Photosynthesis and the intracellular inorganic carbon pool in the bluegreen alga Anabaena variabilis: Response to external CO2 concentration

Sixty years of research on photosynthesis: a personal scientific autobiography

The following scientific autobiography is presented here as a homage to Professor Kazuo Shibata, who is the one who led me to do research in photosynthesis. He had invited me to Riken (The Institute of Physical and Chemical Research), and had launched the Japan-US Collaboration Project on "The Solar Energy Conversion by Means of Photosynthesis" and had invited many international scientists to Riken. My research, under Shibata, started on using a sensitive method for the determination of chlorophyll b, and of SDS-PAGE for the pigment protein complexes of the two photosystems. After Shibata had passed away at the age of 66, I found post-illumination CO2 burst from cyanobacterial cell suspensions. This finding led me to study the CO2-concentrating mechanism (CCM) and the function and structure of NADP(H) dehydrogenase complexes (NDH-I) in cyanobacteria, which were developed after I had moved to Nagoya University, and in several other laboratories in the world after I had retired from Nagoya University.

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

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

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

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

Colonial morphology weakens the response of different inorganic carbon uptake systems to CO2 levels in Microcystis population

Rising atmospheric CO2 concentration negatively impacts aquatic ecosystems and may induce evolutionary changes in the CO2-concentrating mechanism (CCM) of cyanobacteria. As the most notorious freshwater cyanobacteria, Microcystis strains have high phenotypic plasticity to form colonies and blooms in lakes and reservoirs worldwide. However, phenotypic plasticity of Microcystis responses to elevated CO2 is still a major open question. Here, we studied how Microcystis strains with two genotype of inorganic carbon uptake systems, bicA and sbtA, and different colonial morphology response to 200 ppm, 400 ppm, and 800 ppm CO2 levels. The results revealed that sbtA genotypes showed significantly higher specific growth rates, Chl a concentration, and photosynthetic efficiency at 200 ppm CO2, whereas higher specific growth rates, Chl a concentration, and photosynthetic efficiency were found in bicA genotype at 800 ppm CO2. The highest values of specific growth rates, Chl a concentration, Fv/Fm, and maximal net photosynthesis (Pm) were observed in unicellular morphology, followed by small colony and large colonial morphology at all CO2 levels. The values of K0.5 (DIC), K0.5 (CO2), and K0.5 (HCO3-) in the large colonials increased with rising CO2 levels, but these values significantly decreased in the unicellular and small colonials. ANOSIM analysis indicated that colonial morphology reduced significantly inter-group differences between bicA and sbtA genotypes at all CO2 treatments. These results suggest that colonial morphology of Microcystis can weakens the response of different inorganic carbon uptake systems to CO2 levels. Moreover, phenotypic and genotypic plasticity is likely to broaden strongly the fitness of Microcystis from rising atmospheric CO2.

Integrated Approach for Carbon Sequestration and Wastewater Treatment Using Algal–Bacterial Consortia: Opportunities and Challenges

Increasing concentrations of carbon dioxide (CO2), one of the important greenhouse gases, due to combustion of fossil fuels, particularly burning coal, have become the major cause for global warming. As a consequence, many research programs on CO2 management (capture, storage, and sequestration) are being highlighted. Biological sequestration of CO2 by algae is gaining importance, as it makes use of the photosynthetic capability of these aquatic species to efficiently capture CO2 emitted from various industries and converting it into algal biomass as well as a wide range of metabolites such as polysaccharides, amino acids, fatty acids, pigments, and vitamins. In addition, their ability to thrive in rugged conditions such as seawater, contaminated lakes, and even in certain industrial wastewaters containing high organic and inorganic nutrients loads, has attracted the attention of researchers to integrate carbon capture and wastewater treatment. Algae offer a simple solution to tertiary treatments due to their nutrient removal efficiency, particularly inorganic nitrogen and phosphorus uptake. The algal–bacterial energy nexus is an important strategy capable of removing pollutants from wastewater in a synergistic manner. This review article highlights the mechanism involved in biological fixation of CO2 by microalgae, their cultivation systems, factors influencing algal cultivation in wastewater and CO2 uptake, the effect of co-cultivation of algae and bacteria in wastewater treatment systems, and challenges and opportunities.

Quantifying Cyanothece growth under DIC limitation

The photoautotrophic, unicellular N₂-fixer, Cyanothece, is a model organism that has been widely used to study photosynthesis regulation, the structure of photosystems, and the temporal segregation of carbon (C) and nitrogen (N) fixation in light and dark phases of the diel cycle. Here, we present a simple quantitative model and experimental data that together, suggest external dissolved inorganic carbon (DIC) concentration as a major limiting factor for Cyanothece growth, due to its high C-storage requirement. Using experimental data from a parallel laboratory study as a basis, we show that after the onset of the light period, DIC was rapidly consumed by photosynthesis, leading to a sharp drop in the rate of photosynthesis and C accumulation. In N₂-fixing cultures, high rates of photosynthesis in the morning enabled rapid conversion of DIC to intracellular C storage, hastening DIC consumption to levels that limited further uptake. The N₂-fixing condition allows only a small fraction of fixed C for cellular growth since a large fraction was reserved in storage to fuel night-time N₂ fixation. Our model provides a framework for resolving DIC limitation in aquatic ecosystem simulations, where DIC as a growth-limiting factor has rarely been considered, and importantly emphasizes the effect of intracellular C allocation on growth rate that varies depending on the growth environment.

