Evidence for a clade-specific temporal and spatial separation in ribulose bisphosphate carboxylase gene expression in phytoplankton populations off Cape Hatteras and Bermuda

The factors affecting the regulation of photosynthetic carbon fixation in d iverse phytoplankton populations are not yet understood. To this end, we ha ve measured the expression of the gene (rbrL) for the major carbon fixation enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase, in coastal phytop lankton populations off Cape Hatteras and in oligotrophic oceanic picoplank ton near Bermuda. Using gene probes specific for the cyanobacteria/chloroph ytic clade and the chromophytic clade (diatoms, chrysophytes, prymnesiophyt es, and others) of Form 1 rbcL genes ("cyano" and "chromo" probes, respecti vely), we have measured rbcL messenger ribonucleic acid (mRNA) levels in si ze-fractionated coastal waters, in a decktop diel incubator and a Lagrangia n drifter study, and in vertical profiles in stratified, oligotrophic ocean water. In coastal waters influenced by estuarine plumes, an equal distribu tion of carbon fixation between the picoplankton and the micro/nannoplankto n occurred, with cyano rbcL mRNA coinciding with Synechococcus counts in th e <1-mu m fraction, with the majority of the chromo rbcL mRNA expression oc curring in the larger sized phytoplankton fraction. In profiles of oligotro phic oceanic waters, the cyano rbcL mRNA was found in the upper water colum n (similar to 50-m depth) and coincided with peaks in Synechococcus counts. The chrome rbcL mRNA was concentrated at the subsurface chlorophyll a (Chl a) maximum (similar to 85 m) and corresponded to red-fluorescing cell coun ts, thought to be picoeucaryotes and diatoms. Photosynthetic carbon fixatio n and RUBISCO enzyme activity encompassed both cyano and chrome rbcL mRNA p eaks, suggesting a near equal contribution to carbon fixation in the water column by these two phytoplankton clades. Both decktop diel incubator studi es and a Lagrangian drifter study in coastal waters indicated cyano rbcL tr anscription in the morning and chromophytic rbcL transcription in the late afternoon/early evening. Thus, the two major clades of RUBISCO-containing p hytoplankton occupy separate niches in time, space, and cell size in the wa ters off Cape Hatteras, The factors determining such clade-specific niches may include efficiency of nutrient utilization, differences in relative car boxylase/oxygenase activity (tau values) of cyano (Form IB) and chromophyti c (Form LD) RUBISCOs, and differences in pigment composition/adaptation to light regimes. Additionally, we propose that chrome rbcL. mRNA may be indic ative of new production, whereas cyano rbcL mRNA correlates with recycled p roduction in stratified, oligotrophic oceanic environments.