Diversity and community dynamics of protistan microplankton in Sagami Bay revealed by 18S rRNA gene clone analysis

The diversity and short-term changes in the protistan microplankton community from April to June 2006 in Sagami Bay were revealed by 18S rRNA gene clone analysis. A total of 1,076 clones consisted of 68 phylotypes of dinoflagellates, 96 phylotypes of diatoms and 27 phylotypes of other protists affiliated with Ciliophora, Prymnesiomonada, Chlorophyta, Cercozoa, Chytridiomycota, and Heterokonta. Approximately half of all dinoflagellate phylotypes were affiliated with the following genera: Ceratium, Gonyaulax, Gymnodinium, Gyrodinium, Lepidodinium, Neoceratium, Prorocentrum, and Woloszynskia. The other half was classified into seven uncultured groups. These dinoflagellate clones were mostly detected in May, in contrast to the diatom clones, which were detected frequently throughout the study period. Diatoms were diverse and consisted of 14 genera and three uncultured groups. The genera Discostella, Thalassiosira and Skeletonema were dominant in April, May and June, respectively. Species richness (number of phylotypes) and diversity (Shannon-Weiner) of the whole protistan microplankton community were highest in May. This is the first example of a comprehensive molecular biological analysis of protistan microplankton community structure, and the results clearly showed a dynamic shift in the protistan community in coastal waters from April to June in Sagami Bay. The results of a direct comparison between the clone analysis and microscopic observations indicated that the clone analysis had the great advantage of enabling identification of plankton that were morphologically indistinguishable, and to reveal detailed information on the biodiversity of protistan microplankton. This advancement in molecular biological analysis will assist in our understanding of the biodiversity of protistan microplankton.