Physiological responses to seasonality in five species of subtropical coral

Abstract

Coral reefs have evolved to thrive in warm, tropical seas. However, there are coral communities that survive in the subtropics with high seasonal variability in temperature and solar irradiance. Temperature and light stress negatively affects the highly sensitive symbiosis corals share with the dinoflagellate, Symbiodinium. A breakdown of this symbiosis leads to coral bleaching where the internal population of Symbiodinium diminishes such that the animal host is no longer supplied with necessary energy produced through photosynthesis. Corals can alter their metabolisms to contend with temperature stress such as increasing lipid storage during both hot and cold extremes and acclimating their photochemistry to prevent photoinhibition. Very few studies have examined how corals cope with seasonal variation over large temporal scales with high resolution monitoring. Corals that are acclimatized to seasonal variation may offer insights to how certain species will extend or compress their ranges in the face of ocean warming due to climate change. Hong Kong represents an excellent model for studying how corals will cope with climate change because despite the extreme thermal variations, the region hosts a healthy coral community of over 90 species. In this dissertation I examined how five species of scleractinian corals (Acropora samoensis, Galaxea fascicularis, Montipora calyculata, Oulastrea crispata, and Porites lobata) respond to an annual temperature and irradiance regime in Hong Kong from the perspective of the host, symbiont, and the holobiont. Firstly, I genotyped the dominant species of Symbiodinium in 12 of Hong Kong’s more common coral genera because the functional significance of a coral’s dominant Symbiodinium genotype has been shown to influence symbiosis during temperature stress. To assess annual metabolism I continuously monitored corals in a flow through mesocosm. I regularly measured maximum quantum yield using PAM fluorometry, total lipid content in fractions of both host and Symbiodinium tissues, and oxygen evolution ratios of photosynthesis and respiration. Null hypotheses were that aspects of metabolism would not differ with variation in light and temperature or between species. Dominant Symbiodinium genotypes represented low diversity in that corals hosted only three species belonging to Clade C and one species in Clade D. Thermal performance curves for A. samoensis, G. fascicularis, and M. peltiformis suggested that these species exhibit heightened thermal performance at respective temperatures between the coldest and warmest seasons while O. crispata and P. lobata exhibit linear relationships with increasing temperatures, suggesting that these species are able to thrive even during Hong Kong’s thermal extremes. The scope of this data set is novel because these types of metabolic analyses of five coral species across all seasons within the same year have not been measured with such resolution before. Furthermore, the implications of some corals able to tolerate such fluctuations in temperature and irradiance may extend their biogeographical ranges poleward as oceans continue to warm.