A Model for HCO(3) Accumulation and Photosynthesis in the Cyanobacterium Synechococcus sp: Theoretical Predictions and Experimental Observations

Tobacco aquaporin NtAQP1 and human aquaporin hAQP1 contribute to single cell photosynthesis in Synechococcus

BACKGROUND INFORMATION

Aquaporins are H2O-permeable membrane protein pores. However, some aquaporins are also permeable to other substances such as CO2. In higher plants, overexpression of such aquaporins has already led to an enhanced photosynthetic performance due to improved CO2 mesophyll conductance. In this work, we investigated the effects of such aquaporins on unicellular photosynthetically active organisms, specifically cyanobacteria.

RESULTS

Overexpression of aquaporins NtAQP1 or hAQP1 that might have a function to improve CO2 membrane permeability lead to increased photosynthesis rates in the cyanobacterium Synechococcus sp. PCC7002 as concluded by the rate of evolved O2. A shift in the Plastoquinone pool state of the cells supports our findings. Water permeable aquaporins without CO2 permeability, such as NtPIP2;1, do not have this effect.

CONCLUSIONS AND SIGNIFICANCE

We conclude that also in single cell organisms like cyanobacteria, membrane CO2 conductivity could be rate limiting and CO2-porins reduce the respective membrane resistance. We could show that besides the tobacco aquaporin NtAQP1 also the human hAQP1 most likely functions as CO2 diffusion facilitator in the photosynthesis assay.

Prospective bacterial and fungal sources of hyaluronic acid: A review

The unique biological and rheological properties make hyaluronic acid a sought-after material for medicine and cosmetology. Due to very high purity requirements for hyaluronic acid in medical applications, the profitability of streptococcal fermentation is reduced. Production of hyaluronic acid by recombinant systems is considered a promising alternative. Variations in combinations of expressed genes and fermentation conditions alter the yield and molecular weight of produced hyaluronic acid. This review is devoted to the current state of hyaluronic acid production by recombinant bacterial and fungal organisms.

Computational modeling and evolutionary implications of biochemical reactions in bacterial microcompartments

Bacterial microcompartments (BMCs) are protein-encapsulated compartments found across at least 23 bacterial phyla. BMCs contain a variety of metabolic processes that share the commonality of toxic or volatile intermediates, oxygen-sensitive enzymes and cofactors, or increased substrate concentration for magnified reaction rates. These compartmentalized reactions have been computationally modeled to explore the encapsulated dynamics, ask evolutionary-based questions, and develop a more systematic understanding required for the engineering of novel BMCs. Many crucial aspects of these systems remain unknown or unmeasured, such as substrate permeabilities across the protein shell, feasibility of pH gradients, and transport rates of associated substrates into the cell. This review explores existing BMC models, dominated in the literature by cyanobacterial carboxysomes, and highlights potentially important areas for exploration.

N2 fixation in free-floating filaments of Trichodesmium is higher than in transiently suboxic colony microenvironments

To understand the role of micrometer-scale oxygen (O₂ ) gradients in facilitating dinitrogen (N₂ ) fixation, we characterized O₂ dynamics in the microenvironment around free-floating trichomes and colonies of Trichodesmium erythraeum IMS101. Diurnal and spatial variability in O₂ concentrations in the bulk medium, within colonies, along trichomes and within single cells were determined using O₂ optodes, microsensors and model calculations. Carbon (C) and N₂ fixation as well as O₂ evolution and uptake under different O₂ concentrations were analyzed by stable isotope incubations and membrane inlet mass spectrometry. We observed a pronounced diel rhythm in O₂ fluxes, with net O₂ evolution restricted to short periods in the morning and evening, and net O₂ uptake driven by dark respiration and light-dependent O₂ uptake during the major part of the light period. Remarkably, colonies showed lower N₂ fixation and C fixation rates than free-floating trichomes despite the long period of O₂ undersaturation in the colony microenvironment. Model calculations demonstrate that low permeability of the cell wall in combination with metabolic heterogeneity between single cells allows for anoxic intracellular conditions in colonies but also free-floating trichomes of Trichodesmium. Therefore, whereas colony formation must have benefits for Trichodesmium, it does not favor N₂ fixation.

A diurnal flux balance model of Synechocystis sp. PCC 6803 metabolism

Phototrophic organisms such as cyanobacteria utilize the sun's energy to convert atmospheric carbon dioxide into organic carbon, resulting in diurnal variations in the cell's metabolism. Flux balance analysis is a widely accepted constraint-based optimization tool for analyzing growth and metabolism, but it is generally used in a time-invariant manner with no provisions for sequestering different biomass components at different time periods. Here we present CycleSyn, a periodic model of Synechocystis sp. PCC 6803 metabolism that spans a 12-hr light/12-hr dark cycle by segmenting it into 12 Time Point Models (TPMs) with a uniform duration of two hours. The developed framework allows for the flow of metabolites across TPMs while inventorying metabolite levels and only allowing for the utilization of currently or previously produced compounds. The 12 TPMs allow for the incorporation of time-dependent constraints that capture the cyclic nature of cellular processes. Imposing bounds on reactions informed by temporally-segmented transcriptomic data enables simulation of phototrophic growth as a single linear programming (LP) problem. The solution provides the time varying reaction fluxes over a 24-hour cycle and the accumulation/consumption of metabolites. The diurnal rhythm of metabolic gene expression driven by the circadian clock and its metabolic consequences is explored. Predicted flux and metabolite pools are in line with published studies regarding the temporal organization of phototrophic growth in Synechocystis PCC 6803 paving the way for constructing time-resolved genome-scale models (GSMs) for organisms with a circadian clock. In addition, the metabolic reorganization that would be required to enable Synechocystis PCC 6803 to temporally separate photosynthesis from oxygen-sensitive nitrogen fixation is also explored using the developed model formalism.

Toward Multiscale Models of Cyanobacterial Growth: A Modular Approach

Oxygenic photosynthesis dominates global primary productivity ever since its evolution more than three billion years ago. While many aspects of phototrophic growth are well understood, it remains a considerable challenge to elucidate the manifold dependencies and interconnections between the diverse cellular processes that together facilitate the synthesis of new cells. Phototrophic growth involves the coordinated action of several layers of cellular functioning, ranging from the photosynthetic light reactions and the electron transport chain, to carbon-concentrating mechanisms and the assimilation of inorganic carbon. It requires the synthesis of new building blocks by cellular metabolism, protection against excessive light, as well as diurnal regulation by a circadian clock and the orchestration of gene expression and cell division. Computational modeling allows us to quantitatively describe these cellular functions and processes relevant for phototrophic growth. As yet, however, computational models are mostly confined to the inner workings of individual cellular processes, rather than describing the manifold interactions between them in the context of a living cell. Using cyanobacteria as model organisms, this contribution seeks to summarize existing computational models that are relevant to describe phototrophic growth and seeks to outline their interactions and dependencies. Our ultimate aim is to understand cellular functioning and growth as the outcome of a coordinated operation of diverse yet interconnected cellular processes.

Marine phototrophic consortia transfer electrons to electrodes in response to reductive stress

This work studies how extracellular electron transfer (EET) from cyanobacteria-dominated marine microbial biofilms to solid electrodes is affected by the availability of inorganic carbon (Ci). The EET was recorded chronoamperometrically in the form of electrical current by a potentiostat in two identical photo-electrochemical cells using carbon electrodes poised at a potential of +0.6 V versus standard hydrogen electrode under 12/12 h illumination/dark cycles. The Ci was supplied by the addition of NaHCO3 to the medium and/or by sparging CO2 gas. At high Ci conditions, EET from the microbial biofilm to the electrodes was observed only during the dark phase, indicating the occurrence of a form of night-time respiration that can use insoluble electrodes as the terminal electron acceptor. At low or no Ci conditions, however, EET also occurred during illumination suggesting that, in the absence of their natural electron acceptor, some cyanobacteria are able to utilise solid electrodes as an electron sink. This may be a natural survival mechanism for cyanobacteria to maintain redox balance in environments with limiting CO2 and/or high light intensity.

