Reaction-diffusion modeling provides insights into biophysical carbon-concentrating mechanisms in land plants

Carbon-concentrating mechanisms (CCMs) have evolved numerous times in photosynthetic organisms. They elevate the concentration of CO2 around the carbon-fixing enzyme rubisco, thereby increasing CO2 assimilatory flux and reducing photorespiration. Biophysical CCMs, like the pyrenoid-based CCM (PCCM) of Chlamydomonas reinhardtii or carboxysome systems of cyanobacteria, are common in aquatic photosynthetic microbes, but in land plants appear only among the hornworts. To predict the likely efficiency of biophysical CCMs in C3 plants, we used spatially resolved reaction-diffusion models to predict rubisco saturation and light use efficiency. We found that the energy efficiency of adding individual CCM components to a C3 land plant is highly dependent on the permeability of lipid membranes to CO2, with values in the range reported in the literature that are higher than those used in previous modeling studies resulting in low light use efficiency. Adding a complete PCCM into the leaf cells of a C3 land plant was predicted to boost net CO2 fixation, but at higher energetic costs than those incurred by photorespiratory losses without a CCM. Two notable exceptions were when substomatal CO2 levels are as low as those found in land plants that already use biochemical CCMs and when gas exchange is limited, such as with hornworts, making the use of a biophysical CCM necessary to achieve net positive CO2 fixation under atmospheric CO2 levels. This provides an explanation for the uniqueness of hornworts' CCM among land plants and the evolution of pyrenoids multiple times.

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.

Variable carbon isotope fractionation of photosynthetic communities over depth in an open-ocean euphotic zone

Marine particulate organic carbon (POC) contributes to carbon export, food webs, and sediments, but uncertainties remain in its origins. Globally, variations in stable carbon isotope ratios (δ13C values) of POC between the upper and lower euphotic zones (LEZ) indicate either varying aspects of photosynthetic communities or degradative alteration of POC. During summertime in the subtropical north Atlantic Ocean, we find that δ13C values of the photosynthetic product phytol decreased by 6.3‰ and photosynthetic carbon isotope fractionation (εp) increased by 5.6‰ between the surface and the LEZ-variation as large as that found in the geologic record during major carbon cycle perturbations, but here reflecting vertical variation in δ13C values of photosynthetic communities. We find that simultaneous variations in light intensity and phytoplankton community composition over depth may be important factors not fully accounted for in common models of photosynthetic carbon isotope fractionation. Using additional isotopic and cell count data, we estimate that photosynthetic and non-photosynthetic material (heterotrophs or detritus) contribute relatively constant proportions of POC throughout the euphotic zone but are isotopically more distinct in the LEZ. As a result, the large vertical differences in εp result in significant, but smaller, differences in the δ13C values of total POC across the same depths (2.7‰). Vertical structuring of photosynthetic communities and export potential from the LEZ may vary across current and past ocean ecosystems; thus, LEZ photosynthesis may influence the exported and/or sedimentary δ13C values of both phytol and total organic carbon and affect interpretations of εp over geologic time.

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.

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.

Biophysical carbon concentrating mechanisms in land plants: insights from reaction-diffusion modeling

Carbon Concentrating Mechanisms (CCMs) have evolved numerous times in photosynthetic organisms. They elevate the concentration of CO2 around the carbon-fixing enzyme rubisco, thereby increasing CO2 assimilatory flux and reducing photorespiration. Biophysical CCMs, like the pyrenoid-based CCM of Chlamydomonas reinhardtii or carboxysome systems of cyanobacteria, are common in aquatic photosynthetic microbes, but in land plants appear only among the hornworts. To predict the likely efficiency of biophysical CCMs in C3 plants, we used spatially resolved reaction-diffusion models to predict rubisco saturation and light use efficiency. We find that the energy efficiency of adding individual CCM components to a C3 land plant is highly dependent on the permeability of lipid membranes to CO2, with values in the range reported in the literature that are higher than used in previous modeling studies resulting in low light use efficiency. Adding a complete pyrenoid-based CCM into the leaf cells of a C3 land plant is predicted to boost net CO2 fixation, but at higher energetic costs than those incurred by photorespiratory losses without a CCM. Two notable exceptions are when substomatal CO2 levels are as low as those found in land plants that already employ biochemical CCMs and when gas exchange is limited such as with hornworts, making the use of a biophysical CCM necessary to achieve net positive CO2 fixation under atmospheric CO2 levels. This provides an explanation for the uniqueness of hornworts' CCM among land plants and evolution of pyrenoids multiple times.

Impact of kelp forest on seawater chemistry - A review

Kelp forests, globally distributed in cool temperate and polar waters, are renowned for their pivotal role in supporting species diversity and fostering macroalgae productivity. These high-canopy algal ecosystems dynamically influence their surroundings, particularly by altering the physicochemical properties of seawater. This review article aims to underscore the significance of kelp forests in modifying water masses. By serving as effective carbon sinks through the absorption of bicarbonate (HCO3-) and carbon dioxide (CO2) for photosynthesis, kelp forests mitigate nearby acidity levels while enhancing dissolved oxygen concentrations, essential for sustaining diverse marine communities. Additionally, kelp beds have exhibited the need to use inorganic ions (NO3-, NO2-, PO43-) from seawater in order to grow, albeit with associated increases in NH4+ concentrations. Specific examples and findings from relevant studies will be presented to illustrate the profound impact of kelp forests on seawater chemistry, emphasizing their vital role in marine ecosystems.

The carbon-concentrating mechanism of the extremophilic red microalga Cyanidioschyzon merolae

Cyanidioschyzon merolae is an extremophilic red microalga which grows in low-pH, high-temperature environments. The basis of C. merolae's environmental resilience is not fully characterized, including whether this alga uses a carbon-concentrating mechanism (CCM). To determine if C. merolae uses a CCM, we measured CO2 uptake parameters using an open-path infra-red gas analyzer and compared them to values expected in the absence of a CCM. These measurements and analysis indicated that C. merolae had the gas-exchange characteristics of a CCM-operating organism: low CO2 compensation point, high affinity for external CO2, and minimized rubisco oxygenation. The biomass δ13C of C. merolae was also consistent with a CCM. The apparent presence of a CCM in C. merolae suggests the use of an unusual mechanism for carbon concentration, as C. merolae is thought to lack a pyrenoid and gas-exchange measurements indicated that C. merolae primarily takes up inorganic carbon as carbon dioxide, rather than bicarbonate. We use homology to known CCM components to propose a model of a pH-gradient-based CCM, and we discuss how this CCM can be further investigated.

Carbon assimilation in upper subtidal macroalgae is determined by an inverse correlation between Rubisco carboxylation efficiency and CO2 concentrating mechanism effectiveness

Seaweeds have a wide ecophysiological and phylogenetic diversity with species expressing different Rubisco forms that frequently coexist with biophysical CO₂ concentrating mechanisms (CCMs), an adaptation that overcomes the low CO₂ availability and gas diffusion in seawater. Here, we assess the possible coevolution between the Rubisco catalysis and the type and effectiveness of CCMs present in six upper subtidal macroalgal species belonging to three phylogenetic groups of seaweeds. A wide diversity in the Rubisco kinetic traits was found across the analyzed species, although the specificity factor was the only parameter explained by the expressed Rubisco form. Differences in the catalytic trade-offs were found between Rubisco forms, indicating that ID Rubiscos could be better adapted to the intracellular O₂  : CO₂ ratio found in marine organisms during steady-state photosynthesis. The biophysical components of the CCMs also differed among macroalgal species, resulting in different effectiveness to concentrate CO₂ around Rubisco active sites. Interestingly, an inverse relationship was found between the effectiveness of CCMs and the in vitro Rubisco carboxylation efficiency, which possibly led to a similar carboxylation potential across the analyzed macroalgal species. Our results demonstrate a coevolution between Rubisco kinetics and CCMs across phylogenetically distant marine macroalgal species sharing the same environment.

