Abstract
Benthic algae are associated with coral death in the form of stress and disease. It's been proposed that they release exudates, which facilitate invasion of potentially pathogenic microbes at the coral-algal interface, resulting in coral disease. However, the original source of these pathogens remains unknown. This study examined the ability of benthic algae to act as reservoirs of coral pathogens by characterizing surface associated microbes associated with major Caribbean and Indo-Pacific algal species/types and by comparing them to potential pathogens of two dominant coral diseases: White Syndrome (WS) in the Indo-Pacific and Yellow Band Disease (YBD) in the Caribbean. Coral and algal sampling was conducted simultaneously at the same sites to avoid spatial effects. Potential pathogens were defined as those absent or rare in healthy corals, increasing in abundance in healthy tissues adjacent to a disease lesion, and dominant in disease lesions. Potentially pathogenic bacteria were detected in both WS and YBD and were also present within the majority of algal species/types (54 and 100% for WS and YBD respectively). Pathogenic ciliates were associated only with WS and not YBD lesions and these were also present in 36% of the Indo-Pacific algal species. Although potential pathogens were associated with many algal species, their presence was inconsistent among replicate algal samples and detection rates were relatively low, suggestive of low density and occurrence. At the community level, coral-associated microbes irrespective of the health of their host differed from algal-associated microbes, supporting that algae and corals have distinctive microbial communities associated with their tissue. We conclude that benthic algae are common reservoirs for a variety of different potential coral pathogens. However, algal-associated microbes alone are unlikely to cause coral death. Initial damage or stress to the coral via other competitive mechanisms is most likely a prerequisite to potential transmission of these pathogens.
Highlights
Coral diseases have contributed to the regional collapse of important reef-building species worldwide [1,2,3]
At Heron Island, four out of the eleven algal species (Hincksia sp., Hydroclathrus clathrus, Sargassum polycystum and Caulerpa racemosa) showed no bacterial associates, and only a few bacterial ribotypes were detected on Halimeda macroloba (Fig. 2)
To determine whether benthic algae act as a source of coral pathogens, we examined dominant surface microbes associated with a variety of algae species and related them, using the same non culture molecular technique (DGGE), to dominant potentially pathogenic microbes associated with two coral diseases
Summary
Coral diseases have contributed to the regional collapse of important reef-building species worldwide [1,2,3]. In contrast to the above mentioned studies which illustrate numerous direct and indirect effects of algae on coral [32], showed that the presence of macro algae adjacent to corals had no observable effect on coral health and disease prevalence at least in relation to Yellow Band Disease within the Caribbean. In their experiments, macroalgae were not directly touching the coral, which suggests that water borne algal exudates alone are not sufficient to illicit a stress response on the coral
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