Diel rhythms in ribulose-1,5-bisphosphate carboxylase/oxygenase and glutamine synthetase gene expression in a natural population of marine picoplanktonic cyanobacteria (Synechococcus spp.)

Diel periodicity in the expression of key genes involved in carbon and nitr ogen assimilation in marine Synechococcus spp. was investigated in a natura l population growing in the surface waters of a cyclonic eddy in the northe ast Atlantic Ocean. Synechococcus sp. cell concentrations within the upper mixed layer showed a net increase of three- to fourfold during the course o f the experiment (13 to 22 July 1991), the population undergoing approximat ely one synchronous division per day. Consistent with the observed temporal pattern of phycoerythrin (CpeBA) biosynthesis, comparatively little variat ion was found in cpeBA mRNA abundance during either of the diel cycles inve stigated. In marked contrast, the relative abundance of transcripts origina ting from the genes encoding the large subunit of ribulose bisphosphate car boxylase/oxygenase (rbcL) and glutamine synthetase (glnA) showed considerab le systematic temporal variation and oscillated during the course of each d iel cycle in a reciprocal rhythm. Whereas activation of rbcL transcription was clearly not light dependent, expression of glnA appeared sensitive to e ndogenous changes in the physiological demands for nitrogen that arise as a natural consequence of temporal periodicity in photosynthetic carbon assim ilation. The data presented support the hypothesis that a degree of tempora l separation may exist between the most active periods of carbon and nitrog en assimilation in natural populations of marine Synecoccoccus spp.