Correlative adaptation between Rubisco and CO2-concentrating mechanisms in seagrasses

Submerged angiosperms sustain some of the most productive and diverse ecosystems worldwide. However, their carbon acquisition and assimilation mechanisms remain poorly explored, missing an important step in the evolution of photosynthesis during the colonization of aquatic environments by angiosperms. Here we reveal a convergent kinetic adaptation of Rubisco in phylogenetically distant seagrass species that share catalytic efficiencies and CO₂ and O₂ affinities up to three times lower than those observed in phylogenetically closer angiosperms from terrestrial, freshwater and brackish-water habitats. This Rubisco kinetic convergence was found to correlate with the effectiveness of seagrass CO₂-concentrating mechanisms (CCMs), which probably evolved in response to the constant CO₂ limitation in marine environments. The observed Rubisco kinetic adaptation in seagrasses more closely resembles that seen in eukaryotic algae operating CCMs rather than that reported in terrestrial C₄ plants. Our results thus demonstrate a general pattern of co-evolution between Rubisco function and biophysical CCM effectiveness that traverses distantly related aquatic lineages.

Cyanidiales as Polyextreme Eukaryotes

Cyanidiales were named enigmatic microalgae due to their unique polyextreme properties, considered for a very long time unattainable for eukaryotes. Cyanidiales mainly inhabit hot sulfuric springs with high acidity (pH 0-4), temperatures up to 56°C, and ability to survive in the presence of dissolved heavy metals. Owing to the minimal for eukaryotes genome size, Cyanidiales have become one of the most important research objects in plant cell physiology, biochemistry, molecular biology, phylogenomics, and evolutionary biology. They play an important role in studying many aspects of oxygenic photosynthesis and chloroplasts origin. The ability to survive in stressful habitats and the corresponding metabolic pathways were acquired by Cyanidiales from archaea and bacteria via horizontal gene transfer (HGT). Thus, the possibility of gene transfer from prokaryotes to eukaryotes was discovered, which was a new step in understanding of the origin of eukaryotic cell.

Phylogenetic and functional diversity of Chrysophyceae in inland waters

Chrysophyceae are a diverse group of planktonic protists widely distributed in freshwater. They encompass a variety of orders, whereby heterotrophy has evolved independently in several phylogenetic lineages. Therefore, closely related taxa evolved that developed different feeding strategies (photo-, mixo-, heterotrophy). In high-throughput sequencing studies, the Chrysophyceae were usually addressed as a group rather than split in individual phylogenetic orders. Also because of the close relationship of the distinct nutritional strategies, no functional statements were made about nutritional patterns. Based on an extensive phylogenetic tree and phylogenetic placement, we link OTU diversity of Chrysophyceae from 218 freshwater lakes with phylogenetic affiliations. This provides information on the relative importance of lineages affiliated with different nutrition modes. Our study demonstrates that Chrysophyceae are one of the most common groups in freshwaters. We found Chrysophyceae in 213 out of 218 sample sites across Europe and in several sites they belong to the most commonly retrieved taxa. Ochromonadales and a Chrysosacca-Apoikiida clade (including Apoikiida, Chrysosaccales, Chrysastrella) are the most widespread Chrysophyceae groups and show a high degree of OTU diversity. Most detected and assignable OTUs were affiliated with mixotrophic Chrysophyceae. Niche width differs only slightly between members of different clades and between the different trophic modes. We found several OTUs within the Ochromonadales, Synurales, and Chrysosacca-Apoikiida clade, that show a wide distribution and large tolerance ranges concerning ecophysiological factors.

Interactive effects of light, CO2 and temperature on growth and resource partitioning by the mixotrophic dinoflagellate, Karlodinium veneficum

There is little information on the impacts of climate change on resource partitioning for mixotrophic phytoplankton. Here, we investigated the hypothesis that light interacts with temperature and CO₂ to affect changes in growth and cellular carbon and nitrogen content of the mixotrophic dinoflagellate, Karlodinium veneficum, with increasing cellular carbon and nitrogen content under low light conditions and increased growth under high light conditions. Using a multifactorial design, the interactive effects of light, temperature and CO₂ were investigated on K. veneficum at ambient temperature and CO₂ levels (25°C, 375 ppm), high temperature (30°C, 375 ppm CO₂), high CO₂ (30°C, 750 ppm CO₂), or a combination of both high temperature and CO₂ (30°C, 750 ppm CO₂) at low light intensities (LL: 70 μmol photons m^-2 s^-2) and light-saturated conditions (HL: 140 μmol photons m^-2 s^-2). Results revealed significant interactions between light and temperature for all parameters. Growth rates were not significantly different among LL treatments, but increased significantly with temperature or a combination of elevated temperature and CO₂ under HL compared to ambient conditions. Particulate carbon and nitrogen content increased in response to temperature or a combination of elevated temperature and CO₂ under LL conditions, but significantly decreased in HL cultures exposed to elevated temperature and/or CO₂ compared to ambient conditions at HL. Significant increases in C:N ratios were observed only in the combined treatment under LL, suggesting a synergistic effect of temperature and CO₂ on carbon assimilation, while increases in C:N under HL were driven only by an increase in CO₂. Results indicate light-driven variations in growth and nutrient acquisition strategies for K. veneficum that may benefit this species under anticipated climate change conditions (elevated light, temperature and pCO₂) while also affecting trophic transfer efficiency during blooms of this species.

Rate and fate of dissolved organic carbon release by seaweeds: A missing link in the coastal ocean carbon cycle

Dissolved organic carbon (DOC) release by seaweeds (marine macroalgae) is a critical component of the coastal ocean biogeochemical carbon cycle but is an aspect of seaweed carbon physiology that we know relatively little about. Seaweed-derived DOC is found throughout coastal ecosystems and supports multiple food web linkages. Here, we discuss the mechanisms of DOC release by seaweeds and group them into passive (leakage, requires no energy) and active release (exudation, requires energy) with particular focus on the photosynthetic "overflow" hypothesis. The release of DOC from seaweeds was first studied in the 1960s, but subsequent studies use a range of units hindering evaluation: we convert published values to a common unit (μmol C · g DW^-1 · h^-1 ) allowing comparisons between seaweed phyla, functional groups, biogeographic region, and an assessment of the environmental regulation of DOC production. The range of DOC release rates by seaweeds from each phylum under ambient environmental conditions was 0-266.44 μmol C · g DW^-1 · h^-1 (Chlorophyta), 0-89.92 μmol C · g DW^-1 · h^-1 (Ochrophyta), and 0-41.28 μmol C · g DW^-1 · h^-1 (Rhodophyta). DOC release rates increased under environmental factors such as desiccation, high irradiance, non-optimal temperatures, altered salinity, and elevated dissolved carbon dioxide (CO₂ ) concentrations. Importantly, DOC release was highest by seaweeds that were desiccated (<90 times greater DOC release compared to ambient). We discuss the impact of future ocean scenarios (ocean acidification, seawater warming, altered irradiance) on DOC release rates by seaweeds, the role of seaweed-derived DOC in carbon sequestration models, and how they inform future research directions.

