Rosemary Kate Steinberg , Katherine A Dafforn, Emma L Johnston, Tracy Ainsworth

Microbes are essential to the holobiome of all organisms, so when organisms are impacted by environmental stressors the changes to microbial communities can inform on the health state of the host and even cause host disease. In early 2019 marine heatwaves occurred across eastern Australia, resulting in differential bleaching across Lord Howe Island reefs. Here we examined the Symbiodiniaceae and bacteria associated with bleaching susceptible Cladiella sp.1, and Symbiodiniaceae associated with two bleaching resistant octocoral species – Cladiella sp.2 and Xenia cf crassa – during and after marine heatwaves and between two reefs, Sylphs Hole (heavily impacted) and Coral Gardens (mildly impacted). While we found that Cladiella sp.1 Symbiodiniaceae community structure was not altered by bleaching, beta diversity dispersion of Symbiodiniaceae in unbleached colonies was higher at Sylphs Hole than Coral Gardens, suggesting dysbiosis. Unbleached colonies of all species had different Symbiodiniaceae communities between reef sites, and communities were stable through time. Bacterial communities differed between bleached and unbleached Cladiella sp.1, and bleached colonies’ microbial communities changed after bleaching alert levels returned to “no stress”, showing a delayed microbial response to bleaching. Finally, unbleached colonies from Sylphs Hole had higher bacterial alpha diversity compared to colonies at Coral Gardens, which together with possible Symbiodiniaceae dysbiosis suggests that the unbleached colonies at Sylphs Hole were impacted by heatwaves without clear physical changes in the field. So, corals and their microbial communities may be affected at heavily stressed sites regardless of bleaching status.