Algae dictate multiple stressor effects on coral microbiomes

Most studies of stressors focus on the response of traditionally classified organisms via effects on growth, mortality or physiology; however, most species have microbial associates that may mediate the response of a host to the stressor. Additionally, species rarely experience one stressor alone, but instead are influenced by multiple, potentially interacting stressors. We evaluated how coral microbiomes responded to two biotic stressors: the vermetid gastropod, Ceraesignum maximum, and algal turfs, both of which have previously been shown to decrease coral growth, survival and photophysiology. We collected coral mucus from massive Porites colonies in the presence versus absence of both algae and vermetids and sequenced the 16S rRNA gene to characterize the coral surface microbial communities. The presence of algae increased the alpha diversity of the coral microbial community, likely by increasing the relative abundance of rare members of the community. Algae also reduced beta diversity, which we hypothesized was due to algae homogenizing the physical environment. In contrast, vermetids had only small effects on microbial communities, even though vermetids have deleterious effects on coral growth. We previously hypothesized that vermetids would exacerbate algal effects on microbes, but we failed to detect an interaction between vermetids and algae on the coral's microbiome, except for one family, Fusobacteriaceae, which was most abundant in the presence of both stressors. We suggest that algae can affect corals through their effects on microbes, whereas vermetids primarily affect the host directly; these complementary effects may limit the extent to which stressors can interact.