The Bloom-Forming Dinoflagellate Karenia mikimotoi Adopts Different Growth Modes When Exposed to Short or Long Period of Seawater Acidification

Impacts of ocean acidification (OA) on noncalcifying organisms and the possibly responsible mechanism have aroused great research interests with the intensification of global warming. The present study focused on a noxious, noncalcifying, bloom-forming dinoflagellate, Karenia mikimotoi (K. mikimotoi), and its variation of growth patterns exposed to different periods of seawater acidification with stressing gradients was discussed. The dinoflagellates under short-time acidifying stress (2d) with different levels of CO₂ presented significant growth inhibition (p < 0.05). The cell cycle was obviously inhibited at S phase, and the photosynthetic carbon fixation was also greatly suppressed (p < 0.05). Apoptosis was observed and the apoptotic rate increased with the increment of pCO₂. Similar tendencies were observed in the key components of mitochondrial apoptotic pathway (the mitochondrial membrane potential (MMP), Caspase-3 and -9, and Bax/Bcl-2 ratio). However, under prolonged stressing time (8 d and 15 d), the growth of dinoflagellates was recovered or even stimulated, the photosynthetic carbon fixation was significantly increased (p < 0.05), the cell cycle of division presented little difference with those in the control, and no apoptosis was observed (p > 0.05). Besides, acidification adjusted by HCl addition and CO₂ enrichment resulted in different growth performances, while the latter had a more negative impact. The results of present study indicated that (1) the short-time exposure to acidified seawater led to reduced growth performance via inducing apoptosis, blocking of cell cycle, and the alteration in photosynthetic carbon fixation. (2) K. mikimotoi had undergone adaptive changes under long-term exposure to CO₂ induced seawater acidification. This further demonstrated that K. mikimotoi has strong adaptability in the face of seawater acidification, and this may be one of the reasons for the frequent outbreak of red tide. (3) Ions that dissociated by the dissolved CO₂, instead of H⁺ itself, were more important for the impacts induced by the acidification. This work thus provides a new perspective and a possible explanation for the dominance of K. mikimotoi during the occurrence of HABs.

Short-term effects of increased CO2, nitrate and temperature on photosynthetic activity in Ulva rigida (Chlorophyta) estimated by different pulse amplitude modulated fluorometers and oxygen evolution

Short-term effects of pCO2 (700-380 ppm; High carbon (HC) and Low carbon (LC), respectively) and nitrate content (50-5 µM; High nitrogen (HN) and Low nitrogen (LN), respectively on photosynthesis were investigated in Ulva rigida (Chlorophyta) under solar radiation (in-situ) and in the laboratory under artificial light (ex-situ). After six days of incubation at ambient temperature (AT), algae were subjected to a 4 °C temperature increase (AT+4 °C) for 3 d. Both in-situ and ex-situ maximal electron transport rate (ETRmax) and in situ gross photosynthesis (GP), measured by O2 evolution, presented highest values under HCHN, and lowest under HCLN, across all measuring systems. Maximal quantum yield (Fv/Fm), and ETRmax of photosystem (PS) II [ETR(II)max] and PSI [ETR(I)max], decreased under HCLN at AT+4 °C. Ex situ ETR was higher than in situ ETR. At noon, Fv/Fm decreased (indicating photoinhibition), whereas ETR(II)max and maximal non-photochemical quenching (NPQmax) increased. ETR(II)max decreased under AT+ 4 °C in contrast to Fv/Fm, photosynthetic efficiency (α ETR) and saturated irradiance (EK). Thus, U. rigida exhibited a decrease in photosynthesis under acidification, changing LN, and AT+4 °C. These results emphasize the importance of studying the interaction between environmental parameters using in-situ versus ex-situ conditions, when aiming to evaluate the impact of global change on marine macroalgae.

Climate-induced interannual variability and projected change of two harmful algal bloom taxa in Chesapeake Bay, USA

Retrospective analysis of water quality monitoring data reveals strong interannual shifts in the spatial distribution of two harmful algal species (Prorocentrum minimum and Karlodinium veneficum) in eutrophic Chesapeake Bay. A habitat model, based on the temperature and salinity tolerance of the two species as well as their nutrient preferences, provides a good interpretation for the observed seasonal progression and spatial distribution of these taxa. It also points to climate-induced variability in the hydrological forcing as a mechanism driving the interannual shifts in the algal distributions: both P. minimum and K. veneficum shift downstream during wetter years but upstream during dry years. Climate downscaling simulations using the habitat model show upstream shifts of the two species in the estuary and longer blooming seasons by the mid-21st century. Salt intrusion due to sea level rise will raise salinity in the estuary and cause these HAB species to migrate upstream, but increasing winter-spring flows may also drive favorable salinity habitat downstream. Warming leads to longer growing seasons of P. minimum and K. veneficum but may suppress bloom habitat during their respective peak bloom periods.

Inorganic carbon utilization of tropical calcifying macroalgae and the impacts of intensive mariculture-derived coastal acidification on the physiological performance of the rhodolith Sporolithon sp

Fish farming in coastal areas has become an important source of food to support the world's increasing population. However, intensive and unregulated mariculture activities have contributed to changing seawater carbonate chemistry through the production of high levels of respiratory CO₂. This additional CO₂, i.e. in addition to atmospheric inputs, intensifies the effects of global ocean acidification resulting in localized extreme low pH levels. Marine calcifying macroalgae are susceptible to such changes due to their CaCO₃ skeleton. Their physiological response to CO₂-driven acidification is dependent on their carbon physiology. In this study, we used the pH drift experiment to determine the capability of 9 calcifying macroalgae to use one or more inorganic carbon (C_i) species. From the 9 species, we selected the rhodolith Sporolithon sp. as a model organism to investigate the long-term effects of extreme low pH on the physiology and biochemistry of calcifying macroalgae. Samples were incubated under two pH treatments (pH 7.9 = ambient and pH 7.5 = extreme acidification) in a temperature-controlled (26 ± 0.02 °C) room provided with saturating light intensity (98.3 ± 2.50 μmol photons m^-2 s^-1). After the experimental treatment period (40 d), growth rate, calcification rate, nutrient uptake rate, organic content, skeletal CO₃^-2, pigments, and tissue C, N and P of Sporolithon samples were compared. The pH drift experiment revealed species-specific C_i use mechanisms, even between congenerics, among tropical calcifying macroalgae. Furthermore, long-term extreme low pH significantly reduced the growth rate, calcification rate and skeletal CO₃^-2 content by 79%, 66% and 18%, respectively. On the other hand, nutrient uptake rates, organic matter, pigments and tissue C, N and P were not affected by the low pH treatments. Our results suggest that the rhodolith Sporolithon sp. is susceptible to the negative effects of extreme low pH resulting from intensive mariculture-driven coastal acidification.

Inorganic carbon uptake strategies in coralline algae: Plasticity across evolutionary lineages under ocean acidification and warming

Dissolved inorganic carbon (DIC) assimilation is essential to the reef-building capacity of crustose coralline algae (CCA). Little is known, however, about the DIC uptake strategies and their potential plasticity under ongoing ocean acidification (OA) and warming. The persistence of CCA lineages throughout historical oscillations of pCO₂ and temperature suggests that evolutionary history may play a role in selecting for adaptive traits. We evaluated the effects of pCO₂ and temperature on the plasticity of DIC uptake strategies and associated energetic consequences in reef-building CCA from different evolutionary lineages. We simulated past, present, moderate (IPCC RCP 6.0) and high pCO₂ (RCP 8.5) and present and high (RCP 8.5) temperature conditions and quantified stable carbon isotope fractionation (¹³ε), organic carbon content, growth and photochemical efficiency. All investigated CCA species possess CO₂-concentrating mechanisms (CCMs) and assimilate CO₂ via diffusion to varying degrees. Under OA and warming, CCA either increased or maintained CCM capacity, which was associated with overall neutral effects on metabolic performance. More basal taxa, Sporolithales and Hapalidiales, had greater capacity for diffusive CO₂ use than Corallinales. We suggest that CCMs are an adaptation that supports a robust carbon physiology and are likely responsible for the endurance of CCA in historically changing oceans.

