The contribution of stress-tolerant endosymbiotic dinoflagellate Durusdinium to Pocillopora acuta survival in a highly urbanized reef system

Urban coral reefs are regarded as marginal communities that live under localized conditions considered detrimental for coral survival, such as high sediment load. They are also impacted by global environmental changes, especially increases in sea surface temperatures. These conditions can cause sub-optimal performance and may lead to dissociation of the mutualistic symbiosis between the coral host and its endosymbionts (Symbiodiniaceae), which provide the majority of the coral's daily energy budget. While recent studies have explored gene transcriptional responses to extreme conditions using cultured cells of Symbiodiniaceae, it is generally unknown how their responses manifest in hospite. Here, we investigate differential gene expression of endosymbionts hosted by the common reef-building coral Pocillopora acuta, following separate and combined exposures to two major environmental stressors: heat and sediment. We report that Durusdinium largely dominate the Symbiodiniaceae population in P. acuta, which suggests that the observed differential gene expression patterns are mainly responses from this known stress-tolerant endosymbiont genus. Differentially expressed genes were detected in response to heat, and to combined heat and sediment. These genes are associated with various biological processes including apoptosis, cell proliferation, cell-extracellular matrix adhesion, DNA damage repair, lipid catabolism, and lipid homeostasis. Our study provides valuable insights regarding the role of gene regulation by the endosymbiotic dinoflagellates to help maintain health and function of the coral host, which ultimately contributes to the persistence of P. acuta in Singapore's highly urbanized reefs.