Geochemical rate-RNA integrated study:: Ribulose-1,5-bisphosphate carboxylase/oxygenase gene transcription and photosynthetic capacity of planktonic photoautotrophs

A pilot field experiment to assess the relationship between traditional biogeochemical rate measurements and transcriptional activity of microbial populations was carried out at the LEO 15 site off Tuckerton, N.J. Here, we report the relationship between photosynthetic capacity of autotrophic plankton and transcriptional activity of the large subunit gene (rbcL) for ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), the enzyme responsible for primary carbon fixation during photosynthesis. Similar diel patterns of carbon fixation and rbcL gene expression were observed in three of four time series, with maxima for photosynthetic capacity (P-max) and rbcL mRNA occurring between 10 a.m. and 1 p.m.. The lowest P-max and rbcL levels were detected between 6 p.m. and 10:30 p.m.. A significant correlation was found between P-max and form ID rbcL mRNA (R-2 = 0.56) and forms IA and IB (R-2 = 0.41 and 0.47, respectively). The correlation between the abundance of "diatom" rbcL and Pmax mRNA was modest (R-2 = 0.49; n = 12) but improved dramatically (R-2 = 0.97; n = 10) upon removal of two outliers which represented afternoon samples with high P-max but lower mRNA levels. Clone libraries from reverse transcription-PCR-amplified rbcL mRNA indicated the presence of several chromophytic algae (diatoms, prymnesiophytes, and chrysophytes) and some eukaryotic green flagellates. Analogous results were obtained from amplified small rRNA sequences and secondary pigment analysis. These results suggest that diatoms were a major contributor to carbon fixation at LEO 15 at the time of sampling and that photosynthetic carbon fixation was partially controlled by transcriptional regulation of the RubisCO gene.