INORGANIC CARBON REPLETION CONSTRAINS STEADY-STATE LIGHT ACCLIMATION IN THE CYANOBACTERIUM SYNECHOCOCCUS ELONGATUS.

Cyanobacteria show high metabolic plasticity by re-allocating macromolecular resources in response to variations in both environmental inorganic carbon (Ci) and light. We grew cultures of the picoplanktonic cyanobacterium Synechococcus elongatus Nägeli across a 50-fold range of growth irradiance at either a dissolved [Ci] <0.1 mM, sufficient to induce strongly the carbon-concentrating mechanism (CCM) or a dissolved [Ci] of ∼4 mM, sufficient to strongly induce the CCM to basal constitutive activity. There was no detectable growth cost of acclimation to low Ci across the entire range of irradiance and growth was nearly light saturated at 50 l mol photons·m−2·s−1. Cells acclimated to low Ci significantly re-allocated macromolecular resources to support their CCM, while maintaining near homeostatis of metabolic flux per unit photosynthetic complex. Changing growth irradiance also drove re-organization of the photosynthetic machinery to balance excitation flux and metabolic demands, but flux per complex varied widely across the range of tolerable growth irradiances. Across the range of growth irradiance, low Ci cells had significantly less phycocyanin than high Ci cells, which corresponded to a lower PSII absorbance capacity. Furthermore, low Ci cells maintained more PSI per cell−1 than high Ci cells under high growth irradiance. Low Ci cells could therefore maintain more of their PSII reaction centers open at high growth irradiance than could high Ci cells, which experienced a significant PSII closure. Thus, acclimation to growth under high available Ci actually constrained acclimation to high light by restricting electron transport downstream from PSII in S. elongatus. [ABSTRACT FROM AUTHOR]