We have found a diversity of form IA, IB, and ID rbcL phylotypes during previous cruises to the Mississippi River Plume (MRP). Using molecular cloning of RT-PCR amplified rbcL mRNA, we recovered one hundred and sixteen unique rbcL sequences from the eight surface samples; their phylogenetic affiliations are shown below. Most importantly, we observed that the bulk of the eukaryotic, autotrophic picoplankton in our sampling region was composed of diatoms (36 clones), prymnesiophytes (24 clones) and an array of operational taxonomic units (OTUs) closely related to a diverse group of green algae (25 clones). Together these three algal groups accounted for 73% of the species richness observed. The remaining sequences included six eustigmatophytes, nine prochlorophytes, twelve marine A Synechococcus, five silicoflagellates, two sequences related to Trichodesmium thiebautii, three deeply rooted haptophytes and one sequence most closely related to the recently described bolidophytes. With the exception of the two Trichodesmium-like sequences, all form IB clones were related to chlorophytes and no other form IB cyanobacteria were found in our libraries. Form IA containing cyanobacteria (hereafter referred to as alpha-cyanobacteria) belonged either to the marine A cluster of Synechococcus or the high-light adapted clade of Prochlorococcus. A breakdown of our libraries by station and algal group is shown in the table below.
Reference: Wawrik, B. and J.H. Paul. 2004. Phytoplankton community structure and productivity along the axis of the Mississippi River Plume. Aquat. Microb. Ecol. 35:185-196