Diversity of carbon use strategies in a kelp forest community: implications for a high CO2 ocean

Mechanisms for inorganic carbon acquisition in macroalgal assemblages today could indicate how coastal ecosystems will respond to predicted changes in ocean chemistry due to elevated carbon dioxide (CO₂). We identified the proportion of noncalcifying macroalgae with particular carbon use strategies using the natural abundance of carbon isotopes and pH drift experiments in a kelp forest. We also identified all calcifying macroalgae in this system; these were the dominant component of the benthos (by % cover) at all depths and seasons while cover of noncalcareous macroalgae increased at shallower depths and during summer. All large canopy‐forming macroalgae had attributes suggestive of active uptake of inorganic carbon and the presence of a CO₂ concentration mechanism (CCM). CCM species covered, on average, 15–45% of the benthos and were most common at shallow depths and during summer. There was a high level of variability in carbon isotope discrimination within CCM species, probably a result of energetic constraints on active carbon uptake in a low light environment. Over 50% of red noncalcifying species exhibited values below ?30‰ suggesting a reliance on diffusive CO₂ uptake and no functional CCM. Non‐CCM macroalgae covered on average 0–8.9% of rock surfaces and were most common in deep, low light habitats. Elevated CO₂ has the potential to influence competition between dominant coralline species (that will be negatively affected by increased CO₂) and noncalcareous CCM macroalgae (neutral or positive effects) and relatively rare (on a % cover basis) non‐CCM species (positive effects). Responses of macroalgae to elevated CO₂ will be strongly modified by light and any responses are likely to be different at times or locations where energy constrains photosynthesis. Increased growth and competitive ability of noncalcareous macroalgae alongside negative impacts of acidification on calcifying species could have major implications for the functioning of coastal reef systems at elevated CO₂ concentrations.     

INORGANIC CARBON ACQUISITION BY CHRYSOPHYTES1

Twelve species, representing 12 families of the chrysophytes sensu lato, were tested for their ability to take up inorganic carbon. Using the pH‐drift technique, CO₂ compensation points generally varied between 1 and 20 μmol · L^?1 with a mean concentration of 5 μmol · L^?1. Neither pH nor alkalinity affected the CO₂ compensation point. The concentration of oxygen had a relatively minor effect on CO₂‐uptake kinetics, and the mean CO₂ compensation point calculated from the kinetic curves was 3.6 μmol · L^?1 at 10–15 kPa starting oxygen partial pressure and 3.8 μmol · L^?1 at atmospheric starting oxygen partial pressure (21 kPa). Similarly, uptake kinetics were not affected by alkalinity, and hence concentration of bicarbonate. Membrane inlet mass spectrometry (MIMS) in the presence and absence of acetazolamide suggested that external carbonic anhydrase in Dinobryon sertularia Ehrenb. and Synura petersenii Korschikov was either very low or absent. Rates of net HCO₃^? uptake were very low (～5% of oxygen evolution) using MIMS and decreased rather than increased with increasing HCO₃^? concentration, suggesting that it was not a real uptake. The CO₂ compensation points determined by MIMS for CO₂ uptake and oxygen evolution were similar to those determined in pH‐drift and were >1 μmol · L^?1. Overall, the results suggest that chrysophytes as a group lack a carbon‐concentrating mechanism (CCM), or an ability to make use of bicarbonate as an alternative source of inorganic carbon. The possible evolutionary and ecological consequences of this are briefly discussed.     

CO2‐CONCENTRATING MECHANISMS OF THE POTENTIALLY TOXIC DINOFLAGELLATE PROTOCERATIUM RETICULATUM (DINOPHYCEAE,GONYAULACALES)1

