Abstract
Coral reefs of the Central Red Sea display a high degree of endemism, and are increasingly threatened by anthropogenic effects due to intense local coastal development measures. Overfishing and eutrophication are among the most significant local pressures on these reefs, but there is no information available about their potential effects on the associated microbial community. Therefore, we compared holobiont physiology and 16S-based bacterial communities of tissue and mucus of the hard coral Acropora hemprichii after 1 and 16 weeks of in-situ inorganic nutrient enrichment (via fertilizer diffusion) and/or herbivore exclusion (via caging) in an offshore reef of the Central Red Sea. Simulated eutrophication and/or overfishing treatments did not affect coral physiology with respect to coral respiration rates, chlorophyll a content, zooxanthellae abundance, or δ 15N isotopic signatures. The bacterial community of A. hemprichii was rich and uneven, and diversity increased over time in all treatments. While distinct bacterial species were identified as a consequence of eutrophication, overfishing, or both, two bacterial species that could be classified to the genus Endozoicomonas were consistently abundant and constituted two thirds of bacteria in the coral. Several nitrogen-fixing and denitrifying bacteria were found in the coral specimens that were exposed to experimentally increased nutrients. However, no particular bacterial species was consistently associated with the coral under a given treatment and the single effects of manipulated eutrophication and overfishing could not predict the combined effect. Our data underlines the importance of conducting field studies in a holobiont framework, taking both, physiological and molecular measures into account.
Highlights
IntroductionCoral reefs are threatened by global (e.g. ocean acidification, ocean warming) and local (e.g. coral disease outbreaks, overfishing, eutrophication) anthropogenic stressors [1,2,3,4], whereby eutrophication and overfishing are among the two most important local factors affecting coral reef health [3]
Coral reefs are threatened by global and local anthropogenic stressors [1,2,3,4], whereby eutrophication and overfishing are among the two most important local factors affecting coral reef health [3]
Environmental Parameters and Coral Physiology Nutrient enrichment led to significant increases in dissolved inorganic nitrogen (DIN) and in soluble reactive phosphate (SRP) (Figure 1)
Summary
Coral reefs are threatened by global (e.g. ocean acidification, ocean warming) and local (e.g. coral disease outbreaks, overfishing, eutrophication) anthropogenic stressors [1,2,3,4], whereby eutrophication and overfishing are among the two most important local factors affecting coral reef health [3]. Reduced herbivore stocks can lead to prolonged recovery times of corals after disturbances [6], increase crown-of-thorns starfish outbreaks [7], and release macroalgae from their top-down control [3]. Eutrophication affects scleractinian corals indirectly by increasing turf and macroalgae [14,15,16,17], inhibit coral recruitment [18,19], or directly outcompete corals via allelochemicals [20,21,22]. Turf and macroalgae increase microbial activity and trigger changes in the microbial community associated with corals [23,24,25,26] by dissolved organic carbon (DOC) release that can accelerate microbial growth and create a positive feedback loop when corals die and free space for more algae [27,28]
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