Effects on the Pocillopora verrucosa microbiome when in contact with macroalgae under ocean acidification

Abstract

Hard coral cover is in decline and this decline has generally coincided with macroalgal proliferation in coral reefs. Coral degradation can be caused by many variables but this study is focused on potential degradation due to direct competition with allelopathic macroalgae under the effects of future ocean acidification. Allelopathy is the use of chemicals for protection or competitive purposes. It has previously been shown that algae compete with corals through allelopathy, but not if allelopathy causes the microbiome of the coral to enter a diseased state, though there have been several cases of diseased microbiome states observed. As such, it is of interest to determine if the allelopathic competition from algae affects the coral microbiome, leading to a diseased state, and whether these interactions are exaggerated or effected by ocean acidification. We hypothesize that macroalgal allelopathy effects the microbiome of the reef-building coral Pocillopora verrucosa and that these competitive interactions will be affected by the stressor of ocean acidification. We expect the latter because of previous evidence that increased pH causes stress to some species of corals. To test this, we used a pre-established scale of algal allelopathy demonstrated in Rasher et al. (2011) and placed corals and algae in contact under ocean acidification conditions for 3 weeks before samples were processed for microbial taxonomy. The initial analyses have demonstrated no significant differences in the abundances of major microbial taxa compositions for the sampled coral microbiomes when in the presence of the various allelopathic macroalgae, but these are preliminary findings. The data will require finer microbial analysis to determine whether or not there are any significant effects on the coral microbiomes.