Combinatorial use of environmental stresses and genetic engineering to increase ethanol titres in cyanobacteria

Current industrial bioethanol production by yeast through fermentation generates carbon dioxide. Carbon neutral bioethanol production by cyanobacteria uses biological fixation (photosynthesis) of carbon dioxide or other waste inorganic carbon sources, whilst being sustainable and renewable. The first ethanologenic cyanobacterial process was developed over two decades ago using Synechococcus elongatus PCC 7942, by incorporating the recombinant pdc and adh genes from Zymomonas mobilis. Further engineering has increased bioethanol titres 24-fold, yet current levels are far below what is required for industrial application. At the heart of the problem is that the rate of carbon fixation cannot be drastically accelerated and carbon partitioning towards bioethanol production impacts on cell fitness. Key progress has been achieved by increasing the precursor pyruvate levels intracellularly, upregulating synthetic genes and knocking out pathways competing for pyruvate. Studies have shown that cyanobacteria accumulate high proportions of carbon reserves that are mobilised under specific environmental stresses or through pathway engineering to increase ethanol production. When used in conjunction with specific genetic knockouts, they supply significantly more carbon for ethanol production. This review will discuss the progress in generating ethanologenic cyanobacteria through chassis engineering, and exploring the impact of environmental stresses on increasing carbon flux towards ethanol production.

Engineered Accumulation of Bicarbonate in Plant Chloroplasts: Known Knowns and Known Unknowns

Heterologous synthesis of a biophysical CO2-concentrating mechanism (CCM) in plant chloroplasts offers significant potential to improve the photosynthetic efficiency of C3 plants and could translate into substantial increases in crop yield. In organisms utilizing a biophysical CCM, this mechanism efficiently surrounds a high turnover rate Rubisco with elevated CO2 concentrations to maximize carboxylation rates. A critical feature of both native biophysical CCMs and one engineered into a C3 plant chloroplast is functional bicarbonate (HCO3 -) transporters and vectorial CO2-to-HCO3 - converters. Engineering strategies aim to locate these transporters and conversion systems to the C3 chloroplast, enabling elevation of HCO3 - concentrations within the chloroplast stroma. Several CCM components have been identified in proteobacteria, cyanobacteria, and microalgae as likely candidates for this approach, yet their successful functional expression in C3 plant chloroplasts remains elusive. Here, we discuss the challenges in expressing and regulating functional HCO3 - transporter, and CO2-to-HCO3 - converter candidates in chloroplast membranes as an essential step in engineering a biophysical CCM within plant chloroplasts. We highlight the broad technical and physiological concerns which must be considered in proposed engineering strategies, and present our current status of both knowledge and knowledge-gaps which will affect successful engineering outcomes.

Functional reconstitution of a bacterial CO2 concentrating mechanism in Escherichia coli

Many photosynthetic organisms employ a CO₂ concentrating mechanism (CCM) to increase the rate of CO₂ fixation via the Calvin cycle. CCMs catalyze ≈50% of global photosynthesis, yet it remains unclear which genes and proteins are required to produce this complex adaptation. We describe the construction of a functional CCM in a non-native host, achieved by expressing genes from an autotrophic bacterium in an Escherichia coli strain engineered to depend on rubisco carboxylation for growth. Expression of 20 CCM genes enabled E. coli to grow by fixing CO₂ from ambient air into biomass, with growth in ambient air depending on the components of the CCM. Bacterial CCMs are therefore genetically compact and readily transplanted, rationalizing their presence in diverse bacteria. Reconstitution enabled genetic experiments refining our understanding of the CCM, thereby laying the groundwork for deeper study and engineering of the cell biology supporting CO₂ assimilation in diverse organisms.

Harmful algae at the complex nexus of eutrophication and climate change

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

Carboxysome encapsulation of the CO2-fixing enzyme Rubisco in tobacco chloroplasts

A long-term strategy to enhance global crop photosynthesis and yield involves the introduction of cyanobacterial CO2-concentrating mechanisms (CCMs) into plant chloroplasts. Cyanobacterial CCMs enable relatively rapid CO2 fixation by elevating intracellular inorganic carbon as bicarbonate, then concentrating it as CO2 around the enzyme Rubisco in specialized protein micro-compartments called carboxysomes. To date, chloroplastic expression of carboxysomes has been elusive, requiring coordinated expression of almost a dozen proteins. Here we successfully produce simplified carboxysomes, isometric with those of the source organism Cyanobium, within tobacco chloroplasts. We replace the endogenous Rubisco large subunit gene with cyanobacterial Form-1A Rubisco large and small subunit genes, along with genes for two key α-carboxysome structural proteins. This minimal gene set produces carboxysomes, which encapsulate the introduced Rubisco and enable autotrophic growth at elevated CO2. This result demonstrates the formation of α-carboxysomes from a reduced gene set, informing the step-wise construction of fully functional α-carboxysomes in chloroplasts.

