The impact of heat stress, UVB radiation and humic substances on the diversity and structure of prokaryotic communities inhabiting host and environmental coral reef biotopes

Coral cover has declined across the globe over the past few decades. A key contributor to this decline has been the increased prevalence of bleaching events due to heat stress. A number of field observational studies have suggested that the impact of heat stress and intense ultraviolet B (UVB) radiation may be mitigated by the presence of humic substances (HS) in the form of Chromophoric Dissolved Organic Matter (CDOM) in the water column. Here, we investigated the impact of heat stress, UVB radiation, and HS modulation on prokaryotic communities of the finger coral Montipora digitata , the soft coral Sarcophyton glaucum , the sponge Chondrilla sp., and sediment under controlled microcosm conditions. Prokaryotic evenness was significantly lower in M. digitata under UVB exposure, but significantly higher under HS modulation. Evenness was also significantly higher in sediment under HS modulation. Rarefied richness, in turn, was significantly higher in M. digitata and sediment under HS modulation. There were also significant two- and three-way interactions with respect to rarefied richness among heat stress, UVB radiation and HS modulation on the prokaryotic communities of Chondrilla sp. and M. digitata. For example, in M. digitata , HS modulation appeared to mitigate the adverse effects of heat stress and UVB exposure on rarefied richness. Prokaryotic communities from each biotope with and without HS modulation were fitted to a neutral model. Fits to the neutral model varied substantially and were lowest for the soft coral Sarcophyton glaucum (R 2 = 0.547 without HS and R 2 = 0.596 with HS) and highest for the sponge Chondrilla sp. (R 2 = 0.922 without HS and R 2 = 0.949 with HS). In all biotopes the fit to the neutral model was higher under HS modulation. This difference was most pronounced for M. digitata where there was a relatively poor fit without HS modulation (R 2 = 0.303), but a much better fit with HS modulation (R 2 = 0.636). Taken together, these results suggest that HS have biotope-specific effects on prokaryotic diversity and appear to promote neutral community assembly of prokaryotic communities, particularly in M. digitata. HS may, furthermore, help to increase resilience against heat stress and UVB radiation by stimulating the growth of richer and more even prokaryotic assemblages.