The low CO₂ concentration in seawater poses severe restrictions on photosynthesis, especially on those species with form II RUBISCO. We found that the potentially toxic dinoflagellate Protoceratium reticulatum Clap. et J. Lachm. possesses a form II RUBISCO. To cast some light on the mechanisms this organism undergoes to cope with low CO₂ availability, we compared cells grown at atmospheric (370 ppm) and high (5000 ppm) CO₂ concentrations, with respect to a number of physiological parameters related to dissolved inorganic carbon (DIC) acquisition and assimilation. The photosynthetic affinity for DIC was about one order of magnitude lower in cells cultivated at high [CO₂]. End‐point pH‐drift experiments suggest that P. reticulatum was not able to efficiently use HCO₃^? under our growth conditions. Only internal carbonic anhydrase (CA) activity was detected, and its activity decreased by about 60% in cells cultured at high [CO₂]. Antibodies raised against a variety of algal CAs were used for Western blot analysis: P. reticulatum extracts only cross‐reacted with anti‐ß‐CA sera, and the amount of immunoreactive protein decreased in cells grown at high [CO₂]. No pyrenoids were observed under all growth conditions. Our data indicate that P. reticulatum has an inducible carbon‐concentrating mechanism (CCM) that operates in the absence of pyrenoids and with little intracellular CO₂ accumulation. Calculations on the impact of the CA activity to photosynthetic growth [CO₂] suggest that it is an essential component of the CCM of P. reticulatum and is necessary to sustain the photosynthetic rates observed at ambient CO₂.     

LCI1, a Chlamydomonas reinhardtii plasma membrane protein,functions in active CO2 uptake under low CO2

In response to high CO₂ environmental variability, green algae, such as Chlamydomonas reinhardtii, have evolved multiple physiological states dictated by external CO₂ concentration. Genetic and physiological studies demonstrated that at least three CO₂ physiological states, a high CO₂ (0.5–5% CO₂), a low CO₂ (0.03–0.4% CO₂) and a very low CO₂ (< 0.02% CO₂) state, exist in Chlamydomonas. To acclimate in the low and very low CO₂ states, Chlamydomonas induces a sophisticated strategy known as a CO₂‐concentrating mechanism (CCM) that enables proliferation and survival in these unfavorable CO₂ environments. Active uptake of C_i from the environment is a fundamental aspect in the Chlamydomonas CCM, and consists of CO₂ and HCO₃^– uptake systems that play distinct roles in low and very low CO₂ acclimation states. LCI1, a putative plasma membrane C_i transporter, has been linked through conditional overexpression to active C_i uptake. However, both the role of LCI1 in various CO₂ acclimation states and the species of C_i, HCO₃^– or CO₂, that LCI1 transports remain obscure. Here we report the impact of an LCI1 loss‐of‐function mutant on growth and photosynthesis in different genetic backgrounds at multiple pH values. These studies show that LCI1 appears to be associated with active CO₂ uptake in low CO₂, especially above air‐level CO₂, and that any LCI1 role in very low CO₂ is minimal.     

PHOTOSYNTHETIC INORGANIC CARBON ACQUISITION IN AN ACID‐TOLERANT,FREE‐LIVING SPECIES OF COCCOMYXA (CHLOROPHYTA)1

The processes of CO₂ acquisition were characterized for the acid‐tolerant, free‐living chlorophyte alga, CPCC 508. rDNA data indicate an affiliation to the genus Coccomyxa, but distinct from other known members of the genus. The alga grows over a wide range of pH from 3.0 to 9.0. External carbonic anhydrase (CA) was detected in cells grown above pH 5, with the activity increasing marginally from pH 7 to 9, but most of the CA activity was internal. The capacity for HCO₃^? uptake of cells treated with the CA inhibitor acetazolamide (AZA), was investigated by comparing the calculated rate of uncatalyzed CO₂ formation with the rate of photosynthesis. Active bicarbonate transport occurred in cells grown in media above pH 7.0. Monitoring CO₂ uptake and O₂ evolution by membrane‐inlet mass spectrometry demonstrated that air‐grown cells reduced the CO₂ concentration in the medium to an equilibrium concentration of 15 μM, but AZA‐treated cells caused a drop in extracellular CO₂ concentration to a compensation concentration of 27 μM at pH 8.0. CO₂‐pulsing experiments with cells in the light indicated that the cells do not actively take up CO₂. An internal pool of unfixed inorganic carbon was not detected at the CO₂ compensation concentration, probably because of the lack of active CO₂ uptake, but was detectable at times before compensation point was reached. These results indicate that this free‐living Coccomyxa possesses a CO₂‐concentrating mechanism (CCM) due to an active bicarbonate‐uptake system, unlike the Coccomyxa sp. occurring in symbiotic association with lichens.     

