N-125. Community Composition of Picoeukaryotes in Open Ocean and Coastal Waters

M. L. Cuvelier1, E. Demir2, B. J. Binder3, A. Z. Worden2;
1Univ. of Miami - RSMAS, Miami, FL, 2Monterey Bay Aquarium Res. Inst., Moss Landing, CA, 3Univ. of Georgia, Athens, GA.

Picophytoplankton (cells <2 µm) are considered important oceanic primary producers. Phytoplankton community composition within this size fraction is taxonomically broad and includes picoeukaryotes and cyanobacteria. The phylogenetic diversity and population structure of picoeukaryotes have not been well characterized, particularly in the open ocean. To establish the abundance, diversity and biomass of specific picoeukaryote populations, we focused on two open-ocean stations in the Sargasso Sea: the Bermuda Atlantic Time-series Study (BATS) station and north of BATS. In addition, sites along a transect from the Sargasso Sea to Woods Hole, MA were investigated. Flow cytometry (FCM), 18S rRNA gene clone libraries, and fluorescence in situ hybridization (FISH) were used. We performed a comprehensive phylogenetic reconstruction of the retrieved 18S rDNA sequences using maximum likehood methods. Results showed that prymnesiophyte sequences were well represented in the clone libraries, particularly at BATS. However, none had high identity to known cultured prymnesiophytes. Micromonas sequences from the Sargasso clone libraries belong to clade A.A.2, one the 5 clades that have been identified for this genus. 100% of the Ostreococcus sequences from surface (15 m) or deep (70 m) samples belong to a clade of “deep adapted” Ostreococcus. Using published FISH probes, prymnesiophytes appeared to be dominant at BATS but formed a smaller fraction north of BATS, 28% and approximately 10% of the picophytoeukaryotes enumerated by FCM at 15 m and 70 m, respectively. Micromonas appeared to be even less abundant, and surprisingly, Ostreococcus cells were not detected. Application of genus-specific QPCR probes revealed that Ostreococcus was in fact abundant in these samples. In the Northern Sargasso, Ostreococcus may constitute 6-14% and 25-65% of the picophytoeukaryotes enumerated by FCM at 15 m and 70 m, respectively. These results are being contrasted with those for coastal populations. Combined with biomass and primary production measurements, these data allow the relative contribution of specific picoeukaryote populations to be estimated.