Diversity hotspot and unique community structure of foraminifera in the world's deepest marine blue hole-Sansha Yongle Blue Hole

Marine blue holes are precious geological heritages with high scientific research values. They are subsurface voids that are formed during glacial time and are characterized by steep-walled structure and stratified water bodies with oxygen deficit and high levels of H2S in deeper layers. The Sansha Yongle Blue Hole (SYBH), located in the Yongle coral atoll in the South China Sea, has been explored in 2017 and its depth has been estimated at ~300 m making it the deepest marine blue hole reported to date. We have explored the diversity of benthic foraminifera which are a group of calcifying protists that can thrive in low-oxygen or even anoxic environments, due to their capacity to perform denitrification, making them the only facultative anaerobic protist.We collected sediment samples from the top of SYBH until its deepest point and complemented this sampling with a costal polluted habitat of Jiaozhou Bay (JZB) and an abyssal plains of the Northwest Pacific Ocean (NWP). We studied the foraminiferal diversity and community structure with metabarcoding approach. Our results showed that the foraminiferal assemblages in SYBH were vertically structured in response to the abiotic gradients and diversity was higher than in JZB and NWP. The class Globothalamea which is usually not abundant dominated SYBH assemblages, and within Globothalamea, we identified species capable of denitrification in the anoxic zone of SYBH.