Investigating physiological variability across different algal and cnidarian symbioses:possible implications for climate change
Date
2016
Authors
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Publisher
University of Delaware
Abstract
The unique and mutualistic symbioses between scleractinian corals and the dinoflagellate algae Symbiodinium spp. is critical to the overall success and continual growth of many reef corals worldwide. Unfortunately, these symbioses are susceptible to rising oceanic temperature and changes in carbonate chemistry. However, high genetic diversity within the host and symbiont suggests their responses may vary in a species-specific manner, potentially forming coral climate change ‘winners’ and ‘losers’. Here I initially identified potential interactive effects between elevated temperature and pCO2 concentration on the biochemical composition (protein, carbohydrate and lipid content) of the host and symbiont portions within four Pacific coral species and their respective symbionts. Temperature was the principle driver of physiological change and each host + symbiont combination responded to the stress differently, as greater change in biochemical composition was noted within the more thermally tolerant symbioses (M. monastrea and T. reniformis). I extended the question of interactive effects between independent variables by including nutrient concentration as a factor, along with temperature and pCO 2, focusing only on the coral T. reniformis with its symbiont S. trenchii. Temperature remained the leading factor in driving physiological change as net photosynthesis and cellular chlorophyll a increased with temperature under ambient pCO2, whereas temperature related differences in cellular volume were more pronounced under elevated pCO2. Additionally, increased nutrient concentrations mitigated thermal affects under all pCO 2 conditions and suggest significant interactive effects between temperature, pCO2 and nutrient concentrations.
Given the variability in physiological response to both temperature and pCO2 previously observed, I next focused on a better characterization of the unique symbioses established within each host and symbiont combination, including two non-calcifying and symbiotic species. Specifically, I utilized multiple cnidarian symbioses to ask if symbiont type affects translocation of energy rich photosynthate to the host and if this varies with changes in pCO2 and temperature. Two calcifying scleractinian corals (Montipora hirsuta and Pocillopora damicornis) and one non-calcifying coral (Discosoma nummiforme) were exposed to the individual and combined effects of elevated temperature and pCO2 in order to induce a range of physiological states within each symbioses. An inverse relationship between cellular density and net photosynthesis is observed, as were differences in the ratio of photosynthesis cell-1 to carbon translocation cell-1, which appeared to be dependent on the host+symbiont combination. Because anemones represent one of the few cnidarian species where positive effects of elevated pCO2 have been consistently documented, I also measured carbon uptake and translocation along with asexual reproduction within the anemone Exaiptasia pallida under ambient and elevated pCO2 conditions. Additionally we asked whether physiological differences could be detected at the symbiont sub species level, by infecting the host anemones with different S. minutum genotypes. Elevated pCO2 conditions did increase net photosynthesis, carbon incorporation and asexual budding. Subtle differences were also observed across host/symbiont genotypes, placing functional significance on genotypic variance below the species level. I also had the opportunity to extend our comparison of host + symbiont diversity through field studies conducted in Palau. There I investigated the diversity of response strategies to elevated temperature for six congeneric coral species collected from an inshore rock island habitat and an offshore reef-system. Inshore reef corals harbored different symbiont species than their offshore counterparts and likely played a major role in establishing the greater thermal tolerance observed for colonies collected from the warmer inshore reefs. Host dependent differences in symbiont physiology were also observed and affected the overall response to high temperature.
Although symbiont phenotype can certainly provide a major source of adaptive potential for corals as they combat future climate change scenarios, host physiology also remains an important factor in establishing thermal resistance. As a proxy for phenotypic plasticity within the host coral, I quantified epigenetic modification of cytosine residues within the E. pallida genome in response to elevated temperature and across anemones housing B1 vs. D4-5 symbionts. Clear structure in CpG density across functional gene categories was apparent in both the promoter and gene body regions for E. pallida and changes in methylation status occurred in response to both temperature and symbiont species. Interestingly, the average net increase in methylation status observed between low and high temperature and between B1 and D4-5 symbionts are significantly higher within the promoter region as compared to gene introns and exons and may point to the promoter regions as an important target for epigenetic control through DNA methylation.