Coral thermal tolerance: tuning gene expression to resist thermal stress

Abstract

The acclimatization capacity of corals is a critical consideration in the persistence of coral reefs under stresses imposed by global climate change. The stress history of corals plays a role in subsequent response to heat stress, but the transcriptomic changes associated with these plastic changes have not been previously explored. In order to identify host transcriptomic changes associated with acquired thermal tolerance in the scleractinian coral Acropora millepora, corals preconditioned to a sub-lethal temperature of 3°C below bleaching threshold temperature were compared to both non-preconditioned corals and untreated controls using a cDNA microarray platform. After eight days of hyperthermal challenge, conditions under which non-preconditioned corals bleached and preconditioned corals (thermal-tolerant) maintained Symbiodinium density, a clear differentiation in the transcriptional profiles was revealed among the condition examined. Among these changes, nine differentially expressed genes separated preconditioned corals from non-preconditioned corals, with 42 genes differentially expressed between control and preconditioned treatments, and 70 genes between non-preconditioned corals and controls. Differentially expressed genes included components of an apoptotic signaling cascade, which suggest the inhibition of apoptosis in preconditioned corals. Additionally, lectins and genes involved in response to oxidative stress were also detected. One dominant pattern was the apparent tuning of gene expression observed between preconditioned and non-preconditioned treatments; that is, differences in expression magnitude were more apparent than differences in the identity of genes differentially expressed. Our work revealed a transcriptomic signature underlying the tolerance associated with coral thermal history, and suggests that understanding the molecular mechanisms behind physiological acclimatization would be critical for the modeling of reefs in impending climate change scenarios.

Figure 1. Temperature profiles of the thermal treatments to which Acropora millepora fragments were exposed.

Non-preconditioned (NPC) treatment with no pre-stress period prior to exposure to 31°C. Pulse –1 (P1) treament was exposed to a 2-day 28°C pre-stress and returned to ambient temperature for 1 week prior to thermal challenge. Sustained –1 (S1) treatment was exposed to 10 days of 28°C prestress. Sustained –2 treatment was exposed to 14 days of 28°C prestress. Pulse –2 (P2) treament was exposed to a 2-day 28°C pre-stress and returned to ambient temperature for 2 weeks prior to thermal challenge. Sustained –2 treatment was exposed to 14 days of 28°C prestress. Ambient control (C) treatment was not challenged with any increase in temperature. This figure expands upon a smaller dataset originally published by Bellantuono et al. .

Figure 2. Symbiodinium density (algal cells per square centimeter).

Resident Symbiodinium densities at 6 sampling times throughout the course of the experiment; days 23, 27, and 29 represent 2, 6, and 8 days of exposure to 31°C. Asterisks indicate group is significantly different from controls (p<0.001, one-way ANOVA with Tukey HSD, n = 4). A portion of the data presented in this figure was analyzed previously by Bellantuono et al. .

Figure 3. Microarray expression data.

Rows A, B, and C represent gene expression from NPC, S1, and C treatments following 2, 4, and 8 days of 31°C, respectively. Left column contains principal component analysis plots of differentially expressed genes. Center column pie chart illustrated the number and trend of differentially expressed genes, with overlaps indicating differentially expressed genes detected across treatments. Column right indicates fold change differences between shared differentially expressed genes NPC and S1 treatments, both in reference to control.