Macroalgal Extracts Induce Bacterial Assemblage Shifts and Sublethal Tissue Stress in Caribbean Corals

Kathleen M Morrow,Valerie J Paul,Cliff Ross,Raphael Ritson-Williams,Mark R Liles,Vishal Shah

doi:10.1371/journal.pone.0044859

Kathleen M Morrow, Valerie J Paul + Show 4 more

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https://doi.org/10.1371/journal.pone.0044859

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Abstract

Benthic macroalgae can be abundant on present-day coral reefs, especially where rates of herbivory are low and/or dissolved nutrients are high. This study investigated the impact of macroalgal extracts on both coral-associated bacterial assemblages and sublethal stress response of corals. Crude extracts and live algal thalli from common Caribbean macroalgae were applied onto the surface of Montastraea faveolata and Porites astreoides corals on reefs in both Florida and Belize. Denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene amplicons was used to examine changes in the surface mucus layer (SML) bacteria in both coral species. Some of the extracts and live algae induced detectable shifts in coral-associated bacterial assemblages. However, one aqueous extract caused the bacterial assemblages to shift to an entirely new state (Lobophora variegata), whereas other organic extracts had little to no impact (e.g. Dictyota sp.). Macroalgal extracts more frequently induced sublethal stress responses in M. faveolata than in P. astreoides corals, suggesting that cellular integrity can be negatively impacted in selected corals when comparing co-occurring species. As modern reefs experience phase-shifts to a higher abundance of macroalgae with potent chemical defenses, these macroalgae are likely impacting the composition of microbial assemblages associated with corals and affecting overall reef health in unpredicted and unprecedented ways.

Highlights

Reef-building corals harbor diverse eukaryotic and prokaryotic microorganisms that form dynamic mutualistic, parasitic, and commensal associations with the coral host [1,2,3,4,5,6]
Coral Bacterial Assemblages We examined the effect of four macroalgal extracts on the bacterial assemblages associated with the surface mucus layer (SML) of M. faveolata and P. astreoides corals in Belize and Florida using PCR-Denaturing gradient gel electrophoresis (DGGE)
Analysis of DGGE profiles using nonmetric multidimensional scaling (nMDS) and PERMANOVA suggest that neither Dictyota sp. extracts nor treatment controls had a significant effect on bacterial assemblages within the SML of either coral species (Fig. 2A; Table 2A and B)

Summary

Introduction

Reef-building corals harbor diverse eukaryotic and prokaryotic microorganisms that form dynamic mutualistic, parasitic, and commensal associations with the coral host [1,2,3,4,5,6]. Many of the roles that microorganisms play in coral physiology and immune function remain unknown, recent evidence suggests that corals harbor specific and beneficial microbial assemblages [1,3,5,7]. Macroalgae are becoming increasingly abundant competitors of corals (reviewed in [10,11]), and as coral-algal interactions increase, there is greater potential for macroalgae to mediate changes in the coral-associated microorganisms and cause significant coral stress. Macroalgae utilize both physical and biochemical mechanisms to compete with corals and other reef invertebrates (reviewed in [10,12]). While several studies have examined the sublethal stress response of adult corals exposed to anthropogenically-based pollutants [19,20,21] as well as natural global stressors [22], the sublethal stress response of adult corals exposed to macroalgal extracts has not been assessed until now

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