CO2 Concentrating Mechanism In Cyanobacteria Research Articles

Singular adaptations in the carbon assimilation mechanism of the polyextremophile cyanobacterium Chroococcidiopsis thermalis

Cyanobacteria largely contribute to the biogeochemical carbon cycle fixing ~ 25% of the inorganic carbon on Earth. However, the carbon acquisition and assimilation mechanisms in Cyanobacteria are still underexplored regardless of being of great importance for shedding light on the origins of autotropism on Earth and providing new bioengineering tools for crop yield improvement. Here, we fully characterized these mechanisms from the polyextremophile cyanobacterium Chroococcidiopsis thermalis KOMAREK 1964/111 in comparison with the model cyanobacterial strain, Synechococcus sp. PCC6301. In particular, we analyzed the Rubisco kinetics along with the in vivo photosynthetic CO2 assimilation in response to external dissolved inorganic carbon, the effect of CO2 concentrating mechanism (CCM) inhibitors on net photosynthesis and the anatomical particularities of their carboxysomes when grown under either ambient air (0.04% CO2) or 2.5% CO2-enriched air. Our results show that Rubisco from C. thermalis possess the highest specificity factor and carboxylation efficiency ever reported for Cyanobacteria, which were accompanied by a highly effective CCM, concentrating CO2 around Rubisco more than 140-times the external CO2 levels, when grown under ambient CO2 conditions. Our findings provide new insights into the Rubisco kinetics of Cyanobacteria, suggesting that improved Sc/o values can still be compatible with a fast-catalyzing enzyme. The combination of Rubisco kinetics and CCM effectiveness in C. thermalis relative to other cyanobacterial species might indicate that the co-evolution between Rubisco and CCMs in Cyanobacteria is not as constrained as in other phylogenetic groups.

Installing extra bicarbonate transporters in the cyanobacterium Synechocystis sp. PCC6803 enhances biomass production

As a means to improve carbon uptake in the cyanobacterium Synechocystis sp. strain PCC6803, we engineered strains to contain additional inducible copies of the endogenous bicarbonate transporter BicA, an essential component of the CO2-concentrating mechanism in cyanobacteria. When cultured under atmospheric CO2 pressure, the strain expressing extra BicA transporters (BicA+ strain) grew almost twice as fast and accumulated almost twice as much biomass as the control strain. When enriched with 0.5% or 5% CO2, the BicA+ strain grew slower than the control but still showed a superior biomass production. Introducing a point mutation in the large C-terminal cytosolic domain of the inserted BicA, at a site implicated in allosteric regulation of transport activity, resulted in a strain (BicA+(T485G) strain) that exhibited pronounced cell aggregation and failed to grow at 5% CO2. However, the bicarbonate uptake capacity of the induced BicA+(T485G) was twice higher than for the wild-type strain. Metabolic analyses, including phenotyping by synchrotron-radiation Fourier transform Infrared spectromicroscopy, scanning electron microscopy, and lectin staining, suggest that the excess assimilated carbon in BicA+ and BicA+(T485G) cells was directed into production of saccharide-rich exopolymeric substances. We propose that the increased capacity for CO2 uptake in the BicA+ strain can be capitalized on by re-directing carbon flux from exopolymeric substances to other end products such as fuels or high-value chemicals.

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.

An atmospheric stimulus for cyanobacterial-bioinduced calcification ca. 350 million years ago?

