Gradients of three coastal environments off the South China Sea and their impacts on the dynamics of heterotrophic microbial communities.

Heterotrophic fungus-like marine protists are recognized to contribute significantly to the coastal carbon cycling largely due to their high biomass and ability to decompose recalcitrant organic matter. Yet, little is known about their dynamics at polluted coastal environments in the context of heterotrophic microbial communities. Here, we present the dynamics of these protists relative to their heterotrophic counterparts in three different environments, namely Pearl River Estuary (ZJK), Shenzhen Bay (SZW) and Daya Bay (DYW) along the coastline of South China Sea. ZJK and SZW were characterized by low salinity and high N levels with large variations, unlike DYW. However, the average abundance of fungus-like protists did not differ significantly (P > 0.05) among these environments, except that it increased in August (422 ± 264 cells/mL, P < 0.01) over March, May and October. Correlation analysis revealed association of their abundance to different environmental factors, namely dissolved organic N in ZJK (rho = -0.87); NH4+ (rho = 0.64) and Chl a (rho = 0.73) in SZW; and salinity (rho = 0.46), DO (rho = 0.57) and total P (rho = 0.48) in DYW, suggesting distinct influence of trophic conditions. Analysis of their abundance relative to other heterotrophic protists (HP) shows that fungus-like protists display selective advantage over HP in the environment (DWY) with low N levels. Further, the similar biomass fraction (ZJK: 5.97 ± 6.23%, SZW: 5.97 ± 5.28%, and DYW: 12.1 ± 11.4%; P > 0.05) of fungus-like protists relative to heterotrophic bacteria, suggest their invariable contribution to carbon cycling. Thus, dynamics of fungus-like protists in relation to their heterotrophic counterparts is largely regulated by the trophic conditions of coastal environments.