along the central Arabian Sea (64°E), an area profoundly influenced by atmospheric forcing (monsoons) and oxygen minimum zone (OMZ) with high spatial variability. Significantly higher organic carbon (0.97 ± 0.05%) and diatom frustules (5.92 ± 0.57 × 10⁴ valves g^-1) were noticed in the north (21, 19, 15°N) where natural nutrient enrichment via open-ocean upwelling, winter convection, and lateral advection support large diatom-dominated phytoplankton blooms and intense OMZ. Conversely, the south (13, 11°N) depicted significantly lower organic carbon (0.74 ± 0.08%) as well as frustules (4.02 ± 0.87 × 10⁴ valves g^-1) as this area mostly remains nutrient-poor dominated by small-medium-sized phytoplankton. The north was dominated by large-sized diatoms like Coscinodiscus that could escape grazing and sink consequently due to higher ballasting. Furthermore, the presence of the intense OMZ in the north might reduce grazing pressure (low zooplankton stock) and mineralization speed facilitating higher phytodetritus transport. Relatively smaller chain-forming centric (Thalassiosira) and pennate diatoms (Pseudo-nitzschia, Fragilaria, Nitzschia, etc.) were found throughout the transect with higher abundance in the south. The euphotic diatom diversity from the existing literature was compared with the frustule diversity from the sediments suggesting not all diatoms make their way to the abyss. Such distinct spatial north-south variability in diatom frustule size as well as abundance could be attributed to cell size, grazing, and water column mineralization rates related to OMZ.

Phytoplankton growth and community shift over a short-term high-CO2 simulation experiment from the southwestern shelf of India, Eastern Arabian Sea (summer monsoon)

The southwestern shelf water of India (eastern Arabian Sea) experiences high seasonality. This area is one of the understudied regions in terms of phytoplankton response to the projected ocean acidification, particularly, during the summer monsoon when phytoplankton abundance is high. Here we present the results of a short-term simulated ocean acidification experiment (ambient CO₂ 424 µatm; high CO₂, 843, 1138 µatm) on the natural phytoplankton assemblages conducted onboard (R. V. Sindhu Sadhana) during the summer monsoon (Aug 2017). Among the dissolved inorganic nutrients, dissolved silicate (DSi) and nitrate + nitrite levels were quite low (< 2 µM). Phytoplankton biomass did not show any net enhancement after the incubation in any treatment. Both marker pigment analysis and microscopy revealed the dominance of diatoms in the phytoplankton community, and a significant restructuring was noticed over the experimental period. Divinyl chlorophylla (DVChla) containing picocyanobacteria and 19'-hexanoyloxyfucoxanthin (19'HF) containing prymnesiophytes did not show any noticeable change in response to CO₂ enrichment. A CO₂-induced positive growth response was noticed in some diatoms (Guinardia flaccida, Cylindrotheca closterium, and Pseudo-nitzschia sp.) and dinoflagellates (Protoperidinium sp. and Peridinium sp.) indicating their efficiency to quickly acclimatize at elevated CO₂ levels. This is important to note that the positive growth response of toxigenic pennate diatoms like Pseudo-nitzschia as well as a few dinoflagellates at elevated CO₂ levels can be expected in the future-ocean scenario. The proliferation of such non-palatable phytoplankton may impact grazing, the food chain, and carbon cycling in this region.

Improvement of On-Site Sensor for Simultaneous Determination of Phosphate, Silicic Acid, Nitrate plus Nitrite in Seawater

Accurate, on-site determinations of macronutrients (phosphate (PO₄^3-), nitrate (NO₃^-), and silicic acid (H₄SiO₄)) in seawater in real time are essential to obtain information on their distribution, flux, and role in marine biogeochemical cycles. The development of robust sensors for long-term on-site analysis of macronutrients in seawater is a great challenge. Here, we present improvements of a commercial automated sensor for nutrients (including PO₄^3-, H₄SiO₄, and NO₂^- plus NO₃^-), suitable for a variety of aquatic environments. The sensor uses the phosphomolybdate blue method for PO₄^3-, the silicomolybdate blue method for H₄SiO₄ and the Griess reagent method for NO₂^-, modified with vanadium chloride as reducing agent for the determination of NO₃^-. Here, we report the optimization of analytical conditions, including reaction time for PO₄^3- analysis, complexation time for H₄SiO₄ analysis, and analyte to reagent ratio for NO₃^- analysis. The instrument showed wide linear ranges, from 0.2 to 100 μM PO₄^3-, between 0.2 and 100 μM H₄SiO₄, from 0.5 to 100 μM NO₃^-, and between 0.4 and 100 μM NO₂^-, with detection limits of 0.18 μM, 0.15 μM, 0.45 μM, and 0.35 μM for PO₄^3-, H₄SiO₄, NO₃^-, and NO₂^-, respectively. The analyzer showed good precision with a relative standard deviation of 8.9% for PO₄^3-, 4.8% for H₄SiO₄, and 7.4% for NO₂^- plus NO₃^- during routine analysis of certified reference materials (KANSO, Japan). The analyzer performed well in the field during a 46-day deployment on a pontoon in the Kiel Fjord (located in the southwestern Baltic Sea), with a water supply from a depth of 1 m. The system successfully collected 443, 440, and 409 on-site data points for PO₄^3-, Σ(NO₃^- + NO₂^-), and H₄SiO₄, respectively. Time series data agreed well with data obtained from the analysis of discretely collected samples using standard reference laboratory procedures and showed clear correlations with key hydrographic parameters throughout the deployment period.

