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Algae play critical roles in the carbon dioxide (CO 2 ) exchange between the water bodies and the atmosphere. However, the effects of prokaryotic and eukaryotic algae on carbon utilization, CO 2 flux, and the underlying mechanisms remain poorly understood. Therefore, this study investigated the differences in carbon preferences and CO 2 fluxes under different algal dominance days. Our research revealed that dissolved inorganic carbon (DIC) concentration fluctuations had a limited effect on the relative abundance of algae. However, shifts in dominant algal phyla induced changes in DIC, with Cyanobacteria preferring HCO 3 - and Chlorophyta preferring CO 2 . Analysis of the water chemistry balance indicated that the growth of Chlorophyta had a 15.59 times greater effect on CO 2 sinks compared with that of Cyanobacteria. During the Cyanobacteria dominance days, the lower DIC concentration did not result in a reduction in CO 2 emissions. However, increases in the dissolved organic carbon concentration provided a favorable environment for Cyanobacteria, which promoted CO 2 emissions. The CCM model indicated that the growth of Chlorophyta resulted in CO 2 uptake rates at least 3.57 times higher and CO 2 leakage rates up to 0.97 times lower compared to Cyanobacteria, accelerating CO 2 transport into the cell. Overall, CO 2 sink was stronger on Chlorophyta dominance days than on Cyanobacteria dominance days. This study emphasized the influence of algal phyla on CO 2 fluxes, revealing the significant CO 2 sink associated with Chlorophyta. Further research should investigate how to manipulate environmental factors to favor Chlorophyta growth and effectively reduce CO 2 emissions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Hydrogen isotope ratios (δ 2 H) 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 2 H/ 1 H fractionation in algal lipids by systematically manipulating temperature, light, and CO 2 (aq) in continuous cultures of the haptophyte Gephyrocapsa oceanica . We analyze the hydrogen isotope fractionation in alkenones (α alkenone ), a class of acyl lipids specific to this species and other haptophyte algae. We find a strong decrease in the α alkenone with increasing CO 2 (aq) and confirm α alkenone correlates with temperature and light. Based on the known biosynthesis pathways, we develop a cellular model of the δ 2 H 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 2 H-richer intracellular water, increasing α alkenone . Higher chloroplast CO 2 (aq) and temperature shorten NADPH residence time by enhancing the carbon fixation and lipid synthesis rates. The inverse correlation of α alkenone to CO 2 (aq) in our cultures suggests that carbon concentrating mechanisms (CCM) do not achieve a constant saturation of CO 2 at the Rubisco site, but rather that chloroplast CO 2 varies with external CO 2 (aq). The pervasive inverse correlation of α alkenone with CO 2 (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, α alkenone may be a powerful tool to elucidate the carbon limitation of photosynthesis., Competing Interests: Competing interests statement:The authors declare no competing interest.

Heterosigma akashiwo is a raphidophyte known for forming ichthyotoxic blooms. In order to predict the potential impacts of rising CO 2 on H. akashiwo it is necessary to understand the factors influencing growth rates over a range of CO 2 concentrations. Here we examined the physiology and gene expression response of H. akashiwo to concentrations from 200 to 1000ppm CO 2 . Growth rate data were combined from this and previous studies and fit with a CO 2 limitation-inhibition model that revealed an apparent growth optimum around 600-800ppm CO 2 . Physiological changes included a significant increase in C:N ratio at ∼800ppm CO 2 and a significant decrease in hydrogen peroxide concentration at ∼1000ppm. Whole transcriptome sequencing of H. akashiwo revealed sharp distinctions in metabolic pathway gene expression between ∼600 and ∼800ppm CO 2 . Hierarchical clustering by co-expression identified groups of genes with significant correlations to CO 2 and growth rate. Genes with significant differential expression with CO 2 included carbon concentrating mechanism genes such as beta-carbonic anhydrases and a bicarbonate transporter, which may underpin shifts in physiology. Genes involved in cell motility were significantly changed by both elevated CO 2 and growth rate, suggesting that future ocean conditions could modify swimming behavior in this species., (Copyright © 2018 Elsevier GmbH. All rights reserved.)

