CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution

New insights into microalgal-bacterial immobilization systems for wastewater treatment: mechanisms, enhancement strategies, and application prospects

The wastewater treatment based on the symbiosis of microalgae and bacteria has attracted increasing attention for its excellent pollutant removal efficiency, energy savings, and resource recovery. Among them, the microalgae-bacteria immobilization (MABI) system stands out by enhancing the electron transfer efficiency through carrier domain confinement, thereby overcoming bottlenecks of low light energy utilization and challenging biomass recycling. MABI is considered a key breakthrough for advancing engineering applications. However, a comprehensive exploration of MABI systems remains lacking. This review systematically summarizes the latest advancements, covering major immobilization techniques and the intrinsic mechanisms underlying microalgae-bacteria interactions and electron transport. Additionally, it explores enhancement strategies aimed at balancing microbial light energy allocation, optimizing nutrient supply, and constructing complementary ecological niches. The advantages and application prospects of MABI systems are highlighted. The review contributes to structuring the knowledge framework of MABI research and identifies critical gaps for future investigation.

Overexpression of Rboh enhances inorganic carbon acquisition through coordinating with carbonic anhydrase in Pyropia yezoensis

Pyropia yezoensis is an important intertidal economic macroalgae, which is periodically affected by various stresses, such as the limitation of inorganic carbon (Ci) deficiency. Under such environment, the redox homeostasis within the cells of P. yezoensis is seriously affected, and the reactive oxygen species (ROS) signal transduction system would be activated to regulate the photosynthetic activity. Therefore, how P. yezoensis manage ROS to maintain effective photosynthetic carbon fixation has aroused great interest. Here, we characterize transformants overexpressing respiratory burst oxidase homolog (Rboh), an important gene that can actively produce ROS, at the levels of cellular physiology, biochemistry, and transcriptomics. Our data indicated the expression of Rboh significantly increased, accompanied by a significant upregulated expression of alpha-type carbonic anhydrase 3 (αCA3) and increased extracellular carbonic anhydrase activity in the Rboh overexpressing strains. Interestingly, compared with the wild type, the photosynthetic activity of transgenic strains was significantly higher under the low Ci and high light condition, implying that the ROS signal triggered by overexpression of Rboh was involved in regulating the Ci absorption and utilization in P. yezoensis when the Ci source was limited. In summary, this study provided evidence supporting the correlation between the ROS production and the Ci utilization under stress environments in P. yezoensis.

Growth Response and Cell Permeability of the Fish-Killing Phytoflagellate Heterosigma akashiwo Under Projected Climate Conditions

Climate change and anthropogenic alterations in biogeochemical cycles are intensifying the frequency, duration, and potential toxicity of harmful algal blooms (HABs) in marine ecosystems. However, these effects are highly variable and depend on species identity, strain-specific traits, and local environmental conditions. Key drivers include rising sea surface temperatures, changes in salinity resulting from altered precipitation patterns and runoff, and elevated CO2 levels leading to ocean acidification. Heterosigma akashiwo, a euryhaline raphidophyte responsible for the widespread killing of fish, is particularly responsive to these changes. This study investigated the combined effects of temperature, salinity, and CO2 concentration on the growth, yield, and cell membrane permeability of H. akashiwo using a Design of Experiment (DOE) approach. DOE facilitates a detailed and systematic analysis of multifactorial interactions, enabling a deeper understanding of complex relationships while maximizing efficiency and minimizing the use of experimental resources. The results revealed that growth and yield were maximized at higher temperatures and salinities, whereas cell permeability increased under cooler, less saline, and lower CO2 conditions. These findings suggest that projected future ocean conditions may enhance biomass production while potentially reducing cellular permeability and, by extension, toxicity. This study highlights the value of the DOE framework in identifying key interactions among environmental drivers of HABs, offering a practical foundation for future predictive modeling under climate change scenarios.

Boundaries of photosynthesis: adaptations of carbon fixation in extreme environments

Extreme environments challenge fundamental pillars of photosynthesis: light capture and carbon fixation. Organisms thriving in extreme conditions, such as high and low temperatures, extreme pH levels, and high salinity, have evolved remarkable adaptive mechanisms allowing them to sustain photosynthesis. Research into these adaptations has expanded our understanding of the limits and evolution of photosynthesis, while also providing promising biotechnological applications. In this review, we explore the adaptations that tolerant and extremophilic photosynthetic organisms have evolved, overcoming these environmental challenges while maintaining photosynthetic functionality. These adaptations include modifications in photosystems and electron transport chain components, the development of photoprotective mechanisms, the use of unique CO2-concentrating mechanisms (CCMs), and fine-tuning of Rubisco's kinetic properties and concentration. Our aim is to provide the basis for future research in extremophile biology while highlighting its applications in biotechnology.

Optimization of sustainable bioprocessing for the production of high-value biomolecules (chlorophylls and proteins) using Desmodesmus subspicatus

Traditional protein and pigment (e.g., chlorophyll) sources are becoming insufficient due to the rapid rise of the global population in modern civilization. Microalgae offer a promising solution for protein and chlorophyll sources due to their higher productivity than terrestrial plants. This study aims to optimize the cultivation conditions for Desmodesmus subspicatus, a microalgal strain containing ∼60% protein and 4% chlorophyll, to enhance biomass, protein and chlorophyll productivity. A Taguchi Orthogonal Array (TOA) was used for systematic optimization of BG-11 medium components. Further experiments assessed the effects of light intensity and different carbon and nitrogen sources. Under optimized BG-11 conditions, biomass increased 1.3-fold, with protein and chlorophyll productivity rising 2.25 and 1.92-fold, respectively. Supplementation with carbon and nitrogen sources under varying light (84-504 µmol m-2 s-1) further enhanced yields by 1.6-fold. Glycine proved to be the most effective nitrogen source, while cellulose as a carbon source resulted in 2.4-fold higher biomass, 7.3-fold higher protein, and 2.3-fold higher chlorophyll. Cytotoxicity assessment of the extracted chlorophyll revealed over 94% A549 cell viability at concentrations up to 100 µg/mL, confirming its biocompatibility. Therefore, Desmodesmus subspicatus has promise as a sustainable source of proteins and chlorophylls in the nutraceutical and food industries.

Assessment of the short-term salinity effect on algal biofilm through field transfer in the Drâa river (Southeastern Morocco) using metabarcoding and morphological analyses

As a result of human activities and climate change, salinity levels have risen considerably in many of the world's rivers, particularly in arid and semi-arid areas. This freshwater salinization primarily affects microalgal biofilms, the primary producers in aquatic ecosystems. This study is aimed at assessing short-term salinity effects on benthic algal communities in the Drâa river, Morocco, using biofilm field-transfer experiments. Artificial substrates were initially positioned in three sites of the Drâa river with different salinity levels. After 4 weeks, the biofilm-colonized substrates were transferred from one site to another in both directions. After a further 4 weeks, the algal biofilms were sampled to assess their community composition, alpha and beta diversity, and biomass in response to salinity changes using molecular and morphological analyses. Transferring biofilms from low-salt to saline sites significantly reduced biomass but increased it in the reverse transfer. Eliminating certain sensitive microalgae taxa decreased alpha diversity in all biofilm transfers from low-salt sites to the most saline one. Biofilm diversity increased significantly in the opposite transfer. Significant changes were observed in biofilm composition when transferred into saltwater showing an increase in halophilic and euryhaline diatom taxa. In contrast, transfers from saltwater sites to low-salt sites increased oligohaline diatom species. As a result, increasing salinity reduced algal biomass and diversity, while decreasing salinity caused the opposite effect. These results can help predict the salinity variations effects on benthic algae, highlight the potential dangers of increased river salinization, and promote salinity reduction in aquatic ecosystems subject to secondary salinization.

Carbon dioxide removal from triethanolamine solution using living microalgae-loofah biocomposites

Nowadays, the climate change crisis is an urgent matter in which carbon dioxide (CO2) is a major greenhouse gas contributing to global warming. Amine solvents are commonly used for CO2 capture with high efficiency and absorption rates. However, solvent regeneration consumes an extensive amount of energy. One of alternative approaches is amine regeneration through microalgae. Recently, living biocomposites, intensifying traditional suspended cultivation, have been developed. With this technology, immobilizing microalgae on biocompatible materials with binder outperformed the suspended system in terms of CO2 capture rates. In this study, living microalgae-loofah biocomposites with immobilized Scenedesmus acuminatus TISTR 8457 using 5%v/v acrylic medium were tested to remove CO2 from CO2-rich triethanolamine (TEA) solutions. The test using 1 M TEA at various CO2 loading ratios (0.2, 0.4, 0.6, and 0.8 mol CO2/mol TEA) demonstrated that the biocomposites achieved CO2 removal rates 3 to 5 times higher than the suspended cell system over 28 days, with the highest removal observed at the 1 M with 0.4 mol CO2/mol TEA (4.34 ± 0.20 gCO2/gbiomass). This study triggers a new exploration of integration between biological and chemical processes that could elevate the traditional amine-based CO2 capture capabilities. Nevertheless, pilot-scale investigations are necessary to confirm the biocomposites's efficiency.

