Paper Reviewed
Chakravarti, L.J., Buerger, P., Levin, R.A. and van Oppen, M.J.H. 2020. Gene regulation underpinning increased thermal tolerance in a laboratory-evolved coral photosymbiont. Molecular Ecology 29: 1684-1703.

Introducing their study, Chakravarti et al. (2020) note that "coral thermal tolerance limits depend not only on the thermal tolerances of the host animal, but also on that of their symbiotic algae." In the present study, Chakravarti et al. focus on the latter, the thermotolerance of symbiotic algae. More specifically, the team of four Australian researchers "compar[ed] transcriptional responses at ambient (27°C) and bleaching-relevant (31°C) temperatures in a monoclonal, wild-type (WT) strain of Symbiodiniaceae to those of a selected-strain (SS), derived from the same monoclonal culture and experimentally evolved to elevated temperature over 80 generations (2.5 years)." It was their hope that their research might identify genes and functional pathways that could be targeted in future genetic engineering experiments in developing thermally resilient algal symbionts capable of avoiding (i.e., adapting to) temperature-induced coral bleaching stress.

To accomplish their design the scientists cultured cells of the symbiotic algae Cladocopium goreaui in monoculture at 27°C for six months. Thereafter, they increased the temperature in the selected strain in a stepwise fashion to a culture temperature of 31°C over a period of approximately two months (five generations), maintaining the culture at 31°C for a further 80 generations (2.5 years). Upon reaching this point, they authors conducted a reciprocal transplant experiment, subjecting a portion of the thermotolerant culture (the SS strain grown for 80 generations at 31°C) to ambient temperature (27°C) and a portion of the thermosensitive culture (the WT strain grown at 27°C) to bleaching-relevant temperatures (31°C) for 35 days, after which point they conducted transcriptome analyses of the symbiont algae grown under the ambient and transplanted temperature conditions.

And what did the transcriptome analyses reveal?

As described by the authors, "thousands of genes were differentially expressed at a log fold-change of >>8 between the WT and SS over [the] 35 days [of reciprocal] temperature treatment." Specifically, in the bleaching-relevant temperature treatment (31°C) WT cells upregulated genes related to stress response, such as molecular chaperoning, protein repair, protein degradation and DNA repair, while SS cells "exhibited a temporally stable transcriptome response and downregulated many stress response genes that were upregulated by the WT." What is more, Chakravarti et al. report "among the most highly upregulated genes in the SS at 31°C were algal transcription factors and a gene probably of bacterial origin that encodes a type II secretion system protein, suggesting interactions with bacteria may contribute to the increased thermal tolerance of the SS."

Commenting on their findings, the authors write the stable changes in gene expression and phenotype for the SS strain in the 31°C treatment "could result from heritable, epigenetic modifications such as changes in DNA methylation, chromatin modifications and the biogenesis of small noncoding RNAs." And the fact that these heritable, epigenetic modifications were able to occur over a relatively short time period (80 generations or less, or 2.5 years or less) suggests coral symbionts may well be able to adapt/evolve in the short term to rising temperatures and thus confer a degree of thermotolerance upon their coral hosts, helping them to withstand future episodes of temperature-induced coral bleaching. This may especially be the case if scientists act to develop thermally tolerant strains of algal symbionts via genetic engineering experiments, which could be utilized in coral restoration and conservation projects.