Local versus site-level effects of algae on coral microbial communities.

Amy A Briggs,Craig W Osenberg,Anya L Brown

doi:10.1098/rsos.210035

Amy A Briggs, Craig W Osenberg + Show 1 more

Open Access

https://doi.org/10.1098/rsos.210035

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Abstract

Microbes influence ecological processes, including the dynamics and health of macro-organisms and their interactions with other species. In coral reefs, microbes mediate negative effects of algae on corals when corals are in contact with algae. However, it is unknown whether these effects extend to larger spatial scales, such as at sites with high algal densities. We investigated how local algal contact and site-level macroalgal cover influenced coral microbial communities in a field study at two islands in French Polynesia, Mo'orea and Mangareva. At 5 sites at each island, we sampled prokaryotic microbial communities (microbiomes) associated with corals, macroalgae, turf algae and water, with coral samples taken from individuals that were isolated from or in contact with turf or macroalgae. Algal contact and macroalgal cover had antagonistic effects on coral microbiome alpha and beta diversity. Additionally, coral microbiomes shifted and became more similar to macroalgal microbiomes at sites with high macroalgal cover and with algal contact, although the microbial taxa that changed varied by island. Our results indicate that coral microbiomes can be affected by algae outside of the coral's immediate vicinity, and local- and site-level effects of algae can obscure each other's effects when both scales are not considered.

Highlights

Coral reefs around the world are increasingly faced with disturbances such as bleaching events, hurricanes, disease outbreaks, overfishing and eutrophication, which can permit the rapid proliferation of fleshy algae [1,2,3,4,5,6]
This statistical interaction was more extreme than a simple antagonism: it caused the direction of the effects of algal contact to switch from negative to positive at different levels of site-level macroalgal cover. These results indicate that corals free from local competitive interactions with algae could still be affected by algae present elsewhere at a reef site. They reveal that it is difficult to predict how coral microbiomes will respond to competitive interactions with algae without considering algal densities at a larger spatial scale, 14 which could contribute to why studies have reported increases [19,56], decreases [18,67], and no effect [68] of algal contact on coral microbiome alpha and beta diversity
Algal contact and site-level macroalgal cover exhibited antagonistic effects, much like they did with alpha diversity. This antagonistic effect of local algal contact and high site-level macroalgal cover on beta diversity could occur through a similar mechanism as we proposed for alpha diversity

Summary

Introduction

Coral reefs around the world are increasingly faced with disturbances such as bleaching events, hurricanes, disease outbreaks, overfishing and eutrophication, which can permit the rapid proliferation of fleshy algae [1,2,3,4,5,6]. These increases in algae have important consequences for corals, which compete with algae for space, but are much more slow-growing, and take longer to recover when their populations decline. Indirect effects of algae mediated through microbes are increasingly recognized as an important avenue through which algae can harm corals and potentially reduce their abundance [11,12]

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