The reduction of Chlorella vulgaris concentrations through UV-C radiation treatments: A nature-based solution (NBS)

Algal blooms have become a pressing issue in inland freshwater systems on local and global scales. A plausible approach to reducing algae without the use of chemical/biological agents is through the use of UV-C radiation from lamps potentially powered by in situ solar panels to eliminate algae. Yet, the quantitative scientific base has not been established. Our objective is to conduct a controlled experiment to quantify the effectiveness of UV-C radiation on the reduction of Chlorella vulgaris, a common algal species in the Great Lakes region. A full factorial design of three intensities of UV-C radiation (0, 15, and 30W) and three sources of C. vulgaris was constructed to test the corresponding hypotheses. Empirical models were constructed to predict the reductions. UV-C radiation effectively reduced the algal concentration with clear differences by radiation level and source of algal water. Algal concentration decreased exponentially over time, with distinct decreasing trends among the radiation intensities and the samples. With 15W UV-C radiation, algal concentration of three samples were reduced to 75.3%, 51.5%, and 70.0% of the initial level within an hour, respectively. We also found a clear density-dependent reduction rate by UV radiation. Using this information, more efficient treatment systems could be constructed and implemented for cleaning algae-contaminated water.

On the cradle of CCM research: discovery, development, and challenges ahead

Herein, 40 years after its discovery, I briefly and critically survey the development of ideas that propelled research on CO2-concentrating mechanisms (CCMs; a term proposed by Dean Price) of phytoplankton, mainly focusing on cyanobacteria. This is not a comprehensive review on CCM research, but a personal view on the past developments and challenges that lie ahead.

Exploring Components of the CO2-Concentrating Mechanism in Alkaliphilic Cyanobacteria Through Genome-Based Analysis

In cyanobacteria, the CO2-concentrating mechanism (CCM) is a vital biological process that provides effective photosynthetic CO2 fixation by elevating the CO2 level near the active site of Rubisco. This process enables the adaptation of cyanobacteria to various habitats, particularly in CO2-limited environments. Although CCM of freshwater and marine cyanobacteria are well studied, there is limited information on the CCM of cyanobacteria living under alkaline environments. Here, we aimed to explore the molecular components of CCM in 12 alkaliphilic cyanobacteria through genome-based analysis. These cyanobacteria included 6 moderate alkaliphiles; Pleurocapsa sp. PCC 7327, Synechococcus spp., Cyanobacterium spp., Spirulina subsalsa PCC 9445, and 6 strong alkaliphiles (i.e. Arthrospira spp.). The results showed that both groups belong to β-cyanobacteria based on β-carboxysome shell proteins with form 1B of Rubisco. They also contained standard genes, ccmKLMNO cluster, which is essential for β-carboxysome formation. Most strains did not have the high-affinity Na+/HCO3- symporter SbtA and the medium-affinity ATP-dependent HCO3- transporter BCT1. Specifically, all strong alkaliphiles appeared to lack BCT1. Beside the transport systems, carboxysomal β-CA, CcaA, was absent in all alkaliphiles, except for three moderate alkaliphiles: Pleurocapsa sp. PCC 7327, Cyanobacteriumstranieri PCC 7202, and Spirulina subsalsa PCC 9445. Furthermore, comparative analysis of the CCM components among freshwater, marine, and alkaliphilic β-cyanobacteria revealed that the basic molecular components of the CCM in the alkaliphilic cyanobacteria seemed to share more degrees of similarity with freshwater than marine cyanobacteria. These findings provide a relationship between the CCM components of cyanobacteria and their habitats.

Evolution of photorespiration from cyanobacteria to land plants, considering protein phylogenies and acquisition of carbon concentrating mechanisms

Photorespiration and oxygenic photosynthesis are intimately linked processes. It has been shown that under the present day atmospheric conditions cyanobacteria and all eukaryotic phototrophs need functional photorespiration to grow autotrophically. The question arises as to when this essential partnership evolved, i.e. can we assume a coevolution of both processes from the beginning or did photorespiration evolve later to compensate for the generation of 2-phosphoglycolate (2PG) due to Rubisco's oxygenase reaction? This question is mainly discussed here using phylogenetic analysis of proteins involved in the 2PG metabolism and the acquisition of different carbon concentrating mechanisms (CCMs). The phylogenies revealed that the enzymes involved in the photorespiration of vascular plants have diverse origins, with some proteins acquired from cyanobacteria as ancestors of the chloroplasts and others from heterotrophic bacteria as ancestors of mitochondria in the plant cell. Only phosphoglycolate phosphatase was found to originate from Archaea. Notably glaucophyte algae, the earliest branching lineage of Archaeplastida, contain more photorespiratory enzymes of cyanobacterial origin than other algal lineages or land plants indicating a larger initial contribution of cyanobacterial-derived proteins to eukaryotic photorespiration. The acquisition of CCMs is discussed as a proxy for assessing the timing of periods when photorespiratory activity may have been enhanced. The existence of CCMs also had marked influence on the structure and function of photorespiration. Here, we discuss evidence for an early and continuous coevolution of photorespiration, CCMs and photosynthesis starting from cyanobacteria via algae, to land plants.