High-pressure continuous culturing: life at the extreme

Microorganisms adapted to high hydrostatic pressures at depth in the oceans and within the subsurface of Earth's crust represent a phylogenetically diverse community thriving under extreme pressure, temperature, and nutrient availability conditions. To better understand the microbial function, physiological responses, and metabolic strategies at in-situ conditions requires high-pressure (HP) continuous culturing techniques that, although commonly used in bioengineering and biotechnology applications, remain relatively rare in the study of the Earth's microbiomes. Here, we focus on recent developments in the design of HP chemostats, with particular emphasis on adaptations for delivery and sampling of dissolved gases. We present protocols for sterilization, inoculation, agitation, and sampling strategies that minimize cell lysis, applicable to a wide range of chemostat designs.

John Raven, FRS, FRSE: a truly great innovator in plant physiology, photosynthesis and much more

This is a tribute to a truly inspirational plant biologist, Prof. John A. Raven, FRS, FRSE (25th June 1941- 23rd May 2024), who died at the age of 82. He was a leader in the field of evolution and physiology of algae and land plants. His research touched on many areas including photosynthesis, ion transport, carbon utilisation, mineral use, such as silicon, iron and molybdenum, the evolution of phytoplankton, the evolution of root systems, the impact of global change, especially on the acidification of the oceans, carbon gain and water use in early land plants, and ways of detecting extraterrestrial photosynthesis. Beginning his research career in the Botany School, University of Cambridge, John studied ion uptake in a giant algal cell. This was at the time of great strides brought about by Peter Mitchell (1920-1992) in elucidating the role of energy generation in mitochondria and chloroplasts and the coupling of ion transport systems to energy generation. With Enid MacRobbie and Andrew Smith, John pioneered early work on the involvement of ion transport in the growth and metabolism of plant cells.On leaving Cambridge John took up a lectureship at the University of Dundee in 1971, where he was still attached upon his death. His primary focus over the years, with one of us (Paul Falkowski), was on phytoplankton, the photosynthetic microalgae of the oceans. Still, his publication list of 5 books and over 600 scientific papers spans a very broad range. The many highly cited papers (see Table 1) attest to an outstanding innovator, who influenced a multitude of students and coworkers and a very wide readership worldwide. At the personal level, John Raven was a wonderful human being; he had an extraordinary memory, dredging up facts and little-known scientific papers, like a scientific magician, but at the same time making humorous jokes and involving his colleagues in fun and sympathetic appreciation. Table 1 Ten best cited articles (from google scholar) Citations Date Aquatic Photosynthesis, 3rd Edition P.G. Falkowski & J.A. Raven Princeton University Press, 2013 3854 2013 The evolution of modern eukaryotic phytoplankton P.G. Falkowski, M.E. Katz, A.H. Knoll, A. Quigg, J.A. Raven, et al Science 305, 354-360 1790 2004 CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution M. Giordano, J. Beardall & J.A. Raven Annu. Rev. Plant Biol. 56 (1), 99-131 1648 2005 Algae as nutritional food sources: revisiting our understanding M.L. Wells, P. Potin, J.S. Craigie, J.A. Raven, S.S. Merchant, et al Journal of applied phycology 29, 949-982 1527 2017 Plant Nutrient acquisition strategies change with soil age H. Lambers, J.A. Raven, G.R. Shaver & S.E. Smith Trends in ecology & evolution 23, 95-103 1488 2008 Ocean acidification due to increasing atmospheric carbon dioxide J. Raven, K. Caldeira, H. Elderfield, O. Hoegh-Guldberg, P. Liss, et al The Royal Society, Policy Document, June 2005 1470 2005 Phytoplankton in a changing world: cell size and elemental stoichiometry Z.V. Finkel, J. Beardall, K.J. Flynn, A. Quigg, T.A.V. Rees & J.A. Raven Journal of plankton research 32, 119-137 1198 2010 Opportunities for improving phosphorus efficiency in crop plants E.J. Veneklaas, H. Lambers, J. Bragg, P.M. Finnegan, C.E. Lovelock, et al New phytologist 195, 306-320 951 2012 Adaptation of unicellular algae to irradiance: an analysis of strategies K. Richardson, J. Beardall & J.A. Raven New Phytologist 93, 157-191 914 1983 Nitrogen assimilation and transport in vascular land plants in relation to Intracellular pH regulation J.A. Raven & F.A. Smith New Phytologist 76, 415-431 893 1976 Temperature and algal growth J.A. Raven & R.J. Geider New phytologist 110, 441-461 867 1988 The role of trace metals in photosynthetic electron transport in O2 -evolving organisms J.A. Raven, M.C.W. Evans & R.E. Korb Photosynthesis Research 60, 111-150 840 1999.

Progress in culture technology and active substance research on Nostoc sphaeroides Kützing

Nostoc sphaeroides Kützing is a freshwater edible cyanobacterium that is rich in active substances such as polysaccharides, proteins and lipids; it has a variety of pharmacological effects such as antioxidant, anti-inflammatory, antitumor and cholesterol-lowering effects; and is often used as a traditional Chinese medicine with many potential applications in food, cosmetics, medical diagnostics and disease treatment. However, to meet the needs of different fields, such as medicine, there is an urgent need for basic research and technological innovation in culture technology, extraction and preparation of active substances, and the pharmacological mechanism of N. sphaeroides. This paper reviews the pharmacological effects of N. sphaeroides active substances, discusses current culture techniques and methods for extracting active components, and outlines the challenges encountered in cultivating and industrializing N. sphaeroides while discussing future development trends. © 2024 Society of Chemical Industry.

Expression and localization of two β-carbonic anhydrases in Bienertia, a single-cell C4 plant

Carbonic anhydrases (CAs) are ubiquitous enzymes that catalyze reversibly both the hydration and dehydration reactions of CO2 and HCO3-, respectively. Higher plants contain many different isoforms of CAs that can be classified into α-, β- and γ-type subfamilies. β-type CAs play a key role in the CO2-concentrating mechanism, thereby contributing to efficient photosynthesis in the C4 plants in addition to many other biochemical reactions in plant metabolism. Here, we characterized at the molecular, cellular and biochemical levels two β-type CAs in Bienertia sinuspersici, a plant that operates a C4 carbon concentrating mechanism within individual cells without the Kranz anatomy. These two β-type CAs (BsCAβs), named BsCAβ1 and BsCAβ2, in Bienertia were strongly induced along with maturation of leaves. Both BsCAβ1 and BsCAβ2 existed as a dimeric form in vivo but showed differential localization. BsCAβ2 was localized exclusively to the plasma membrane in Bienertia and when expressed heterologously in the C3 Arabidopsis. In contrast, BsCAβ1 largely localized to the cytosol together with a portion to the plasma membrane (PM) in both plants. BsCAβ2 had two cysteine residues at the N-terminal region for palmitoylation and their substitution with serine residues led to a change in the localization from the plasma membrane (PM) to the cytosol. Thus, we propose that BsCAβ2 localizes to the PM using a lipid moiety added posttranslationally plays a role in conversion of cytosolic CO2 into HCO3- as part of the CO2-concentrating mechanism, thereby contributing to the single-cell C4 photosynthesis in Bienertia.

Cellular heterogeneity in metabolism and associated microbiome of a non-model phytoflagellate

Single-cell transcriptomics is a key tool for unravelling metabolism and tissue diversity in model organisms. Its potential for elucidating the ecological roles of microeukaryotes, especially non-model ones, remains largely unexplored. This study employed the Smart-seq2 protocol on Ochromonas triangulata, a microeukaryote lacking a reference genome, showcasing how transcriptional states align with two distinct growth phases: a fast-growing phase and a slow-growing phase. Besides the two expected expression clusters, each corresponding to either growth phase, a third transcriptional state was identified across both growth phases. Metabolic mapping revealed a boost of photosynthetic activity in the fast growth over the slow growth stage, as well as downregulation trend in pathways associated with ribosome functioning, CO2 fixation, and carbohydrate catabolism characteristic of the third transcriptional state. In addition, carry-over rRNA reads recapitulated the taxonomic identity of the target while revealing distinct bacterial communities, in co-culture with the eukaryote, each associated with distinct transcriptional states. This study underscores single-cell transcriptomics as a powerful tool for characterizing metabolic states in microeukaryotes without a reference genome, offering insights into unknown physiological states and individual-level interactions with different bacterial taxa. This approach holds broad applicability to describe the ecological roles of environmental microeukaryotes, culture-free, and reference-free, surpassing alternative methods like metagenomics or metatranscriptomics.

