| Summary: | Natural Science Foundation of China [NSFC 40925018, 41176112]; National Basic Research Program ('973' program) of China [2009CB 421203]; Ocean Public Welfare Scientific Research Project, State Oceanic Administration People's Republic of China [201005015, 201105021, GASI-03-01-02-03] Photosynthetic picoeukaryotes (PPEs) can be important primary producers in the oligotrophic ocean and coastal waters at certain times of the year. In this study, we investigated the abundance and biodiversity of picoplankton, focusing on PPEs in the South China Sea (SCS) from the Pearl River estuary to SCS basin in January 2010, when the northeast monsoon prevailed. PPE abundance was quantified using fluorescent in situ hybridization associated tyramide signal amplification, and the biodiversity at 5 selected stations was determined using small subunit ribosomal RNA gene (18S rDNA) clone libraries. Our results showed that PPEs were most abundant in the Pearl River estuary (up to 8500 cells ml(-1)), and that mamiellophycean picoplanktonic green algae, such as Micromonas, Ostreococcus and Bathycoccus, showed peaks of abundance in slope or coastal waters. The 18S rDNA phylogeny revealed that most of the PPEs belonged to prasinophytes, affiliating to 4 clades (Clade IX, Clade V, Clade VII, and Mamiellophyceae). Phytoplankton pigment analysis clearly showed the difference in picophytoplankton community structure along the environmental gradient provided by the selected stations. Among the PPEs, prasinophytes and prymnesiophytes accounted for 18.7 and 41.5%, respectively, of the chlorophyll a biomass. Putting all the data together, we describe a complete picture of PPE assemblages along the coast-offshore gradient, showing that the prasinophytes and prymnesiophytes appear to be the key PPE components in this subtropical-tropical marginal sea.
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