Rubisco and carbon-concentrating mechanism co-evolution across chlorophyte and streptophyte green algae

Green algae expressing a carbon-concentrating mechanism (CCM) are usually associated with a Rubisco-containing micro-compartment, the pyrenoid. A link between the small subunit (SSU) of Rubisco and pyrenoid formation in Chlamydomonas reinhardtii has previously suggested that specific RbcS residues could explain pyrenoid occurrence in green algae. A phylogeny of RbcS was used to compare the protein sequence and CCM distribution across the green algae and positive selection in RbcS was estimated. For six streptophyte algae, Rubisco catalytic properties, affinity for CO₂ uptake (K_0.5 ), carbon isotope discrimination (δ¹³ C) and pyrenoid morphology were compared. The length of the βA-βB loop in RbcS provided a phylogenetic marker discriminating chlorophyte from streptophyte green algae. Rubisco kinetic properties in streptophyte algae have responded to the extent of inducible CCM activity, as indicated by changes in inorganic carbon uptake affinity, δ¹³ C and pyrenoid ultrastructure between high and low CO₂ conditions for growth. We conclude that the Rubisco catalytic properties found in streptophyte algae have coevolved and reflect the strength of any CCM or degree of pyrenoid leakiness, and limitations to inorganic carbon in the aquatic habitat, whereas Rubisco in extant land plants reflects more recent selective pressures associated with improved diffusive supply of the terrestrial environment.

Insights on the Functions and Ecophysiological Relevance of the Diverse Carbonic Anhydrases in Microalgae

Carbonic anhydrases (CAs) exist in all kingdoms of life. They are metalloenzymes, often containing zinc, that catalyze the interconversion of bicarbonate and carbon dioxide-a ubiquitous reaction involved in a variety of cellular processes. So far, eight classes of apparently evolutionary unrelated CAs that are present in a large diversity of living organisms have been described. In this review, we focus on the diversity of CAs and their roles in photosynthetic microalgae. We describe their essential role in carbon dioxide-concentrating mechanisms and photosynthesis, their regulation, as well as their less studied roles in non-photosynthetic processes. We also discuss the presence in some microalgae, especially diatoms, of cambialistic CAs (i.e., CAs that can replace Zn by Co, Cd, or Fe) and, more recently, a CA that uses Mn as a metal cofactor, with potential ecological relevance in aquatic environments where trace metal concentrations are low. There has been a recent explosion of knowledge about this well-known enzyme with exciting future opportunities to answer outstanding questions using a range of different approaches.

Evolutionary trends in RuBisCO kinetics and their co-evolution with CO2 concentrating mechanisms

RuBisCO-catalyzed CO₂ fixation is the main source of organic carbon in the biosphere. This enzyme is present in all domains of life in different forms (III, II, and I) and its origin goes back to 3500 Mya, when the atmosphere was anoxygenic. However, the RuBisCO active site also catalyzes oxygenation of ribulose 1,5-bisphosphate, therefore, the development of oxygenic photosynthesis and the subsequent oxygen-rich atmosphere promoted the appearance of CO₂ concentrating mechanisms (CCMs) and/or the evolution of a more CO₂ -specific RuBisCO enzyme. The wide variability in RuBisCO kinetic traits of extant organisms reveals a history of adaptation to the prevailing CO₂ /O₂ concentrations and the thermal environment throughout evolution. Notable differences in the kinetic parameters are found among the different forms of RuBisCO, but the differences are also associated with the presence and type of CCMs within each form, indicative of co-evolution of RuBisCO and CCMs. Trade-offs between RuBisCO kinetic traits vary among the RuBisCO forms and also among phylogenetic groups within the same form. These results suggest that different biochemical and structural constraints have operated on each type of RuBisCO during evolution, probably reflecting different environmental selective pressures. In a similar way, variations in carbon isotopic fractionation of the enzyme point to significant differences in its relationship to the CO₂ specificity among different RuBisCO forms. A deeper knowledge of the natural variability of RuBisCO catalytic traits and the chemical mechanism of RuBisCO carboxylation and oxygenation reactions raises the possibility of finding unrevealed landscapes in RuBisCO evolution.

Harmful algae at the complex nexus of eutrophication and climate change

Climate projections suggest-with substantial certainty-that global warming >1.5 °C will occur by mid-century (2050). Population is also projected to increase, amplifying the demands for food, fuel, water and sanitation, which, in turn, escalate nutrient pollution. Global projections of nutrient pollution, however, are less certain than those of climate as there are regionally decreasing trends projected in Europe, and stabilization of nutrient use in North America and Australia. In this review of the effects of eutrophication and climate on harmful algae, some of the complex, subtle, and non-intuitive effects and interactions on the physiology of both harmful and non-harmful taxa are emphasized. In a future ocean, non-harmful diatoms may be disproportionately stressed and mixotrophs advantaged due to changing nutrient stoichiometry and forms of nutrients, temperature, stratification and oceanic pH. Modeling is advancing, but there is much yet to be understood, in terms of physiology, biogeochemistry and trophodynamics and how both harmful and nonharmful taxa may change in an uncertain future driven by anthropogenic activities.

Subtropical freshwater phytoplankton show a greater response to increased temperature than to increased pCO2

Global increases in atmospheric CO₂ and temperatures will impact aquatic systems, with freshwater habitats being affected. Some studies suggest that these conditions will promote cyanobacterial dominance. There is a need for a clearer picture of how algal species and strains within species will respond to higher temperatures and CO₂, especially in combination. This study examined two chlorophytes (Monoraphidium and Staurastrum), and two strains of the cyanobacterium Raphidiopsis raciborskii (straight S07 and coiled C03), to determine how the combination of higher temperature and CO₂ levels will affect their growth and maximum cell concentrations. Continuous cultures were used to compare the steady state cell concentrations at 28 °C and 30 °C, and CO₂ partial pressures (pCO₂), 400 and 750 ppm for all cultures, and in addition 1000 ppm at 28 °C for R. raciborskii strains. This study showed that, for all species, water temperature had a greater effect than higher pCO₂ on cell concentrations. There were clear differences in response between the chlorophyte species, with Monoraphidium preferring 28 °C and Staurastrum preferring 30 °C. There were also differences in response of the R. raciborskii strains to increasing temperature and pCO₂, with S07 having a greater increase in cell concentration. Genome analysis of R. raciborskii showed that the straight strain has five additional carbon acquisition genes (β-CA, chpY, cmpB, cmpD and NdhD4), indicative of increased carbon metabolism. These differences in the strains' response to elevated pCO₂ will lead to changes in the species population structure and distribution in the water column. This study shows that it is important to examine the effects of both pCO₂ and temperature, and to consider strain variation, to understand how species composition of natural systems may change under future climate conditions.