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.

Cellular inorganic carbon fluxes in Trichodesmium: a combined approach using measurements and modelling

To predict effects of climate change on phytoplankton, it is crucial to understand how their mechanisms for carbon acquisition respond to environmental conditions. Aiming to shed light on the responses of extra- and intracellular inorganic C (Ci) fluxes, the cyanobacterium Trichodesmium erythraeum IMS101 was grown with different nitrogen sources (N2 vs NO3 (-)) and pCO2 levels (380 vs 1400 µatm). Cellular Ci fluxes were assessed by combining membrane inlet mass spectrometry (MIMS), (13)C fractionation measurements, and modelling. Aside from a significant decrease in Ci affinity at elevated pCO2 and changes in CO2 efflux with different N sources, extracellular Ci fluxes estimated by MIMS were largely unaffected by the treatments. (13)C fractionation during biomass production, however, increased with pCO2, irrespective of the N source. Strong discrepancies were observed in CO2 leakage estimates obtained by MIMS and a (13)C-based approach, which further increased under elevated pCO2. These offsets could be explained by applying a model that comprises extracellular CO2 and HCO3 (-) fluxes as well as internal Ci cycling around the carboxysome via the CO2 uptake facilitator NDH-14. Assuming unidirectional, kinetic fractionation between CO2 and HCO3 (-) in the cytosol or enzymatic fractionation by NDH-14, both significantly improved the comparability of leakage estimates. Our results highlight the importance of internal Ci cycling for (13)C composition as well as cellular energy budgets of Trichodesmium, which ought to be considered in process studies on climate change effects.

The minimal CO2-concentrating mechanism of Prochlorococcus spp. MED4 is effective and efficient

As an oligotrophic specialist, Prochlorococcus spp. has streamlined its genome and metabolism including the CO2-concentrating mechanism (CCM), which serves to elevate the CO2 concentration around Rubisco. The genomes of Prochlorococcus spp. indicate that they have a simple CCM composed of one or two HCO3(-) pumps and a carboxysome, but its functionality has not been examined. Here, we show that the CCM of Prochlorococcus spp. is effective and efficient, transporting only two molecules of HCO3(-) per molecule of CO2 fixed. A mechanistic, numerical model with a structure based on the CCM components present in the genome is able to match data on photosynthesis, CO2 efflux, and the intracellular inorganic carbon pool. The model requires the carboxysome shell to be a major barrier to CO2 efflux and shows that excess Rubisco capacity is critical to attaining a high-affinity CCM without CO2 recovery mechanisms or high-affinity HCO3(-) transporters. No differences in CCM physiology or gene expression were observed when Prochlorococcus spp. was fully acclimated to high-CO2 (1,000 µL L(-1)) or low-CO2 (150 µL L(-1)) conditions. Prochlorococcus spp. CCM components in the Global Ocean Survey metagenomes were very similar to those in the genomes of cultivated strains, indicating that the CCM in environmental populations is similar to that of cultured representatives.

Bicarbonate uptake via an anion exchange protein is the main mechanism of inorganic carbon acquisition by the giant kelp Macrocystis pyrifera (Laminariales, Phaeophyceae) under variable pH

Macrocystis pyrifera is a widely distributed, highly productive, seaweed. It is known to use bicarbonate (HCO3 (-) ) from seawater in photosynthesis and the main mechanism of utilization is attributed to the external catalyzed dehydration of HCO3 (-) by the surface-bound enzyme carbonic anhydrase (CAext ). Here, we examined other putative HCO3 (-) uptake mechanisms in M. pyrifera under pHT 9.00 (HCO3 (-) : CO2  = 940:1) and pHT 7.65 (HCO3 (-) : CO2  = 51:1). Rates of photosynthesis, and internal CA (CAint ) and CAext activity were measured following the application of AZ which inhibits CAext , and DIDS which inhibits a different HCO3 (-) uptake system, via an anion exchange (AE) protein. We found that the main mechanism of HCO3 (-) uptake by M. pyrifera is via an AE protein, regardless of the HCO3 (-) : CO2 ratio, with CAext making little contribution. Inhibiting the AE protein led to a 55%-65% decrease in photosynthetic rates. Inhibiting both the AE protein and CAext at pHT 9.00 led to 80%-100% inhibition of photosynthesis, whereas at pHT 7.65, passive CO2 diffusion supported 33% of photosynthesis. CAint was active at pHT 7.65 and 9.00, and activity was always higher than CAext , because of its role in dehydrating HCO3 (-) to supply CO2 to RuBisCO. Interestingly, the main mechanism of HCO3 (-) uptake in M. pyrifera was different than that in other Laminariales studied (CAext -catalyzed reaction) and we suggest that species-specific knowledge of carbon uptake mechanisms is required in order to elucidate how seaweeds might respond to future changes in HCO3 (-) :CO2 due to ocean acidification.

Energy costs of carbon dioxide concentrating mechanisms in aquatic organisms

Minimum energy (as photon) costs are predicted for core reactions of photosynthesis, for photorespiratory metabolism in algae lacking CO2 concentrating mechanisms (CCMs) and for various types of CCMs; in algae, with CCMs; allowance was made for leakage of CO2 from the internal pool. These predicted values are just compatible with the minimum measured photon costs of photosynthesis in microalgae and macroalgae lacking or expressing CCMs. More energy-expensive photorespiration, for example for organisms using Rubiscos with lower CO2-O2 selectivity coefficients, would be less readily accommodated within the lowest measured photon costs of photosynthesis by algae lacking CCMs. The same applies to the cases of CCMs with higher energy costs of active transport of protons or inorganic carbon species, or greater allowance for significant leakage from the accumulated intracellular pool of CO2. High energetic efficiency can involve a higher concentration of catalyst to achieve a given rate of reaction, adding to the resource costs of growth. There are no obvious mechanistic interpretations of the occurrence of CCMs algae adapted to low light and low temperatures using the rationales adopted for the occurrence of C4 photosynthesis in terrestrial flowering plants. There is an exception for cyanobacteria with low-selectivity Form IA or IB Rubiscos, and those dinoflagellates with low-selectivity Form II Rubiscos, for which very few natural environments have high enough CO2:O2 ratios to allow photosynthesis in the absence of CCMs.

Comparing the in vivo function of α-carboxysomes and β-carboxysomes in two model cyanobacteria

The carbon dioxide (CO2)-concentrating mechanism of cyanobacteria is characterized by the occurrence of Rubisco-containing microcompartments called carboxysomes within cells. The encapsulation of Rubisco allows for high-CO2 concentrations at the site of fixation, providing an advantage in low-CO2 environments. Cyanobacteria with Form-IA Rubisco contain α-carboxysomes, and cyanobacteria with Form-IB Rubisco contain β-carboxysomes. The two carboxysome types have arisen through convergent evolution, and α-cyanobacteria and β-cyanobacteria occupy different ecological niches. Here, we present, to our knowledge, the first direct comparison of the carboxysome function from α-cyanobacteria (Cyanobium spp. PCC7001) and β-cyanobacteria (Synechococcus spp. PCC7942) with similar inorganic carbon (Ci; as CO2 and HCO3-) transporter systems. Despite evolutionary and structural differences between α-carboxysomes and β-carboxysomes, we found that the two strains are remarkably similar in many physiological parameters, particularly the response of photosynthesis to light and external Ci and their modulation of internal ribulose-1,5-bisphosphate, phosphoglycerate, and Ci pools when grown under comparable conditions. In addition, the different Rubisco forms present in each carboxysome had almost identical kinetic parameters. The conclusions indicate that the possession of different carboxysome types does not significantly influence the physiological function of these species and that similar carboxysome function may be possessed by each carboxysome type. Interestingly, both carboxysome types showed a response to cytosolic Ci, which is of higher affinity than predicted by current models, being saturated by 5 to 15 mm Ci. This finding has bearing on the viability of transplanting functional carboxysomes into the C3 chloroplast.

