Okinawa is the southwesternmost part of Japan and represents the northern limit of coral reefs that are influenced by the Kuroshio warm current. We have been studying two coral reefs off Sesoko Island, in northern Okinawa, to evaluate coral reef environments since 2014. In this study, we applied a metagenomic approach to analyze the seasonal composition of picophytoplankton at these two reef sites (Sesoko Minami and Ishikawabaru). The two sites were selected for their differences: Sesoko Minami faces the open ocean and experiences relatively small land effects, while Ishikawabaru experiences relatively high land effects and is surrounded by barrier reefs. Seawater samples were collected from these two sites and filtered through a 1.6 μm pore filter followed by a 0.2 μm membrane filter. DNA was extracted from the 0.2 μm filters, and the V1-V2 region of the 16S rRNA gene (260 bp) was amplified before sequencing using next-generation sequencing techniques. Cyanobacteria or chloroplast sequences were selected for further metabarcording analyses. Interestingly, almost no differences were indicated between the two sites and more than 95% of the picophytoplankton at both sampling sites was composed of only 5 to 7 OTUs. The seasonal composition of picophytoplankton clearly changed; in summer (Jul 2014, May to Jul 2015, and May to Aug 2016) OTU 1 (Synechococcus sp. NC007513) was most abundant. OTU 4 (Prochlorococcus marinus NC009091) also reached its maximum abundance in summer (May to Aug 2016), while OTU 6 (Bathycoccous prasinos FN563099) increased in winter. OTU 8 (Micromonas commoda RCC299 FJ858267) was present at every sampling period and accounted for 5–20% of the total picophytoplankton contig numbers. Our results indicate that seasonal changes clearly influence fluctuations in picophytoplankton community compositions.
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