Cyanobacterial calcification is promoted by CO2-concentrating mechanisms (CCMs) developed in response to photosynthetic carbon limitation. Changes in atmospheric composition (CO2 fall, O2 rise) near the Devonian–Mississippian transition (ca. 360 Ma) were sufficiently large to induce CCMs in cyanobacteria. Cyanobacterial sheath calcification significantly increased during the Mississippian, ca. 325–355 Ma. It is proposed that these atmospheric changes triggered cyanobacteria to induce CCMs—previously developed during a large CO2 decline in the Proterozoic—and that this promoted their calcification. CCMs in phytoplankton stimulate primary productivity by increasing photosynthetic efficiency and ameliorating carbon limitation. Phytoplankton community restructuring in favor of groups that possessed effective CCMs but had poor body-fossil records, such as picoplanktic cyanobacteria, could account for Late Devonian acritarch decline and the subsequent apparent scarcity of phytoplankton in the late Paleozoic (the so-called phytoplankton blackout). This is supported by biomarkers indicating an increase in cyanobacteria at the Devonian–Mississippian transition and by carbon isotope values and black shale deposition that, despite acritarch decline, reflect increased primary productivity. The Mississippian episode of cyanobacterial calcification was relatively short lived. Calcification declined ca. 325 Ma, before the end of the Mississippian, as a continued decline in CO2 lowered seawater carbonate saturation. The induction of cyanobacterial CCMs, triggered by Late Devonian change to a relatively low CO2 and high O2 atmosphere, has probably persisted to the present day, but well-developed calcification in marine cyanobacteria has been restricted to intervals of elevated carbonate saturation state.

Analysis of PSI and PSII driven electron transportduring steady-state photosynthesis in the cyanobacteriumSynechococcus PCC7942

Photosynthetic electron transport is required for the transport of CO2 and HCO3- in Synechococcus PCC 7942. This transport supports the CO2 concentrating mechanism in cyanobacteria. The roles that linear and cyclic electron flow play in the energization of these processes is unclear. This study describes a system for measuring the quantum yields of photochemistry in both photosystems and, thus, the extent of electron flow, during steady-state cyanobacterial photosynthesis. These measurements were made in a custom-built cuvette coupled by a membrane inlet to a mass spectrometer enabling the concomitant measurement of photosynthetic gas exchange. In wild-type cells, the relationship between f PSI and f PSII was found to be linear between the CO2 compensation point and to approximately ¼ Vmax. The ratio of f PSI/f PSII was always greater than 1 and increased as net photosynthesis approached zero. This was suggestive of significant cyclic electron transport around PSI which became more pronounced as the rate of Ci supply decreased. Whether CO2 or HCO3- was used to drive photosynthesis the relationship between f PSI and f PSII was the same indicative of similar rates of linear and cyclic electron transport. Since we found no greater engagement of f PSI in the presence of enhanced CO2 supply we could not provide evidence for a model of CO2 transport coupled exclusively to cyclic electron transport. A double mutant (ndhD3-/ndhD4-) of Synechococcus PCC7942 in which CO2 transport is absent showed an alteration in electron transport properties relative to wild-type cells.

ラン藻のＣＯ２濃縮機構

ラン藻のＣＯ２濃縮機構

Selection and analysis of mutants of the CO2-concentrating mechanism in cyanobacteria

The selection and analysis of mutants of the CO2-concentrating mechanism in cyanobacteria have greatly advanced our understanding of the physiological and genetic components of this mechanism. This paper reviews the processes for the creation of mutants and the properties of mutants that have been produced so far. In addition to this, consideration is given to developing new mutant selection protocols, based on our current knowledge of the operation of the CO2-concentrating mechanism. As a result, new screens are suggested for the isolation of transport mutants that are defective in either HCO3− or CO2 transport activity, and putative carboxysomal mutants that have altered carbonic anhydrase activities. The procedures for physiological analysis of mutants are also reviewed, with a conclusion that there is a great need for the further development of in vitro assays, particularly for the inorganic carbon transport process and carboxysome function. Particular consideration is given to the in vitro carboxysome assay, and it is concluded that many of the properties of a carboxysomal Rubisco may be difficult to resolve owing to increases in carboxysomal leakiness during isolation and the higher ratio of [CO2] to [HCO3−] experienced during assays compared with the in vitro situation. Finally, consideration is given to the genetic analysis of cyanobacterial mutants and the progress that will need to be made in the future, discovering what are the functions of the proteins coded for by the genes that are isolated. Key words: cyanobacteria, mutants, photosynthesis, carboxysome, CO2-concentrating mechanism.