Abstract:

Seamounts are often identified as ecological hotspots, where physical environment, such as ocean currents, can create upwelling and downwelling, enhancing vertical mixing and potentially increasing species diversity and biological productivity. This study focuses on Xianbei, a shallow seamount located in the oligotrophic waters of the northern South China Sea, with its summit 208 m below the surface. Near the seamount, two eddies rotate clockwise and counterclockwise, with possible upwelling and downwelling regions occurring on the sides of the seamount, affecting the composition of phytoplankton. Picocyanobacteria, particularly Prochlorococcus and Synechococcus, are important primary producers, contributing approximately 25% of oceanic primary production. They are diverse and abundant in the open ocean. Prochlorococcus can be divided into high-light (HL) and low-light (LL) clades, occupying different depths along the water column. Synechococcus possesses additional light-harvesting complexes called phycobilisomes (PBS), and distinct pigment types allow them to survive in a wide range of environments. This research aims to understand the niche partitioning of different ecotypes of Prochlorococcus and Synechococcus around the seamount and how the hydrodynamics surrounding the seamount affect their community structure. To investigate the composition and diversity of the ecotypes of Synechococcus and Prochlorococcus, third generation of high-throughput sequencing of the 16S-23S rRNA internal transcribed spacer region (ITS) was used. Flow cytometry results indicate that Prochlorococcus has a maximum abundance on the upstream side of the seamount between 50 m and 100 m, while Synechococcus also has a relatively high abundance on the upstream side from the surface to 100 m. Limited results from a testing station indicated that the downstream side of the seamount is mainly occupied by the High-Light II clade of Prochlorococcus and Clade II of Synechococcus. However, further analysis is required to conclusively determine the niche partitioning in this region.

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