CO2 concentrating mechanisms in cyanobacteria: molecular components,their diversity and evolution

Cyanobacteria have evolved an extremely effective single-cell CO(2) concentrating mechanism (CCM). Recent molecular, biochemical and physiological studies have significantly extended current knowledge about the genes and protein components of this system and how they operate to elevate CO(2) around Rubisco during photosynthesis. The CCM components include at least four modes of active inorganic carbon uptake, including two bicarbonate transporters and two CO(2) uptake systems associated with the operation of specialized NDH-1 complexes. All these uptake systems serve to accumulate HCO(3)(-) in the cytosol of the cell, which is subsequently used by the Rubisco-containing carboxysome protein micro-compartment within the cell to elevate CO(2) around Rubisco. A specialized carbonic anhydrase is also generally present in this compartment. The recent availability of at least nine cyanobacterial genomes has made it possible to begin to undertake comparative genomics of the CCM in cyanobacteria. Analyses have revealed a number of surprising findings. Firstly, cyanobacteria have evolved two types of carboxysomes, correlated with the form of Rubisco present (Form 1A and 1B). Secondly, the two HCO(3)(-) and CO(2) transport systems are distributed variably, with some cyanobacteria (Prochlorococcus marinus species) appearing to lack CO(2) uptake systems entirely. Finally, there are multiple carbonic anhydrases in many cyanobacteria, but, surprisingly, several cyanobacterial genomes appear to lack any identifiable CA genes. A pathway for the evolution of CCM components is suggested.     

The roles of carbonic anhydrases in photosynthetic CO2 concentrating mechanisms

Cyanobacteria, algae, aquatic angiosperms and higher plants have all developed their own unique versions of photosynthetic CO₂ concentrating mechanisms (CCMs) to aid Rubisco in efficient CO₂ capture. An important aspect of all CCMs is the critical roles that the specialised location and function that various carbonic anhydrase enzymes play in the overall process, participating the interconversion of CO₂ and HCO₃ ⁻ species both inside and outside the cell. This review examines what we currently understand about the nature of the carbonic anhydrase enzymes, their localisation and roles in the various CCMs that have been studied in detail. This revised version was published online in August 2006 with corrections to the Cover Date.     

Carbon dioxide concentrating mechanism in Chlamydomonas reinhardtii: inorganic carbon transport and CO2 recapture

Many microalgae are capable of acclimating to CO(2) limited environments by operating a CO(2) concentrating mechanism (CCM), which is driven by various energy-coupled inorganic carbon (Ci; CO(2) and HCO(3)(-)) uptake systems. Chlamydomonas reinhardtii (hereafter, Chlamydomonas), a versatile genetic model organism, has been used for several decades to exemplify the active Ci transport in eukaryotic algae, but only recently have many molecular details behind these Ci uptake systems emerged. Recent advances in genetic and molecular approaches, combined with the genome sequencing of Chlamydomonas and several other eukaryotic algae have unraveled some unique characteristics associated with the Ci uptake mechanism and the Ci-recapture system in eukaryotic microalgae. Several good candidate genes for Ci transporters in Chlamydomonas have been identified, and a few specific gene products have been linked with the Ci uptake systems associated with the different acclimation states. This review will focus on the latest studies on characterization of functional components involved in the Ci uptake and the Ci-recapture in Chlamydomonas.     

Biochemical and biophysical CO2 concentrating mechanisms in two species of freshwater macrophyte within the genus Ottelia (Hydrocharitaceae)

Two freshwater macrophytes, Ottelia alismoides and O. acuminata, were grown at low (mean 5 μmol L^?1) and high (mean 400 μmol L^?1) CO₂ concentrations under natural conditions. The ratio of PEPC to RuBisCO activity was 1.8 in O. acuminata in both treatments. In O. alismoides, this ratio was 2.8 and 5.9 when grown at high and low CO₂, respectively, as a result of a twofold increase in PEPC activity. The activity of PPDK was similar to, and changed with, PEPC (1.9-fold change). The activity of the decarboxylating NADP-malic enzyme (ME) was very low in both species, while NAD-ME activity was high and increased with PEPC activity in O. alismoides. These results suggest that O. alismoides might perform a type of C₄ metabolism with NAD-ME decarboxylation, despite lacking Kranz anatomy. The C₄-activity was still present at high CO₂ suggesting that it could be constitutive. O. alismoides at low CO₂ showed diel acidity variation of up to 34 μequiv g^?1 FW indicating that it may also operate a form of crassulacean acid metabolism (CAM). pH-drift experiments showed that both species were able to use bicarbonate. In O. acuminata, the kinetics of carbon uptake were altered by CO₂ growth conditions, unlike in O. alismoides. Thus, the two species appear to regulate their carbon concentrating mechanisms differently in response to changing CO₂. O. alismoides is potentially using three different concentrating mechanisms. The Hydrocharitaceae have many species with evidence for C₄, CAM or some other metabolism involving organic acids, and are worthy of further study.     