Detection of a variable intracellular acid-labile carbon pool in Thalassiosira weissflogii (Heterokontophyta) and Emiliania huxleyi (Haptophyta) in response to changes in the seawater carbon system

Accumulation of an intracellular pool of carbon (C(i) pool) is one strategy by which marine algae overcome the low abundance of dissolved CO2 (CO2 (aq) ) in modern seawater. To identify the environmental conditions under which algae accumulate an acid-labile C(i) pool, we applied a (14) C pulse-chase method, used originally in dinoflagellates, to two new classes of algae, coccolithophorids and diatoms. This method measures the carbon accumulation inside the cells without altering the medium carbon chemistry or culture cell density. We found that the diatom Thalassiosira weissflogii [(Grunow) G. Fryxell & Hasle] and a calcifying strain of the coccolithophorid Emiliania huxleyi [(Lohmann) W. W. Hay & H. P. Mohler] develop significant acid-labile C(i) pools. C(i) pools are measureable in cells cultured in media with 2-30 µmol l(-1) CO2 (aq), corresponding to a medium pH of 8.6-7.9. The absolute C(i) pool was greater for the larger celled diatoms. For both algal classes, the C(i) pool became a negligible contributor to photosynthesis once CO2 (aq) exceeded 30 µmol l(-1) . Combining the (14) C pulse-chase method and (14) C disequilibrium method enabled us to assess whether E. huxleyi and T. weissflogii exhibited thresholds for foregoing accumulation of DIC or reduced the reliance on bicarbonate uptake with increasing CO2 (aq) . We showed that the C(i) pool decreases with higher CO2 :HCO3 (-) uptake rates.

The effect of rock composition on cyanobacterial weathering of crystalline basalt and rhyolite

The weathering of volcanic rocks contributes significantly to the global silicate weathering budget, effecting carbon dioxide drawdown and long-term climate control. The rate of chemical weathering is influenced by the composition of the rock. Rock-dwelling micro-organisms are known to play a role in changing the rate of weathering reactions; however, the influence of rock composition on bio-weathering is unknown. Cyanobacteria are known to be a ubiquitous surface taxon in volcanic rocks. In this study, we used a selection of fast and slow growing cyanobacterial species to compare microbial-mediated weathering of bulk crystalline rocks of basaltic and rhyolitic composition, under batch conditions. Cyanobacterial growth caused an increase in the pH of the medium and an acceleration of rock dissolution compared to the abiotic controls. For example, Anabaena cylindrica increased the linear release rate (R(i)(l)) of Ca, Mg, Si and K from the basalt by more than fivefold (5.21-12.48) and increased the pH of the medium by 1.9 units. Although A. cylindrica enhanced rhyolite weathering, the increase in R(i)(l) was less than threefold (2.04-2.97) and the pH increase was only 0.83 units. The R(i)(l) values obtained with A. cylindrica were at least ninefold greater with the basalt than the rhyolite, whereas in the abiotic controls, the difference was less than fivefold. Factors accounting for the slower rate of rhyolite weathering and lower biomass achieved are likely to include the higher content of quartz, which has a low rate of weathering and lower concentrations of bio-essential elements, such as, Ca, Fe and Mg, which are known to be important in controlling cyanobacterial growth. We show that at conditions where weathering is favoured, biota can enhance the difference between low and high Si-rock weathering. Our data show that cyanobacteria can play a significant role in enhancing rock weathering and likely have done since they evolved on the early Earth.

Modelling cyanobacteria: from metabolism to integrative models of phototrophic growth

Cyanobacteria are phototrophic microorganisms of global importance and have recently attracted increasing attention due to their capability to convert sunlight and atmospheric CO(2) directly into organic compounds, including carbon-based biofuels. The utilization of cyanobacteria as a biological chassis to generate third-generation biofuels would greatly benefit from an increased understanding of cyanobacterial metabolism and its interplay with other cellular processes. In this respect, metabolic modelling has been proposed as a way to overcome the traditional trial and error methodology that is often employed to introduce novel pathways. In particular, flux balance analysis and related methods have proved to be powerful tools to investigate the organization of large-scale metabolic networks-with the prospect of predicting modifications that are likely to increase the yield of a desired product and thereby to streamline the experimental progress and avoid futile avenues. This contribution seeks to describe the utilization of metabolic modelling as a research tool to understand the metabolism and phototrophic growth of cyanobacteria. The focus of the contribution is on a mathematical description of the metabolic network of Synechocystis sp. PCC 6803 and its analysis using constraint-based methods. A particular challenge is to integrate the description of the metabolic network with other cellular processes, such as the circadian clock, the photosynthetic light reactions, carbon concentration mechanism, and transcriptional regulation-aiming at a predictive model of a cyanobacterium in silico.

Adaptive signals in algal Rubisco reveal a history of ancient atmospheric carbon dioxide

Rubisco, the most abundant enzyme on the Earth and responsible for all photosynthetic carbon fixation, is often thought of as a highly conserved and sluggish enzyme. Yet, different algal Rubiscos demonstrate a range of kinetic properties hinting at a history of evolution and adaptation. Here, we show that algal Rubisco has indeed evolved adaptively during ancient and distinct geological periods. Using DNA sequences of extant marine algae of the red and Chromista lineage, we define positive selection within the large subunit of Rubisco, encoded by rbcL, to occur basal to the radiation of modern marine groups. This signal of positive selection appears to be responding to changing intracellular concentrations of carbon dioxide (CO(2)) triggered by physiological adaptations to declining atmospheric CO(2). Within the ecologically important Haptophyta (including coccolithophores) and Bacillariophyta (diatoms), positive selection occurred consistently during periods of falling Phanerozoic CO(2) and suggests emergence of carbon-concentrating mechanisms. During the Proterozoic, a strong signal of positive selection after secondary endosymbiosis occurs at the origin of the Chromista lineage (approx. 1.1 Ga), with further positive selection events until 0.41 Ga, implying a significant and continuous decrease in atmospheric CO(2) encompassing the Cryogenian Snowball Earth events. We surmise that positive selection in Rubisco has been caused by declines in atmospheric CO(2) and hence acts as a proxy for ancient atmospheric CO(2).

Bacterial microcompartments

Bacterial microcompartments (BMCs) are organelles composed entirely of protein. They promote specific metabolic processes by encapsulating and colocalizing enzymes with their substrates and cofactors, by protecting vulnerable enzymes in a defined microenvironment, and by sequestering toxic or volatile intermediates. Prototypes of the BMCs are the carboxysomes of autotrophic bacteria. However, structures of similar polyhedral shape are being discovered in an ever-increasing number of heterotrophic bacteria, where they participate in the utilization of specialty carbon and energy sources. Comparative genomics reveals that the potential for this type of compartmentalization is widespread across bacterial phyla and suggests that genetic modules encoding BMCs are frequently laterally transferred among bacteria. The diverse functions of these BMCs suggest that they contribute to metabolic innovation in bacteria in a broad range of environments.

