Coral reefs are based on the symbiotic relationship between corals and photosynthetic dinoflagellates of the genus Symbiodinium
, also known as zooxanthellae, and exist within a narrow temperature range. The optimum temperature for adult scleractinian corals is between 25°C and 29.0°C [1
]. As climate change becomes an increasing threat to the biosphere, corals are among the first organisms to suffer from the consequences of global warming [2
]. Heat stress in reef-building corals affects the coral hosts and their algal symbionts, but the relative sensitivity of both to thermal stress is uncertain [3
]. The first visual sign of heat stress to the coral holobiont (i.e. host, symbionts, and associated microorganisms) is bleaching, i.e. the loss of photosynthetic symbionts [4
Studies on heat stress in adult corals have shown that processes such as Ca2+
homeostasis, cytoskeletal organization, cell death, calcification, metabolism, protein synthesis, and heat shock protein activity are affected among others [5
]. Many of the identified genes from these studies code for known stress-responsive proteins that are shared among eukaryotes. Furthermore, it has been shown that an increase in temperature leads to oxidative stress in corals [10
], with evidence pointing towards photosystem II of the algal symbiont as the main source of reactive oxygen species (ROS) [11
Larvae play an important role in coral reef ecosystems as they form the starting point of the bentho-pelagic lifecycle of a coral [14
]. From a molecular and genetic perspective, coral embryos/larvae represent an interesting system, as many species initially lack endosymbionts. Hence, it is possible to measure the effect of temperature on corals without the confounding factor of symbionts and their different physiologies. Studies on coral larvae show that increasing temperatures affect fertilization, embryogenesis, development, survival, and settlement [15
]. However, molecular studies that assess transcriptome-wide changes in gene expression upon increasing temperatures in coral embryos and larvae have not yet been published.
In this study, we exposed newly fertilized azooxanthellate coral embryos of the Caribbean species Montastraea faveolata (Cnidaria, Anthozoa, Hexacorallia) to a range of temperatures: 1) a permissive temperature of 27.5°C that is known to be non-stressful; 2) 29.0°C, which is a normal summer seawater temperature in the Caribbean Sea during the spawning period; and 3) an elevated temperature of 31.5°C, which has been observed during the late summer of bleaching years such as 2005. Transcriptomic changes were assayed with microarrays at 12 and 48 hours after fertilization. Based on our analysis of differentially expressed genes we devised a model that proposes that genes that play a role in system perturbation, system maintenance, and system regulation are affected upon heat stress. This study is the first transcriptome-wide analysis of heat stress in coral embryos and our data provide first insights into the relevant genes and adaptive capabilities of coral embryos in light of projected increases in seawater temperatures.