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.     

Algal evolution in relation to atmospheric CO2: carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles

Oxygenic photosynthesis evolved at least 2.4 Ga; all oxygenic organisms use the ribulose bisphosphate carboxylase-oxygenase (Rubisco)-photosynthetic carbon reduction cycle (PCRC) rather than one of the five other known pathways of autotrophic CO(2) assimilation. The high CO(2) and (initially) O(2)-free conditions permitted the use of a Rubisco with a high maximum specific reaction rate. As CO(2) decreased and O(2) increased, Rubisco oxygenase activity increased and 2-phosphoglycolate was produced, with the evolution of pathways recycling this inhibitory product to sugar phosphates. Changed atmospheric composition also selected for Rubiscos with higher CO(2) affinity and CO(2)/O(2) selectivity correlated with decreased CO(2)-saturated catalytic capacity and/or for CO(2)-concentrating mechanisms (CCMs). These changes increase the energy, nitrogen, phosphorus, iron, zinc and manganese cost of producing and operating Rubisco-PCRC, while biosphere oxygenation decreased the availability of nitrogen, phosphorus and iron. The majority of algae today have CCMs; the timing of their origins is unclear. If CCMs evolved in a low-CO(2) episode followed by one or more lengthy high-CO(2) episodes, CCM retention could involve a combination of environmental factors known to favour CCM retention in extant organisms that also occur in a warmer high-CO(2) ocean. More investigations, including studies of genetic adaptation, are needed.     

Inorganic carbon concentrating mechanisms in relation to the biology of algae

Significant advances have recently been made in our understanding of the mechanism of inorganic carbon transport in algae and, especially, cyanobacteria with inorganic carbon concentrating mechanisms (CCMs). Furthermore, the role of CCMs in increasing the rate of photosynthesis in air-equilibrated solutions is also quite well understood. However, less often considered is how the presence (or absence) of a CCM relates to the biology of algae. This mini-review relates the occurrence of algal CCMs to phylogeny, life form, life history, and interactions with other organisms. While some patterns can be seen, the occurrence of CCMs in relation to the overall biology of the algae needs more investigation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of CO(2), O(2), and Light on Photosynthesis and Photorespiration in Wheat

Unidirectional O₂ fluxes were measured with ¹⁸O₂ in a whole plant of wheat cultivated in a controlled environment. At 2 or 21% O₂, O₂ uptake was maximum at 60 microliters per liter CO₂. At lower CO₂ concentrations, it was strongly inhibited, as was photosynthetic O₂ evolution. At 2% O₂, there remained a substantial O₂ uptake, even at high CO₂ level; the O₂ evolution was inhibited at CO₂ concentrations under 330 microliters per liter. The O₂ uptake increased linearly with light intensity, starting from the level of dark respiration. No saturation was observed at high light intensities. No significant change in the gas-exchange patterns occurred during a long period of the plant life. An adaptation to low light intensities was observed after 3 hours illumination. These results are interpreted in relation to the functioning of the photosynthetic apparatus and point to a regulation by the electron acceptors and a specific action of CO₂. The behavior of the O₂ uptake and the study of the CO₂ compensation point seem to indicate the persistence of mitochondrial respiration during photosynthesis.     

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).     

Cyanobacterial calcification, carbon dioxide concentrating mechanisms, and Proterozoic–Cambrian changes in atmospheric composition

