Alien vs. predator: bacterial challenge alters coral microbiomes unless controlled by predators.

Coral microbiomes are known to play important roles in organismal health, response to environmental stress, and resistance to disease. The coral microbiome contains diverse assemblages of resident bacteria, ranging from defensive and metabolic symbionts to opportunistic bacteria that may turn harmful in compromised hosts. However, little is known about how these bacterial interactions influence the mechanism and controls of overall structure, stability, and function of the microbiome. We sought to test how coral microbiome dynamics were affected by interactions between two bacteria: , a known temperature-dependent pathogen of some corals, and , a unique bacterial predator of and other gram-negative bacteria. We challenged reef-building coral with in the presence or absence of predators, and monitored microbial community dynamics with 16S rRNA gene profiling time-series. inoculation increased the mean relative abundance of by greater than 35% from the 4 to 8 hour time point, but not in the 24 & 32 hour time points. However, strong secondary effects of the challenge were also observed for the rest of the microbiome such as increased richness (observed species), and reduced stability (increased beta-diversity). Moreover, after the transient increase in two lineages of bacteria ( and ) increased in coral tissues, suggesting that challenge opens niche space for these known opportunists. increased from 6.99% (±0.05 SEM) to a maximum mean relative abundance of 48.75% (±0.14 SEM) in the final time point and from <0.001% to 3.656%. predators are commonly present at low-abundance on coral surfaces. Based on the keystone role of predators in many ecosystems, we hypothesized that predators might help protect corals by consuming foreign or "alien" gram negative bacteria. inoculation also altered the microbiome but to a lesser degree than , and were never detected after inoculation. Simultaneous challenge with both and predatory eliminated the increase in , ameliorated changes to the rest of the coral microbiome, and prevented the secondary blooms of opportunistic and seen in the challenge. These data suggest that, under certain circumstances, host-associated bacterial predators may mitigate the ability of other bacteria to destabilize the microbiome.