Micro- and macrodiversity in rbcL sequences in ambient phytoplankton populations from the southeastern Gulf of Mexico

Ribulose-1,5-diphosphate carboxylase/oxygenase (RuBisCO) large subunit genes (rbcL) were obtained by amplification and cloning of 554 or 614 bp sequences of indigenous phytoplankton populations at 2 stations in the southeastern Gulf of Mexico. One station (Stn 4) was located in a low salinity, high chlorophyll plume (the 'Green River') which has previously been shown to contain elevated levels of Form IA rbcL mRNA while the other (Stn 7) was in oligotrophic, oceanic water. A diversity of rbcL sequences was obtained, spanning 3 of the 4 evolutionary clades of Form I RuBisCOs. Six nucleotide sequences obtained from Stn 4 were closely related (92 to 96% similar) to the Form IA-containing Prochlorococcus GP2. Flow cytometry and pigment analysis indicated that Prochlorococcus was abundant at this site. Other sequences found included a Form IB rbcL closely related to prasino-phytes, and Form ID sequences related to prymnesiophytes, diatoms, and pelagophytes. One sequence was nearly identical to the pelagophyte, Pelagomonas calceolata. At Stn 7, sequences were obtained that were more deeply rooted, and less similar to rbcLs in existing databases (77 to 83 % similar), and no Form IA rbcLs were detected. HPLC pigment signatures and flow cytometry data were consistent with the forms obtained by cloning, The similarity of the 6 Prochlorococcus GP2-like sequences (93 to 98%) is consistent with the phenomenon of molecular microdiversity as found at other loci in marine land other environmental) microorganisms.