Adapting from Low to High: An Update to CO2-Concentrating Mechanisms of Cyanobacteria and Microalgae

Abstract

The intracellular accumulation of inorganic carbon (Ci) by microalgae and cyanobacteria under ambient atmospheric CO2 levels was first documented in the 80s of the 20th Century. Hence, a third variety of the CO2-concentrating mechanism (CCM), acting in aquatic photoautotrophs with the C3 photosynthetic pathway, was revealed in addition to the then-known schemes of CCM, functioning in CAM and C4 higher plants. Despite the low affinity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) of microalgae and cyanobacteria for the CO2 substrate and low CO2/O2 specificity, CCM allows them to perform efficient CO2 fixation in the reductive pentose phosphate (RPP) cycle. CCM is based on the coordinated operation of strategically located carbonic anhydrases and CO2/HCO3− uptake systems. This cooperation enables the intracellular accumulation of HCO3−, which is then employed to generate a high concentration of CO2 molecules in the vicinity of Rubisco’s active centers compensating up for the shortcomings of enzyme features. CCM functions as an add-on to the RPP cycle while also acting as an important regulatory link in the interaction of dark and light reactions of photosynthesis. This review summarizes recent advances in the study of CCM molecular and cellular organization in microalgae and cyanobacteria, as well as the fundamental principles of its functioning and regulation.

Inorganic carbon sensing and signalling in cyanobacteria

“…The BCT1 subunits are encoded by the cmp operon (Omata et al 1999). In addition, a few auxiliary complexes, including the specialized NDH‐1 complex Mnh and the Na + ‐H + antiporter NhaS3, whose function is required for Na + gradient formation, and the PcxA proton pump, which maintains the cellular pH, are supporting bicarbonate influx (Kupriyanova et al 2023).…”

Modeling bacterial microcompartment architectures for enhanced cyanobacterial carbon fixation

“…The cyanobacterial CCM has been a focus of multiple studies for the elucidation of structure, function, and its integration into cellular metabolism ( Kupriyanova et al., 2023 ). Carbon assimilation in cyanobacteria begins with the uptake and accumulation of inorganic carbon sources within the cytoplasm ( Figure 1A ).…”

Impact of Carbon Fixation, Distribution and Storage on the Production of Farnesene and Limonene in Synechocystis PCC 6803 and Synechococcus PCC 7002

“…Cyanobacteria fix CO 2 using the Calvin-Benson-Bassham (CBB) pathway in which the key enzyme ribulose bisphosphate carboxylase/oxygenase (RubisCO) catalyzes the addition of one molecule of CO 2 to one molecule of ribulose biphosphate produced by the other key enzyme phosphoribulokinase (PRK). In parallel, they have developed the carboxysome subcellular compartment (assembled from various shell proteins such as CcmK3 and CcmK4) to encapsulate RubisCO in a CO 2 -rich environment that favors its carbon-fixing (carboxylase) activity over its detrimental oxygenase activity (for reviews see [4,5]). A part of the photosynthetically fixed carbon is used by the methylerythritol-4-phosphate (MEP) pathway to produce DMAPP and IPP (Figure 1), which are transformed by a geranylgeranyl pyrophosphate synthase (CrtE) into GPP, FPP and GGPP, which is used by the carotene hydroxylases (CrtR and CruF) to produce chlorophyll and carotenoids [2,3,6].…”

“…For this purpose, an interesting target is the carbon concentrating mechanism (CCM) that cyanobacteria use to grow in the low CO 2 concentration of their natural aquatic biotopes. The CCM system uses the carboxysome subcellular compartment, assembled from various Ccm shell proteins, which encapsulates the RubisCO enzyme in a CO 2 -rich environment favoring its carbon-fixing (carboxylase) activity over its detrimental oxygenase activity (for reviews see [4,5]). As a biocontainment strategy, previous workers have deleted carboxysome genes of S.7002 and S.7942 GMO to impose a high-CO 2 requirement phenotype (HCR) preventing their escape from their cultivation photobioreactors [41,42].…”