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.

Regulation of CO2 Concentrating Mechanism in Cyanobacteria

In this chapter, we mainly focus on the acclimation of cyanobacteria to the changing ambient CO2 and discuss mechanisms of inorganic carbon (Ci) uptake, photorespiration, and the regulation among the metabolic fluxes involved in photoautotrophic, photomixotrophic and heterotrophic growth. The structural components for several of the transport and uptake mechanisms are described and the progress towards elucidating their regulation is discussed in the context of studies, which have documented metabolomic changes in response to changes in Ci availability. Genes for several of the transport and uptake mechanisms are regulated by transcriptional regulators that are in the LysR-transcriptional regulator family and are known to act in concert with small molecule effectors, which appear to be well-known metabolites. Signals that trigger changes in gene expression and enzyme activity correspond to specific "regulatory metabolites" whose concentrations depend on the ambient Ci availability. Finally, emerging evidence for an additional layer of regulatory complexity involving small non-coding RNAs is discussed.

Redox changes accompanying inorganic carbon limitation in Synechocystis sp. PCC 6803

Inorganic carbon (Ci) is the major sink for photosynthetic reductant in organisms capable of oxygenic photosynthesis. In the absence of abundant Ci, the cyanobacterium Synechocystis sp. strain PCC6803 expresses a high affinity Ci acquisition system, the CO2-concentrating mechanisms (CCM), controlled by the transcriptional regulator CcmR and the metabolites NADP+ and α-ketoglutarate, which act as co-repressors of CcmR by modulating its DNA binding. The CCM thus responds to internal cellular redox changes during the transition from Ci-replete to Ci-limited conditions. However, the actual changes in the metabolic state of the NADPH/NADP+ system that occur during the transition to Ci-limited conditions remain ill-defined. Analysis of changes in the redox state of cells experiencing Ci limitation reveals systematic changes associated with physiological adjustments and a trend towards the quinone and NADP pools becoming highly reduced. A rapid and persistent increase in F0 was observed in cells reaching the Ci-limited state, as was the induction of photoprotective fluorescence quenching. Systematic changes in the fluorescence induction transients were also observed. As with Chl fluorescence, a transient reduction of the NADPH pool ('M' peak), is assigned to State 2→State 1 transition associated with increased electron flow to NADP+. This was followed by a characteristic decline, which was abolished by Ci limitation or inhibition of the Calvin-Benson-Bassham (CBB) cycle and is thus assigned to the activation of the CBB cycle. The results are consistent with the proposed regulation of the CCM and provide new information on the nature of the Chl and NADPH fluorescence induction curves.

Ecophysiology of gelatinous Nostoc colonies: unprecedented slow growth and survival in resource-poor and harsh environments

BACKGROUND

The cyanobacterial genus Nostoc includes several species forming centimetre-large gelatinous colonies in nutrient-poor freshwaters and harsh semi-terrestrial environments with extended drought or freezing. These Nostoc species have filaments with normal photosynthetic cells and N2-fixing heterocysts embedded in an extensive gelatinous matrix of polysaccharides and many other organic substances providing biological and environmental protection. Large colony size imposes constraints on the use of external resources and the gelatinous matrix represents extra costs and reduced growth rates.

SCOPE

The objective of this review is to evaluate the mechanisms behind the low rates of growth and mortality, protection against environmental hazards and the persistence and longevity of gelatinous Nostoc colonies, and their ability to economize with highly limiting resources.

CONCLUSIONS

Simple models predict the decline in uptake of dissolved inorganic carbon (DIC) and a decline in the growth rate of spherical freshwater colonies of N. pruniforme and N. zetterstedtii and sheet-like colonies of N. commune in response to a thicker diffusion boundary layer, lower external DIC concentration and higher organic carbon mass per surface area (CMA) of the colony. Measured growth rates of N. commune and N. pruniforme at high DIC availability comply with general empirical predictions of maximum growth rate (i.e. doubling time 10-14 d) as functions of CMA for marine macroalgae and as functions of tissue thickness for aquatic and terrestrial plants, while extremely low growth rates of N. zetterstedtii (i.e. doubling time 2-3 years) are 10-fold lower than model predictions, either because of very low ambient DIC and/or an extremely costly colony matrix. DIC uptake is limited by diffusion at low concentrations for all species, although they exhibit efficient HCO3(-) uptake, accumulation of respiratory DIC within the colonies and very low CO2 compensation points. Long light paths and light attenuation by structural substances in large Nostoc colonies cause lower quantum efficiency and assimilation number and higher light compensation points than in unicells and other aquatic macrophytes. Extremely low growth and mortality rates of N. zetterstedtii reflect stress-selected adaptation to nutrient- and DIC-poor temperate lakes, while N. pruniforme exhibits a mixed ruderal- and stress-selected strategy with slow growth and year-long survival prevailing in sub-Arctic lakes and faster growth and shorter longevity in temperate lakes. Nostoc commune and its close relative N. flagelliforme have a mixed stress-disturbance strategy not found among higher plants, with stress selection to limiting water and nutrients and disturbance selection in quiescent dry or frozen stages. Despite profound ecological differences between species, active growth of temperate specimens is mostly restricted to the same temperature range (0-35 °C; maximum at 25 °C). Future studies should aim to unravel the processes behind the extreme persistence and low metabolism of Nostoc species under ambient resource supply on sediment and soil surfaces.

