Contact- and Water-Mediated Interactions With an Allelopathic Macroalga Drive Distinct Coral Microbiome and Metabolome.

Macroalgal proliferation constitutes a major threat to coral reef resilience. Macroalgae can affect corals by altering their microbiome and metabolome. However, our understanding of the spatial scale of these effects and the influence of environmental factors is limited. We conducted a manipulative field experiment to investigate how interaction types (direct contact and close proximity) with the allelopathic macroalga Dictyota bartayresiana and prevailing water current influence the microbiome and metabolome of the coral Pocillopora acuta and its near-surface seawater. Coral tissue damage was spatially constrained to the algal contact zone. Direct contact caused significant increases in harmful bacteria at the expense of beneficial ones in side coral fragments. Non-significant changes were observed within the microbiome of apex fragments, suggesting a resistance of the coral holobiont to colony-wide microbial colonisation. The coral metabolome responded to both algal contact and proximity. We detected several compounds potentially relevant for oxidative stress mitigation and coral defence. This metabolomic response was similar between apex and side fragments, supportive of a colony-wide metabolomic response. In the near-surface coral seawater, only a microbial response to algal contact was detected. We conclude that coral holobionts are capable of colony-wide metabolomic responses to maintain homeostasis against macroalgal competitors.