Meta-analysis reveals enhanced growth of marine harmful algae from temperate regions with warming and elevated CO2 levels

Elevated pCO₂ and warming may promote algal growth and toxin production, and thereby possibly support the proliferation and toxicity of harmful algal blooms (HABs). Here, we tested whether empirical data support this hypothesis using a meta-analytic approach and investigated the responses of growth rate and toxin content or toxicity of numerous marine and estuarine HAB species to elevated pCO₂ and warming. Most of the available data on HAB responses towards the two tested climate change variables concern dinoflagellates, as many members of this phytoplankton group are known to cause HAB outbreaks. Toxin content and toxicity did not reveal a consistent response towards both tested climate change variables, while growth rate increased consistently with elevated pCO₂ . Warming also led to higher growth rates, but only for species isolated at higher latitudes. The observed gradient in temperature growth responses shows the potential for enhanced development of HABs at higher latitudes. Increases in growth rates with more CO₂ may present an additional competitive advantage for HAB species, particularly as CO₂ was not shown to enhance growth rate of other non-HAB phytoplankton species. However, this may also be related to the difference in representation of dinoflagellate and diatom species in the respective HAB and non-HAB phytoplankton groups. Since the proliferation of HAB species may strongly depend on their growth rates, our results warn for a greater potential of dinoflagellate HAB development in future coastal waters, particularly in temperate regions.

Future CO2-induced seawater acidification mediates the physiological performance of a green alga Ulva linza in different photoperiods

Photoperiods have an important impact on macroalgae living in the intertidal zone. Ocean acidification also influences the physiology of macroalgae. However, little is known about the interaction between ocean acidification and photoperiod on macroalgae. In this study, a green alga Ulva linza was cultured under three different photoperiods (L: D = 8:16, 12:12, 16:8) and two different CO₂ levels (LC, 400 ppm; HC, 1,000 ppm) to investigate their responses. The results showed that relative growth rate of U. linza increased with extended light periods under LC but decreased at HC when exposed to the longest light period of 16 h compared to 12 h. Higher CO₂ levels enhanced the relative growth rate at a L: D of 8:16, had no effect at 12:12 but reduced RGR at 16:8. At LC, the L: D of 16:8 significantly stimulated maximum quantum yield (Yield). Higher CO₂ levels enhanced Yield at L: D of 12:12 and 8:16, had negative effect at 16:8. Non-photochemical quenching (NPQ) increased with increasing light period. High CO₂ levels did not affect respiration rate during shorter light periods but enhanced it at a light period of 16 h. Longer light periods had negative effects on Chl a and Chl b content, and high CO₂ level also inhibited the synthesis of these pigments. Our data demonstrate the interactive effects of CO₂ and photoperiod on the physiological characteristics of the green tide macroalga Ulva linza and indicate that future ocean acidification may hinder the stimulatory effect of long light periods on growth of Ulva species.

Rubisco carboxylation kinetics and inorganic carbon utilization in polar versus cold-temperate seaweeds

Despite the high productivity and ecological importance of seaweeds in polar coastal regions, little is known about their carbon utilization mechanisms, especially the kinetics of the CO2-fixing enzyme Rubisco. We analyzed Rubisco carboxylation kinetics at 4 °C and 25 °C in 12 diverse polar seaweed species (including cold-temperate populations of the same species) and the relationship with their ability to use bicarbonate, by using 13C isotope discrimination and pH drift experiments. We observed a large variation in Rubisco carboxylation kinetics among the selected species, although no correlation was found between either the Michaelis-Menten constant for CO2 (Kc) or Rubisco content per total soluble protein ([Rubisco]/[TSP]) and the ability to use bicarbonate for non-green seaweeds. This study reports intraspecific Rubisco cold adaptation by means of either higher Rubisco carboxylation turnover rate (kcatc) and carboxylase efficiency (kcatc/Kc) at 4 °C or higher [Rubisco]/[TSP] in some of the analyzed species. Our data point to a widespread ability for photosynthetic bicarbonate usage among polar seaweeds, despite the higher affinity of Rubisco for CO2 and higher dissolved CO2 concentration in cold seawater. Moreover, the reported catalytic variation within form ID Rubisco might avert the canonical trade-off previously observed between Kc and kcatc for plant Rubiscos.

Responses of macroalgae to CO2 enrichment cannot be inferred solely from their inorganic carbon uptake strategy

Increased plant biomass is observed in terrestrial systems due to rising levels of atmospheric CO2, but responses of marine macroalgae to CO2 enrichment are unclear. The 200% increase in CO2 by 2100 is predicted to enhance the productivity of fleshy macroalgae that acquire inorganic carbon solely as CO2 (non-carbon dioxide-concentrating mechanism [CCM] species-i.e., species without a carbon dioxide-concentrating mechanism), whereas those that additionally uptake bicarbonate (CCM species) are predicted to respond neutrally or positively depending on their affinity for bicarbonate. Previous studies, however, show that fleshy macroalgae exhibit a broad variety of responses to CO2 enrichment and the underlying mechanisms are largely unknown. This physiological study compared the responses of a CCM species (Lomentaria australis) with a non-CCM species (Craspedocarpus ramentaceus) to CO2 enrichment with regards to growth, net photosynthesis, and biochemistry. Contrary to expectations, there was no enrichment effect for the non-CCM species, whereas the CCM species had a twofold greater growth rate, likely driven by a downregulation of the energetically costly CCM(s). This saved energy was invested into new growth rather than storage lipids and fatty acids. In addition, we conducted a comprehensive literature synthesis to examine the extent to which the growth and photosynthetic responses of fleshy macroalgae to elevated CO2 are related to their carbon acquisition strategies. Findings highlight that the responses of macroalgae to CO2 enrichment cannot be inferred solely from their carbon uptake strategy, and targeted physiological experiments on a wider range of species are needed to better predict responses of macroalgae to future oceanic change.

Carbonate Disequilibrium in the External Boundary Layer of Freshwater Chrysophytes: Implications for Contaminant Uptake

The interplay between biological and chemical reactions in the freshwater phytoplankton phycosphere and the resulting modulations of contaminant speciation and uptake is poorly characterized. Here we modeled the effect of algal C and N uptake on carbonate cycling and speciation of selected contaminants in the phycosphere (external boundary layer) of chrysophytes, a key phytoplankton group in oligotrophic systems. We calculated an enrichment in H⁺ concentration relative to that in the bulk solution (pH 7.0) of approximately 40% or a depletion of approximately 30% for NH₄⁺ or NO₃^--grown cells, respectively, at the algal membrane surface of a 5-μm radius cell. Such changes are mainly due to direct H⁺ uptake or release at the plasmalemma if NO₃^- or NH₄⁺ is the N source, respectively. Due to these pH changes in the external boundary layer, competition between H⁺ and metals for uptake is enhanced, for NH₄⁺-grown cells which contributes to a decrease in potential metal uptake. Our model suggests that the uptake of protonated weakly acidic organic acids (HA) is greater in NH₄⁺-grown cells compared to that in NO₃^--grown cells. The account of chemical reactions in the algal external boundary layer could improve ecological risk assessments for a wide range of contaminants.

Macroalgal response to a warmer ocean with higher CO2 concentration

Primary production and respiration rates were studied for six seaweed species (Cystoseira abies-marina, Lobophora variegata, Pterocladiella capillacea, Canistrocarpus cervicornis, Padina pavonica and Corallina caespitosa) from Subtropical North-East Atlantic, to estimate the combined effects of different pH and temperature levels. Macroalgal samples were cultured at temperature and pH combinations ranging from current levels to those predicted for the next century (19, 21, 23, 25 °C, pH: 8.1, 7.7 and 7.4). Decreased pH had a positive effect on short-term production of the studied species. Raised temperatures had a more varied and species dependent effect on short term primary production. Thermophilic algae increased their production at higher temperatures, while temperate species were more productive at lower or present temperature conditions. Temperature also affected algal respiration rates, which were higher at low temperature levels. The results suggest that biomass and productivity of the more tropical species in coastal ecosystems would be enhanced by future ocean conditions.