The role of external carbonic anhydrase in photosynthesis during growth of the marine diatom Chaetoceros muelleri

Carbon dioxide concentrating mechanisms (CCMs) act to improve the supply of CO₂ at the active site of ribulose‐1,5‐bisphosphate carboxylase/oxygenase. There is substantial evidence that in some microalgal species CCMs involve an external carbonic anhydrase (CA_ext) and that CA_ext activity is induced by low CO₂ concentrations in the growth medium. However, much of this work has been conducted on cells adapted to air‐equilibrium concentrations of CO₂, rather than to changing CO₂ conditions caused by growing microalgal populations. We investigated the role of CA_ext in inorganic carbon (C_i) acquisition and photosynthesis at three sampling points during the growth cycle of the cosmopolitan marine diatom Chaetoceros muelleri. We observed that CA_ext activity increased with decreasing C_i, particularly CO₂, concentration, supporting the idea that CA_ext is modulated by external CO₂ concentration. Additionally, we found that the contribution of CA_ext activity to carbon acquisition for photosynthesis varies over time, increasing between the first and second sampling points before decreasing at the last sampling point, where external pH was high. Lastly, decreases in maximum quantum yield of photosystem II (F_v/F_m), chlorophyll, maximum relative electron transport rate, light harvesting efficiency (α) and maximum rates of C_i‐ saturated photosynthesis (V_max) were observed over time. Despite this decrease in photosynthetic capacity an up‐regulation of CCM activity, indicated by a decreasing half‐saturation constant for CO₂ (K_0.5CO₂), occurred over time. The flexibility of the CCM during the course of growth in C. muelleri may contribute to the reported dominance and persistence of this species in phytoplankton blooms.     

Strategies of dissolved inorganic carbon use in macroalgae across a gradient of terrestrial influence: implications for the Great Barrier Reef in the context of ocean acidification

Macroalgae are generally used as indicators of coral reef status; thus, understanding the drivers and mechanisms leading to increased macroalgal abundance are of critical importance. Ocean acidification (OA) due to elevated carbon dioxide (CO₂) concentrations has been suggested to stimulate macroalgal growth and abundance on reefs. However, little is known about the physiological mechanisms by which reef macroalgae use CO₂ from the bulk seawater for photosynthesis [i.e., (1) direct uptake of bicarbonate (HCO₃ ^?) and/or CO₂ by means of carbon concentrating mechanisms (CCM) and (2) the diffusive uptake of CO₂], which species could benefit from increased CO₂ or which habitats may be more susceptible to acidification-induced algal proliferations. Here, we provide the first quantitative examination of CO₂-use strategies in coral reef macroalgae and provide information on how the proportion of species and the proportional abundance of species utilising each of the carbon acquisition strategies varies across a gradient of terrestrial influence (from inshore to offshore reefs) in the Great Barrier Reef (GBR). Four macroalgal groups were identified based on their carbon uptake strategies: (1) CCM-only (HCO₃ ^? only users); (2) CCM-HCO₃ ^?/CO₂ (active uptake HCO₃ ^? and/or CO₂ use); (3) Non-CCM species (those relying on diffusive CO₂ uptake); and (4) Calcifiers. δ¹³C values of macroalgae, confirmed by pH drift assays, show that diffusive CO₂ use is more prevalent in deeper waters, possibly due to low light availability that limits activity of CCMs. Inshore shallow reefs had a higher proportion of CCM-only species, while reefs further away from terrestrial influence and exposed to better water quality had a higher number of non-CCM species than inshore and mid-shelf reefs. As non-CCM macroalgae are more responsive to increased seawater CO₂ and OA, reef slopes of the outer reefs are probably the habitats most vulnerable to the impacts of OA. Our results suggest a potentially important role of carbon physiology in structuring macroalgal communities in the GBR.     

