Heterotrophy can save corals from oxidative stress and bleaching

Scleractinian corals acquire autotrophic nutrients via the photosynthetic activity of their symbionts and the subsequent transfer of photosynthates. Zooplankton predation by the animal (heterotrophy) is an additional food source. Under stress events, corals loose their symbionts, a phenomena known as bleaching, which eventually leads to starvation, unless corals increase their heterotrophic capacities. Molecular mechanisms by which heterotrophy sustains metabolism in stressed corals remain elusive. Here for the first time, we identify specific genes expressed in heterotrophically fed and unfed corals maintained under normal and light-stress conditions inducing bleaching. Physiological parameters and gene expression profiling showed ominously that fed corals better resisted the stress than unfed corals, by presenting less oxidative damage and protein/DNA degradation. Light stressed and unfed/starved corals (HLS) up-regulated by 140 and 13 times two genes (CP2U1 and CP1A2), which belong to the Cytochrome P450 superfamily, while these genes remained almost unchanged in fed corals (HLF). Other genes of redox regulation, DNA damage response, molecular chaperones, and protein degradation were also up-regulated in HLS corals, presenting higher bleaching, and strong decrease of the photosynthesis performance compared to HLF corals. Several pivotal genes associated with the calcification apparatus such as carbonic anhydrases, calcium-transporting ATPase, calcium channel subunit, and bone morphogenetic proteins (BMPs), were significantly down-regulated only in HLS corals. A parallel decrease in the calcification rates of these later corals was also observed. All together, these results show clearly that heterotrophy helps preventing oxidative stress in corals, and thus avoid the cascade of metabolic problems downstream this stress. 4-condition experiment: low light-unfed/starved (LLS), low light fed (LLF), high light unfed/starved (HLS) and high light fed (HLF) treatments on Red Sea coral Stylophora pistillata. Data from four to six biological replicates and one to two technical replicates for each biological one, were used for each treatment.