Nutrient pathways and their susceptibility to past and future change in the Eurasian Arctic Ocean

Climate change is altering nutrient cycling within the Arctic Ocean, having knock-on effects to Arctic ecosystems. Primary production in the Arctic is principally nitrogen-limited, particularly in the western Pacific-dominated regions where denitrification exacerbates nitrogen loss. The nutrient status of the eastern Eurasian Arctic remains under debate. In the Barents Sea, primary production has increased by 88% since 1998. To support this rapid increase in productivity, either the standing stock of nutrients has been depleted, or the external nutrient supply has increased. Atlantic water inflow, enhanced mixing, benthic nitrogen cycling, and land-ocean interaction have the potential to alter the nutrient supply through addition, dilution or removal. Here we use new datasets from the Changing Arctic Ocean program alongside historical datasets to assess how nitrate and phosphate concentrations may be changing in response to these processes. We highlight how nutrient dynamics may continue to change, why this is important for regional and international policy-making and suggest relevant research priorities for the future.

The 79°N Glacier cavity modulates subglacial iron export to the NE Greenland Shelf

Approximately half of the freshwater discharged from the Greenland and Antarctic Ice Sheets enters the ocean subsurface as a result of basal ice melt, or runoff draining via the grounding line of a deep ice shelf or marine-terminating glacier. Around Antarctica and parts of northern Greenland, this freshwater then experiences prolonged residence times in large cavities beneath floating ice tongues. Due to the inaccessibility of these cavities, it is unclear how they moderate the freshwater associated supply of nutrients such as iron (Fe) to the ocean. Here, we show that subglacial dissolved Fe export from Nioghalvfjerdsbrae (the '79°N Glacier') is decoupled from particulate inputs including freshwater Fe supply, likely due to the prolonged ~162-day residence time of Atlantic water beneath Greenland's largest floating ice-tongue. Our findings indicate that the overturning rate and particle-dissolved phase exchanges in ice cavities exert a dominant control on subglacial nutrient supply to shelf regions.

Impacts of stratigraphic heterogeneity and release pathway on the transport of bacterial cells in porous media

In order to manage and control the pathogen release from waste streams of various municipal, industrial, and agricultural pollution sources, it is crucial to investigate the impact of release pathways of such contaminants on their fate and transport in groundwater, especially in respect to natural heterogeneities encountered in aquifers. In this laboratory scale study, we investigate the impacts of different release scenarios of Escherichia coli bacteria, including spatially distributed surface recharge and single-point deep injection, as well as mono-pulse and continuous injection on the transport of Escherichia coli within both single-layered and multilayer aquifers. The results demonstrate earlier arrival of bacteria breakthrough curve (BTC) than conservative solute within a single-layer system with textural and continuum scale heterogeneities, attributed to size exclusion mechanism and preferential flow paths. Size exclusion may be responsible for multiple peaked BTCs observed in all cases of mono-pulse injection of bacteria through both single layer and multi-layer systems. The higher breakthrough of bacteria suspension introduced through a distributed source compared to the point source injection at the same flow rate (19% and 53% in middle and top layers, respectively) suggests that natural hydrologic events such as storm may be more influential in the transport of pathogens in soils than point injections of bacteria in engineering applications such as bioremediation. Moreover, our results reveal that the concentration of the semi-steady state breakthrough formed under distributed and continuous injection condition increases significantly with an increase in the recharge flow rate. This would suggest that a variation in hydrologic conditions can significantly mobilize pathogens which are already deposited in soils.

The Importance of Protein Phosphorylation for Signaling and Metabolism in Response to Diel Light Cycling and Nutrient Availability in a Marine Diatom

Diatoms are major contributors to global primary production and their populations in the modern oceans are affected by availability of iron, nitrogen, phosphate, silica, and other trace metals, vitamins, and infochemicals. However, little is known about the role of phosphorylation in diatoms and its role in regulation and signaling. We report a total of 2759 phosphorylation sites on 1502 proteins detected in Phaeodactylum tricornutum. Conditionally phosphorylated peptides were detected at low iron (n = 108), during the diel cycle (n = 149), and due to nitrogen availability (n = 137). Through a multi-omic comparison of transcript, protein, phosphorylation, and protein homology, we identify numerous proteins and key cellular processes that are likely under control of phospho-regulation. We show that phosphorylation regulates: (1) carbon retrenchment and reallocation during growth under low iron, (2) carbon flux towards lipid biosynthesis after the lights turn on, (3) coordination of transcription and translation over the diel cycle and (4) in response to nitrogen depletion. We also uncover phosphorylation sites for proteins that play major roles in diatom Fe sensing and utilization, including flavodoxin and phytotransferrin (ISIP2A), as well as identify phospho-regulated stress proteins and kinases. These findings provide much needed insight into the roles of protein phosphorylation in diel cycling and nutrient sensing in diatoms.