Form IA rbcL transcripts associated with a low salinity/high chlorophyll plume ('Green River') in the eastern Gulf of Mexico

Coastal plumes of low salinity water that extend hundreds of kilometers offshore into oligotrophic waters are often found in the Gulf of Mexico. To characterize one such feature, a series of photoautotrophic activity and biomass parameters were measured at 2 stations in the eastern Gulf of Mexico, including pigments by high performance liquid chromatography (HPLC), autotrophic picoplankton abundance by flow cytometry, photoautotrophic C-14-HCO3- fixation, and Ribulose-1,5-diphosphate carboxylase large subunit gene (rbcL) transcriptional activity. One sampling site (Stn 4) was in a 15 m deep, low salinity (29.8 ppt) plume 242 km west of Tampa Bay. This feature contained relatively high chlorophyll a (chl a) concentrations, carbon fixation rates, and Synechococcus cell abundance (8.6 x 10(4) cells ml(-1)) at 3 m depth and a relatively shallow (45 m depth) subsurface chlorophyll a maximum (SCM). We also found a high level (1.1 ng l(-1)) of Form IA rbcL mRNA in the surface water as determined by probing with a 1.1 kb Synechococcus WH7803 rbcL probe. Form IA rbcLs have been found to occur mainly in chemosynthetic autotrophic bacteria but have recently been described in Synechococcus WH7803 as well as in Prochlorococcus GP2. In contrast, a nearby station outside of the plume (Stn 7) had a SCM at 83 m, lower chi a, Synechococcus cell counts, and carbon fixation rates in the surface waters. The amount of Form IA rbcL was only about 3% of the concentration found in the surface waters of Stn 4. Both stations had an abundance of Prochlorococcus cells (> 10(5) ml(-1)) at intermediate depths (20 to 70 m). The picoeucaryote community occurred principally below the Prochlorococcus community, coinciding with the SCM, and was composed of diatoms, prymnesiophytes, and pelagophytes as determined by HPLC pigment analysis. This report represents the first description of Form IA rbcL transcriptional activity in the marine environment, and indicates that Form IA rbcL-containing picoplankton (like Prochlorococcus GP2 and Synechococcus WH7803) may be important in the primary production of low salinity, surface water plumes of the Gulf of Mexico.