Introducing an algal carbon‐concentrating mechanism into higher plants: location and incorporation of key components

Many eukaryotic green algae possess biophysical carbon‐concentrating mechanisms (CCMs) that enhance photosynthetic efficiency and thus permit high growth rates at low CO₂ concentrations. They are thus an attractive option for improving productivity in higher plants. In this study, the intracellular locations of ten CCM components in the unicellular green alga Chlamydomonas reinhardtii were confirmed. When expressed in tobacco, all of these components except chloroplastic carbonic anhydrases CAH3 and CAH6 had the same intracellular locations as in Chlamydomonas. CAH6 could be directed to the chloroplast by fusion to an Arabidopsis chloroplast transit peptide. Similarly, the putative inorganic carbon (Ci) transporter LCI1 was directed to the chloroplast from its native location on the plasma membrane. CCP1 and CCP2 proteins, putative Ci transporters previously reported to be in the chloroplast envelope, localized to mitochondria in both Chlamydomonas and tobacco, suggesting that the algal CCM model requires expansion to include a role for mitochondria. For the Ci transporters LCIA and HLA3, membrane location and Ci transport capacity were confirmed by heterologous expression and H¹⁴CO₃^‐ uptake assays in Xenopus oocytes. Both were expressed in Arabidopsis resulting in growth comparable with that of wild‐type plants. We conclude that CCM components from Chlamydomonas can be expressed both transiently (in tobacco) and stably (in Arabidopsis) and retargeted to appropriate locations in higher plant cells. As expression of individual Ci transporters did not enhance Arabidopsis growth, stacking of further CCM components will probably be required to achieve a significant increase in photosynthetic efficiency in this species.     

CO2‐concentrating mechanisms in three southern hemisphere strains of Emiliania huxleyi

Rising global CO₂ is changing the carbonate chemistry of seawater, which is expected to influence the way phytoplankton acquire inorganic carbon. All phytoplankton rely on ribulose‐bisphosphate carboxylase oxygenase (RUBISCO) for assimilation of inorganic carbon in photosynthesis, but this enzyme is inefficient at present day CO₂ levels. Many algae have developed a range of energy demanding mechanisms, referred to as carbon concentrating mechanisms (CCMs), which increase the efficiency of carbon acquisition. We investigated CCM activity in three southern hemisphere strains of the coccolithophorid Emiliania huxleyi W. W. Hay & H. P. Mohler. Both calcifying and non‐calcifying strains showed strong CCM activity, with HCO₃^? as a preferred source of photosynthetic carbon in the non‐calcifying strain, but a higher preference for CO₂ in the calcifying strains. All three strains were characterized by the presence of pyrenoids, external carbonic anhydrase (CA) and high affinity for CO₂ in photosynthesis, indicative of active CCMs. We postulate that under higher CO₂ levels cocco‐lithophorids will be able to down‐regulate their CCMs, and re‐direct some of the metabolic energy to processes such as calcification. Due to the expected rise in CO₂ levels, photosynthesis in calcifying strains is expected to benefit most, due to their use of CO₂ for carbon uptake. The non‐calcifying strain, on the other hand, will experience only a 10% increase in HCO₃^?, thus making it less responsive to changes in carbonate chemistry of water.     

Differential effects of ocean acidification on carbon acquisition in two bloom‐forming dinoflagellate species

Dinoflagellates represent a cosmopolitan group of phytoplankton with the ability to form harmful algal blooms. Featuring a Ribulose‐1,5‐bisphosphate carboxylase/oxygenase (RubisCO) with very low CO₂ affinities, photosynthesis of this group may be particularly prone to carbon limitation and thus benefit from rising atmospheric CO₂ partial pressure (pCO₂) under ocean acidification (OA). Here, we investigated the consequences of OA on two bloom‐forming dinoflagellate species, the calcareous Scrippsiella trochoidea and the toxic Alexandrium tamarense. Using dilute batch incubations, we assessed growth characteristics over a range of pCO₂ (i.e. 180–1200 µatm). To understand the underlying physiology, several aspects of inorganic carbon acquisition were investigated by membrane‐inlet mass spectrometry. Our results show that both species kept growth rates constant over the tested pCO₂ range, but we observed a number of species‐specific responses. For instance, biomass production and cell size decreased in S. trochoidea, while A. tamarense was not responsive to OA in these measures. In terms of oxygen fluxes, rates of photosynthesis and respiration remained unaltered in S. trochoidea whereas respiration increased in A. tamarense under OA. Both species featured efficient carbon concentrating mechanisms (CCMs) with a CO₂‐dependent contribution of HCO₃^? uptake. In S. trochoidea, the CCM was further facilitated by exceptionally high and CO₂‐independent carbonic anhydrase activity. Comparing both species, a general trade‐off between maximum rates of photosynthesis and respective affinities is indicated. In conclusion, our results demonstrate effective CCMs in both species, yet very different strategies to adjust their carbon acquisition. This regulation in CCMs enables both species to maintain growth over a wide range of ecologically relevant pCO₂.     