A tale of two mixotrophic chrysophytes: Insights into the metabolisms of two Ochromonas species (Chrysophyceae) through a comparison of gene expression

Ochromonas spp. strains CCMP1393 and BG-1 are phagotrophic phytoflagellates with different nutritional strategies. Strain CCMP1393 is an obligate phototroph while strain BG-1 readily grows in continuous darkness in the presence of bacterial prey. Growth and gene expression of strain CCMP1393 were investigated under conditions allowing phagotrophic, mixotrophic, or phototrophic nutrition. The availability of light and bacterial prey led to the differential expression of 42% or 45–59% of all genes, respectively. Data from strain CCMP1393 were compared to those from a study conducted previously on strain BG-1, and revealed notable differences in carbon and nitrogen metabolism between the 2 congeners under similar environmental conditions. Strain BG-1 utilized bacterial carbon and amino acids through glycolysis and the tricarboxylic acid cycle, while downregulating light harvesting and carbon fixation in the Calvin cycle when both light and bacteria were available. In contrast, the upregulation of genes related to photosynthesis, light harvesting, chlorophyll synthesis, and carbon fixation in the presence of light and prey for strain CCMP1393 implied that this species is more phototrophic than strain BG-1, and that phagotrophy may have enhanced phototrophy. Cellular chlorophyll a content was also significantly higher in strain CCMP1393 supplied with bacteria compared to those without prey. Our results thus point to very different physiological strategies for mixotrophic nutrition in these closely related chrysophyte species.

Acquisition and metabolism of carbon in the Ochrophyta other than diatoms

The acquisition and assimilation of inorganic C have been investigated in several of the 15 clades of the Ochrophyta other than diatoms, with biochemical, physiological and genomic data indicating significant mechanistic variation. Form ID Rubiscos in the Ochrophyta are characterized by a broad range of kinetics values. In spite of relatively high K0.5CO2 and low CO2 : O2 selectivity, diffusive entry of CO2 occurs in the Chrysophyceae and Synurophyceae. Eustigmatophyceae and Phaeophyceae, on the contrary, have CO2 concentrating mechanisms, usually involving the direct or indirect use of [Formula: see text] This variability is possibly due to the ecological contexts of the organism. In brown algae, C fixation generally takes place through a classical C3 metabolism, but there are some hints of the occurrence of C4 metabolism and low amplitude CAM in a few members of the Fucales. Genomic data show the presence of a number of potential C4 and CAM genes in Ochrophyta other than diatoms, but the other core functions of many of these genes give a very limited diagnostic value to their presence and are insufficient to conclude that C4 photosynthesis is present in these algae.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Effect of CO2-induced seawater acidification on growth, photosynthesis and inorganic carbon acquisition of the harmful bloom-forming marine microalga, Karenia mikimotoi

Karenia mikimotoi is a widespread, toxic and non-calcifying dinoflagellate, which can release and produce ichthyotoxins and hemolytic toxins affecting the food web within the area of its bloom. Shifts in the physiological characteristics of K. mikimotoi due to CO2-induced seawater acidification could alter the occurrence, severity and impacts of harmful algal blooms (HABs). Here, we investigated the effects of elevated pCO2 on the physiology of K. mikimotoi. Using semi-continuous cultures under controlled laboratory conditions, growth, photosynthesis and inorganic carbon acquisition were determined over 4-6 week incubations at ambient (390ppmv) and elevated pCO2 levels (1000 ppmv and 2000 ppmv). pH-drift and inhibitor-experiments suggested that K. mikimotoi was capable of acquiring HCO3-, and that the utilization of HCO3- was predominantly mediated by anion-exchange proteins, but that HCO3- dehydration catalyzed by external carbonic anhydrase (CAext) only played a minor role in K. mikimotoi. Even though down-regulated CO2 concentrating mechanisms (CCMs) and enhanced gross photosynthetic O2 evolution were observed under 1000 ppmv CO2 conditions, the saved energy did not stimulate growth of K. mikimotoi under 1000 ppmv CO2, probably due to the increased dark respiration. However, significantly higher growth and photosynthesis [in terms of photosynthetic oxygen evolution, effective quantum Yield (Yield), photosynthetic efficiency (α), light saturation point (Ek) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity] were observed under 2000 ppmv CO2 conditions. Furthermore, elevated pCO2 increased the photo-inhibition rate of photosystem II (β) and non-photochemical quenching (NPQ) at high light. We suggest that the energy saved through the down-regulation of CCMs might lead to the additional light stress and photo-damage. Therefore, the response of this species to elevated CO2 conditions will be determined by more than regulation and efficiency of CCMs.

Ocean acidification and kelp development: Reduced pH has no negative effects on meiospore germination and gametophyte development of Macrocystis pyrifera and Undaria pinnatifida

The absorption of anthropogenic CO2 by the oceans is causing a reduction in the pH of the surface waters termed ocean acidification (OA). This could have substantial effects on marine coastal environments where fleshy (non-calcareous) macroalgae are dominant primary producers and ecosystem engineers. Few OA studies have focused on the early life stages of large macroalgae such as kelps. This study evaluated the effects of seawater pH on the ontogenic development of meiospores of the native kelp Macrocystis pyrifera and the invasive kelp Undaria pinnatifida, in south-eastern New Zealand. Meiospores of both kelps were released into four seawater pH treatments (pHT 7.20, extreme OA predicted for 2300; pHT 7.65, OA predicted for 2100; pHT 8.01, ambient pH; and pHT 8.40, pre-industrial pH) and cultured for 15 d. Meiospore germination, germling growth rate, and gametophyte size and sex ratio were monitored and measured. Exposure to reduced pHT (7.20 and 7.65) had positive effects on germling growth rate and gametophyte size in both M. pyrifera and U. pinnatifida, whereas, higher pHT (8.01 and 8.40) reduced the gametophyte size in both kelps. Sex ratio of gametophytes of both kelps was biased toward females under all pHT treatments, except for U. pinnatifida at pHT 7.65. Germling growth rate under OA was significantly higher in M. pyrifera compared to U. pinnatifida but gametophyte development was equal for both kelps under all seawater pHT treatments, indicating that the microscopic stages of the native M. pyrifera and the invasive U. pinnatifida will respond similarly to OA.

The possible evolution and future of CO2-concentrating mechanisms

CO2-concentrating mechanisms (CCMs), based either on active transport of inorganic carbon (biophysical CCMs) or on biochemistry involving supplementary carbon fixation into C4 acids (C4 and CAM), play a major role in global primary productivity. However, the ubiquitous CO2-fixing enzyme in autotrophs, Rubisco, evolved at a time when atmospheric CO2 levels were very much higher than today and O2 was very low and, as CO2 and O2 approached (by no means monotonically), today's levels, at some time subsequently many organisms evolved a CCM that increased the supply of CO2 and decreased Rubisco oxygenase activity. Given that CO2 levels and other environmental factors have altered considerably between when autotrophs evolved and the present day, and are predicted to continue to change into the future, we here examine the drivers for, and possible timing of, evolution of CCMs. CCMs probably evolved when CO2 fell to 2-16 times the present atmospheric level, depending on Rubisco kinetics. We also assess the effects of other key environmental factors such as temperature and nutrient levels on CCM activity and examine the evidence for evolutionary changes in CCM activity and related cellular processes as well as limitations on continuity of CCMs through environmental variations.

