Abstract

Cyanobacteria (blue–green algae) have evolved a remarkable environmental adaptation for survival at limiting CO₂ concentrations. The adaptation is known as a CO₂ concentrating mechanism, and functions to actively transport and accumulate inorganic carbon (Ci; HCO₃^– and CO₂) within the cell. Thereafter, this Ci pool is utilised to provide elevated CO₂ concentrations around the primary CO₂ fixing enzyme, Rubisco, which is encapsulated in a unique micro-compartment known as the carboxysome. Recently, significant progress has been gained in understanding the different types of Ci transport in cyanobacteria. This semi-review centres on the model cyanobacterium, Synechococcus sp. PCC7942, which possesses at least four distinct modes of Ci uptake when grown under Ci limitation, each possessing a high degree of functional redundancy. The four modes so far identified are: (i) BCT1, an inducible, high affinity HCO₃^– transporter of the bacterial ATP binding cassette transporter family, encoded by cmpABCD; (ii) a constitutive, Na⁺-dependent HCO₃^– transport system that can be allosterically activated (possibly by phosphorylation) in as little as 10 min; (iii) and (iv) two CO₂ uptake systems, one constitutive and the other inducible, based on specialised forms of thylakoid-based, type 1, NAD(P)H dehydrogenase complexes (NDH-1). Here, we forward a speculative model that proposes that two unique proteins, ChpX and ChpY, possess CO₂ hydration activity in the light, and when coupled to photosynthetic electron transport through the two specialised NDH-1 complexes, result in net hydration of CO₂ to HCO₃^– as a crucial component of the CO₂ uptake process.