Carboxysomal carbonic anhydrases: Structure and role in microbial CO2 fixation

Cyanobacteria and some chemoautotrophic bacteria are able to grow in environments with limiting CO(2) concentrations by employing a CO(2)-concentrating mechanism (CCM) that allows them to accumulate inorganic carbon in their cytoplasm to concentrations several orders of magnitude higher than that on the outside. The final step of this process takes place in polyhedral protein microcompartments known as carboxysomes, which contain the majority of the CO(2)-fixing enzyme, RubisCO. The efficiency of CO(2) fixation by the sequestered RubisCO is enhanced by co-localization with a specialized carbonic anhydrase that catalyzes dehydration of the cytoplasmic bicarbonate and ensures saturation of RubisCO with its substrate, CO(2). There are two genetically distinct carboxysome types that differ in their protein composition and in the carbonic anhydrase(s) they employ. Here we review the existing information concerning the genomics, structure and enzymology of these uniquely adapted carbonic anhydrases, which are of fundamental importance in the global carbon cycle.

EFFECTS OF MODERATE HEAT STRESS AND DISSOLVED INORGANIC CARBON CONCENTRATION ON PHOTOSYNTHESIS AND RESPIRATION OF SYMBIODINIUM SP. (DINOPHYCEAE) IN CULTURE AND IN SYMBIOSIS(1)

The influence of temperature and inorganic carbon (Ci ) concentration on photosynthesis was examined in whole corals and samples of cultured symbiotic dinoflagellates (Symbiodinium sp.) using combined measurements from a membrane inlet mass spectrometer and chl a fluorometer. In whole corals, O2 production at 26°C was significantly limited at Ci concentrations below ambient seawater (∼2.2 mM). Further additions of Ci up to ∼10 mM caused no further stimulation of oxygenic photosynthesis. Following exposure to 30°C (2 d), net oxygen production decreased significantly in whole corals, as a result of reduced production of photosynthetically derived oxygen rather than increased host consumption. Whole corals maintained a rate of oxygen evolution around eight times lower than cultured Symbiodinium sp. at inorganic carbon concentrations <2 mM, but cultures displayed greater levels of photoinhibition following heat treatment (30°C, 2 d). Whole corals and cultured zooxanthellae differed considerably in their responses to Ci concentration and moderate heat stress, demonstrating that cultured Symbiodinium make an incongruous model for those in hospite. Reduced net oxygen evolution, in whole corals, under conditions of low Ci (<2 mM) has been interpreted in terms of possible sink limitation leading to increased nonphotochemical energy dissipation. The advantages of combined measurement of net gas exchange and fluorometry offered by this method are discussed.

Historical perspective on microalgal and cyanobacterial acclimation to low- and extremely high-CO(2) conditions

Reports in the 1970s from several laboratories revealed that the affinity of photosynthetic machinery for dissolved inorganic carbon (DIC) was greatly increased when unicellular green microalgae were transferred from high to low-CO(2) conditions. This increase was due to the induction of carbonic anhydrase (CA) and the active transport of CO(2) and/or HCO(3) (-) which increased the internal DIC concentration. The feature is referred to as the 'CO(2)-concentrating mechanism (CCM)'. It was revealed that CA facilitates the supply of DIC from outside to inside the algal cells. It was also found that the active species of DIC absorbed by the algal cells and chloroplasts were CO(2) and/or HCO(3) (-), depending on the species. In the 1990s, gene technology started to throw light on the molecular aspects of CCM and identified the genes involved. The identification of the active HCO(3) (-) transporter, of the molecules functioning for the energization of cyanobacteria and of CAs with different cellular localizations in eukaryotes are examples of such successes. The first X-ray structural analysis of CA in a photosynthetic organism was carried out with a red alga. The results showed that the red alga possessed a homodimeric beta-type of CA composed of two internally repeating structures. An increase in the CO(2) concentration to several percent results in the loss of CCM and any further increase is often disadvantageous to cellular growth. It has recently been found that some microalgae and cyanobacteria can grow rapidly even under CO(2) concentrations higher than 40%. Studies on the mechanism underlying the resistance to extremely high CO(2) concentrations have indicated that only algae that can adopt the state transition in favor of PS I could adapt to and survive under such conditions. It was concluded that extra ATP produced by enhanced PS I cyclic electron flow is used as an energy source of H(+)-transport in extremely high-CO(2) conditions. This same state transition has also been observed when high-CO(2) cells were transferred to low CO(2) conditions, indicating that ATP produced by cyclic electron transfer was necessary to accumulate DIC in low-CO(2) conditions.

Bicarbonate binding activity of the CmpA protein of the cyanobacterium Synechococcus sp. strain PCC 7942 involved in active transport of bicarbonate

The cmpABCD operon of the cyanobacterium Synechococcus sp. strain PCC 7942 encodes an ATP-binding cassette transporter involved in HCO(3)(-) uptake. The three genes, cmpBCD, encode membrane components of an ATP-binding cassette transporter, whereas cmpA encodes a 42-kDa cytoplasmic membrane protein, which is 46.5% identical to the membrane-anchored substrate-binding protein of the nitrate/nitrite transporter. Equilibrium dialysis analysis using H(14)CO(3)(-) showed that a truncated CmpA protein lacking the N-terminal 31 amino acids, expressed in Escherichia coli cells as a histidine-tagged soluble protein, specifically binds inorganic carbon (CO(2) or HCO(3)(-)). The addition of the recombinant CmpA protein to a buffer caused a decrease in the concentration of dissolved CO(2) because of the binding of inorganic carbon to the protein. The decrease in CO(2) concentration was accelerated by the addition of carbonic anhydrase, indicating that HCO(3)(-), but not CO(2), binds to the protein. Mass spectrometric measurements of the amounts of unbound and bound HCO(3)(-) in CmpA solutions containing low concentrations of inorganic carbon revealed that CmpA binds HCO(3)(-) with high affinity (K(d) = 5 microm). A similar dissociation constant was obtained by analysis of the competitive inhibition of the CmpA protein on the carboxylation of phosphoenolpyruvate by phosphoenolpyruvate carboxylase at limiting concentrations of HCO(3)(-). These findings showed that the cmpA gene encodes the substrate-binding protein of the HCO(3)(-) transporter.

The diversity and coevolution of Rubisco, plastids, pyrenoids, and chloroplast-based CO2-concentrating mechanisms in algae

Algae have adopted two primary strategies to maximize the performance of Rubisco in photosynthetic CO2 fixation. This has included either the development of a CO2-concentrating mechanism (CCM), based at the level of the chloroplast, or the evolution of the kinetic properties of Rubisco. This review examines the potential diversity of both Rubisco and chloroplast-based CCMs across algal divisions, including both green and nongreen algae, and seeks to highlight recent advances in our understanding of the area and future areas for research. Overall, the available data show that Rubisco enzymes from algae have evolved a higher affinity for CO2 when the algae have adopted a strategy for CO2 fixation that does not utilise a CCM. This appears to be true of both Green and Red Form I Rubisco enzymes found in green and nongreen algae, respectively. However, the Red Form I Rubisco enzymes present in nongreen algae appear to have reduced oxygenase potential at air level of O2. This has resulted in a photosynthetic physiology with a reduced potential to be inhibited by O2 and a reduced need to deal with photorespiration. In the limited number of microalgae that have been examined, there is a strong correlation between the existence of a high-affinity CCM physiology and the presence of pyrenoids in all algae, highlighting the potential importance of these chloroplast Rubisco-containing bodies. However, in macroalgae, there is greater diversity in the apparent relationships between pyrenoids and chloroplast features and the CCM physiology that the species shows. There are many examples of microalgae and macroalgae with variations in the presence and absence of pyrenoids as well as single and multiple chloroplasts per cell. This occurs in both green and nongreen algae and should provide ample material for extending studies in this area. Future research into the function of the pyrenoid and other chloroplast features, such as thylakoids, in the operation of a chloroplast-based CCM needs to be addressed in a diverse range of algal species. This should be approached together with assessment of the coevolution of Rubisco, particularly the evolution of Red Form I Rubisco enzymes, which appear to achieve superior kinetic characteristics when compared with the Rubisco of C3 higher plants, which are derived from green algal ancestors.Key words: Rubisco, CO2-concentrating mechanism, carbonic anhydrase, aquatic photosynthesis, algae, pyrenoids, inorganic carbon.

