Award: OCE-1220478

This study examined the effects of ocean acidification, the decline in pH that results as increasing levels of anthropogenic CO2 in the atmosphere equilibrate with the surface layers of the ocean. The first deep-water measurements of pH and aragonite saturation state (a measure of the relative availability of the mineral that corals use to make their skeleton) revealed that many deep-sea corals are living very close to the limits of what they can tolerate. The principal aim of this project was to characterize the response of the deep-water coral Lophelia pertusa to the effects of ocean acidification. The first component of this work involved continuing our monitoring of ongoing ocean acidification in the deep Gulf of Mexico. Here, corals were collected and preserved at depth to examine their current physiological state and to compare differences in gene expression from sites with different aragonite saturation states. The second component of this project was to transplant live corals to the laboratory and maintain them in recirculating aquaria as part of a series of controlled experiments to determine the physiological (e.g., survivorship and growth) and gene expression responses to known aragonite saturation states. All together, this study provided our first comprehensive view of ocean acidification and deep-sea corals in US waters. Since the initiation of this award, we have conducted several laboratory experiments on Lophelia pertusa. Using corals collected on research cruises prior to the start of the study, we completed a series of short-term (2 weeks) experiments on L. pertusa at a range of pH values. These experiments showed that different individual corals had different responses to low pH. We also included respirometry, growth, and feeding, and one of the graduate students on the study traveled to Sweden to complete the same experiments on the L. pertusa colonies from those waters. Surprisingly, corals from these populations responded very differently to lower pH, with the Gulf of Mexico corals slowing their metabolism and reducing feeding and growth rates, while the corals from the Tisler reef in Sweden elevated their feeding rate to provide the resources necessary to maintain calcification at low pH. To assess whether L. pertusa is capable of acclimating to acidified conditions over longer time scales, we completed a six-month experiment to monitor changes in calcification and gene expression under control (pHT=7.9, Ωarag=1.5) and highly acidified (pHT=7.6, Ωarag=0.8) conditions. In 2014, we collected fresh coral colonies from the Gulf of Mexico for use in this experiment, and established two large experimental aquaria capable of being precisely controlled for temperature (±0.5°), salinity (±0.5 psu), pH (±0.04), and total alkalinity (±50 µmol kg-1). The corals were fragmented and six different genotypes were placed in each tank. The most significant finding of this study was that different coral colonies (genotypes) responded differently to lower pH and saturation state, providing a genetic basis for resilience to OA and the potential for adaptation. This was corroborated by the RNAseq results, which revealed the greatest variability among genotypes rather than among pH treatments. These results provide insight into the energetic mechanisms that currently allow L. pertusa to calcify at low saturation states, as well as its long-term potential for growth and survival under future acidification scenarios. This study, representing the first characterization of the carbonate system in the deep Gulf of Mexico, has greatly expanded our understanding of the genetic and physiological mechanisms underlying the response of scleractinian corals to ocean acidification. The discovery of a significant genetic basis to the response to low pH and saturation state may be the most significant single finding of the project. This is a subject that is still being debated in the community, but the results of our series of experiments clearly show that, for this species anyway, there is ample genetic variability to provide an adaptive capacity to deal with ongoing ocean change. This study also represents the first published transcriptome of the "lab rat" of deep-sea corals, Lophelia pertusa, and the first to examine changes in gene expression with altered pH. The major finding here was the significant difference in the transcriptomic response of the different genotypes, mirroring the differences in the physiological responses measured among the same genotypes. The identification of the genes underlying these responses will inform future research on shallow-water corals, since this system has the advantage of removing interference from the eukaryotic zoothanthellae that complicate the analysis of these data in scleractinian corals that rely on photosynthesis. We have also collaborated on a documentary film about ocean acidification and deep-sea corals, which provided uniquely personal views of the inner workings of a large oceanographic research cruise. This film, "Acid Horizon" is currently in the application stages for major film festivals around the country and internationally and should appear in one or more of these venues in 2018. Last Modified: 10/25/2017 Submitted by: Rob J Kulathinal