Dominance of Sulfur-Oxidizing Bacteria, Thiomicrorhabdus, in the Waters Affected by a Shallow-Sea Hydrothermal Plume

The shallow-sea hydrothermal vent at Guishan Islet, located off the coast of Taiwan, serves as a remarkable natural site for studying microbial ecology in extreme environments. In April 2019, we investigated the composition of prokaryotic picoplankton communities, their gene expression profiles, and the dissolved inorganic carbon uptake efficiency. Our results revealed that the chemolithotrophs Thiomicrorhabdus spp. contributed to the majority of primary production in the waters affected by the hydrothermal vent plume. The metatranscriptomic analysis aligned with the primary productivity measurements, indicating the significant gene upregulations associated with carboxysome-mediated carbon fixation in Thiomicrorhabdus. Synechococcus and Prochlorococcus served as the prokaryotic photoautotrophs for primary productivity in the waters with lower influence from hydrothermal vent emissions. Thiomicrorhabdus and picocyanobacteria jointly provided organic carbon for sustaining the shallow-sea hydrothermal vent ecosystem. In addition to the carbon fixation, the upregulation of genes involved in the SOX (sulfur-oxidizing) pathway, and the dissimilatory sulfate reduction indicated that energy generation and detoxification co-occurred in Thiomicrorhabdus. This study improved our understanding of the impacts of shallow-sea hydrothermal vents on the operation of marine ecosystems and biogeochemical cycles.

Effects of DCA gene overexpression in transgenic Dunaliella parva

Duplicated carbonic anhydrase was a member of carbonic anhydrase family. The ubiquitous carbonic anhydrase (CA) plays important roles in plants. However, the role of CA in Dunaliella parva remains to be further explored. In this study, a member of the CAs family was isolated from D. parva and named DpDCA. It was transferred into D. parva by genetic engineering techniques. The carotenoid, total carbohydrate, starch, protein, and oil contents of transgenic D. parva increased by 16.31%, 31.68%, 43.97%, 52.91%, and 12.32%, compared to control. Perhaps DpDCA affected photosynthesis through cis-element. Therefore, this study lays a foundation for studying the function of DpDCA gene in D. parva.

Biological and Nutritional Applications of Microalgae

Microalgae are photosynthetic microorganisms that have a rapid growth cycle and carbon fixation ability. They have diverse cellular structures, ranging from prokaryotic cyanobacteria to more complex eukaryotic forms, which enable them to thrive in a variety of environments and support biomass production. They utilize both photosynthesis and heterotrophic pathways, indicating their ecological importance and potential for biotechnological applications. Reproducing primarily through asexual means, microalgae have complex cell cycles that are crucial for their growth and ability to adapt to changing conditions. Additionally, microalgae possess bioactive compounds that make them both nutritious and functional. Thanks to their content of proteins, lipids, carbohydrates, vitamins, and minerals, they play an important role in the development of functional food products, particularly by enhancing nutritional content and product quality. Furthermore, studies have demonstrated that algae and algal bioactive compounds support cardiovascular health, immune function, and gut health, especially in relation to obesity and other metabolic diseases. They also contribute to skin health and cognitive functions, including memory. This review article explores the biological, nutritional, and functional properties of microalgae based on the studies conducted.

Resilience of Phytoplankton and Microzooplankton Communities under Ocean Alkalinity Enhancement in the Oligotrophic Ocean

Ocean alkalinity enhancement (OAE) is currently discussed as a potential negative emission technology to sequester atmospheric carbon dioxide in seawater. Yet, its potential risks or cobenefits for marine ecosystems are still mostly unknown, thus hampering its evaluation for large-scale application. Here, we assessed the impacts OAE may have on plankton communities, focusing on phytoplankton and microzooplankton. In a mesocosm study in the oligotrophic subtropical North Atlantic, we investigated the response of a natural plankton community to CO2-equilibrated OAE across a gradient from ambient alkalinity (2400 μmol kg-1) to double (4800 μmol kg-1). Abundance and biomass of phytoplankton and microzooplankton were insensitive to OAE across all size classes (pico, nano and micro), nutritional modes (autotrophic, mixotrophic and heterotrophic) and taxonomic groups (cyanobacteria, diatoms, haptophytes, dinoflagellates, and ciliates). Consequently, plankton communities under OAE maintained their natural chlorophyll a levels, size structure, taxonomic composition and biodiversity. These findings suggest a high tolerance of phytoplankton and microzooplankton to CO2-equilibrated OAE in the oligotrophic ocean. However, alternative application schemes involving more drastic perturbations in water chemistry and nutrient-rich ecosystems require further investigation. Nevertheless, our study on idealized OAE will help develop an environmentally safe operating space for this climate change mitigation solution.

Effects of pH, Temperature, and Light on the Inorganic Carbon Uptake Strategies in Early Life Stages of Macrocystis pyrifera (Ochrophyta, Laminariales)

The responses of seaweed species to increased CO2 and lowered pH (Ocean Acidification: OA) depend on their carbon concentrating mechanisms (CCMs) and inorganic carbon (Ci) preferences. However, few studies have described these mechanisms in the early life stages of seaweeds or assessed the effects of OA and its interactions with other environmental drivers on their functionality and photophysiology. Our study evaluated the effects of pH, light (PAR), temperature, and their interactions on the Ci uptake strategies and photophysiology in the early stages of Macrocystis pyrifera. Gametophytes were cultivated under varying pH (7.80 and 8.20), light (20 and 50 µmol photons m-2s-1), and temperature (12 and 16 °C) conditions for 25 days. We assessed photophysiological responses and CCMs (in particular, the extracellular dehydration of HCO3- to CO2 mediated by the enzyme carbonic anhydrase (CA) and direct HCO3- uptake via an anion exchange port). This study is the first to describe the Ci uptake strategies in gametophytes of M. pyrifera, demonstrating that their primary CCM is the extracellular conversion of HCO3- to CO2 mediated by CA. Additionally, our results indicate that decreased pH can positively affect their photosynthetic efficiency and maximum quantum yield; however, this response is dependent on the light and temperature conditions.

Regulation of seawater dissolved carbon pools by environmental changes in Ulva prolifera originating sites: A new perspective on the contribution of U. prolifera to the seawater carbon sink function

The Ulva prolifera bloom is considered one of the most serious ecological disasters in the Yellow Sea in the past decade, forming a carbon sink in its source area within a short period but becoming a carbon source at its destination. To explore the effects of different environmental changes on seawater dissolved carbon pools faced by living U. prolifera in its originating area, U. prolifera were cultured in three sets with different light intensity (54, 108, and 162 μmol m-2 s-1), temperature (12, 20, and 28 °C) and nitrate concentration gradients (25, 50, and 100 μmol L-1). The results showed that moderate light (108 μmol m-2 s-1), temperature (20 °C), and continuous addition of exogenous nitrate significantly enhanced the absorption of dissolved inorganic carbon (DIC) in seawater by U. prolifera and most promoted its growth. Under the most suitable environment, the changes in the seawater carbonate system were mainly dominated by biological production and denitrification, with less influence from aerobic respiration. Facing different environmental changes, U. prolifera continuously changed its carbon fixation mode according to tissue δ13C results, with the changes in the concentrations of various components of DIC in seawater, especially the fluctuation of HCO3- and CO2 concentrations. Enhanced light intensity of 108 μmol m-2 s-1 could shift the carbon fixation pathway of U. prolifera towards the C4 pathway compared to temperature and nitrate stimulation. Environmental conditions at the origin determined the amount of dissolved carbon fixed by U. prolifera. Therefore, more attention should be paid to the changes in marine environmental conditions at the origin of U. prolifera, providing a basis for scientific management of U. prolifera.

