Competition between coral and algal holobionts

Coral reefs around the world have suffered devastating losses of reef building corals with a concomitant increase in benthic algae. While it is clear that a variety of local and global disturbances play a role in the replacement of corals by algae, the mechanisms behind this transition are not. Space is limited on coral reefs, and competition between corals and benthic algae plays a major role in shaping the composition of the benthos. The objective of this dissertation was to investigate the dynamics of coral-algae competition, with a focus on how different algae affect coral physiology and their associated microbes, and how these small-scale dynamics influence the distribution of corals and algae on pristine to degraded coral reefs. I found significant differences in the composition and outcomes of coral-algae interactions across reefs; corals were consistently better competitors against crustose coralline algae (CCA), but were damaged by turf algae on inhabited but not uninhabited reefs, suggesting that competition dynamics are affected by human activity. Physiological investigations of coral interactions with four common types of benthic algae (CCA, calcareous macroalgae, fleshy macroalgae, and turf algae) demonstrated that all algae except CCA cause net heterotrophy and disruption of coral tissue and pigments along the interaction border. These effects were negated by antibiotics, indicating that disruption of coral health during algal competition is mediated by microbes. These same algae were found to harbor highly diverse bacterial and eukaryotic microbial communities, but their competitive interfaces with corals hosted a community of microbes distinct from either side. Turf algae borders had a large proportion of potential pathogens, while fleshy macroalgae led to an increase of bacterial carbohydrate utilization metabolisms. This suggests that turf and fleshy macroalgae compete with corals by stimulating bacterial growth and respiration and promoting the invasion of opportunistic pathogens on corals, leading to coral mortality and freeing space for the alga. This dynamic appears to be amplified by human disturbances such as overfishing and eutrophication, which remove the top-down and bottom-up controls on alga