Photosynthetic uptake of inorganic carbon can raise the pH adjacent to cyanobacterial cells, promoting CaCO₃ precipitation. This effect is enhanced by CO₂ concentrating mechanisms that actively transport into cells for carbon fixation. CO₂ concentrating mechanisms presumably developed in response to atmospheric decrease in CO₂ and increase in O₂ over geological timescales. In present‐day cyanobacteria, CO₂ concentrating mechanisms are induced when the atmospheric partial pressure of CO₂ (p_CO2) falls below ～0.4%. Reduction in p_CO2 during the Proterozoic may have had two successive effects on cyanobacterial calcification. First, fall in p_CO2 below ～1% (33 times present atmospheric level, PAL) resulted in lower dissolved inorganic carbon (DIC) concentrations that reduced pH buffering sufficiently for isolated CaCO₃ crystals to begin to nucleate adjacent to cyanobacterial cells. As a result, blooms of planktic cyanobacteria induced precipitated ‘whitings’ of carbonate mud in the water column whose sedimentary accumulation began to dominate carbonate platforms ～1400–1300 Ma. Second, fall in p_CO2 below ～0.4% (10 PAL) induced CO₂‐concentrating mechanisms that further increased pH rise adjacent to cells and promoted in vivo cyanobacterial sheath calcification. Crossing of this second threshold is indicated in the fossil record by the appearance of Girvanella 750–700 Ma. Coeval acquisition of CO₂ concentrating mechanisms by planktic cyanobacteria further stimulated whiting production. These inferences, that p_CO2 fell below ～1%～1400–1300 Ma and below ～0.4% 750–700 Ma, are consistent with empirical and modelled palaeo‐atmosphere estimates. Development of CO₂ concentrating mechanisms was probably temporarily slowed by global cooling ～700–570 Ma that favoured diffusive entry of CO₂ into cells. Lower levels of temperature and DIC at this time would have reduced seawater carbonate saturation state, also hindering cyanobacterial calcification. It is suggested that as Earth emerged from ‘Snowball’ glaciations in the late Neoproterozoic, global warming and O₂ rise reactivated the development of CO₂ concentrating mechanisms. At the same time, rising levels of temperature, calcium ions and DIC increased seawater carbonate saturation state, stimulating widespread cyanobacterial in vivo sheath calcification in the Early Cambrian. This biocalcification event promoted rapid widespread development of calcified cyanobacterial reefs and transformed benthic microbial carbonate fabrics.     

Estimation of Photorespiration Based on the Initial Rate of Postillumination CO(2) Release: II. Effects of O(2), CO(2), and Temperature

An open system associated with an infrared gas analyzer was employed to study transients in CO₂ exchange generated upon darkening preilluminated leaf discs of tobacco (Nicotiana tabacum vars John Williams Broadleaf and Havana Seed). An empirical formula presented previously enabled prediction of the analyzer response under nonsteady state conditions as a function of time and of the leaf CO₂ exchange rate. A computer was used to evaluate parameters of the leaf CO₂ release rate to provide an estimate of the initial rate of postillumination CO₂ evolution and to produce maximal agreement between predicted and observed analyzer responses. In 21% O₂, the decline in rate of CO₂ evolution upon darkening followed first order kinetics. Initial rates of CO₂ evolution following darkening were relatively independent of the prior ambient CO₂ concentrations. However, rates of photorespiration expressed as a fraction of net photosynthesis declined rapidly with increasing external CO₂ concentration at 21% O₂. Under normal atmospheric conditions, photorespiration was 45 to 50% of the net CO₂ fixation rate at 32°C and high irradiance. The rapid initial CO₂ evolution observed upon darkening at 21% O₂ was absent in 3% O₂. Rates of photorespiration under normal atmospheric concentrations of CO₂ and O₂ as measured by the postillumination burst were highly dependent upon temperature (observed activation energy = 30.1 kilocalories per mole). The results are discussed with respect to previously published estimates of photorespiration in C₃ leaf tissue.     

Estimation of Photorespiration of Green Plants and of their Mesophyll Resistance to CO2 Uptake

An estimate of photorespiration is obtained from the relationshipbetween the net exchange of CO₂ of the leaf and the internalCO₂ concentration, i.e. within the mesophyll intercellular spaces.The latter is obtained by calculation, taking into account thecombined epidermal and boundary-layer resistances between thebulk atmosphere and the mesophyll intercellular spaces. Thelinear part of this relationship (at low CO₂ concentrations)is extrapolated to zero internal concentration, at which noneof the intercellular photorespired CO₂ is available for reassimilation.The calculated output of CO₂ under such conditions providesan estimate of photorespiration, but, by failing to take intoaccount intracellular reassimilation of photorespired CO₂ underestimatesactual photorespiration. As the slope of this linear relationshiprepresents the mesophyll (intracellular) resistance to CO₂ uptake,this procedure was used to recalculate published data on effectsof light intensity and of oxygen concentration on net photosynthesis.The analysis showed that increased oxygen concentration anddecreased light intensity reduced photosynthesis largely byincreasing mesophyll resistance to CO₂ uptake. It is suggestedthat the CO₂ compensation point () is a function of both photorespiration(L) and mesophyll resistance (r_m): = L. r_m.     