Species differences in carbon drawdown during marine phytoplankton growth

Ocean alkalinity enhancement (OAE) has been proposed as a mitigation method for negative carbon emission. Its effects on marine phytoplankton communities would depend on species differences in tolerance to high pH, which results from phytoplankton photosynthetic drawdown of dissolved inorganic carbon (DIC). In this study, 20 marine phytoplankton species were grown in sealed batch cultures and DIC, pH and chlorophyll a (Chl-a) were measured at the peaks of biomass. These results revealed a wide range of species differences. The drawdown DIC (ΔDIC) vs. increases in pH (ΔpH) graph resembled a Michaelis-Menten curve: significantly linear for ΔDIC < ~1000 μM and starting to plateau at ΔDIC > 1000 μM. This indicated that two mechanisms were operating: CO2 limitation at ΔpH < 1.41 and biologically-mediated precipitation-CO2 released carbon uptake at ΔpH > 1.41. These findings suggest that the potential effects of OAE on the phytoplankton communities would depend on the species differences in oceans.

Elucidating the role of primary and secondary sphere Zn2+ ligands in the cyanobacterial CO2 uptake complex NDH-14: The essentiality of arginine in zinc coordination and catalysis

Inorganic carbon uptake in cyanobacteria is facilitated by an energetically intensive CO2-concentrating mechanism (CCM). Specialized Type-1 NDH complexes function as a part of this mechanism to couple photosynthetic energy generated by redox reactions of the electron transport chain (ETC) to CO2 hydration. This active site of CO2 hydration incorporates an arginine side chain as a Zn ligand, diverging from the typical histidine and/or cysteine residues found in standard CAs. In this study, we focused on mutating three amino acids in the active site of the constitutively expressed NDH-14 CO2 hydration complex in Synechococcus sp. PCC7942: CupB-R91, which acts as a zinc ligand, and CupB-E95 and CupB-H89, both of which closely interact with the arginine ligand. These mutations aimed to explore how they affect the unusual metal ligation by CupB-R91 and potentially influence the unusual catalytic process. The most severe defects in activity among the targeted residues are due to a substitution of CupB-R91 and the ionically interacting E95 since both proved essential for the structural stability of the CupB protein. On the other hand, CupB-H89 mutations show a range of catalytic phenotypes indicating a role of this residue in the catalytic mechanism of CO2-hydration, but no evidence was obtained for aberrant carbonic anhydrase activity that would have indicated uncoupling of the CO2-hydration activity from proton pumping. The results are discussed in terms of possible alternative CO2 hydration mechanisms.

Response to the CO2 concentrating mechanisms and transcriptional time series analysis of Ulva prolifera under inorganic carbon limitation

Ulva prolifera is a dominant species in green tides and has been affecting marine ecosystem for many years. Due to the low availability of CO2 in the environment, U. prolifera utilizes the CO2 concentrating mechanisms (CCMs) to increase intracellular inorganic carbon concentration. However, the transcriptional response mechanism and temporal changes of U. prolifera CCMs based on transcriptomics have not been thoroughly described. Therefore, we induced U. prolifera CCMs in a low CO2 environment to explore the dynamic regulation of CCMs expression under inadequate inorganic carbon supply. The results showed that inorganic carbon limitation increased the inorganic carbon affinity of U. prolifera, upregulating CCMs. The first 24 h of inorganic carbon environmental changes were the most active period for U. prolifera's expression regulation. U. prolifera gradually achieved a new steady state by regulating metabolic processes such as nucleic acids, energy, and ethylene-activated signaling pathways. In the carbon fixation system of U. prolifera, there are characteristics of both biophysical and biochemical CCMs. After 24 h of inorganic carbon limitation, the biophysical CCMs becomes more effective under conditions of inorganic carbon depletion. This study aids in exploring the CCMs of U. prolifera and their evolution in response to environmental changes.

Diatom pyrenoids are encased in a protein shell that enables efficient CO2 fixation

Pyrenoids are subcompartments of algal chloroplasts that increase the efficiency of Rubisco-driven CO2 fixation. Diatoms fix up to 20% of global CO2, but their pyrenoids remain poorly characterized. Here, we used in vivo photo-crosslinking to identify pyrenoid shell (PyShell) proteins, which we localized to the pyrenoid periphery of model pennate and centric diatoms, Phaeodactylum tricornutum and Thalassiosira pseudonana. In situ cryo-electron tomography revealed that pyrenoids of both diatom species are encased in a lattice-like protein sheath. Single-particle cryo-EM yielded a 2.4-Å-resolution structure of an in vitro TpPyShell1 lattice, which showed how protein subunits interlock. T. pseudonana TpPyShell1/2 knockout mutants had no PyShell sheath, altered pyrenoid morphology, and a high-CO2 requiring phenotype, with reduced photosynthetic efficiency and impaired growth under standard atmospheric conditions. The structure and function of the diatom PyShell provide a molecular view of how CO2 is assimilated in the ocean, a critical ecosystem undergoing rapid change.

Diverse inorganic carbon uptake strategies in Antarctic seaweeds: Revealing species-specific responses and implications for Ocean Acidification

Seaweeds are important components of coastal benthic ecosystems along the Western Antarctic Peninsula (WAP), providing refuge, food, and habitat for numerous associated species. Despite their crucial role, the WAP is among the regions most affected by global climate change, potentially impacting the ecology and physiology of seaweeds. Elevated atmospheric CO2 concentrations have led to increased dissolved inorganic carbon (Ci) with consequent declines in oceanic pH and alterations in seawater carbonate chemistry, known as Ocean Acidification (OA). Seaweeds possess diverse strategies for Ci uptake, including CO2 concentrating mechanisms (CCMs), which may distinctly respond to changes in Ci concentrations. Conversely, some seaweeds do not operate CCMs (non-CCM species) and rely solely on CO2. Nevertheless, our understanding of the status and functionality of Ci uptake strategies in Antarctic seaweeds remains limited. Here, we investigated the Ci uptake strategies of seaweeds along a depth gradient in the WAP. Carbon isotope signatures (δ13C) and pH drift assays were used as indicators of the presence or absence of CCMs. Our results reveal variability in CCM occurrence among algal phyla and depths ranging from 0 to 20 m. However, this response was species specific. Among red seaweeds, the majority relied solely on CO2 as an exogenous Ci source, with a high percentage of non-CCM species. Green seaweeds exhibited depth-dependent variations in CCM status, with the proportion of non-CCM species increasing at greater depths. Conversely, brown seaweeds exhibited a higher prevalence of CCM species, even in deep waters, indicating the use of CO2 and HCO3-. Our results are similar to those observed in temperate and tropical regions, indicating that the potential impacts of OA on Antarctic seaweeds will be species specific. Additionally, OA may potentially increase the abundance of non-CCM species relative to those with CCMs.

The occurrence of positive selection on BicA transporter of Microcystis aeruginosa

The rapid growth of cyanobacteria, particularly Microcystis aeruginosa, poses a significant threat to global water security. The proliferation of toxic Microcystis aeruginosa raises concerns due to its potential harm to human health and socioeconomic impacts. Dense blooms contribute to spatiotemporal inorganic carbon depletion, promoting interest in the roles of carbon-concentrating mechanisms (CCMs) for competitive carbon uptake. Despite the importance of HCO3- transporters, genetic evaluations and functional predictions in M. aeruginosa remain insufficient. In this study, we explored the diversity of HCO3- transporters in the genomes of 46 strains of M. aeruginosa, assessing positive selection for each. Intriguingly, although the Microcystis BicA transporter became a partial gene in 23 out of 46 genomic strains, we observed significant positive sites. Structural analyses, including predicted 2D and 3D models, confirmed the structural conservation of the Microcystis BicA transporter. Our findings suggest that the Microcystis BicA transport likely plays a crucial role in competitive carbon uptake, emphasizing its ecological significance. The ecological function of the Microcystis BicA transport in competitive growth during cyanobacterial blooms raises important questions. Future studies require experimental confirmation to better understand the role of the Microcysits BicA transporter in cyanobacterial blooms dynamics.

The biological functions of microcystins

Microcystins are potent hepatotoxins predominantly produced by bloom-forming freshwater cyanobacteria (e.g., Microcystis, Planktothrix, Dolichospermum). Microcystin biosynthesis involves large multienzyme complexes and tailoring enzymes encoded by the mcy gene cluster. Mutation, recombination, and deletion events have shaped the mcy gene cluster in the course of evolution, resulting in a large diversity of microcystin congeners and the natural coexistence of toxic and non-toxic strains. The biological functions of microcystins and their association with algal bloom formation have been extensively investigated over the past decades. This review synthesizes recent advances in decoding the biological role of microcystins in carbon/nitrogen metabolism, antioxidation, colony formation, and cell-to-cell communication. Microcystins appear to adopt multifunctional roles in cyanobacteria that reflect the adaptive plasticity of toxic cyanobacteria to changing environments.

