Macroalgae may interrupt important cues for coral larval settlement

Coral reefs are becoming increasingly dominated by fleshy macroalgae. Recovery of reefs is dependent on the recruitment of new corals and the presence of certain macroalgae impedes the settlement of coral larvae directly through physical and chemical competition. However, there may be a third impediment to the successful recruitment of coral larvae on algal-dominated reefs, namely the interruption of bacterially produced settlement cues. Fo

r some corals, the settlement process appears to be dependent on the presence of chemical cues produced by biofilm bacteria. To determine if macroalgae impact bacterial communities on settlement substrata, we examined the effects of two species of macroalgae commonly found on Caribbean reefs (Halimeda opuntia and Dictyota sp.) on the bacterial communities associated with the crustose coralline alga (CCA) Hydrolithon boergesenii. We attached either a live clump of algae or a fake aquarium plant to CCA pieces and placed them in individual flow-through chambers (n = 5) on the reef. After 48 hours, we sampled the biofilm communities on the CCA and the algae and analyzed them using next-generation sequencing. Both algae caused a shift in the bacterial community found on the surface of the CCA; however H. opuntia had the greatest impact. Organic extracts of H. opuntia affected growth of bacterial strains isolated from the surface of H. boergesenii in laboratory assays and the compound halimedatetraacetate demonstrated antibiotic activity against several strains. This study demonstrates that macroalgae can alter biofilm bacterial communities, some of which may provide chemical cues necessary for coral larval settlement.