The response of marine phytoplankton to the ongoing increase in atmospheric pCO 2 reflects the consequences of both increased CO 2 concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii, we explored the effects of varying pCO 2 and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H 2 18 on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In short-term experiments (~3 min), we also observed no effects of pCO 2 on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In short-term experiments (~3 min), we also observed no effects of pCO 2 or pH, even under limiting light intensity. We surmise that in T. weissflogii, it is the photosynthetic production of NADPH and ATP, rather than the CO 2 -saturation of Rubisco that controls the rate of photosynthesis at low irradiance. In short-term experiments, we observed a slightly higher respiration rate at low pH at the onset of the dark period, possibly reflecting the energy used for exporting H + and maintaining pH homeostasis. Based on what is known of the biochemistry of marine phytoplankton, our results are likely generalizable to other diatoms and a number of other eukaryotic species. The direct effects of ocean acidification on growth, photosynthesis and respiration in these organisms should be small over the range of atmospheric pCO 2 predicted for the twenty-first century.

Many autotrophic microorganisms are likely to adapt to scarcity in dissolved inorganic carbon (DIC; CO 2 + HCO 3 - + CO 3 2- ) with CO 2 concentrating mechanisms (CCM) that actively transport DIC across the cell membrane to facilitate carbon fixation. Surprisingly, DIC transport has been well studied among cyanobacteria and microalgae only. The deep-sea vent gammaproteobacterial chemolithoautotroph Thiomicrospira crunogena has a low-DIC inducible CCM, though the mechanism for uptake is unclear, as homologs to cyanobacterial transporters are absent. To identify the components of this CCM, proteomes of T. crunogena cultivated under low- and high-DIC conditions were compared. Fourteen proteins, including those comprising carboxysomes, were at least 4-fold more abundant under low-DIC conditions. One of these proteins was encoded by Tcr_0854 ; strains carrying mutated copies of this gene, as well as the adjacent Tcr_0853 , required elevated DIC for growth. Strains carrying mutated copies of Tcr_0853 and Tcr_0854 overexpressed carboxysomes and had diminished ability to accumulate intracellular DIC. Based on reverse transcription (RT)-PCR, Tcr_0853 and Tcr_0854 were cotranscribed and upregulated under low-DIC conditions. The Tcr_0853 -encoded protein was predicted to have 13 transmembrane helices. Given the mutant phenotypes described above, Tcr_0853 and Tcr_0854 may encode a two-subunit DIC transporter that belongs to a previously undescribed transporter family, though it is widespread among autotrophs from multiple phyla. IMPORTANCE DIC uptake and fixation by autotrophs are the primary input of inorganic carbon into the biosphere. The mechanism for dissolved inorganic carbon uptake has been characterized only for cyanobacteria despite the importance of DIC uptake by autotrophic microorganisms from many phyla among the Bacteria and Archaea In this work, proteins necessary for dissolved inorganic carbon utilization in the deep-sea vent chemolithoautotroph T. crunogena were identified, and two of these may be able to form a novel transporter. Homologs of these proteins are present in 14 phyla in Bacteria and also in one phylum of Archaea , the Euryarchaeota Many organisms carrying these homologs are autotrophs, suggesting a role in facilitating dissolved inorganic carbon uptake and fixation well beyond the genus Thiomicrospira ., (Copyright © 2017 American Society for Microbiology.)

As all plastids that have been investigated so far can be traced back to endosymbiotic uptake of cyanobacteria by heterotrophic host cells, they accordingly show a high similarity regarding photosynthesis, which includes both the photosystems and the biochemical reactions around the CO2 fixation via the Calvin-Bassham cycle. Major differences between the different algal and plant groups may include the presence or absence of carbon concentrating mechanisms, pyrenoids, Rubisco activases, carbonic anhydrases as well as differences in the regulation of the Calvin-Bassham cycle. This review describes the diversity of primary carbon fixation steps in algae and plants and the respective regulatory mechanisms., (Copyright © 2014 Elsevier GmbH. All rights reserved.)