Natural xenobiotics to prevent cyanobacterial and algal growth in freshwater: contrasting efficacy of tannic acid, gallic acid, and gramine

Allelochemical action against planktonic phototrophs is one central issue in freshwater ecology and quality management. To determine some basic mechanisms of this toxic action, we exposed the coccal green alga, Desmodesmus armatus, and the coccal cyanobacterium, Microcystis aeruginosa, in a batch culture well-supplied with carbon dioxide to increasing concentrations of the polyphenols tannic acid and gallic acid and the alkaloid gramine. The phototrophs were checked after 2d and at the end of the culture for biomass-based growth rates, cell volume, maximum quantum yield of photosystem II (ΦPSIImax), chlorophyll a content (chla) after 2d and at the end of the culture, and lipid peroxidation only at the end of the culture. During the culture, the pH rose from 7.64 to 10.95, a pH characteristic of eutrophic freshwater bodies during nuisance algal blooms. All xenobiotics reduced the growth rate, ΦPSIImax, and chla during the first 2d with M. aeruginosa being more sensitive to the polyphenols than D. armatus. The efficacy of the polyphenols declined with increasing pH, indicating potential polymerization and corresponding reduced bioavailability of the polyphenols. In contrast to the polyphenols, gramine increased its toxic action over time, independent of the prevailing pH. All exposures caused slight to severe lipid peroxidation (LPO) in the phototrophs. Hence, one mechanism of growth inhibition may be oxidative stress-mediated reduction in photosynthesis. The presented results suggest that in successful field trials with leachate, the prevailing environmental conditions may inactivate polyphenols and xenobiotics other than polyphenols may be more effective.

Photophysiological responses of marine diatoms to elevated CO2 and decreased pH: a review

Diatoms dominate nearly half of current oceanic productivity, so their responses to ocean acidification are of general concern regarding future oceanic carbon sequestration. Community, mesocosm and laboratory studies show a range of diatom growth and photophysiological responses to increasing pCO2. Nearly 20 studies on effects of elevated pCO2 on diatoms have shown stimulations, no effects or inhibitions of growth rates. These differential responses could result from differences in experimental setups, cell densities, levels of light and temperature, but also from taxon-specific physiology. Generally, ocean acidification treatments of lowered pH with elevated CO2 stimulate diatom growth under low to moderate levels of light, but lead to growth inhibition when combined with excess light. Additionally, diatom cell sizes and their co-varying metabolic rates can influence responses to increasing pCO2 and decreasing pH, although cell size effects are confounded with taxonomic specificities in cell structures and metabolism. Here we summarise known diatom growth and photophysiological responses to increasing pCO2 and decreasing pH, and discuss some reasons for the diverse responses observed across studies.

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.

CO2-concentrating mechanism in cyanobacterial photosynthesis: organization, physiological role, and evolutionary origin

The cellular and molecular organization of the CO2-concentrating mechanism (CCM) of cyanobacteria is reviewed. The primary processes of uptake, translocation, and accumulation of inorganic carbon (Ci) near the active site of carbon assimilation by the enzyme ribulose-1,5-bisphosphate carboxylase in the C3 cycle in cyanobacteria are described as one of the specialized forms of CO2 concentration which occurs in some photoautotrophic cells. The existence of this form of CO2 concentration expands our understanding of photosynthetic Ci assimilation. The means of supplying Ci to the C3 cycle in cyanobacteria is not by simple diffusion into the cell, but it is the result of coordinated functions of high-affinity systems for the uptake of CO2 and bicarbonate, as well as intracellular CO2/HCO3 (-) interconversions by carbonic anhydrases. These biochemical events are under genetic control, and they serve to maintain cellular homeostasis and adaptation to CO2 limitation. Here we describe the organization of the CCM in cyanobacteria with a special focus on the CCM of relict halo- and alkaliphilic cyanobacteria of soda lakes. We also assess the role of the CCM at the levels of the organism, the biosphere, and evolution.

CO(2)-cAMP-responsive cis-elements targeted by a transcription factor with CREB/ATF-like basic zipper domain in the marine diatom Phaeodactylum tricornutum

Expression controls of the carbon acquisition system in marine diatoms in response to environmental factors are an essential issue to understand the changes in marine primary productivity. A pyrenoidal β-carbonic anhydrase, PtCA1, is one of the most important candidates to investigate the control mechanisms of the CO(2) acquisition system in the marine diatom Phaeodactylum tricornutum. A detailed functional assay was carried out on the putative core regulatory region of the ptca1 promoter using a β-glucuronidase reporter in P. tricornutum cells under changing CO(2) conditions. A set of loss-of-function assays led to the identification of three CO(2)-responsive elements, TGACGT, ACGTCA, and TGACGC, at a region -86 to -42 relative to the transcription start site. Treatment with a cyclic (c)AMP analog, dibutyryl cAMP, revealed these three elements to be under the control of cAMP; thus, we designated them, from 5' to 3', as CO(2)-cAMP-Responsive Element1 (CCRE1), CCRE2, and CCRE3. Because the sequence TGACGT is known to be a typical target of human Activating Transcription Factor6 (ATF6), we searched for genes containing a basic zipper (bZIP) region homologous to that of ATF6 in the genome of P. tricornutum. Gel-shift assays using CCRE pentamers as labeled probes showed that at least one candidate of bZIP proteins, PtbZIP11, bound specifically to CCREs. A series of gain-of-function assays with CCREs fused to a minimal promoter strongly suggested that the alternative combination of CCRE1/2 or CCRE2/3 at proper distances from the minimal promoter is required as a potential target of PtbZIP11 for an effective CO(2) response of the ptca1 gene.

