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phytoplankton, Tampa Bay


Oceanic phytoplankton are known to fix CO2 primarily through the action of the enzyme ribulose-1,5-bisphosphate carboxylase (RuBPCase). The amino acid and nucleotide sequence of the large subunit of this enzyme have been conserved across the evolution of the chlorophytic plants (from cyanobacteria to higher plants via green algae) with approximately 80 and 70 % homology at the amino acid and nucleotide levels respectively. To understand the molecular regulation of this enzyme in phytoplankton, we have measured levels of the RuBPCase large subunit (rbcL) mRNA and DNA, in combination with rates of photosynthetic CO2 fixation, autofluorescent cell counts, and chlorophyll a in natural phytoplankton communities of Tampa Bay (Florida, USA) and the southeastern Gulf of Mexico. We measured rbcL mRNA, rbcL DNA, and the ratio of rbcL mRNA to rbcL DNA by extracting RNA and DNA and probing the extracts with the Synechococcus PCC 6301 rbcL gene as a probe. Additionally, rbcL mRNA was amplified from certain samples using a reverse transcriptase-linked polymerase chain reaction procedure. In a transect from Tampa Bay seaward, levels of rbcL mRNA decreased 3- to 8-fold from the estuarine environment to the offshore environment, and followed similar trends as photosynthetic CO2 fixation, picocyanobacterial counts, and chlorophyll a. In an offshore vertical profile, the subsurface maximum in the rbcL mRNA/DNA ratio coincided with the 60 m maximum in photosynthetic assimilation rates. In a diel study using 150 1 of offshore water in a deck-top incubator, the rbcL mRNA/DNA ratio was nearly an order of magnitude greater (76 ng mRNA ng-1 DNA) during the light period than in the dark (17 ng mRNA ng-1 DNA). Likewise, the photosynthetic rate (P(B)) at constant illumination was highest during daylight hours (6.2 mug C mug-1 chl a h-1) and lowest at night (1.9 mug C mug-1 chl a h-1). These results indicate that (1) carbon fixation in oceanic phytoplankton may be transcriptionally regulated at the level of the RuBPCase gene and (2) that the quantitation of particular target mRNAs and DNAs is an effective means to study the regulation of conserved gene functions in natural water column microbial populations.

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Marine Ecology Progress Series, v. 101, p. 55-65

© Inter-Research 1993