Carbon-concentrating mechanisms in seagrasses

Seagrasses are unique angiosperms that carry out growth and reproduction submerged in seawater. They occur in at least three families of the Alismatales. All have chloroplasts mainly in the cells of the epidermis. Living in seawater, the supply of inorganic carbon (Ci) to the chloroplasts is diffusion limited, especially under unstirred conditions. Therefore, the supply of CO2 and bicarbonate across the diffusive boundary layer on the outer side of the epidermis is often a limiting factor. Here we discuss the evidence for mechanisms that enhance the uptake of Ci into the epidermal cells. Since bicarbonate is plentiful in seawater, a bicarbonate pump might be expected; however, the evidence for such a pump is not strongly supported. There is evidence for a carbonic anhydrase outside the outer plasmalemma. This, together with evidence for an outward proton pump, suggests the possibility that local acidification leads to enhanced concentrations of CO2 adjacent to the outer tangential epidermal walls, which enhances the uptake of CO2, and this could be followed by a carbon-concentrating mechanism (CCM) in the cytoplasm and/or chloroplasts. The lines of evidence for such an epidermal CCM are discussed, including evidence for special 'transfer cells' in some but not all seagrass leaves in the tangential inner walls of the epidermal cells. It is concluded that seagrasses have a CCM but that the case for concentration of CO2 at the site of Rubisco carboxylation is not proven.

Ecological imperatives for aquatic CO2-concentrating mechanisms

In aquatic environments, the concentration of inorganic carbon is spatially and temporally variable and CO2 can be substantially oversaturated or depleted. Depletion of CO2 plus low rates of diffusion cause inorganic carbon to be more limiting in aquatic than terrestrial environments, and the frequency of species with a CO2-concentrating mechanism (CCM), and their contribution to productivity, is correspondingly greater. Aquatic photoautotrophs may have biochemical or biophysical CCMs and exploit CO2 from the sediment or the atmosphere. Though partly constrained by phylogeny, CCM activity is related to environmental conditions. CCMs are absent or down-regulated when their increased energy costs, lower CO2 affinity, or altered mineral requirements outweigh their benefits. Aquatic CCMs are most widespread in environments with low CO2, high HCO3-, high pH, and high light. Freshwater species are generally less effective at inorganic carbon removal than marine species, but have a greater range of ability to remove carbon, matching the environmental variability in carbon availability. The diversity of CCMs in seagrasses and marine phytoplankton, and detailed mechanistic studies on larger aquatic photoautotrophs are understudied. Strengthening the links between ecology and CCMs will increase our understanding of the mechanisms underlying ecological success and will place mechanistic studies in a clearer ecological context.

Photosynthetic use of inorganic carbon in deep-water kelps from the Strait of Gibraltar

Mechanisms of inorganic carbon assimilation were investigated in the four deep-water kelps inhabiting sea bottoms at the Strait of Gibraltar; these species are distributed at different depths (Saccorhiza polysiches at shallower waters, followed by Laminaria ochroleuca, then Phyllariopsis brevipes and, at the deepest bottoms, Phyllariopsis purpurascens). To elucidate the capacity to use HCO3(-) as a source of inorganic carbon for photosynthesis in the kelps, different experimental approaches were used. Specifically, we measured the irradiance-saturated gross photosynthetic rate versus pH at a constant dissolved inorganic carbon (DIC) concentration of 2 mM, the irradiance-saturated apparent photosynthesis (APS) rate versus DIC, the total and the extracellular carbonic anhydrase (CAext), the observed and the theoretical photosynthetic rates supported by the spontaneous dehydration of HCO3(-) to CO2, and the δ(13)C signature in tissues of the algae. While S. polyschides and L. ochroleuca showed photosynthetic activity at pH 9.5 (around 1.0 µmol O2 m(-2) s(-1)), the activity was close to zero in both species of Phyllariopsis. The APS versus DIC was almost saturated for the DIC values of natural seawater (2 mM) in S. polyschides and L. ochroleuca, but the relationship was linear in P. brevipes and P. purpurascens. The four species showed total and CAext activities but the inhibition of the CAext originated the observed photosynthetic rates at pH 8.0 to be similar to the theoretical rates that could be supported by the spontaneous dehydration of HCO3(-). The isotopic (13)C signatures ranged from -17.40 ± 1.81 to -21.11 ± 1.73 ‰ in the four species. Additionally, the δ(13)C signature was also measured in the deep-water Laminaria rodriguezii growing at 60-80 m, showing even a more negative value of -26.49 ± 1.25 ‰. All these results suggest that the four kelps can use HCO3(-) as external carbon source for photosynthesis mainly by the action of external CAext, but they also suggest that the species inhabiting shallower waters show a higher capacity than the smaller kelps living in deeper waters. In fact, the photosynthesis in the two Phyllariopsis species could be accomplished by the spontaneous dehydration of HCO3(-) to CO2. These differences in the capacity to use HCO3(-) in photosynthesis among species could be important considering the increasing levels of atmospheric CO2 predicted for the near future.

The ins and outs of CO2

It is difficult to distinguish influx and efflux of inorganic C in photosynthesizing tissues; this article examines what is known and where there are gaps in knowledge. Irreversible decarboxylases produce CO2, and CO2 is the substrate/product of enzymes that act as carboxylases and decarboxylases. Some irreversible carboxylases use CO2; others use HCO3(-). The relative role of permeation through the lipid bilayer versus movement through CO2-selective membrane proteins in the downhill, non-energized, movement of CO2 is not clear. Passive permeation explains most CO2 entry, including terrestrial and aquatic organisms with C3 physiology and biochemistry, terrestrial C4 plants and all crassulacean acid metabolism (CAM) plants, as well as being part of some mechanisms of HCO3(-) use in CO2 concentrating mechanism (CCM) function, although further work is needed to test the mechanism in some cases. However, there is some evidence of active CO2 influx at the plasmalemma of algae. HCO3(-) active influx at the plasmalemma underlies all cyanobacterial and some algal CCMs. HCO3(-) can also enter some algal chloroplasts, probably as part of a CCM. The high intracellular CO2 and HCO3(-) pools consequent upon CCMs result in leakage involving CO2, and occasionally HCO3(-). Leakage from cyanobacterial and microalgal CCMs involves up to half, but sometimes more, of the gross inorganic C entering in the CCM; leakage from terrestrial C4 plants is lower in most environments. Little is known of leakage from other organisms with CCMs, though given the leakage better-examined organisms, leakage occurs and increases the energetic cost of net carbon assimilation.