Summary: C2 photosynthesis is a carbon concentrating mechanism that can increase net CO2 assimilation by capturing, concentrating and re‐assimilating CO2 released by photorespiration. Empirical and modelling studies indicate that C2 plants assimilate more carbon than C3 plants under high temperature, bright light, and low CO2 conditions. I argue that engineering C2 photosynthesis into C3 crops is a promising approach to improve photosynthetic performance under these – and temporally heterogeneous – environments, and review the modifications that may re‐create a C2 phenotype in C3 plants. Although a C2 engineering program would encounter many of the same challenges faced by C4 engineering programmes, the simpler leaf anatomical requirements make C2 engineering a feasible approach to improve crops in the medium term. [ABSTRACT FROM AUTHOR]

Aquatic photosynthetic microorganisms, cyanobacteria and microalgae, account for almost half of the world's photosynthesis. They absorb carbon di oxide (CO2) as the major substrate to support photosynthesis, the beginning of energy flow into living organisms and one of the primary processes comprising the global carbon cycle. Among all photosynthetic mechanisms, inorganic carbon transport into ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is one of the major limiting steps in photosynthetic carbon fixation which involves active transport of HCO3−, CO2and/or H+, or an energized biochemical mechanism. In fact, a unique system “carbon concentrating mechanism (CCM)” manages the inorganic carbon assimilation, accumulation of CO2around RuBisCO, and utilization in algal cells. However, the information on mechanism of CO2uptake and fixation inside the algal cells is limited. In order to make strategies for enhancement of CO2fixation, understanding of CCM is crucial. Thus, this review provides an overview of advances in CCM research, the comparative state of the art and reports on the CO2uptake model in cyanobacteria and microalgae. The review also discusses the challenges and future perspectives associated with algal CCM research. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the catalytic activity and inhibition of the β-class carbonic anhydrase (CA, EC 4.2.1.1) CahB1, from the relict cyanobacterium Coleofasciculus chthonoplastes (previously denominated Microcoleus chthonoplastes). The enzyme showed good activity as a catalyst for the CO2 hydration, with a k cat of 2.4×105 s−1 and a k cat/K m of 6.3×107 M−1 s−1. A range of inorganic anions and small molecules were investigated as inhibitors of CahB1. Perchlorate and tetrafluoroborate did not inhibit the enzyme (K Is >200mM) whereas selenate and selenocyanide were ineffective inhibitors too, with K Is of 29.9–48.61mM. The halides, pseudohalides, carbonate, bicarbonate, trithiocarbonate and a range of heavy metal ions-containing anions were submillimolar–millimolar inhibitors (K Is in the range of 0.15–0.90mM). The best CahB1 inhibitors were N,N-diethyldithiocarbamate, sulfamate, sulfamide, phenylboronic acid and phenylarsonic acid, with K Is in the range of 8–75μM, whereas acetazolamide inhibited the enzyme with a K I of 76nM. This is the first kinetic and inhibition study of a cyanobacterial CA. As these enzymes are widespread in many cyanobacteria, being crucial for the carbon concentrating mechanism which assures substrate to RubisCO for the CO2 fixation by these organisms, a detailed kinetic/inhibition study may be essential for a better understanding of this superfamily of metalloenzymes and for potential biotechnological applications in biomimetic CO2 capture processes. [Copyright &y& Elsevier]

Estimates of the effect of increased global atmospheric CO2 levels on oceanic primary productivity depend on the physiological responses of contemporary phytoplankton populations. However, microalgal populations will possibly adapt to rising CO2 levels in such a way that they become genetically different from contemporary populations. The unknown properties of these future populations introduce an undefined error into predictions of C pool dynamics, especially the presence and size of the biological C pump. To address the bias in predictions introduced by evolution, we measured the kinetics of CO2 uptake in populations of Chlamydomonas reinhardtii that had been selected for growth at high CO2 for 1000 generations. Following selection at high CO2, the populations were unable to induce high-affinity CO2 uptake, and one line had a lower rate of net CO2 uptake. We attribute this to conditionally neutral mutations in genes affecting the C concentrating mechanism (CCM). Lower affinity CO2 uptake, in addition to smaller population sizes, results in a significant reduction in net CO2 uptake of about 38% relative to contemporary populations under the same conditions. This shows how predictions about the properties of communities in the future can be influenced by the effect of natural selection. [ABSTRACT FROM AUTHOR]

