Indirect effects of algae on coral: algae‐mediated, microbe‐induced coral mortality

Abstract

Declines in coral cover are generally associated with increases in the abundance of fleshy algae. In many cases, it remains unclear whether algae are responsible, directly or indirectly, for coral death or whether they simply settle on dead coral surfaces. Here, we show that algae can indirectly cause coral mortality by enhancing microbial activity via the release of dissolved compounds. When coral and algae were placed in chambers together but separated by a 0.02 mum filter, corals suffered 100% mortality. With the addition of the broad-spectrum antibiotic ampicillin, mortality was completely prevented. Physiological measurements showed complementary patterns of increasing coral stress with proximity to algae. Our results suggest that as human impacts increase and algae become more abundant on reefs a positive feedback loop may be created whereby compounds released by algae enhance microbial activity on live coral surfaces causing mortality of corals and further algal growth.

Macroalgal terpenes function as allelopathic agents against reef corals

“…Visual bleaching and photosynthetic efficiency were correlated for all three corals (r = −0.80 to −0.96, P < 0.001 for all comparisons) (Fig. S1); thus, PAM fluorometry measurements are indicative of visual bleaching but are less subjective (17,18,21,22). The most resistant coral was Montipora digitata; for this coral, Dictyota bartayresiana, Galaxaura filamentosa, and Chlorodesmis fastigiata caused significant bleaching and suppression of photosynthetic efficiency, whereas Liagora sp.…”

“…Recent laboratory studies demonstrated that macroalgae near but not in contact with corals triggered coral mortality and suggested that algae release water-soluble compounds that kill corals indirectly by stimulation of harmful coral-associated microbes (17). Field studies indicate that benthic algae release hydrophilic molecules, such as dissolved organic carbon (DOC), capable of fueling these interactions (36,37).…”

“…However, for most of these studies, it is unclear whether the findings are particular to the macroalgal and coral species tested or are common to algal-coral interactions in general and could thus transform the way ecologists and resource managers view processes driving phase-shifts on coral reefs. Despite recent studies demonstrating the potential importance of chemically mediated algalcoral competition (17)(18)(19) and its increasing impact as the result of ocean acidification (20), no algal compounds mediating these interactions have been identified.…”

“…However, the consequences of and mechanisms driving most algal-coral interactions remain poorly understood. Recent field studies suggest that macroalgae may damage corals by (i) shading and abrasion (15), (ii) vectoring of coral disease (16), (iii) release of water-soluble compounds that stimulate harmful, coral-associated microbes (17), or (iv) transfer of hydrophobic allelochemicals by direct contact (18). However, for most of these studies, it is unclear whether the findings are particular to the macroalgal and coral species tested or are common to algal-coral interactions in general and could thus transform the way ecologists and resource managers view processes driving phase-shifts on coral reefs.…”

Baía de todos os santos: aspectos oceanográficos

“…Na BTS o maior problema é a grande quantidade de alga filamentosa, organismo que impede que as larvas plânulas (larvas dos corais) assentem-se nos recifes (Birkeland, 1977). Estas algas podem ainda abrigar bactérias que causam doenças nos corais (Smith et al, 2006). Doenças que foram constatadas (observação pessoal) e necessitam ser estudadas na BTS.…”