Mechanistic characterization of dissolved inorganic phosphorus in water during the red tide

The eutrophication of water, such as excessive nitrogen and phosphorus, are closely associated with the outbreak of red tide. However, the response of dissolved inorganic phosphorus (DIP) to red tide remained unclear in water. In this study, three species of diatoms capable of causing red tides were cultured in simulated seawater with different concentrations of DIP. The changes of biomass, chlorophyll a concentration and the carbon stable isotope composition of microalgae, the DIP concentration and pH of the culture medium were compared among the experimental groups. In addition, correlation verification was used to test the correlation between the change of DIP concentration and other indicators. The results showed that in the experimental period, the DIP concentration of each experimental group decreased significantly first, and the concentration dropped to less than 40% of the initial level. After that, the pH of the medium, the biomass, chlorophyll a concentration and carbon stable isotope composition of the microalgae showed varying degrees of increase, and then stabilized or decreased. These also marked the outbreak of red tide. Moreover, the correlation test showed that there was a correlation between them and the change of DIP concentration. Therefore, by exploring the relationship between the change of DIP concentration in water and the occurrence of red tide, this study provides a possible direction for the current prediction of red tide, and provides a basis for further investigation of the occurrence mechanism of red tide.

Effects of carbon limitation and carbon fertilization on karst lake-reservoir productivity

Nitrogen and phosphorus are universally recognized as limiting elements in the eutrophication processes affecting the majority of the world's lakes, reservoirs, and coastal ecosystems. However, despite extensive research spanning several decades, critical questions in eutrophication science remain unanswered. For example, there is still much to understand about the interactions between carbon limitation and ecosystem stability, and the availability of carbon components adds significant complexity to aquatic resource management. Mounting evidence suggests that aqueous CO2 could be a limiting factor, influencing the structure and succession of aquatic plant communities, especially in karstic lake and reservoir ecosystems. Moreover, the fertilization effect of aqueous CO2 has the potential to enhance carbon sequestration and phosphorus removal. Therefore, it is important to address these uncertainties to achieve multiple positive outcomes, including improved water quality and increased carbon sinks in karst lakes and reservoirs.

Enhancing CO2 fixation by microalgae in a Photobioreactor: Molecular mechanisms with exogenous carbonic anhydrase

Microalgae biotechnology holds great potential for mitigating CO2 emissions, yet faces challenges in commercialization due to suboptimal photosynthetic efficiency. This study presents an innovative approach to improve CO2 mass transfer efficiency in microalgae using carbonic anhydrase (CA) in an internal LED flexible air-lift photobioreactor. Optimal conditions initial inoculation with 3.55 × 106 cells/mL and 20 % CO2 concentration, complemented by white LED lighting in Chlorella sp. CA regulated intracellular composition, enhancing chlorophyll, lipid, and protein contents. Metabolomics revealed elevated malic and succinic acids, associated with increased Ribulose 1,5-bisphosphate carboxylase oxygenase (RuBisCO) and Acetoacetyl coenzyme A (Acetyl-CoA) activities, facilitating efficient carbon fixation. CA also mitigated cellular oxidative stress by reducing reactive oxygen species (ROS). Furthermore, CA improved extracellular electron acceptor with currents surpassed CK. This CA-based microalgae biotechnology provides a foundation for future commercial applications, addressing CO2 emissions.

Monocultures vs. polyculture of microalgae: unveiling physiological changes to facilitate growth in ammonium rich-medium

Due to the increasing production of wastewater from human activities, the use of algal consortia for phytoremediation has become well-established over the past decade. Understanding how interspecific interactions and cultivation modes (monocultures vs. polyculture) influence algal growth and behaviour is a cutting-edge topic in both fundamental and applied science. Ammonium-rich growth media were used to challenge the monocultures of Auxenochlorella protothecoides, Chlamydomonas reinhardtii and Tetradesmus obliquus, as well as their polyculture; NO3 - was also used as the sole nitrogen chemical form in control cultures. The study primarily compared the growth, carbon and nitrogen metabolisms, and protein content of the green microalgae monocultures to those of their consortium. Overall, the cultivation mode significantly affected all the measured parameters. Notably, at 50 mM NH4 +, the assimilation rates of carbon and nitrogen were at least twice as high as those in the monoculture counterparts, and the protein content was three times more abundant.Additionally, the consortium's response to NH4 + toxicity was investigated by observing a linear relationship between the indicator of tolerance to NH4 + nutrition and the N isotopic signature. The study highlighted a high degree of acclimation through metabolic flexibility and diversity, as well as species abundance plasticity in the consortium, resulting in a functional resilience that would otherwise have been unattainable by the respective monocultures.

Usage of Chlorella and diverse microalgae for CO2 capture - towards a bioenergy revolution

To address climate change threats to ecosystems and the global economy, sustainable solutions for reducing atmospheric carbon dioxide (CO2) levels are crucial. Existing CO2 capture projects face challenges like high costs and environmental risks. This review explores leveraging microalgae, specifically the Chlorella genus, for CO2 capture and conversion into valuable bioenergy products like biohydrogen. The introduction section provides an overview of carbon pathways in microalgal cells and their role in CO2 capture for biomass production. It discusses current carbon credit industries and projects, highlighting the Chlorella genus's carbon concentration mechanism (CCM) model for efficient CO2 sequestration. Factors influencing microalgal CO2 sequestration are examined, including pretreatment, pH, temperature, irradiation, nutrients, dissolved oxygen, and sources and concentrations of CO2. The review explores microalgae as a feedstock for various bioenergy applications like biodiesel, biooil, bioethanol, biogas and biohydrogen production. Strategies for optimizing biohydrogen yield from Chlorella are highlighted. Outlining the possibilities of further optimizations the review concludes by suggesting that microalgae and Chlorella-based CO2 capture is promising and offers contributions to achieve global climate goals.

The genomic potential of photosynthesis in piconanoplankton is functionally redundant but taxonomically structured at a global scale

Carbon fixation is a key metabolic function shaping marine life, but the underlying taxonomic and functional diversity involved is only partially understood. Using metagenomic resources targeted at marine piconanoplankton, we provide a reproducible machine learning framework to derive the potential biogeography of genomic functions through the multi-output regression of gene read counts on environmental climatologies. Leveraging the Marine Atlas of Tara Oceans Unigenes, we investigate the genomic potential of primary production in the global ocean. The latter is performed by ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) and is often associated with carbon concentration mechanisms in piconanoplankton, major marine unicellular photosynthetic organisms. We show that the genomic potential supporting C4 enzymes and RUBISCO exhibits strong functional redundancy and important affinity toward tropical oligotrophic waters. This redundancy is taxonomically structured by the dominance of Mamiellophyceae and Prymnesiophyceae in mid and high latitudes. These findings enhance our understanding of the relationship between functional and taxonomic diversity of microorganisms and environmental drivers of key biogeochemical cycles.

Influence of multi-stressor combinations of pCO2, temperature, and salinity on the toxicity of Heterosigma akashiwo (Raphidophyceae), a fish-killing flagellate

Climate change and global warming have led to more frequent harmful algal blooms in the last decade. Among these blooms, Heterosigma akashiwo, a golden-brown phytoflagellate, is one of the 40 species with a high potential to form harmful blooms, leading to significant fish mortality. Climate change leads to rising atmospheric and ocean temperatures. These changes, along with altered rainfall patterns and meltwater input, can cause fluctuations in ocean salinity. Elevated atmospheric carbon dioxide (CO2) levels increase water acidity as oceans absorb CO2. This study investigated the effects of temperature, salinity, and CO2 levels on lipid production, hemolytic activity, and toxicity of H. akashiwo using the design of experiment approach, which can be used to investigate the effect of two or more factors on the same response simultaneously in a precise manner with fewer experiments and materials but in a larger region of the factor space. The lipid content was measured using a high-throughput Nile Red method, and the highest level of lipid content was detected at 25°C, a salinity of 30, and a CO2 concentration of 400 ppm. Hemolytic activity was assessed using rabbit blood erythrocytes in a 96-well plate, and the optimal conditions for achieving the highest hemolytic activity were determined at 15°C, a salinity of 10, and a CO2 concentration of 400 ppm. As the chemical structure of the toxin is not known, we used the toxicity against the cell line RTgill-W1 as the cell toxicity proxy. The maximum toxicity was identified at 15°C, a salinity of 10, and a CO2 level of 700 ppm.