Driving Forces for Bicarbonate Transport in the Cyanobacterium Synechococcus R-2 (PCC 7942)

Air-grown Synechococcus R-2 (PCC 7942) cultures grown in BG-11 medium are very alkaline (outside pH is 10.0) and use HCO3- as their inorganic carbon source. The cells showed a dependence on Na+ for photosynthesis, but low Na+ conditions (1 mol m-3) were sufficient to support saturating photosynthesis. The intracellular dissolved inorganic carbon in the light was greater than 20 mol m-3 in both low-Na+ conditions and in BG-11 medium containing the usual [Na+] (24 mol m-3, designated high-Na+ conditions). The electrochemical potential for HCO3- in the light was in excess of 25 kJ mol-1, even in high-Na+ conditions. The Na+-motive force was greater than -12 kJ mol-1 under both Na+ conditions. On thermodynamic grounds, an Na+-driven co-port process would need to have a stoichiometry of 2 or greater ([greater than or equal to]2Na+ in/HCO3-1 in), but we show that Na+ or K+ fluxes cannot be linked to HCO3- transport. Na+ and K+ fluxes were unaffected by the presence or absence of dissolved inorganic carbon. In low-Na+ conditions, Na+ fluxes are too low to support the observed net 14C-carbon fixation rate. Active transport of HCO3- hyperpolarizes (not depolarizes) the membrane potential.

Carbonic anhydrase(s) associated with lichens: in vivo activities, possible locations and putative roles

The activity of the enzyme carbonic anhydrase (CA; E.C. 4.2.1.1) was examined in eight species of lichen in vivo by measurements of ¹⁸ O exchange from doubly labelled CO₂ (¹³ C¹⁸ O¹⁸ O) to water, with the aim of determining whether this enzyme is present in lichens and if so to what extent it participates in physiological processes. The lichens were chosen to represent different mycobiont (Heterodera, Leptogium, Lobaria, Peltigera, Pseudocyphellaria, Xantoparmelia) and photobiont (Coccomyxa, Dictyochloropsis, Myrmecia, Nostoc, Trebouxia) genera as well as varying morphology. All lichens were found to possess considerable activity of CA, which varied by a factor of about four between the species when compared on a chlorophyl basis, and by u factor of about 10 when related to the weight. Incubation of lichen tholli with ethoxyzolamide (EZA) and acetazolamide (AZA), which inhibit intracellular and extracellular CA respectively, showed that intracellular CA was present in all of the species. Incubation with EZA also resulted in inhibition of photosynthetic CO₂ , uptake in all species suggesting a role of internal CA in the CO₂ , acquisition process in all the investigated photobiont genera. A part of the CA activity was inhibited by AZA in all except two of the four cyanobacterial Nostoc lichens, indicating the possible presence of extracellular forms of CA. This CA also appeared to be involved in the acquisition of CO₂ , as AZA inhibited photosynthetic CO₂ uptake in these lichens. The CA inhibitors also affected CO₂ efflux in the dark, i.e. fungal respiration, which was inhibited by both AZA and EZA in six of the species, suggesting that a proportion of the CA activity might be located in the mycobiont.

Quantitative evaluation of the role of a putative CO2-scavenging entity in the cyanobacterial CO2-concentrating mechanism

This paper assesses the contribution of a postulated CO2-scavenging system to the efficient operation of the CO2-concentrating mechanism (CCM) in cyanobacteria. A quantitative model for the CCM is presented which incorporates an energy-dependent carbonic anhydrase-like entity located at or near the inner surface of the plasma membrane. This entity, which converts CO2 to HCO3- against the thermodynamic potential, scavenges CO2 leaking outward from the carboxysomes, and, further, converts CO2 entering from the medium to HCO3-, thus maintaining an inward diffusion gradient along which CO2 enters passively. The model resembles our earlier models in postulating that CO2 and HCO3- are not at equilibrium throughout the greater part of the cell, and that CO2 is generated in high concentration at carbonic anhydrase sites within the carboxysomes. The model further takes into account the concentric thylakoid membranes which surround the carboxysomes, and events in the periplasmic space and the unstirred layer surrounding the cell. Implications of the predicted steady state fluxes of CO2 and HCO3-, and of their steady state concentrations in various cellular compartments, are discussed. The plasma membrane carbonic anhydrase-like activity lowers the photosynthetic Km for external Ci, as well as decreasing the inorganic C 'leak', but it may not save on energy expenditure.

Photosynthesis and photorespiration in a mutant of the cyanobacterium Synechocystis PCC 6803 lacking carboxysomes

A mutant of the cyanobacterium Synechocystis PCC 6803 was obtained by replacing the gene of the carboxylation enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) with that of the photosynthetic bacterium Rhodospirillum rubrum. This mutant consequently lacks carboxysomes - the protein complexes in which the original enzyme is packed. It is incapable of growing at atmospheric CO2 levels and has an apparent photosynthetic affinity for inorganic carbon (Ci) which is 1000 times lower than that of the wild type, yet it accumulates more Ci than the wild type. The mutant appears to be defective in its ability to utilize the intracellular Ci pool for photosynthesis. Unlike the carboxysomal carboxylase activity of Rubisco, which is almost insensitive to inhibition by O2 in vitro, the soluble enzyme is competitively inhibited by O2. The photosynthetic rate and Ci compensation point of the wild type were hardly affected by low O2 levels. Above 100 μM O2, however, both parameters became inhibited. The CO2 compensation point of the mutant was linearly dependent on O2 concentration. The higher sensitivity of the mutant to O2 inhibition than that expected from in-vitro kinetics parameters of Rubisco, indicates a low capacity to recycle photorespiratory metabolites to Calvin-cycle intermediates.

High Affinity Transport of CO(2) in the Cyanobacterium Synechococcus UTEX 625

The active transport of CO(2) in Synechococcus UTEX 625 was measured by mass spectrometry under conditions that preclude HCO(3) (-) transport. The substrate concentration required to give one half the maximum rate for whole cell CO(2) transport was determined to be 0.4 +/- 0.2 micromolar (mean +/- standard deviation; n = 7) with a range between 0.2 and 0.66 micromolar. The maximum rates of CO(2) transport ranged between 400 and 735 micromoles per milligram of chlorophyll per hour with an average rate of 522 for seven experiments. This rate of transport was about three times greater than the dissolved inorganic carbon saturated rate of photosynthetic O(2) evolution observed under these conditions. The initial rate of chlorophyll a fluorescence quenching was highly correlated with the initial rate of CO(2) transport (correlation coefficient = 0.98) and could be used as an indirect method to detect CO(2) transport and calculate the substrate concentration required to give one half the maximum rate of transport. Little, if any, inhibition of CO(2) transport was caused by HCO(3) (-) or by Na(+)-dependent HCO(3) (-) transport. However, (12)CO(2) readily interfered with (13)CO(2) transport. CO(2) transport and Na(+)-dependent HCO(3) (-) transport are separate, independent processes and the high affinity CO(2) transporter is not only responsible for the initial transport of CO(2) into the cell but also for scavenging any CO(2) that may leak from the cell during ongoing photosynthesis.