Structure and function of LCI1: a plasma membrane CO2 channel in the Chlamydomonas CO2 concentrating mechanism

Microalgae and cyanobacteria contribute roughly half of the global photosynthetic carbon assimilation. Faced with limited access to CO₂ in aquatic environments, which can vary daily or hourly, these microorganisms have evolved use of an efficient CO₂ concentrating mechanism (CCM) to accumulate high internal concentrations of inorganic carbon (C_i) to maintain photosynthetic performance. For eukaryotic algae, a combination of molecular, genetic and physiological studies using the model organism Chlamydomonas reinhardtii, have revealed the function and molecular characteristics of many CCM components, including active C_i uptake systems. Fundamental to eukaryotic C_i uptake systems are C_i transporters/channels located in membranes of various cell compartments, which together facilitate the movement of C_i from the environment into the chloroplast, where primary CO₂ assimilation occurs. Two putative plasma membrane C_i transporters, HLA3 and LCI1, are reportedly involved in active C_i uptake. Based on previous studies, HLA3 clearly plays a meaningful role in HCO₃^? transport, but the function of LCI1 has not yet been thoroughly investigated so remains somewhat obscure. Here we report a crystal structure of the full‐length LCI1 membrane protein to reveal LCI1 structural characteristics, as well as in vivo physiological studies in an LCI1 loss‐of‐function mutant to reveal the C_i species preference for LCI1. Together, these new studies demonstrate LCI1 plays an important role in active CO₂ uptake and that LCI1 likely functions as a plasma membrane CO₂ channel, possibly a gated channel.     

NO MECHANISTIC DEPENDENCE OF PHOTOSYNTHESIS ON CALCIFICATION IN THE COCCOLITHOPHORID EMILIANIA HUXLEYI (HAPTOPHYTA)1

There is still considerable uncertainty about the relationship between calcification and photosynthesis. It has been suggested that since calcification in coccolithophorids is an intracellular process that releases CO₂, it enhances photosynthesis in a manner analogous to a carbon‐concentrating mechanism (CCM). The ubiquitous, bloom‐forming, and numerically abundant coccolithophorid Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler was studied in nutrient‐replete, pH and [CO₂] controlled, continuous cultures (turbidostats) under a range of [Ca²⁺] from 0 to 9 mM. We examined the long‐term, fully acclimated photosynthesis‐light responses and analyzed the crystalline structure of the coccoliths using SEM. The E. huxleyi cells completely lost their coccosphere when grown in 0 [Ca²⁺], while thin, undercalcified and brittle coccoliths were evident at 1 mM [Ca²⁺]. Coccoliths showed increasing levels of calcification with increasing [Ca²⁺]. More robust coccoliths were noted, with no discernable differences in coccolith morphology when the cells were grown in either 5 or 9 mM (ambient seawater) [Ca²⁺]. In contrast to calcification, photosynthesis was not affected by the [Ca²⁺] in the media. Cells showed no correlation of their light‐dependent O₂ evolution with [Ca²⁺], and in all [Ca²⁺]‐containing turbidostats, there were no significant differences in growth rate. The results show unequivocally that as a process, photosynthesis in E. huxleyi is mechanistically independent from calcification.     