Carboxysomes: cyanobacterial RubisCO comes in small packages

Cyanobacteria (as well as many chemoautotrophs) actively pump inorganic carbon (in the form of HCO(3)(-)) into the cytosol in order to enhance the overall efficiency of carbon fixation. The success of this approach is dependent upon the presence of carboxysomes-large, polyhedral, cytosolic bodies which sequester ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) and carbonic anhydrase. Carboxysomes seem to function by allowing ready passage of HCO(3)(-) into the body, but hindering the escape of evolved CO(2), promoting the accumulation of CO(2) in the vicinity of RubisCO and, consequently, efficient carbon fixation. This selectivity is mediated by a thin shell of protein, which envelops the carboxysome's enzymatic core and uses narrow pores to control the passage of small molecules. In this review, we summarize recent advances in understanding the organization and functioning of these intriguing, and ecologically very important molecular machines.

Algal and aquatic plant carbon concentrating mechanisms in relation to environmental change

Carbon dioxide concentrating mechanisms (also known as inorganic carbon concentrating mechanisms; both abbreviated as CCMs) presumably evolved under conditions of low CO(2) availability. However, the timing of their origin is unclear since there are no sound estimates from molecular clocks, and even if there were, there are no proxies for the functioning of CCMs. Accordingly, we cannot use previous episodes of high CO(2) (e.g. the Palaeocene-Eocene Thermal Maximum) to indicate how organisms with CCMs responded. Present and predicted environmental change in terms of increased CO(2) and temperature are leading to increased CO(2) and HCO(3)(-) and decreased CO(3)(2-) and pH in surface seawater, as well as decreasing the depth of the upper mixed layer and increasing the degree of isolation of this layer with respect to nutrient flux from deeper waters. The outcome of these forcing factors is to increase the availability of inorganic carbon, photosynthetic active radiation (PAR) and ultraviolet B radiation (UVB) to aquatic photolithotrophs and to decrease the supply of the nutrients (combined) nitrogen and phosphorus and of any non-aeolian iron. The influence of these variations on CCM expression has been examined to varying degrees as acclimation by extant organisms. Increased PAR increases CCM expression in terms of CO(2) affinity, whilst increased UVB has a range of effects in the organisms examined; little relevant information is available on increased temperature. Decreased combined nitrogen supply generally increases CO(2) affinity, decreased iron availability increases CO(2) affinity, and decreased phosphorus supply has varying effects on the organisms examined. There are few data sets showing interactions amongst the observed changes, and even less information on genetic (adaptation) changes in response to the forcing factors. In freshwaters, changes in phytoplankton species composition may alter with environmental change with consequences for frequency of species with or without CCMs. The information available permits less predictive power as to the effect of the forcing factors on CCM expression than for their overall effects on growth. CCMs are currently not part of models as to how global environmental change has altered, and is likely to further alter, algal and aquatic plant primary productivity.

The expression of a carbon concentrating mechanism in Chlamydomonas acidophila under variable phosphorus, iron, and CO2 concentrations

The CO(2) acquisition was analyzed in Chlamydomonas acidophila at pH 2.4 in a range of medium P and Fe concentrations and at high and low CO(2) condition. The inorganic carbon concentrating factor (CCF) was related to cellular P quota (Q(p)), maximum CO(2)-uptake rate by photosynthesis (V(max,O2)), half saturation constant for CO(2) uptake (K(0.5)), and medium Fe concentration. There was no effect of the medium Fe concentration on the CCF. The CCF increased with increasing Q(p) in both high and low CO(2) grown algae, but maximum Q(p) was 6-fold higher in the low CO(2) cells. In high CO(2) conditions, the CCF was low, ranging between 0.8 and 3.5. High CCF values up to 9.1 were only observed in CO(2)-limited cells, but P- and CO(2)-colimited cells had a low CCF. High CCF did not relate with a low K(0.5) as all CO(2)-limited cells had a low K(0.5) (<4 μM CO(2)). High C(i)-pools in cells with high Q(p) suggested the presence of an active CO(2)-uptake mechanism. The CCF also increased with increasing V(max,O2) which reflect an adaptation to the nutrient in highest demand (CO(2)) under balanced growth conditions. It is proposed that the size of the CCF in C. acidophila is more strongly related to porter density for CO(2) uptake (reflected in V(max,O2)) and less- to high-affinity CO(2) uptake (low K(0.5)) at balanced growth. In addition, high CCF can only be realized with high Q(p).