Properties of a Mutant from Synechocystis PCC6803 Resistant to Acetazolamide, an Inhibitor of Carbonic Anhydrase

A spontaneous mutant of the cyanobacterium Synechocystis PCC6803 was isolated for its resistance to acetazolamide, an inhibitor of carbonic anhydrase. The mutant showed a deficiency in oxygen exchange between CO(2) and H(2)O, a lower level of stable internal CO(2) pool and a decreased capacity to adapt its photosynthetic affinity under limited inorganic carbon regime. The initial rate of uptake of inorganic carbon was identical to that of wild-type cells. It is demonstrated that the mutation affects the carbonic anhydrase activity. This could result from either of two impairments: a deficiency in the enzyme activity detectable by mass spectrometric determinations, or a modification of the cellular compartment in which the enzyme is located, preventing its activity.

Glycolaldehyde Inhibits CO(2) Fixation in the Cyanobacterium Synechococcus UTEX 625 without Inhibiting the Accumulation of Inorganic Carbon or the Associated Quenching of Chlorophyll a Fluorescence

When studying active CO(2) and HCO(3) (-) transport by cyanobacteria, it is often useful to be able to inhibit concomitant CO(2) fixation. We have found that glycolaldehyde was an efficient inhibitor of photosynthetic CO(2) fixation in Synechococcus UTEX 625. Glycolaldehyde did not inhibit inorganic carbon accumulation due to either active CO(2) or HCO(3) (-) transport. When glycolaldehyde (10 millimolar) was added to rapidly photosynthesizing cells, CO(2) fixation was stopped within 15 seconds. The quenching of chlorophyll a fluorescence remained high (</= 82% control) when CO(2) fixation was completely blocked by glycolaldehyde. This quenching was relieved upon the addition of a glucose oxidase oxygentrap. This is consistent with our previous finding that q-quenching in the absence of CO(2) fixation was due to O(2) photoreduction. Photosynthetic CO(2) fixation was also inhibited by d,l,-glyceraldehyde but a sixfold higher concentration was required. Glycolaldehyde acted much more rapidly than iodoacetamide (15 seconds versus 300 seconds) and did not cause the onset of net O(2) evolution often observed with iodoacetamide. Glycolaldehyde will be a useful inhibitor when it is required to study CO(2) and HCO(3) (-) transport without the complication of concomitant CO(2) fixation.

Isolation and Characterization of High CO(2)-Requiring-Mutants of the Cyanobacterium Synechococcus PCC7942 : Two Phenotypes that Accumulate Inorganic Carbon but Are Apparently Unable to Generate CO(2) within the Carboxysome

A total of 24 high CO(2)-requiring-mutants of the cyanobacterium Synechococcus PCC7942 have been isolated and partially characterized. These chemically induced mutants are able to grow at 1% CO(2), on agar media, but are incapable of growth at air levels of CO(2). All the mutants were able to accumulate inorganic carbon (C(i)) to levels similar to or higher than wild type cells, but were apparently unable to generate intracellular CO(2). On the basis of the rate of C(i) release following a light (5 minutes) --> dark transition two extreme phenotypes (fast and slow release mutants) and a number of ;intermediate' mutants (normal release) were identified. Compared to wild-type cells, Type I mutants had the following characteristics: fast C(i) release, normal internal C(i) pool, normal carbonic anhydrase (CA) activity in crude extracts, reduced internal exchange of (18)O from (18)O-labeled CO(2), 1% CO(2) requirement for growth in liquid media, normal affinity of carboxylase for CO(2), and long, rod-like carboxysomes. Type II mutants had the following characteristics: slow C(i) release, increased internal C(i) pool, normal CA activity in crude extracts, normal internal (18)O exchange, a 3% CO(2) requirement for growth in liquid media, high carboxylase activity, normal affinity of carboxylase for CO(2), and normal carboxysome structure but increased in numbers per cell. Both mutant phenotypes appear to have genetic lesions that result in an inability to convert intracellular HCO(3) (-) to CO(2) inside the carboxysome. The features of the type I mutants are consistent with a scenario where carboxysomal CA has been mistargeted to the cytosol. The characteristics of the type II phenotype appear to be most consistent with a scenario where CA activity is totally missing from the cell except for the fact that cell extracts have normal CA activity. Alternatively the type II mutants may have a lesion in their capacity for H(+) import during photosynthesis.

Expression of Human Carbonic Anhydrase in the Cyanobacterium Synechococcus PCC7942 Creates a High CO(2)-Requiring Phenotype : Evidence for a Central Role for Carboxysomes in the CO(2) Concentrating Mechanism

Active human carbonic anhydrase II (HCAII) protein was expressed in the cyanobacterium Synechococcus PCC7942 by means of transformation with the bidirectional expression vector, pCA. This expression was driven by the bacterial Tac promoter and was regulated by the IacIQ repressor protein, which was expressed from the same plasmid. Expression levels reached values of around 0.3% of total cell protein and this protein appeared to be entirely soluble in nature and located within the cytosol of the cell. The expression of this protein has dramatic effects on the photosynthetic physiology of the cell. Induction of expression of carbonic anhydrase (CA) activity in both high dissolved inorganic carbon (C(i)) and low C(i) grown cells leads the creation of a high C(i) requiring phenotype causing: (a) a dramatic increase in the K(0.5) (C(i)) for photosynthesis, (b) a loss of the ability to accumulate internal C(i), and (c) a decrease in the lag between the initial C(i) accumulation following illumination and the efflux of CO(2) from the cells. In addition, the effects of the expressed CA can largely be reversed by the carbonic anhydrase inhibitor ethoxyzolamide. As a result of the above findings, it is concluded that the CO(2) concentrating mechanism in Synechococcus PCC7942 is largely dependent on (a) the absence of CA activity from the cytosol, and (b) the specific localization of CA activity in the carboxysome. A theoretical model of photosynthesis and C(i) accumulation is developed in which the carboxysome plays a central role as both the site of CO(2) generation from HCO(3(-) ) and a resistance barrier to CO(2) efflux from the cell. There is good qualitative agreement between this model and the measured physiological effects of expressed cytosolic CA in Synechococcus cells.

Selective and Reversible Inhibition of Active CO(2) Transport by Hydrogen Sulfide in a Cyanobacterium

The active transport of CO(2) in the cyanobacterium Synechococcus UTEX 625 was inhibited by H(2)S. Treatment of the cells with up to 150 micromolar H(2)S + HS(-) at pH 8.0 had little effect on Na(+)-dependent HCO(3) (-) transport or photosynthetic O(2) evolution, but CO(2) transport was inhibited by more than 90%. CO(2) transport was restored when H(2)S was removed by flushing with N(2). At constant total H(2)S + HS(-) concentrations, inhibition of CO(2) transport increased as the ratio of H(2)S to HS(-) increased, suggesting a direct role for H(2)S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H(2)S and Na(+) -dependent HCO(3) (-) transport, the extracellular CO(2) concentration rose considerably above its equilibrium level, but was maintained far below its equilibrium level in the absence of H(2)S. The inhibition of CO(2) transport, therefore, revealed an ongoing leakage from the cells of CO(2) which was derived from the intracellular dehydration of HCO(3) (-) which itself had been recently transported into the cells. Normally, leaked CO(2) is efficiently transported back into the cell by the CO(2) transport system, thus maintaining the extracellular CO(2) concentration near zero. It is suggested that CO(2) transport not only serves as a primary means of inorganic carbon acquisition for photosynthesis but also serves as a means of recovering CO(2) lost from the cell. A schematic model describing the relationship between the CO(2) and HCO(3) (-) transport systems is presented.