Dissolved nitrogen uptake versus nitrogen fixation: Mode of nitrogen acquisition affects stable isotope signatures of a diazotrophic cyanobacterium and its grazer

Field studies suggest that changes in the stable isotope ratios of phytoplankton communities can be used to track changes in the utilization of different nitrogen sources, i.e., to detect shifts from dissolved inorganic nitrogen (DIN) uptake to atmospheric nitrogen (N2) fixation by diazotrophic cyanobacteria as an indication of nitrogen limitation. We explored changes in the stable isotope signature of the diazotrophic cyanobacterium Trichormus variabilis in response to increasing nitrate (NO3-) concentrations (0 to 170 mg L-1) under controlled laboratory conditions. In addition, we explored the influence of nitrogen utilization at the primary producer level on trophic fractionation by studying potential changes in isotope ratios in the freshwater model Daphnia magna feeding on the differently grown cyanobacteria. We show that δ 15N values of the cyanobacterium increase asymptotically with DIN availability, from -0.7 ‰ in the absence of DIN (suggesting N2 fixation) to 2.9 ‰ at the highest DIN concentration (exclusive DIN uptake). In contrast, δ 13C values of the cyanobacterium did not show a clear relationship with DIN availability. The stable isotope ratios of the consumer reflected those of the differently grown cyanobacteria but also revealed significant trophic fractionation in response to nitrogen utilization at the primary producer level. Nitrogen isotope turnover rates of Daphnia were highest in the absence of DIN as a consequence of N2 fixation and resulting depletion in 15N at the primary producer level. Our results highlight the potential of stable isotopes to assess nitrogen limitation and to explore diazotrophy in aquatic food webs.

Acidification alleviates the inhibition of hyposaline stress on physiological performance of tropical seagrass Thalassia hemprichii

Since the Industrial Revolution, increasing atmospheric CO2 concentrations have had a substantial negative impact influence on coastal ecosystems because of direct effects including ocean acidification and indirect effects such as extreme rainfall events. Using a two-factor crossover indoor simulation experiment, this study examined the combined effects of acidification and hyposaline stress on Thalassia hemprichii. Seawater acidification increased the photosynthetic pigment content of T. hemprichii leaves and promoted seagrass growth rate. Hyposaline stress slowed down seagrass growth and had an impact on the osmotic potential and osmoregulatory substance content of seagrass leaves. Acidification and salinity reduction had significant interaction effects on the photosynthesis rate, photosynthetic pigment content, chlorophyll fluorescence parameters, and osmotic potential of T. hemprichii, but not on the growth rate. Overall, these findings have shown that the hyposaline stress inhibitory effect on the T. hemprichii physiological performance and growth may be reduced by acidification.

Multifaceted Dinoflagellates and the Marine Model Prorocentrum cordatum

BACKGROUND

Dinoflagellates are a monophyletic group within the taxon Alveolata, which comprises unicellular eukaryotes. Dinoflagellates have long been studied for their organismic and morphologic diversity as well as striking cellular features. They have a main size range of 10-100 µm, a complex "cell covering", exceptionally large genomes (∼1-250 Gbp with a mean of 50,000 protein-encoding genes) spread over a variable number of highly condensed chromosomes, and perform a closed mitosis with extranuclear spindles (dinomitosis). Photosynthetic, marine, and free-living Prorocentrum cordatum is a ubiquitously occurring, bloom-forming dinoflagellate, and an emerging model system, particularly with respect to systems biology.

SUMMARY

Focused ion beam/scanning electron microscopy (FIB/SEM) analysis of P. cordatum recently revealed (i) a flattened nucleus with unusual structural features and a total of 62 tightly packed chromosomes, (ii) a single, barrel-shaped chloroplast devoid of grana and harboring multiple starch granules, (iii) a single, highly reticular mitochondrion, and (iv) multiple phosphate and lipid storage bodies. Comprehensive proteomics of subcellular fractions suggested (i) major basic nuclear proteins to participate in chromosome condensation, (ii) composition of nuclear pores to differ from standard knowledge, (iii) photosystems I and II, chloroplast complex I, and chlorophyll a-b binding light-harvesting complex to form a large megacomplex (>1.5 MDa), and (iv) an extraordinary richness in pigment-binding proteins. Systems biology-level investigation of heat stress response demonstrated a concerted down-regulation of CO2-concentrating mechanisms, CO2-fixation, central metabolism, and monomer biosynthesis, which agrees with reduced growth yields.

KEY MESSAGES

FIB/SEM analysis revealed new insights into the remarkable subcellular architecture of P. cordatum, complemented by proteogenomic unraveling of novel nuclear structures and a photosynthetic megacomplex. These recent findings are put in the wider context of current understanding of dinoflagellates.

Enhanced biomass production and harvesting efficiency of Chlamydomonas reinhardtii under high-ammonium conditions by powdered oyster shell

Chlamydomonas reinhardtii prefers ammonium (NH4+) as a nitrogen source, but its late-stage growth under high-NH4+ concentrations (0.5 ∼ 1 g/L) is retarded due to medium acidification. In this study, oyster shell powders were shown to increase the tolerance of C. reinhardtii to NH4+ supplementation at 0.7 g/L in TAP medium in 1-L bubble-column bioreactors, resulting in a 22.9 % increase in biomass production, 62.1 % rise in unsaturated fatty acid accumulation, and 19.2 % improvement in harvesting efficiency. Powdered oyster shell mitigated medium acidification (pH 7.2-7.8) and provided dissolved inorganic carbon up to 8.02 × 103 μmol/L, facilitating a 76.3 % NH4+ consumption, release of up to 189 mg/L of Ca2+, a 42.1 % reduction in ζ-potential and 27.7 % increase in flocculation activity of microalgae cells. This study highlights a promising approach to utilize powdered oyster shell as a liming agent, supplement carbon source, and bio-flocculant for enhancing biomass production and microalgae harvesting in NH4+-rich environments.

Hydrogen isotope fractionation is controlled by CO2 in coccolithophore lipids

Hydrogen isotope ratios (δ2H) represent an important natural tracer of metabolic processes, but quantitative models of processes controlling H-fractionation in aquatic photosynthetic organisms are lacking. Here, we elucidate the underlying physiological controls of 2H/1H fractionation in algal lipids by systematically manipulating temperature, light, and CO2(aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica. We analyze the hydrogen isotope fractionation in alkenones (αalkenone), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the αalkenone with increasing CO2(aq) and confirm αalkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ2H of algal acyl lipids to evaluate processes contributing to these controls on fractionation. Simulations show that longer residence times of NADPH in the chloroplast favor a greater exchange of NADPH with 2H-richer intracellular water, increasing αalkenone. Higher chloroplast CO2(aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of αalkenone to CO2(aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO2 at the Rubisco site, but rather that chloroplast CO2 varies with external CO2(aq). The pervasive inverse correlation of αalkenone with CO2(aq) in the modern and preindustrial ocean also suggests that natural populations may not attain a constant saturation of Rubisco with the CCM. Rather than reconstructing growth water, αalkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis.

Evidence of a putative CO2 delivery system to the chromatophore in the photosynthetic amoeba Paulinella

The photosynthetic amoeba, Paulinella provides a recent (ca. 120 Mya) example of primary plastid endosymbiosis. Given the extensive data demonstrating host lineage-driven endosymbiont integration, we analysed nuclear genome and transcriptome data to investigate mechanisms that may have evolved in Paulinella micropora KR01 (hereinafter, KR01) to maintain photosynthetic function in the novel organelle, the chromatophore. The chromatophore is of α-cyanobacterial provenance and has undergone massive gene loss due to Muller's ratchet, but still retains genes that encode the ancestral α-carboxysome and the shell carbonic anhydrase, two critical components of the biophysical CO2 concentrating mechanism (CCM) in cyanobacteria. We identified KR01 nuclear genes potentially involved in the CCM that arose via duplication and divergence and are upregulated in response to high light and downregulated under elevated CO2. We speculate that these genes may comprise a novel CO2 delivery system (i.e., a biochemical CCM) to promote the turnover of the RuBisCO carboxylation reaction and counteract photorespiration. We posit that KR01 has an inefficient photorespiratory system that cannot fully recycle the C2 product of RuBisCO oxygenation back to the Calvin-Benson cycle. Nonetheless, both these systems appear to be sufficient to allow Paulinella to persist in environments dominated by faster-growing phototrophs.

Algal-bacterial consortium promotes carbon sink formation in saline environment

INTRODUCTION

The level of atmospheric CO2 has continuously been increasing and the resulting greenhouse effects are receiving attention globally. Carbon removal from the atmosphere occurs naturally in various ecosystems. Among them, saline environments contribute significantly to the global carbon cycle. Carbonate deposits in the sediments of salt lakes are omnipresent, and the biological effects, especially driven by halophilic microalgae and bacteria, on carbonate formation remain to be elucidated.

