Sensing of inorganic carbon limitation in Synechococcus PCC7942 is correlated with the size of the internal inorganic carbon pool and involves oxygen

Freshwater cyanobacteria are subjected to large seasonal fluctuations in the availability of nutrients, including inorganic carbon (Ci). We are interested in the regulation of the CO₂-concentrating mechanism (CCM) in the model freshwater cyanobacterium Synechococcus sp. strain PCC7942 in response to Ci limitation; however, the nature of Ci sensing is poorly understood. We monitored the expression of high-affinity Ci-transporter genes and the corresponding induction of a high-affinity CCM in Ci-limited wild-type cells and a number of CCM mutants. These genotypes were subjected to a variety of physiological and pharmacological treatments to assess whether Ci sensing might involve monitoring of fluctuations in the size of the internal Ci pool or, alternatively, the activity of the photorespiratory pathway. These modes of Ci sensing are congruent with previous results. We found that induction of a high-affinity CCM correlates most closely with a depletion of the internal Ci pool, but that full induction of this mechanism also requires some unresolved oxygen-dependent process.