The effects of climate change on the Acropora aspera holobiont

Corals host a community of microbes, comprised of endosymbiotic algae, Symbiodinium, bacteria, Archaea, and viruses. This coral/microbe assemblage is known as the coral holobiont. The mutualistic nutrient exchange between coral host and Symbiodinium is arguably the most critical and therefore most studied of these associations; energy supplied by the algae contributes the majority of corals' daily energy requirements. The loss of Symbiodinium during coral bleaching can potentially be fatal to the host. Unfortunately, mass bleaching episodes have become more frequent with intensifying global climate change. Anthropogenic CO₂ emissions are increasing the concentration of atmospheric CO₂, which causes a greenhouse effect resulting in elevated sea surface temperature. These anomalously high temperatures cause the holobiont to become stressed and triggers expulsion of Symbiodinium. Concomitant with temperature rise is the acidification of the surface ocean, caused by uptake of atmospheric CO₂. The interactive effects of these two factors on coral holobiont physiology and community dynamics are largely unknown. Symbiodinium in vitro and in hospite exhibit significant photosynthetic dysfunction above 34°C, which coincides with the bleaching temperature of Acropora aspera. Expression of targeted key photosynthetic and stress response genes of Symbiodinium (measured using quantitative PCR) remains unchanged by heat stress, irrespective of symbiosis status (isolated or in hospite) and pCO₂. The stress response of Symbiodinium, therefore, likely relies on some other mechanism, possibly post-translational modification of transcripts or shifts in the metabolome. Conversely, transcript levels of key carbon metabolism genes of A. aspera are significantly altered by a synergistic effect of pCO₂ and bleaching stress, despite relatively small increases in treatment pCO₂ compared to the diurnal fluctuations experienced by these corals in the reef flat environment. This suggests the physiological effects of ocean acidification will be felt by mid-century, much sooner than predictions made for calcifying processes. Although 34°C bleaching stress resulted in mass expulsion of Symbiodinium in both ambient pCO₂ and elevated pCO₂ treatments, there was no apparent change in the bacterial community. This contradicts many studies that have found an increase in Vibrio in bleached corals. These results suggest a relatively static nature of the A. aspera bacterial community, implicating that shifts in the bacterial assemblage do not contribute to the environmental response of this holobiont. The ability of corals to acclimate to changing temperature regimes was demonstrated in A. aspera. Exposure to sub-lethal temperature stress prior to a bleaching-level stress conferred an acclimatory response in Symbiodinium non-photochemical quenching of chlorophyll fluorescence and A. aspera antioxidant and heat shock protein gene expression, which was not observed in repeated bleaching-level stresses. The results of this thesis suggest Acropora aspera transcriptional regulation and Symbiodinium photophysiology are the key factors in environmental response and Symbiodinium transcriptional regulation and bacterial community dynamics play a limited role in temperature and acidification response.