Abstract
The surface mucus layer of reef-building corals supports feeding, sediment clearing, and protection from pathogenic invaders. As much as half of the fixed carbon supplied by the corals’ photosynthetic symbionts is incorporated into expelled mucus. It is therefore reasonable to expect that coral bleaching (disruption of the coral–algal symbiosis) would affect mucus production. Since coral mucus serves as an important nutrient source for the entire reef community, this could have substantial ecosystem-wide consequences. In this study, we examined the effects of heat stress-induced coral bleaching on the composition and antibacterial properties of coral mucus. In a controlled laboratory thermal challenge, stressed corals produced mucus with higher protein (β = 2.1, p < 0.001) and lipid content (β = 15.7, p = 0.02) and increased antibacterial activity (likelihood ratio = 100, p < 0.001) relative to clonal controls. These results are likely explained by the expelled symbionts in the mucus of bleached individuals. Our study suggests that coral bleaching could immediately impact the nutrient flux in the coral reef ecosystem via its effect on coral mucus.
Highlights
Rising sea surface temperature has increased the global risk of coral bleaching, the breakdown of the symbiosis between a coral host and its algal symbiont, to alarming levels (Hughes et al, 2018)
We found no difference in the quantity of mucus produced by stressed corals compared to healthy corals after four days of heat stress or after a six-day recovery period (Fig. 2), suggesting that the quantity of mucus produced by a coral is relatively unaffected by thermal stress and that mucus stores cannot be replenished within a week
Our results show that thermal stress does not significantly affect the volume of mucus produced by A. cervicornis immediately following a bleaching event
Summary
Rising sea surface temperature has increased the global risk of coral bleaching, the breakdown of the symbiosis between a coral host and its algal symbiont, to alarming levels (Hughes et al, 2018). The direct impacts of bleaching on the animal host and algal symbiont are well studied. Coral bleaching has been shown to downregulate the expression of genes related to host immunity (Pinzón et al, 2015) and alter host metabolism (Kenkel, Meyer & Matz, 2013; Rodrigues & Grottoli, 2007). Sustained coral bleaching often leads to host mortality and subsequent shifts in the reef community structure (Harriott, 1985). Coral bleaching and mortality causes shifts in reef-fish assemblage structure and alters recruitment success (Booth & Beretta, 2002; Richardson et al, 2018)
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