Use of Carbon Oxysulfide, a Structural Analog of CO(2), to Study Active CO(2) Transport in the Cyanobacterium Synechococcus UTEX 625

Carbon oxysulfide (carbonyl sulfide, COS) is a close structural analog of CO(2). Although hydrolysis of COS (to CO(2) and H(2)S) does occur at alkaline pH (>9), at pH 8.0 the rate of hydrolysis is slow enough to allow investigation of COS as a possible substrate and inhibitor of the active CO(2) transport system of Synechococcus UTEX 625. A light-dependent uptake of COS was observed that was inhibited by CO(2) and the ATPase inhibitor diethylstilbestrol. The COS taken up by the cells could not be recovered when the lights were turned off or when acid was added. It was concluded that most of the COS taken up was hydrolyzed by intracellular carbonic anhydrase. The production of H(2)S was observed and COS removal from the medium was inhibited by ethoxyzolamide. Bovine erythrocyte carbonic anhydrase catalysed the stoichiometric hydrolysis of COS to H(2)S. The active transport of CO(2) was inhibited by COS in an apparently competitive manner. When Na(+)-dependent HCO(3) (-) transport was allowed in the presence of COS, the extracellular [CO(2)] rose considerably above the equilibrium level. This CO(2) appearing in the medium was derived from the dehydration of transported HCO(3) (-) and was leaked from the cells. In the presence of COS the return to the cells of this leaked CO(2) was inhibited. These results showed that the Na(+)-dependent HCO(3) (-) transport was not inhibited by COS, whereas active CO(2) transport was inhibited. When COS was removed by gassing with N(2), a normal pattern of CO(2) uptake was observed. The silicone fluid centrifugation method showed that COS (100 micromolar) had little effect upon the initial rate of HCO(3) (-) transport or CO(2) fixation. The steady state rate of CO(2) fixation was, however, inhibited about 50% in the presence of COS. This inhibition can be at least partially explained by the significant leakage of CO(2) from the cells that occurred when CO(2) uptake was inhibited by COS. Neither CS(2) nor N(2)O acted like COS. It is concluded that COS is an effective and selective inhibitor of active CO(2) transport.

Correlation between Carbonic Anhydrase Activity and Inorganic Carbon Internal Pool in Strain Synechocystis PCC 6174

Existence of an internal carbonic anhydrase was demonstrated in the cyanobacterium Synechocystis PCC 6714. The enzyme, present at a low specific activity, was inducible by limitation in inorganic carbon and inhibited both in vivo and in vitro by acetazolamide. The internal inorganic carbon pool as determined by mass spectrometry, was similarly modulated by the actual inorganic carbon growth regime; its building up was also sensitive to acetazolamide. A possible role of carbonic anhydrase in inorganic carbon metabolism regulation through the control of the dimension and nature of the inorganic carbon pool is discussed.

The Relationship between Ribulose Bisphosphate Concentration, Dissolved Inorganic Carbon (DIC) Transport and DIC-Limited Photosynthesis in the Cyanobacterium Synechococcus leopoliensis Grown at Different Concentrations of Inorganic Carbon

To examine the factors which limit photosynthesis and their role in photosynthetic adaptation to growth at low dissolved inorganic carbon (DIC), Synechococcus leopoliensis was grown at three concentrations (as signified by brackets) of DIC, high (1000-1800 micromolar), intermediate (200-300 micromolar), and low (10-20 micromolar). In all cell types photosynthesis varied from being ribulose bisphosphate (RuBP)-saturated at low external [DIC] to RuBP-limited at high external [DIC]. The maximum rate of photosynthesis (P(max)) was achieved when the internal concentration of RuBP fell below the active site density of RuBP carboxylase/oxygenase (Rubisco). At rates of photosynthesis below P(max), photosynthetic capacity was limited by the ability of the cell to transport inorganic carbon and to supply CO(2) to Rubisco. Adaptation to low DIC was reflected by a decrease in the [DIC] required to half-saturate photosynthesis. Simultaneous mass-spectrometric measurement of rates of photosynthesis and DIC transport showed that the initial slope of the photosynthesis versus [DIC] curve is identical to the initial slope of the DIC transport versus [DIC] curve. This provided evidence that the enhanced capacity for DIC transport which occurs upon adaptation to low [DIC] was responsible for the increase in the initial slope of the photosynthesis versus [DIC] curve and therefore the decrease in the half saturation constant of photosynthesis with respect to DIC. Levels of RuBP and in vitro Rubisco activity varied only slightly between high and intermediate [DIC] grown cells but fell significantly (65-70%) in low [DIC] grown cells. Maximum rates of photosynthesis followed a similar pattern with P(max) only slightly lower in intermediate [DIC] grown cells than in high [DIC] grown cells, but much lower in low [DIC] grown cells. The changing response of photosynthesis to [DIC] during adaptation to low DIC, may be explained by the interaction between DIC-transport limited and [RuBP]-limited photosynthesis.

Nature of the light-induced h efflux and na uptake in cyanobacteria

We investigated the nature of the light-induced, sodium-dependent acidification of the medium and the uptake of sodium by Synechococcus. The rate of acidification (net H(+) efflux) was strongly and specifically stimulated by sodium. The rates of acidification and sodium uptake were strongly affected by the pH of the medium; the optimal pH for both processes being in the alkaline pH range. Net proton efflux was severely inhibited by inhibitors of adenosine triphosphatase activity, energy transfer, and photosynthetic electron transport, but was not affected by the presence of inorganic carbon (C(i)). Light and C(i) stimulated the uptake of sodium, but the stimulation by C(i) was observed only when C(i) was present at the time sodium was provided. Amiloride, a potent inhibitor of Na(+)/H(+) antiport and Na(+) channels, stimulated the rate of acidification but inhibited the rate of sodium uptake. It is suggested that acidification might stem from the activity of a light dependent proton excreting adenosine triphosphatase, while sodium transport seems to be mediated by both Na(+)/H(+) antiport and Na(+) uniport.

Active CO(2) Transport by the Green Alga Chlamydomonas reinhardtii

Mass spectrometric measurements of dissolved free (13)CO(2) were used to monitor CO(2) uptake by air grown (low CO(2)) cells and protoplasts from the green alga Chlamydomonas reinhardtii. In the presence of 50 micromolar dissolved inorganic carbon and light, protoplasts which had been washed free of external carbonic anhydrase reduced the (13)CO(2) concentration in the medium to close to zero. Similar results were obtained with low CO(2) cells treated with 50 micromolar acetazolamide. Addition of carbonic anhydrase to protoplasts after the period of rapid CO(2) uptake revealed that the removal of CO(2) from the medium in the light was due to selective and active CO(2) transport rather than uptake of total dissolved inorganic carbon. In the light, low CO(2) cells and protoplasts incubated with carbonic anhydrase took up CO(2) at an apparently low rate which reflected the uptake of total dissolved inorganic carbon. No net CO(2) uptake occurred in the dark. Measurement of chlorophyll a fluorescence yield with low CO(2) cells and washed protoplasts showed that variable fluorescence was mainly influenced by energy quenching which was reciprocally related to photosynthetic activity with its highest value at the CO(2) compensation point. During the linear uptake of CO(2), low CO(2) cells and protoplasts incubated with carbonic anhydrase showed similar rates of net O(2) evolution (102 and 108 micromoles per milligram of chlorophyll per hour, respectively). The rate of net O(2) evolution (83 micromoles per milligram of chlorophyll per hour) with washed protoplasts was 20 to 30% lower during the period of rapid CO(2) uptake and decreased to a still lower value of 46 micromoles per milligram of chlorophyll per hour when most of the free CO(2) had been removed from the medium. The addition of carbonic anhydrase at this point resulted in more than a doubling of the rate of O(2) evolution. These results show low CO(2) cells of Chlamydomonas are able to transport both CO(2) and HCO(3) (-) but CO(2) is preferentially removed from the medium. The external carbonic anhydrase is important in the supply to the cells of free CO(2) from the dehydration of HCO(3) (-).