Rubisco mutagenesis provides new insight into limitations on photosynthesis and growth in Synechocystis PCC6803

Orthophosphate (Pi) stimulates the activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) while paradoxically inhibiting its catalysis. Of three Pi-binding sites, the roles of the 5P- and latch sites have been documented, whereas that of the 1P-site remained unclear. Conserved residues at the 1P-site of Rubisco from the cyanobacterium Synechocystis PCC6803 were substituted and the kinetic properties of the enzyme derivatives and effects on cell photosynthesis and growth were examined. While Pi-stimulated Rubisco activation diminished for enzyme mutants T65A/S and G404A, inhibition of catalysis by Pi remained unchanged. Together with previous studies, the results suggest that all three Pi-binding sites are involved in stimulation of Rubisco activation, whereas only the 5P-site is involved in inhibition of catalysis. While all the mutations reduced the catalytic turnover of Rubisco (K(cat)) between 6- and 20-fold, the photosynthesis and growth rates under saturating irradiance and inorganic carbon (Ci) concentrations were only reduced 40-50% (in the T65A/S mutants) or not at all (G404A mutant). Analysis of the mutant cells revealed a 3-fold increase in Rubisco content that partially compensated for the reduced K(cat) so that the carboxylation rate per chlorophyll was one-third of that in the wild type. Correlation between the kinetic properties of Rubisco and the photosynthetic rate (P(max)) under saturating irradiance and Ci concentrations indicate that a >60% reduction in K(cat) can be tolerated before P(max) in Synechocystsis PCC6803 is affected. These results indicate that the limitation of Rubisco activity on the rate of photosynthesis in Synechocystis is low. Determination of Calvin cycle metabolites revealed that unlike in higher plants, cyanobacterial photosynthesis is constrained by phosphoglycerate reduction probably due to limitation of ATP or NADPH.

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.

Formation of macromolecular complexes of carbonic anhydrases in the chloroplast of a marine diatom by the action of the C-terminal helix

A beta-type carbonic anhydrase, PtCA1, of the marine diatom Phaeodactylum tricornutum was previously shown to be present in the chloroplast as clumped particles on the girdle lamellae. A series of deletions was carried out on the PtCA1 gene, ptca1, at regions encoding N- or C-terminal domains of the mature PtCA1. These deletion constructs were fused with the EGFP [enhanced GFP (green fluorescent protein)] gene, egfp, introduced and expressed in the cells of P. tricornutum. All three types of N-terminal deletions, Delta52-63, Delta64-75 and Delta76-87 relative to the initiation methionine, showed little interference with the particle formation of the PtCA1::GFP fusion protein. Similarly, one of the three types of C-terminal deletions, Delta253-262, was silent. However, the remaining two C-terminal deletions, Delta263-272 and Delta273-282 relative to the initiation methionine, were strongly inhibitory to the particle formation of PtCA1. The C-terminal 263-282 region comprises five hydrophobic amino acids, Met(263), Leu(266), Ile(269), Leu(272) and Leu(275), which were predicted to form a hydrophobic cluster on the C-terminal alpha-helix. Each or all five of these hydrophobic residues were replaced with a hydrophilic residue with a side chain of similar size and structure, glutamate. Particle formations of PtCA1 were moderately inhibited by substitutions of Met(263), Leu(266) and Ile(269) but more evidently by substitutions of Leu(272) and Leu(275). Finally, substitutions of all five hydrophobic residues resulted in an efficient inhibition of particle formation and the GFP signal was totally dispersed throughout the stroma area. These results strongly suggest that the amphipathic C-terminal helix of PtCA1 plays an essential role in the formation of the macromolecular protein complex.

Cyanobacteria as potential options for environmental sustainability - promises and challenges

Cyanobacteria represent an ancient group of photosynthetic prokaryotes, whose ubiquity, metabolic flexibility and adaptive abilities have made them a subject of research worldwide. These structurally simple organisms combine in themselves interesting facets of plant and bacterial metabolism, which is amenable to genetic exploitation. Despite their globally recognized significance in the sustenance of fertility in rice based cropping systems, they have not been tapped for their extraordinary repertoire of activities, especially their beneficial role as biological agents in remediation and amelioration of soil and water environment and as sinks for greenhouse gases. The information available on these aspects and future lines of research for more efficient utilization of these microorganisms is presented.

Acclimation of wild-type cells and CO2-insensitive mutants of the green alga Chlorella ellipsoidea to elevated [CO2]