C4 Photosynthesis (The CO2-Concentrating Mechanism and Photorespiration)

Despite previous reports of no apparent photorespiration in C4 plants based on measurements of gas exchange under 2 versus 21% O2 at varying [CO2], photosynthesis in maize (Zea mays) shows a dual response to varying [O2]. The maximum rate of photosynthesis in maize is dependent on O2 (approximately 10%). This O2 dependence is not related to stomatal conductance, because measurements were made at constant intercellular CO2 concentration (Ci); it may be linked to respiration or pseudocyclic electron flow. At a given Ci, increasing [O2] above 10% inhibits both the rate of photosynthesis, measured under high light, and the maximum quantum yield, measured under limiting light ([phi]CO2). The dual effect of O2 is masked if measurements are made under only 2 versus 21% O2. The inhibition of both photosynthesis and [phi]CO2 by O2 (measured above 10% O2) with decreasing Ci increases in a very similar manner, characteristically of O2 inhibition due to photorespiration. There is a sharp increase in O2 inhibition when the Ci decreases below 50 [mu]bar of CO2. Also, increasing temperature, which favors photorespiration, causes a decrease in [phi]CO2 under limiting CO2 and 40% O2. By comparing the degree of inhibition of photosynthesis in maize with that in the C3 species wheat (Triticum aestivum) at varying Ci, the effectiveness of C4 photosynthesis in concentrating CO2 in the leaf was evaluated. Under high light, 30[deg]C, and atmospheric levels of CO2 (340 [mu]bar), where there is little inhibition of photosynthesis in maize by O2, the estimated level of CO2 around ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the bundle sheath compartment was 900 [mu]bar, which is about 3 times higher than the value around Rubisco in mesophyll cells of wheat. A high [CO2] is maintained in the bundle sheath compartment in maize until Ci decreases below approximately 100 [mu]bar. The results from these gas exchange measurements indicate that photorespiration occurs in maize but that the rate is low unless the intercellular [CO2] is severely limited by stress.     

Photorespiration in Air and High CO(2)-Grown Chlorella pyrenoidosa

Oxygen inhibition of photosynthesis and CO₂ evolution during photorespiration were compared in high CO₂-grown and air-grown Chlorella pyrenoidosa, using the artificial leaf technique at pH 5.0. High CO₂ cells, in contrast to air-grown cells, exhibited a marked inhibition of photosynthesis by O₂, which appeared to be competitive and similar in magnitude to that in higher C₃ plants. With increasing time after transfer to air, the photosynthetic rate in high CO₂ cells increased while the O₂ effect declined. Photorespiration, measured as the difference between ¹⁴CO₂ and ¹²CO₂ uptake, was much greater and sensitive to O₂ in high CO₂ cells. Some CO₂ evolution was also present in air-grown algae; however, it did not appear to be sensitive to O₂. True photosynthesis was not affected by O₂ in either case. The data indicate that the difference between high CO₂ and air-grown algae could be attributed to the magnitude of CO₂ evolution. This conclusion is discussed with reference to the oxygenase reaction and the control of photorespiration in algae.     

Human breathing patterns on mouthpiece or face mask during air, CO2, or low O2

Steady-state breathing patterns on mouthpiece and noseclip (MP) and face mask (MASK) during air and chemostimulated breathing were obtained from pneumotachometer flow. On air, all 10 subjects decreased frequency (f) and increased tidal volume (VT) on MP relative to that on MASK without changing ventilation (VE), mean inspiratory flow (VT/TI), or mean expiratory flow (VT/TE). On elevated CO2 and low O2, MP exaggerated the increase in VE, f, and VT/TE due to profoundly shortened TE. On elevated CO2, MASK exaggerated VT increase with little change in f. Increased VE and VT/TI were thus due to increased VT. During low O2 on MASK, both VT and f increased. During isocapnia, shortened TE accounted for increased f; during hypocapnia, increased f was related primarily to shortened TI. Thus the choice of a mouthpiece or face mask differentially alters breathing pattern on air and all components of ventilatory responses to chemostimuli. In addition, breathing apparatus effects are not a simple consequence of a shift from oronasal to oral breathing, since a noseclip under the mask did not change breathing pattern from that on mask alone.