Estimates of the effect of increased global atmospheric CO2 levels on oceanic primary productivity depend on the physiological responses of contemporary phytoplankton populations. However, microalgal populations will possibly adapt to rising CO2 levels in such a way that they become genetically different from contemporary populations. The unknown properties of these future populations introduce an undefined error into predictions of C pool dynamics, especially the presence and size of the biological C pump. To address the bias in predictions introduced by evolution, we measured the kinetics of CO2 uptake in populations of Chlamydomonas reinhardtii that had been selected for growth at high CO2 for 1000 generations. Following selection at high CO2, the populations were unable to induce high-affinity CO2 uptake, and one line had a lower rate of net CO2 uptake. We attribute this to conditionally neutral mutations in genes affecting the C concentrating mechanism (CCM). Lower affinity CO2 uptake, in addition to smaller population sizes, results in a significant reduction in net CO2 uptake of about 38% relative to contemporary populations under the same conditions. This shows how predictions about the properties of communities in the future can be influenced by the effect of natural selection. [ABSTRACT FROM AUTHOR]

The interactive effects of inorganic carbon status, temperature and light on chlorosis induced by nitrogen deficiency, and the roles of Clp proteases in this process were investigated. In wild-type cultures grown in high or ambient CO2, following transfer to media lacking combined nitrogen, phycocyanin per cell dropped primarily through dilution of the pigment through cell division, and also suffered variable degrees of net degradation. When grown at high CO2 (5%), chlorophyll (Chl) suffered net degradation to a greater extent than phycocyanin. In marked contrast, growth at ambient CO2 resulted in Chl per cell dropping through dilution. Conditions that drove net Chl degradation in the wild-type resulted in little or no net Chl degradation in aclpPIinactivation mutant, with Chl content dropping largely through growth dilution in the mutant. The chlorotic response of aclpPIIinactivation strain was nearly the same as that of wild-type, although phycocyanin degradation may have been slightly accelerated in the former. [ABSTRACT FROM AUTHOR]

ABSTRACT Physiological properties of photosynthesis were determined in the marine diatom, Phaeodactylum tricornutum UTEX640, during acclimation from 5% CO2 to air and related to H2CO3 dissociation kinetics and equilibria in artificial seawater. The concentration of dissolved inorganic carbon at half maximum rate of photosynthesis (K0·5[DIC]) value in high CO2-grown cells was 1009 mmol m-3 but was reduced three-fold by the addition of bovine carbonic anhydrase (CA), whereas in air-grown cells K0·5[DIC] was 71 mmol m-3, irrespective of the presence of CA. The maximum rate of photosynthesis (Pmax) values varied between 300 and 500 μmol O2 mg Chl-1 h-1 regardless of growth pCO2. Bicarbonate dehydration kinetics in artificial seawater were re-examined to evaluate the direct HCO3- uptake as a substrate for photosynthesis. The uncatalysed CO2 formation rate in artificial seawater of 31·65°/oo of salinity at pH 8·2 and 25 °C was found to be 0·6 mmol m-3 min-1 at 100 mmol m-3 DIC, which is 53·5 and 7·3 times slower than the rates of photosynthesis exhibited in air- and high CO2-grown cells, respectively. These data indicate that even high CO2-grown cells of P. tricornutum can take up both CO2 and HCO3- as substrates for photosynthesis and HCO3- use improves dramatically when the cells are grown in air. Detailed time courses were obtained of changes in affinity for DIC during the acclimation of high CO2-grown cells to air. The development of high-affinity photosynthesis started after a 2–5 h lag period, followed by a steady increase over the next 15 h. This acclimation time course is the slowest to be described so far. High CO2-grown cells were transferred to controlled DIC conditions, at which the concentrations of each DIC species could be defined, and were allowed to acclimate for more than 36 h. The K0·5[DIC] values in acclimated cells appeared to be correlated only with [CO2(aq)] in the medium but not to HCO3-, CO32-, total [DIC] or the pH of the medium and indicate that the critical signal regulating the affinity of cells for DIC in the marine diatom, P. tricornutum, is [CO2(aq)] in the medium. [ABSTRACT FROM AUTHOR]