Regulation of ribulose bisphosphate carboxylase gene expression in natural phytoplankton communities .1. Diel rhythms

Marine phytoplankton fix carbon dioxide primarily through the action of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), the first enzyme in the Calvin Cycle. Although the regulation of this enzyme has been studied in algal cultures and higher plants, little is known regarding RubisCO regulation in natural phytoplankton populations. To determine ii natural communities of phytoplankton utilize transcriptional regulation to control RubisCO expression, we investigated the diel relationship between C-14 carbon fixation and RubisCO large subunit (rbcL) transcript levels, rbcL DNA, chlorophyll a, autofluorescent cell counts and bacterial direct counts for natural communities of the southeastern Gulf of Mexico. Studies were performed with natural phytoplankton populations in Lagrangian studies or deck-top incubators as well as with a Prochlorococcus isolate in culture from the North Pacific Ocean. rbcL mRNA and DNA were detected by stringent hybridization using an antisense rbcL RNA gene probe originating from Synechococcus sp. PCC 6391. For natural communities, carbon fixation maxima (similar to 0.1 to 0.7 pg C l(-1) h(-1)) occurred from early morning to mid afternoon, with minimal values at night. Peak levels of rbcL mRNA (3.6 to 22.2 ng l(-1)) almost always coincided with the time of maximum carbon fixation and were positively correlated. When natural phytoplankton populations or the Prochlorococcus culture were maintained in continuous illumination for 24 h, the same diel pattern of RubisCO regulation was observed, implying that diel entrained rhythms in rbcL transcription occurred. The results thus indicate that natural phytoplankton communities appear to regulate carbon fixation at least in part by transcriptional control of RubisCO synthesis and that such patterns appear to be rhythmic in nature.