CO(2)-insensitive mutants of the green alga Chlorella ellipsoidea were previously shown to be unable to repress an inorganic carbon-concentrating mechanism (CCM) when grown under 5% CO(2). When air-grown, wild-type (WT) cells were transferred to 5% CO(2), an abrupt drop of P(max) to 43% the original level of air-grown cells was observed within the initial 12 h. Photosynthetic affinities of WT cells to dissolved inorganic carbon (DIC) were maintained at high levels for the initial 4 d of acclimation, and then decreased gradually to lower levels over the next 6 d. In contrast to WT cells, the CO(2)-insensitive mutant, ENU16, exhibited a constant P(max) at maximum levels and a low K(1/2)[DIC] throughout the acclimation period. The rapid P(max) drop within 12 h of acclimation in WT cells was significantly reduced by treatment with 0.5 mm of 6-ethoxybenzothiazole-2-sulphonamide (EZA), a specific membrane-permeable inhibitor of carbonic anhydrase (CA), suggesting the participation of internal CAs in the temporary drop in P(max) in WT cells. WT and ENU16 cells were grown in controlled equilibrium [CO(2)], and the photosynthetic rate of each acclimated cell type was measured under equilibrated growth [DIC] conditions. In WT cells acclimated to 0.14-0.4% [CO(2)], K(1/2)[DIC] values increased as [CO(2)] increased, and the photosynthetic rates at growth DIC conditions were shown to decrease to about 70% the P(max) level in this intermediate [CO(2)] range. Such decreases in the net photosynthetic rates were not observed in ENU16. These results suggest that algal primary production could be depressed significantly under moderately enriched CO(2) conditions as a result of acquiring intermediate affinities for DIC because of their sensitive responses to changes in the ambient [CO(2)].

The thylakoid carbonic anhydrase associated with photosystem II is the component of inorganic carbon accumulating system in cells of halo- and alkaliphilic cyanobacterium Rhabdoderma lineare

The organization of carbonic anhydrase (CA) system in halo- and alkaliphilic cyanobacterium Rhabdoderma lineare was studied by Western blot analysis and immunocytochemical electron microscopy. The presence of putative extracellular alpha-CA of 60 kDa in the glycocalyx, forming a tight sheath around the cell, and of two intracellular beta-CA is reported. We show for the first time that the beta-CA of 60 kDa is expressed constitutively and associated with polypeptides of photosystem II (beta-CA-PS II). Another soluble beta-CA of 25 kDa was induced in low-bicarbonate medium. Induction of synthesis of the latter beta-CA was accompanied by an increase in the intracellular pool of inorganic carbon, which suggests an important role of this enzyme in the functioning of a CO(2)-concentrating mechanism.

CO2 sensing at ocean surface mediated by cAMP in a marine diatom

Marine diatoms are known to be responsible for about a quarter of global primary production and their photosynthesis is sustained by inorganic carbon-concentrating mechanisms and/or C(4) metabolism. Activities of the inorganic carbon-concentrating mechanism are attenuated under enriched [CO(2)]; however, impacts of this factor on primary productivity and the molecular mechanisms of CO(2) responses in marine diatoms are unknown. In this study, transgenic cells were generated of the marine diatom Phaeodactylum tricornutum by the introduction of a beta-glucuronidase reporter gene under the control of an intrinsic CO(2)-responsive promoter, which is the sequence between -80 to +61 relative to the transcription start site of a chloroplastic-carbonic anhydrase gene, ptca1, obtained from P. tricornutum. The activity of the ptca1 promoter was effectively repressed in air-level CO(2) by treating cells with a 1.0 mm cAMP analog, dibutyryl cAMP, or a cAMP phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. Deletion of the intrinsic cAMP-response element from the ptca1 promoter caused a lack of repression of the reporter gene uidA, even under elevated [CO(2)] and a null phenotype to the strong repressive effects of dibutyryl cAMP and 3-isobutyl-1-methylxanthine on the ptca1 promoter. Deletion of the cAMP-response element was also shown to cause derepression of the uidA reporter gene in the dark. These results indicate that the cytosolic cAMP level increases under elevated [CO(2)] and represses the ptca1 promoter. This strongly suggests the participation of cAMP metabolism, presumably at the cytosolic level, in controlling CO(2)-acquisition systems under elevated [CO(2)] at the ocean surface in a marine diatom.

A putative sensor kinase, Hik31, is involved in the response of Synechocystis sp. strain PCC 6803 to the presence of glucose

The reason(s) for glucose sensitivity in certain cyanobacterial strains is poorly understood. Inactivation of genes encoding the putative sensor kinase Hik31 in Synechocystis sp. strain PCC 6803 resulted in a mutant unable to grow in the presence of D-glucose. Sensitivities to D-glucose, its analogue 2-deoxy-D-glucose, and fructose, were alleviated in mutants in which glcP, encoding the glucose transporter, was inactivated. These data indicate that permeation of these substrates is required to inflict cell death. The mutant Deltahik31, and the glucose-sensitive strain of Synechocystis, do not possess glucokinase activity, although a transcript originating from glk, encoding glucokinase, is present. Inactivation of glk led to severe sensitivity to glucose, indicating that the presence of glucose itself, within the cells, inflicted this sensitivity. On the other hand, sensitivity to 2-deoxy-D-glucose was lower in Deltaglk, thus distinguishing between the effect of glucose itself and that of its analogue, which, in the absence of glucokinase activity, may not be phosphorylated. Addition of glucose led to a small rise in glucose-6-phosphate dehydrogenase activity in the wild type, but constitutive activity was observed in the Deltahik31 mutant regardless of the presence of glucose. Microarray analyses showed only small changes in the abundance of global transcripts in Synechocystis following glucose addition, but the transcription levels of several genes, including icfG, but not glk, were strongly affected by inactivation of hik31. The mechanism(s) whereby Hik31 is involved in glucose sensing and response is discussed.