The impact of fluctuating light on the dinoflagellate Prorocentrum micans depends on NO3(-) and CO2 availability

Increasing atmospheric pCO2 and its dissolution into oceans leads to ocean acidification and warming, which reduces the thickness of upper mixing layer (UML) and upward nutrient supply from deeper layers. These events may alter the nutritional conditions and the light regime to which primary producers are exposed in the UML. In order to better understand the physiology behind the responses to the concomitant climate changes factors, we examined the impact of light fluctuation on the dinoflagellate Prorocentrum micans grown at low (1 μmol L(-1)) or high (800 μmol L(-1)) [NO3(-)] and at high (1000 μatm) or low (390 μatm, ambient) pCO2. The light regimes to which the algal cells were subjected were (1) constant light at a photon flux density (PFD) of either 100 (C100) or 500 (C500) μmol m(-2) s(-1) or (2) fluctuating light between 100 or 500 μmol photons m(-2) s(-1) with a frequency of either 15 (F15) or 60 (F60) min. Under continuous light, the initial portion of the light phase required the concomitant presence of high CO2 and NO3(-) concentrations for maximum growth. After exposure to light for 3h, high CO2 exerted a negative effect on growth and effective quantum yield of photosystem II (F'(v)/F'(m)). Fluctuating light ameliorated growth in the first period of illumination. In the second 3h of treatment, higher frequency (F15) of fluctuations afforded high growth rates, whereas the F60 treatment had detrimental consequences, especially when NO3(-) concentration was lower. F'(v)/F'(m) respondent differently from growth to fluctuating light: the fluorescence yield was always lower than at continuous light at 100 μmol m(-2) s(-1), and always higher at 500 μmol m(-2) s(-1). Our data show that the impact of atmospheric pCO2 increase on primary production of dinoflagellate depends on the availability of nitrate and the irradiance (intensity and the frequency of irradiance fluctuations) to which the cells are exposed. The impact of global change on oceanic primary producers would therefore be different in waters with different chemical and physical (mixing) properties.

High prevalence of diffusive uptake of CO2 by macroalgae in a temperate subtidal ecosystem

Productivity of most macroalgae is not currently considered limited by dissolved inorganic carbon (DIC), as the majority of species have CO2-concentrating mechanisms (CCM) allowing the active uptake of DIC. The alternative, diffusive uptake of CO2 (non-CCM), is considered rare (0-9% of all macroalgal cover in a given ecosystem), and identifying species without CCMs is important in understanding factors controlling inorganic carbon use by eukaryotic algae. CCM activity has higher energetic requirements than diffusive CO2 uptake, therefore when light is low, CCM activity is reduced in favour of diffusive CO2 uptake. We hypothesized that the proportional cover of macroalgae without CCMs (red and green macroalgae) would be low (<10%) across four sites in Tasmania, southern Australia at two depths (4-5 and 12-14 m); the proportion of species lacking CCMs would increase with decreasing depth; the δ(13)C values of macroalgae with CCMs would be more depleted with depth. We found the proportion of non-CCM species ranged from 0 to 90% and included species from all three macroalgal phyla: 81% of red (59 species), 14% of brown (three species) and 29% of green macroalgae (two species). The proportion of non-CCM species increased with depth at three of four sites. 35% of species tested had significantly depleted δ(13)C values at deeper depths. Non-CCM macroalgae are more abundant in some temperate reefs than previously thought. If ocean acidification benefits non-CCM species, the ramifications for subtidal macroalgal assemblages could be larger than previously considered.

Abiotic versus biotic drivers of ocean pH variation under fast sea ice in McMurdo Sound, Antarctica

Ocean acidification is expected to have a major effect on the marine carbonate system over the next century, particularly in high latitude seas. Less appreciated is natural environmental variation within these systems, particularly in terms of pH, and how this natural variation may inform laboratory experiments. In this study, we deployed sensor-equipped moorings at 20 m depths at three locations in McMurdo Sound, comprising deep (bottom depth>200 m: Hut Point Peninsula) and shallow environments (bottom depth ∼25 m: Cape Evans and New Harbor). Our sensors recorded high-frequency variation in pH (Hut Point and Cape Evans only), tide (Cape Evans and New Harbor), and water mass properties (temperature and salinity) during spring and early summer 2011. These collective observations showed that (1) pH differed spatially both in terms of mean pH (Cape Evans: 8.009±0.015; Hut Point: 8.020±0.007) and range of pH (Cape Evans: 0.090; Hut Point: 0.036), and (2) pH was not related to the mixing of two water masses, suggesting that the observed pH variation is likely not driven by this abiotic process. Given the large daily fluctuation in pH at Cape Evans, we developed a simple mechanistic model to explore the potential for biotic processes – in this case algal photosynthesis – to increase pH by fixing carbon from the water column. For this model, we incorporated published photosynthetic parameters for the three dominant algal functional groups found at Cape Evans (benthic fleshy red macroalgae, crustose coralline algae, and sea ice algal communities) to estimate oxygen produced/carbon fixed from the water column underneath fast sea ice and the resulting pH change. These results suggest that biotic processes may be a primary driver of pH variation observed under fast sea ice at Cape Evans and potentially at other shallow sites in McMurdo Sound.

On-line stable isotope gas exchange reveals an inducible but leaky carbon concentrating mechanism in Nannochloropsis salina

Carbon concentrating mechanisms (CCMs) are common among microalgae, but their regulation and even existence in some of the most promising biofuel production strains is poorly understood. This is partly because screening for new strains does not commonly include assessment of CCM function or regulation despite its fundamental role in primary carbon metabolism. In addition, the inducible nature of many microalgal CCMs means that environmental conditions should be considered when assessing CCM function and its potential impact on biofuels. In this study, we address the effect of environmental conditions by combining novel, high frequency, on-line (13)CO2 gas exchange screen with microscope-based lipid characterization to assess CCM function in Nannochloropsis salina and its interaction with lipid production. Regulation of CCM function was explored by changing the concentration of CO2 provided to continuous cultures in airlift bioreactors where cell density was kept constant across conditions by controlling the rate of media supply. Our isotopic gas exchange results were consistent with N. salina having an inducible "pump-leak" style CCM similar to that of Nannochloropsis gaditana. Though cells grew faster at high CO2 and had higher rates of net CO2 uptake, we did not observe significant differences in lipid content between conditions. Since the rate of CO2 supply was much higher for the high CO2 conditions, we calculated that growing cells bubbled with low CO2 is about 40 % more efficient for carbon capture than bubbling with high CO2. We attribute this higher efficiency to the activity of a CCM under low CO2 conditions.

Carbon allocation and element composition in four Chlamydomonas mutants defective in genes related to the CO2 concentrating mechanism

Four mutants of Chlamydomonas reinhardtii with defects in different components of the CO2 concentrating mechanism (CCM) or in Rubisco activase were grown autotrophically at high pCO2 and then transferred to low pCO2, in order to study the role of different components of the CCM on carbon allocation and elemental composition. To study carbon allocation, we measured the relative size of the main organic pools by Fourier Transform Infrared spectroscopy. Total reflection X-ray fluorescence was used to analyze the elemental composition of algal cells. Our data show that although the organic pools increased their size at high CO2 in all strains, their stoichiometry was highly homeostatic, i.e., the ratios between carbohydrates and proteins, lipid and proteins, and carbohydrates and lipids, did not change significantly. The only exception was the wild-type 137c, in which proteins decreased relative to carbohydrates and lipids, when the cells were transferred to low CO2. It is noticeable that the two wild types used in this study responded differently to the transition from high to low CO2. Malfunctions of the CCM influenced the concentration of several elements, somewhat altering cell elemental stoichiometry: especially the C/P and N/P ratios changed appreciably in almost all strains as a function of the growth CO2 concentration, except in 137c and the Rubisco activase mutant rca1. In strain cia3, defective in the lumenal carbonic anhydrase (CA), the cell quotas of P, S, Ca, Mn, Fe, and Zn were about 5-fold higher at low CO2 than at high CO2. A Principle Components Analysis showed that, mostly because of its elemental composition, cia3 behaved in a substantially different way from all other strains, at low CO2. The lumenal CA thus plays a crucial role, not only for the correct functioning of the CCM, but also for element utilization. Not surprisingly, growth at high CO2 attenuated differences among strains.