Environmentally driven transcriptional regulation in a table top coral: the response to a midday low tide

Corals living in back-reef environments regularly experience fluctuations in temperature, pH, and oxygen, yet we do not know what physiological mechanisms allow them to deal with this variation. The back-reef around Ofu Island in American Samoa can have daily temperature fluctuations of up to 6°C and pH can vary by 0.58 units within a day. We used transcriptomics to monitor coral physiology in the reef as the tide changed environmental conditions from day to day. We sampled three colonies of Acropora hyacinthus once a day for 17 consecutive days. Transcriptomes for each day were sequenced and we identified modules of co-expressed genes in the host and symbiont. There are large portions of the A. hyacinthus and Symbiodinium transcriptomes that are co-regulated and stable though time regardless of environment, and have expression level differences between colonies. Interestingly, transcription of these modules is more stable in the host than in the symbiont. We also see transcriptional variability from day-to-day in A. hyacinthus and Symbiodinium. We looked for strong associations between module expression patterns and changes in the environment. One coral transcriptional module had a very significant relationship to temperature, pH, and dissolved oxygen saturation. At the same time, some Symbiodinium genes showed a response to environmental shifts; however, even though coral expression was more stable than Symbiodinium expression across most of the transcriptome, the coral response to the environment was greater than the Symbiodinium response. By looking at gene expression at short temporal scales, we see that corals have quick transcriptomic reactions to tidally driven environmental shifts—demonstrating they are highly responsive to their environment and may be stressed by temporary extremes.