OBJECTIVES

The present study aims to characterize the carbonates formed in saline environments and demonstrate the mechanisms underlying biological-driven CO2 removal via microalgal-bacterial consortium.

METHODS

The carbonates naturally formed in saline environments were collected and analyzed. Two saline representative organisms, the photosynthetic microalga Dunaliella salina and its mutualistic halophilic bacteria Nesterenkonia sp. were isolated from the inhabiting saline environment and co-cultivated to study their biological effects on carbonates precipitation and isotopic composition. During this process, electrochemical parameters and Ca2+ flux, and expression of genes related to CaCO3 formation were analyzed. Genome sequencing and metagenomic analysis were conducted to provide molecular evidence.

RESULTS

The results showed that natural saline sediments are enriched with CaCO3 and enrichment of genes related to photosynthesis and ureolysis. The co-cultivation stimulated 54.54% increase in CaCO3 precipitation and significantly promoted the absorption of external CO2 by 49.63%. A pH gradient was formed between the bacteria and algae culture, creating 150.22 mV of electronic potential, which might promote Ca2+ movement toward D. salina cells. Based on the results of lab-scale induction and 13C analysis, a theoretical calculation indicates a non-negligible amount of 0.16 and 2.3 Tg C/year carbon sequestration in China and global saline lakes, respectively.

CONCLUSION

The combined effects of these two typical representative species have contributed to the carbon sequestration in saline environments, by promoting Ca2+ influx and increase of pH via microalgal and bacterial metabolic processes.

Individual and combined impacts of carbon dioxide enrichment, heatwaves, flow velocity variability, and fine sediment deposition on stream invertebrate communities

Climate change and land-use change are widely altering freshwater ecosystem functioning and there is an urgent need to understand how these broad stressor categories may interact in future. While much research has focused on mean temperature increases, climate change also involves increasing variability of both water temperature and flow regimes and increasing concentrations of atmospheric CO2, all with potential to alter stream invertebrate communities. Deposited fine sediment is a pervasive land-use stressor with widespread impacts on stream invertebrates. Sedimentation may be managed at the catchment scale; thus, uncovering interactions with these three key climate stressors may assist mitigation of future threats. This is the first experiment to investigate the individual and combined effects of enriched CO2, heatwaves, flow velocity variability, and fine sediment on realistic stream invertebrate communities. Using 128 mesocosms simulating small stony-bottomed streams in a 7-week experiment, we manipulated dissolved CO2 (ambient; enriched), fine sediment (no sediment; 300 g dry sediment), temperature (ambient; two 7-day heatwaves), and flow velocity (constant; variable). All treatments changed community composition. CO2 enrichment reduced abundances of Orthocladiinae and Chironominae and increased Copepoda abundance. Variable flow velocity had only positive effects on invertebrate abundances (7 of 13 common taxa and total abundance), in contrast to previous experiments showing negative impacts of reduced velocity. CO2 was implicated in most stressor interactions found, with CO2 × sediment interactions being most common. Communities forming under enriched CO2 conditions in sediment-impacted mesocosms had ~20% fewer total invertebrates than those with either treatment alone. Copepoda abundances doubled in CO2-enriched mesocosms without sediment, whereas no CO2 effect occurred in mesocosms with sediment. Our findings provide new insights into potential future impacts of climate change and land use in running freshwaters, in particular highlighting the potential for elevated CO2 to interact with fine sediment deposition in unpredictable ways.

Chromosome-scale assembly of the streamlined picoeukaryote Picochlorum sp. SENEW3 genome reveals Rabl-like chromatin structure and potential for C4 photosynthesis

Genome sequencing and assembly of the photosynthetic picoeukaryotic Picochlorum sp. SENEW3 revealed a compact genome with a reduced gene set, few repetitive sequences, and an organized Rabl-like chromatin structure. Hi-C chromosome conformation capture revealed evidence of possible chromosomal translocations, as well as putative centromere locations. Maintenance of a relatively few selenoproteins, as compared to similarly sized marine picoprasinophytes Mamiellales, and broad halotolerance compared to others in Trebouxiophyceae, suggests evolutionary adaptation to variable salinity environments. Such adaptation may have driven size and genome minimization and have been enabled by the retention of a high number of membrane transporters. Identification of required pathway genes for both CAM and C4 photosynthetic carbon fixation, known to exist in the marine mamiellale pico-prasinophytes and seaweed Ulva, but few other chlorophyte species, further highlights the unique adaptations of this robust alga. This high-quality assembly provides a significant advance in the resources available for genomic investigations of this and other photosynthetic picoeukaryotes.

Effects of HCO3- and CO2 conversion rates on carbon assimilation strategies in marine microalgae: Implication by stable carbon isotope analysis of fatty acids

Marine microalgae are an essential component of marine plankton and critical primary producers, playing a vital role in marine ecosystems. The seawater carbonate system is a dynamic equilibrium system, and changes in any component can alter the carbonate balance. In CO2-concentrating mechanisms (CCMs), carbonic anhydrase (CA) regulates CO2 concentration by catalyzing the interconversion between CO2 and HCO3-. Therefore, limiting the activity of extracellular carbonic anhydrase (exCA) alters the rate at which carbonate equilibrium is reached and further affects the carbon assimilation process in microalgae. In this study, two different microalgae, Phaeodactylum tricornutum and Nannochloropsis oceanica, were selected to investigate the effects of changes in the carbonate system on photosynthetic carbon assimilation in microalgae by inhibiting exCA activity with acetazolamide (AZ). Inhibition of exCA activity reduces specific growth rates and photosynthetic efficiency of microalgae. The total alkalinity, HCO3- concentration, and CO2 concentration of the cultures increased with the decrease of pH, but the changes of the ribulose 1,5- bisphosphate carboxylase/oxygenase (Rubisco) activities of the two microalgae were different. In addition, the two microalgae possessed different lipid and carbohydrate synthesis strategies, but both restricted triacylglycerol (TAG) synthesis. Meanwhile, the microalgal cells had to utilize more 13CO2 when HCO3- and CO2 conversion rates were limited and restricted. This led to the continuous accumulation of 13C in fatty acids and the elevation of δ13CFAs. In conclusion, our study provides a new perspective on the role of microalgae in correlating carbonate changes with photosynthetic carbon assimilation strategies under mechanistic constraints on inorganic carbon utilization.

Antagonistic interactions of the dinoflagellate Alexandrium catenella under simultaneous warming and acidification

There is a concern that harmful algal bloom (HAB) species may increase under climate change. Yet, we lack understanding of how ecological interactions will be affected under ocean warming and acidification (OWA) conditions. We tested the antagonistic effects of three strains of the dinoflagellate HAB species Alexandrium catenella on three target species (the chlorophyte Tetraselmis sp., the cryptomonad Rhodomonas salina, and the diatom Thalassiosira weissflogii) at various biomass ratios between species, at ambient (16 °C and 400 µatm CO2) and OWA (20 °C and 2000 µatm CO2) conditions. In these experiments the Alexandrium strains had been raised under OWA conditions for ∼100 generations. All three non-HAB species increased their growth rate under OWA relative to ambient conditions. Growth rate inhibition was evident for R. salina and Tetraselmis sp. under OWA conditions, but not under ambient conditions. These negative effects were exacerbated at higher concentrations of Alexandrium relative to non-HAB species. By contrast, T. weissflogii showed positive growth in the presence of two strains of Alexandrium under ambient conditions, whereas growth was unaffected under OWA. Contrary to our expectations, A. catenella had a slight negative response in the presence of the diatom. These results demonstrate that Alexandrium exerts higher antagonistic effects under OWA compared to ambient conditions, and these effects are species-specific and density dependent. These negative effects may shift phytoplankton community composition under OWA conditions.