In deep shade, elevated CO2 increases the vigor of tropical climbing plants

Climbing plants have profound influences on tropical forest dynamics and may take particular advantage from atmospheric CO₂ enrichment, thus potentially enhancing tree turnover. Here we test the effect of a four‐step CO₂‐enrichment on growth of three typical Yucatan (Mexico) climbers, across two low photon flux densities, representing typical understory situations. In pairs of two, species of Gonolobus (Asclepiadaceae), Ceratophytum (Bignoniaceae) and Thinouia (Sapindaceae) were grown on Yucatan forest soil in growth cabinets, which simulated the diurnal climate variation. Biomass increased non‐linearly in response to CO₂ enrichment from 280 (preindustrial) to 420 ppm and 560 ppm, but then (700 ppm) leveled off. The relative effect of CO₂‐enrichment between the two lower (280–420 ppm) CO₂ concentrations was 63% at low light (LL == 42 µmol m^2?2 s^2?1), compared to 37% at high light (HL = 87 µmol m^2?2 s^2?1). This overall response of species pairs was the combined effect of linear and non‐linear responses of the individual species across CO₂ treatments. Plant biomass was 61% larger in HL compared to LL. The species‐specific response depended on the neighbor, a species grew with h, irrespective of plant size. Stem length increased, but not consistently across species and light conditions. Specific stem length (SSL, length per dry mass) declined non‐linearly in all three species as CO₂ concentration increased (more pronounced at LL than at HL). SLA (leaf area per unit leaf dry mass) became lower as CO₂ concentration increased (more pronounced in HL). Enhanced vigor of climbers under elevated CO₂ as documented here may accelerate tropical forest dynamics and lead to greater abundance of early succesional tree species. This could reduce forest carbon stocking in the long run.     

CARBON‐USE STRATEGIES IN MACROALGAE: DIFFERENTIAL RESPONSES TO LOWERED PH AND IMPLICATIONS FOR OCEAN ACIDIFICATION1

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO₂. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO₂ and HCO₃^? increase; CO₃^2? decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO₂ gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO₃^?]. We measured the short‐term photosynthetic responses of five macroalgal species with various carbon‐use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO₂ gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO₂, HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO₃^? increased their use of CO₂ in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO₂ did so exclusively in all treatments. Although CO₂ is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO₂ is less energetically expensive than active HCO₃^? uptake, and so HCO₃^?‐using macroalgae may benefit in future seawater with elevated CO₂.     

Inorganic carbon acquisition by two Antarctic macroalgae,Porphyra endiviifolium (Rhodophyta: Bangiales) and Palmaria decipiens (Rhodophyta: Palmariales)

Aspects of the physiology of two rhodophyte macroalgae from the Antarctic, Palmaria decipiens and Porphyra endiviifolium, were examined. Both species showed low light compensation points and I _k values. Measurements of the dissolved inorganic carbon dependent kinetics of oxygen evolution gave values for K _0.5 (CO₂) of 10.5 and 3.7 μM for Palmaria and Porphyra respectively. These values are lower than expected from the kinetic properties of ribulose 1,5-bisphosphate carboxylase oxygenase and imply that the two species are capable of the active transport and accumulation of dissolved inorganic carbon by a “CO₂ concentrating mechanism”. Both organisms are able to use bicarbonate from the bulk medium. These features are similar to those found in temperate species and thus, despite the low photon flux, low temperatures and consequent elevated CO₂ concentrations in seawater at air-equilibration, the Antarctic rhodophytes examined appeared not to have a diminished capacity for transport of dissolved inorganic carbon and internal CO₂ concentration. Accepted: 24 October 1998     

Carbon acquisition characteristics of six microalgal species isolated from a subtropical reservoir: potential implications for species succession