Ethoxyzolamide Inhibition of CO(2)-Dependent Photosynthesis in the Cyanobacterium Synechococcus PCC7942

Cells of the cyanobacterium, Synechococcus PCC7942, grown under high inorganic carbon (C(i)) conditions (1% CO(2); pH 8) were found to be photosynthetically dependent on exogenous CO(2). This was judged by the fact that they had a similar photosynthetic affinity for CO(2) (K(0.5)[CO(2)] of 3.4-5.4 micromolar) over the pH range 7 to 9 and that the low photosynthetic affinity for C(i) measured in dense cell suspensions was improved by the addition of exogenous carbonic anhydrase (CA). The CA inhibitor, ethoxyzolamide (EZ), was shown to reduce photosynthetic affinity for CO(2) in high C(i) cells. The addition of 200 micromolar EZ to high C(i) cells increased K(0.5)(CO(2)) from 4.6 micromolar to more than 155 micromolar at pH 8.0, whereas low C(i) cells (grown at 30 microliters CO(2) per liter of air) were less sensitive to EZ. EZ inhibition in high and low C(i) cells was largely relieved by increasing exogenous C(i) up to 100 millimolar. Lipid soluble CA inhibitors such as EZ and chlorazolamide were shown to be the most effective inhibitors of CO(2) usage, whereas water soluble CA inhibitors such as methazolamide and acetazolamide had little or no effect. EZ was found to cause a small drop in photosystem II activity, but this level of inhibition was not sufficient to explain the large effect that EZ had on CO(2) usage. High C(i) cells of Anabaena variabilis M3 and Synechocystis PCC6803 were also found to be sensitive to 200 micromolar EZ. We discuss the possibility that the inhibitory effect of EZ on CO(2) usage in high C(i) cells of Synechococcus PCC7942 may be due to inhibition of a ;CA-like' function associated with the CO(2) utilizing C(i) pump or due to inhibition of an internal CA activity, thus affecting CO(2) supply to ribulose bisphosphate carboxylase-oxygenase.

Carbonic Anhydrase Activity Associated with the Cyanobacterium Synechococcus PCC7942

Intact cells and crude homogenates of high (1% CO(2)) and low dissolved inorganic carbon (C(i)) (30-50 microliters per liter of CO(2)) grown Synechococcus PCC7942 have carbonic anhydrase (CA)-like activity, which enables them to catalyze the exchange of (18)O from CO(2) to H(2)O. This activity was studied using a mass spectrometer coupled to a cuvette with a membrane inlet system. Intact high and low C(i) cells were found to contain CA activity, separated from the medium by a membrane which is preferentially permeable to CO(2). This activity is most apparent in the light, where (18)O-labeled CO(2) species are being taken up by the cells but the effluxing CO(2) has lost most of its label to water. In the dark, low C(i) cells catalyze the depletion of the (18)O enrichment of CO(2) and this activity is inhibited by both ethoxyzolamide and 2-(trifluoromethoxy)carbonyl cyanide. This may occur via a common inhibition of the C(i) pump and the C(i) pump is proposed as a potential site for the exchange of (18)O. CA activity was measurable in homogenates of both cell types but was 5- to 10-fold higher in low C(i) cells. This was inhibited by ethoxyzolamide with an I(50) of 50 to 100 micromolar in both low and high C(i) cells. A large proportion of the internal CA activity appears to be pelletable in nature. This pelletability is increased by the presence of Mg(2+) in a manner similar to that of ribulose bisphosphate carboxylase-oxygenase activity and chlorophyll (thylakoids) and may be the result of nonspecific aggregation. Separation of crude homogenates on sucrose gradients is consistent with the notion that CA and ribulose bisphosphate carboxylase-oxygenase activity may be associated with the same pelletable fraction. However, we cannot unequivocally establish that CA is located within the carboxysome. The sucrose gradients show the presence of separate soluble and pelletable CA activity. This may be due to the presence of separate forms of the enzyme or may arise from the same pelletable association which is unstable during extraction.

Ethoxyzolamide Inhibition of CO(2) Uptake in the Cyanobacterium Synechococcus PCC7942 without Apparent Inhibition of Internal Carbonic Anhydrase Activity

In high inorganic carbon grown (1% CO(2) [volume/volume]) cells of the cyanobacterium Synechococcus PCC7942, the carbonic anhydrase (CA) inhibitor, ethoxyzolamide (EZ), was found to inhibit the rate of CO(2) uptake and to reduce the final internal inorganic carbon (C(i)) pool size reached. The relationship between CO(2) fixation rate and internal C(i) concentration in high C(i) grown cells was little affected by EZ. This suggests that in intact cells internal CA activity was unaffected by EZ. High C(i) grown cells readily took up CO(2) but had little or no capacity for HCO(3) (-) uptake. These cells appear to possess a CO(2) utilizing C(i) pump that has a CA-like function associated with the transport step such that HCO(3) (-) is the species delivered to the cell interior. This CA-like step may be the site of inhibition by EZ. Low C(i) grown cells possess both CO(2) uptake and HCO(3) (-) uptake activities and EZ inhibited both activities to a similar degree, suggesting that a common step in CO(2) and HCO(3) (-) uptake (such as the C(i) pump) may have been affected. The inhibitor had no apparent effect on internal CO(2)/HCO(3) (-) equilibria (internal CA function) in low C(i) grown cells.

Is there a role for the 42 kilodalton polypeptide in inorganic carbon uptake by cyanobacteria?

Cyanobacterial cells accumulate substantial amounts of a membrane-associated 42 kilodalton polypeptide during adaptation to low CO(2) conditions. The role of this polypeptide in the process of adaptation and in particular in the large increase in the ability to accumulate inorganic carbon (C(i)), which accompanies this process, is not yet understood. We have isolated a mutant Synechococcus PCC7942 that does not accumulate the 42 kilodalton polypeptide. The mutant requires a high-CO(2) concentration for growth and exhibits a very low apparent photosynthetic affinity for extracellular C(i). The latter might be attributable to the observed defective ability of the mutant to utilize the intracellular C(i) pool for photosynthesis. The 42 kilodalton polypeptide does not appear to participate directly in the active transport of C(i), since the difference between the observed capabilities for CO(2) and HCO(3) (-) uptake of the mutant and the wild type is not sufficient to account for their different growth and photosynthetic performance. Furthermore, high CO(2)-grown wild-type cells, where we could not detect the 42 kilodalton polypeptide, transported CO(2) faster than the mutant. An analysis of the curves relating the rate of accumulation of C(i) to the concentration of CO(2) or HCO(3) (-) supplied, in the presence or absence of carbonic anhydrase, indicated that under the experimental conditions used here, CO(2) was the preferred C(i) species taken up by Synechococcus.