Physiological-biochemical responses and transcriptomic analysis reveal the effects and mechanisms of sulfamethoxazole on the carbon fixation function of Chlorella pyrenoidosa

The occurrence of sulfamethoxazole (SMX) is characterized by low concentration and pseudo-persistence. However, the toxic effects and mechanisms of SMX, especially for low concentration and long-term exposure, are still not clear. This study investigated the effects and mechanisms of SMX on carbon fixation-related biological processes of Chlorella pyrenoidosa at population, physiological-biochemical, and transcriptional levels. Results showed that 1-1000 μg/L SMX significantly inhibited the dry weight and carbon fixation rate of C. pyrenoidosa during 21 d. The upregulation of superoxide dismutase (SOD) and catalase (CAT) activities, as well as the accumulation of malondialdehyde (MDA) demonstrated that SMX posed oxidative damage to C. pyrenoidosa. SMX inhibited the activity of carbonic anhydrase (CA), and consequently stimulated the activity of Rubisco. Principal component analysis (PCA) revealed that SMX concentration was positively correlated with Rubisco and CAT while exposure time was negatively correlated with CA. Transcriptional analysis showed that the synthesis of chlorophyll-a was stabilized by regulating the diversion of protoporphyrin IX and the chlorophyll cycle. Meanwhile, multiple CO2 compensation mechanisms, including photorespiratory, C4-like CO2 compensation and purine metabolism pathways were triggered in response to the CO2 requirements of Rubisco. This study provides a scientific basis for the comprehensive assessment of the ecological risk of SMX.

The diversity and coevolution of Rubisco and CO2 concentrating mechanisms in marine macrophytes

The kinetic properties of Rubisco, the most important carbon-fixing enzyme, have been assessed in a small fraction of the estimated existing biodiversity of photosynthetic organisms. Until recently, one of the most significant gaps of knowledge in Rubisco kinetics was marine macrophytes, an ecologically relevant group including brown (Ochrophyta), red (Rhodophyta) and green (Chlorophyta) macroalgae and seagrasses (Streptophyta). These organisms express various Rubisco types and predominantly possess CO2 -concentrating mechanisms (CCMs), which facilitate the use of bicarbonate for photosynthesis. Since bicarbonate is the most abundant form of dissolved inorganic carbon in seawater, CCMs allow marine macrophytes to overcome the slow gas diffusion and low CO2 availability in this environment. The present review aims to compile and integrate recent findings on the biochemical diversity of Rubisco and CCMs in the main groups of marine macrophytes. The Rubisco kinetic data provided demonstrate a more relaxed relationship among catalytic parameters than previously reported, uncovering a variability in Rubisco catalysis that has been hidden by a bias in the literature towards terrestrial vascular plants. The compiled data indicate the existence of convergent evolution between Rubisco and biophysical CCMs across the polyphyletic groups of marine macrophytes and suggest a potential role for oxygen in shaping such relationship.

The interactions between olivine dissolution and phytoplankton in seawater: Potential implications for ocean alkalinization

Ocean alkalinity enhancement, one of the ocean-based CO2 removal techniques, has the potential to assist us in achieving the goal of carbon neutrality. Olivine is considered the most promising mineral for ocean alkalinization enhancement due to its theoretically high CO2 sequestration efficiency. Olivine dissolution has been predicted to alter marine phytoplankton communities, however, there is still a lack of experimental evidence. The olivine dissolution process in seawater can be influenced by a range of factors, including biotic factors, which have yet to be explored. In this study, we cultivated two diatoms and one coccolithophore with and without olivine particles to investigate their interactions with olivine dissolution. Our findings demonstrate that olivine dissolution promoted the growth of all phytoplankton species, with the highly silicified diatom Thalassiosira pseudonana benefiting the most. This was probably due to the highly silicified diatom having a higher silicate requirement and, therefore, growing more quickly when silicate was released during olivine dissolution. Based on the structural characteristics and chemical compositions on the exterior surface of olivine particles, T. pseudonana was found to promote olivine dissolution by inhibiting the formation of the amorphous SiO2 layer on the surface of olivine and therefore enhancing the stoichiometric dissolution of olivine. However, the positive effects of T. pseudonana on olivine dissolution were not observed in the coccolithophore Gephyrocapsa oceanica or the non-silicate obligate diatom Phaeodactylum tricornutum. This study provides the first experimental evidence of the interaction between phytoplankton and olivine dissolution, which has important implications for ocean alkalinization research.

Features of the microalga Raphidocelis subcapitata: physiology and applications

The microalga Raphidocelis subcapitata was isolated from the Nitelva River (Norway) and subsequently deposited in the collection of the Norwegian Institute of Water Research as "Selenastrum capricornutum Printz". This freshwater microalga, also known as Pseudokirchneriella subcapitata, acquired much of its notoriety due to its high sensitivity to different chemical species, which makes it recommended by different international organizations for the assessment of ecotoxicity. However, outside this scope, R. subcapitata continues to be little explored. This review aims to shed light on a microalga that, despite its popularity, continues to be an "illustrious" unknown in many ways. Therefore, R. subcapitata taxonomy, phylogeny, shape, size/biovolume, cell ultra-structure, and reproduction are reviewed. The nutritional and cultural conditions, chronological aging, and maintenance and preservation of the alga are summarized and critically discussed. Applications of R. subcapitata, such as its use in aquatic toxicology (ecotoxicity assessment and elucidation of adverse toxic outcome pathways) are presented. Furthermore, the latest advances in the use of this alga in biotechnology, namely in the bioremediation of effluents and the production of value-added biomolecules and biofuels, are highlighted. To end, a perspective regarding the future exploitation of R. subcapitata potentialities, in a modern concept of biorefinery, is outlined. KEY POINTS: • An overview of alga phylogeny and physiology is critically reviewed. • Advances in alga nutrition, cultural conditions, and chronological aging are presented. • Its use in aquatic toxicology and biotechnology is highlighted.

Cool-edge populations of the kelp Ecklonia radiata under global ocean change scenarios: strong sensitivity to ocean warming but little effect of ocean acidification

Kelp forests are threatened by ocean warming, yet effects of co-occurring drivers such as CO2 are rarely considered when predicting their performance in the future. In Australia, the kelp Ecklonia radiata forms extensive forests across seawater temperatures of approximately 7-26°C. Cool-edge populations are typically considered more thermally tolerant than their warm-edge counterparts but this ignores the possibility of local adaptation. Moreover, it is unknown whether elevated CO2 can mitigate negative effects of warming. To identify whether elevated CO2 could improve thermal performance of a cool-edge population of E. radiata, we constructed thermal performance curves for growth and photosynthesis, under both current and elevated CO2 (approx. 400 and 1000 µatm). We then modelled annual performance under warming scenarios to highlight thermal susceptibility. Elevated CO2 had minimal effect on growth but increased photosynthesis around the thermal optimum. Thermal optima were approximately 16°C for growth and approximately 18°C for photosynthesis, and modelled performance indicated cool-edge populations may be vulnerable in the future. Our findings demonstrate that elevated CO2 is unlikely to offset negative effects of ocean warming on the kelp E. radiata and highlight the potential susceptibility of cool-edge populations to ocean warming.

Distinctive microfossil supports early Paleoproterozoic rise in complex cellular organisation

The great oxidation event (GOE), ~2.4 billion years ago, caused fundamental changes to the chemistry of Earth's surface environments. However, the effect of these changes on the biosphere is unknown, due to a worldwide lack of well-preserved fossils from this time. Here, we investigate exceptionally preserved, large spherical aggregate (SA) microfossils permineralised in chert from the c. 2.4 Ga Turee Creek Group in Western Australia. Field and petrographic observations, Raman spectroscopic mapping, and in situ carbon isotopic analyses uncover insights into the morphology, habitat, reproduction and metabolism of this unusual form, whose distinctive, SA morphology has no known counterpart in the fossil record. Comparative analysis with microfossils from before the GOE reveals the large SA microfossils represent a step-up in cellular organisation. Morphological comparison to extant micro-organisms indicates the SAs have more in common with coenobial algae than coccoidal bacteria, emphasising the complexity of this microfossil form. The remarkable preservation here provides a unique window into the biosphere, revealing an increase in the complexity of life coinciding with the GOE.

Anthropogenic nitrogen pollution inferred by stable isotope records of crustose coralline algae

Since reef ecosystems can offer intricate habitats for various marine organisms, calcified reefs may contain valuable long-term environmental data. This study investigated stable isotopic composition of marine organisms from the Taoyuan and Linshanbi crustose coralline algae (CCA) reef ecosystems to understand sewage pollution. CCA samples from Taoyuan (Palaeo Xin A: ∼1000 years old and Palaeo G: ∼7000 years old) and Linshanbi (Palaeo L: ∼7000 years old and modern CCA) had significantly lower δ15N values (2.5-5.6 ‰) compared to modern CCA from Taoyuan (10.2 ± 1.2 ‰). Intertidal organisms from the Taoyuan CCA reef also showed higher δ15N values than those from Linshanbi CCA reef, indicating anthropogenic stress in both ecosystems. Long-term pollution monitoring and effective strategies to mitigate sewage pollution are recommended for these CCA reef ecosystems.