Driving Factors of Dimethylated Sulfur Compounds in the Pearl River Estuary and Its Adjacent Coastal Waters: The Nonnegligible Role of Synechococcus

Estuaries serve significant roles in the global dimethylated sulfur compounds (DSCs) cycle, however, the driving factors and biological contribution still need to be elucidated. A field investigation was conducted alongside diluted water in the Pearl River Estuary (PRE) and its adjacent coastal waters to explore the factors controlling the spatial variabilities of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP), as well as the role of phytoplankton in DSCs emissions in estuaries. Higher values of DSCs were observed in the freshwater-seawater mixing areas. Furthermore, the influx of the Pearl River diluted water (PRDW) caused a shift in higher DMSP and DMS values from nearshore to offshore. Dimethylated sulfur compounds did not show clear correlations with Chl-a, while a positive relationship existed between DSCs and certain eukaryotic phytoplankton species, such as dinoflagellates. Additionally, this study emphasized the crucial role of prokaryotic phytoplankton, mainly Synechococcus, in DSCs production. The abundance of Synechococcus exhibited significant positive correlations with DMS and DMSP, compared with eukaryotic phytoplankton. These results indicated that Synechococcus may be an essential biological factor driving DSCs production in estuaries. Overall, this study provides new insights to clarify the nonnegligible role of Synechococcus in the biogenic sulfur cycle, in which its contribution must be explored in greater detail in future studies.