Diel patterns of regulation of rbcL transcription in a cyanobacterium and a prymnesiophyte

Diel patterns of rbcl transcription, ribulose-1,5-bisphosphate carboxylase/ oxygenase (RubisCO) activity, and whole cell carbon fixation were compared in the marine cyanobacterium Synechococcus PCC7002 and the chromophytic pry mnesiophyte Pavlova gyrans. Both organisms were grown on a 12:12 light-dark cycle,,vith the light period starting at 0700. Strong diel patterns in the se three aspects of carbon fixation occurred in both organisms, with maxima l levels in the light period and minima in the dark. In Synechococcus, maxi mal rbcl. transcript abundance occurred at noon and was followed by rapid d isappearance. RubisCO enzyme activity and whole cell carbon fixation were e levated at 1600, and they disappeared over the next 8 hours. In contrast, i n Pavlova rbcL transcript abundance was maximal at 1600, and it was maintai ned at 66% of this level into the dark period (2000). Whole cell carbon fix ation and RubisCO activity were elevated into the dark period (at 2000), be ing 77% and 81%, respectively, of the maximum. A similar diel pattern of cy anobacterial-like and chromophyte-like,bd transcription has been observed i n natural phytoplankton populations. These studies suggest that chromophyte s are more adapted to take advantage of carbon fixation late in the day tha n cyanophytes.