Gene expression patterns of the coral in response to contact with macroalgae.

Contact with macroalgae often causes coral mortality, but the roles of abrasion versus shading versus allelopathy in these interactions are rarely clear and effects on gene expression are unknown. Identification of gene expression changes within corals in response to contact with macroalgae can provide insight into the mode of action of allelochemicals, as well as reveal transcriptional strategies of the coral that mitigate damage from this competitive interaction, enabling the coral to survive. Gene expression responses of the coral after long-term (20 d) direct contact with macroalgae (, and ) and short-term (1 h and 24 h) exposure to thalli and their hydrophobic extract were assessed. After 20 d of exposure, thalli elicited no significant change in visual bleaching or zooxanthellae PSII quantum yield within nubbins, but stimulated the greatest alteration in gene expression of all treatments. and caused significant visual bleaching of coral nubbins and reduced the PSII quantum yield of associated zooxanthellae after 20 d, but elicited fewer changes in gene expression relative to at day 20. To evaluate initial molecular processes leading to reduction of zooxanthella PSII quantum yield, visual bleaching, and coral death, short-term exposures to thalli and hydrophobic extracts were conducted; these interactions revealed protein degradation and significant changes in catalytic and metabolic activity within 24 h of contact. These molecular responses are consistent with the hypothesis that allelopathic interactions lead to alteration of signal transduction and an imbalance between reactive oxidant species production and antioxidant capabilities within the coral holobiont. This oxidative imbalance results in rapid protein degradation and eventually to apoptosis and/or necrosis when compensatory transcriptional action by the coral holobiont insufficiently mitigates damage by the allelochemicals of .