Fitness and energetic consequences of introducing reef-building corals to reefs with distinct seawater pH regimes

Environmental variability is predicted to promote stress tolerance by selecting for phenotypic plasticity within populations, yet the influence of environmental pH dynamics on coral resilience to acidification and thermal stress is not well understood. Here we tested the phenotypic performance of 20 thermally resilient genotypes of each of two major reef-building coral species in Hawaii following reciprocal transplantation between two reefs with either relatively stable vs. variable diel seawater pH dynamics. Key performance indicators included survivorship, calcification, biomass, metabolism, reproductive output, and acute thermal stress response. Corals showed rapid physiological acclimatization to the local reef environment, matching performance of native genets for traits ranging from growth and survival to tissue biomass and metabolic rates. Site of transplantation significantly impacted coral performance, with both species showing higher overall fitness at the variable pH reef regardless of site of origin. This difference is potentially due to higher flow rates at the variable reef, which may be stimulating coral metabolic activity by increasing nutrient delivery and waste removal. Interestingly, corals did not show evidence of local adaptation, and were in fact maladapted to the stable pH reef. We also observed genotype-specific responses within species, with genets exhibiting a range of phenotypes within each site, and only some genets exhibiting phenotypic plasticity. Interestingly, acclimatization did not result in tradeoffs with thermal tolerance, as the acute thermal stress response of both species was not affected by acclimatization to a novel pH regime. Our ongoing work is testing how environmental pH history influences coral intracellular acid/base homeostasis during thermal stress as we begin to elucidate the cellular mechanisms driving these observed differences in coral performance.