CO ₂ levels in freshwater systems can fluctuate widely, potentially influencing photosynthetic rates and growth of phytoplankton. Given the right conditions, this can lead to bloom formation and affect water quality. This study investigated the acquisition of dissolved inorganic carbon (DIC ) by six species of microalgae, a cyanobacterium Cylindrospermopsis raciborskii , the diatoms Cyclotella sp., Nitzschia sp., and the green algae Stichococcus sp., Staurastrum sp., and Monoraphidium sp., all isolated from a subtropical reservoir in Australia. Carbon acquisition characteristics, specifically the affinity for DIC , internal pH , and internal DIC concentrations were measured. Affinities for CO ₂ () ranged between 0.7 and 6 μM CO ₂. This was considerably lower than air‐equilibrated surface water CO ₂ concentrations, and below reported affinities for CO ₂ of RuBisCO suggesting operation of active carbon dioxide concentrating mechanisms (CCM s) in all species. Internal pH was lowest for Cyclotella sp. at 7.19, and highest for Staurastrum sp., at 7.71. At 180 μM external DIC , ratios of internal:external CO ₂ ranged from 2.5 for Nitzschia sp. to 14 in C. raciborskii . Internal HCO ₃^? concentration showed a linear relationship with surface area to biovolume ratio (SA :Vol). We hypothesized that species with a higher SA :Vol suffer more from diffusive escape of CO ₂, thus storage of DIC as bicarbonate is favored in these strains. For C. raciborskii , under stratified summer conditions, its strong CCM , and resilient photosynthetic characteristics may contribute to its bloom forming capacity.     

Climate change and ocean acidification effects on seagrasses and marine macroalgae

Although seagrasses and marine macroalgae (macro‐autotrophs) play critical ecological roles in reef, lagoon, coastal and open‐water ecosystems, their response to ocean acidification (OA) and climate change is not well understood. In this review, we examine marine macro‐autotroph biochemistry and physiology relevant to their response to elevated dissolved inorganic carbon [DIC], carbon dioxide [CO₂], and lower carbonate [CO₃^2?] and pH. We also explore the effects of increasing temperature under climate change and the interactions of elevated temperature and [CO₂]. Finally, recommendations are made for future research based on this synthesis. A literature review of >100 species revealed that marine macro‐autotroph photosynthesis is overwhelmingly C₃ (≥ 85%) with most species capable of utilizing HCO₃^?; however, most are not saturated at current ocean [DIC]. These results, and the presence of CO₂‐only users, lead us to conclude that photosynthetic and growth rates of marine macro‐autotrophs are likely to increase under elevated [CO₂] similar to terrestrial C₃ species. In the tropics, many species live close to their thermal limits and will have to up‐regulate stress‐response systems to tolerate sublethal temperature exposures with climate change, whereas elevated [CO₂] effects on thermal acclimation are unknown. Fundamental linkages between elevated [CO₂] and temperature on photorespiration, enzyme systems, carbohydrate production, and calcification dictate the need to consider these two parameters simultaneously. Relevant to calcifiers, elevated [CO₂] lowers net calcification and this effect is amplified by high temperature. Although the mechanisms are not clear, OA likely disrupts diffusion and transport systems of H⁺ and DIC. These fluxes control micro‐environments that promote calcification over dissolution and may be more important than CaCO₃ mineralogy in predicting macroalgal responses to OA. Calcareous macroalgae are highly vulnerable to OA, and it is likely that fleshy macroalgae will dominate in a higher CO₂ ocean; therefore, it is critical to elucidate the research gaps identified in this review.     

Elevated CO2 enhances nitrogen fixation and growth in the marine cyanobacterium Trichodesmium

The increases in atmospheric pCO₂ over the last century are accompanied by higher concentrations of CO₂(aq) in the surface oceans. This acidification of the surface ocean is expected to influence aquatic primary productivity and may also affect cyanobacterial nitrogen (N)‐fixers (diazotrophs). No data is currently available showing the response of diazotrophs to enhanced oceanic CO₂(aq). We examined the influence of pCO₂ [preindustrial∼250 ppmv (low), ambient∼400, future∼900 ppmv (high)] on the photosynthesis, N fixation, and growth of Trichodesmium IMS101. Trichodesmium spp. is a bloom‐forming cyanobacterium contributing substantial inputs of ‘new N’ to the oligotrophic subtropical and tropical oceans. High pCO₂ enhanced N fixation, C : N ratios, filament length, and biomass of Trichodesmium in comparison with both ambient and low pCO₂ cultures. Photosynthesis and respiration did not change significantly between the treatments. We suggest that enhanced N fixation and growth in the high pCO₂ cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms (CCM) required under low and ambient pCO₂. Thus, in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO₂ to increase N fixation and growth of Trichodesmium. Other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.