Spatiotemporal Distribution And Correlation With Heterotrophic Microorganisms Of Labyrinthulomycete Protists In South China Sea

Labyrinthulomycete protists are well-known heterotrophic fungi-like marine protists that have long been considered to play an important role in coastal ecosystems.While,their distribution,diversity and ecological function in open ocean remain largely unknown.In this study,we presented a widely and large-scale study of their spatiotemporal abundance and diversity in different coastal and oceanic habitats of South China Sea.The results showed Labyrinthulomycetes were widely distributed in the South China Sea.Their horizontal distribution showed the trend of esturine area>offshore>open ocean.Their abundance was positively correlated with the levels of inorganic nitrogen and phosphorus in the coastal environment.While in the open ocean,their abundance was likely more related to the organic particulate concentration which down from the up layers.Though their average abundance below the 200 m layer(～10⁴ cells L^-1)were much lower than the coastal area(10⁵-10⁶ cells L^-1),their relative biomass compared with bacterioplankton were increased with depth and finally approached or exceed bacteria below the 1000 m depth.It strongly supports the major role of Labyrinthulomycete protists in secondary production of the deep ocean.High-throughput sequencing analyses revealed high diversity of these protists in the South China Sea.Aplanochytrium was the dominant of Labyrinthulomycetes genera in each habitats of the South China Sea.Thraustochytrium and Aurantiochytrium was the dominant genus of open ocean and coastal area,respectively.It indicated that different groups of Labyrinthulomycetes have different responses to habitats.NMDS and pairwise permutation MANOVAs analysis further showed significant（p<0.05）habitat segregation in their community structure.Finally,further ecological network analysis indicated that Labyrinthulomycetes protists have different cooperative and competative relationships with bacterioplamkton in different habitats.This cooperative competition relationship was mainly mediated by their cooperative and competition with heterotrophic bacterioplankton clades of SAR116,SAR11,SAR324 and member of Flavobacterium.Moreover,their relationships with bacterioplankton displayed different pattern habitats above or below the euphotic layer.Thus,the Labyrinthulomycetes have flexible survival strategies and may play different ecological functions in different habitats.