Coral reefs constitute some of the most prolific and diverse ecosystems on our planet, but also among the most threatened.

This thesis investigates the effects of environmental stressors on corals’ metabolism and symbiont diversity. Paper I shows that the coral Turbinaria mesenterina withstood a single stressor while a combination of two stressors (decreased salinity and increased seawater temperature) lead to decreased metabolism. Increased seawater temperature in combination with two stressors (enhanced nutrients and decreased salinity) lead to rapid mortality of all specimens. Paper II shows that chronic stress in combination with increased seawater temperature affects coral species differently. Porites lutea did not show any difference in response to temperature increase, regardless of environmental disturbance history, while Galaxea fascicularis’ metabolism was negatively affected in chronically disturbed corals but not in corals from less disturbed areas. The main explanation for the difference in response between the two species is different compositions of endosymbionts as found in paper III. P. lutea only harboured the symbiont C15, regardless of environment, whilst D1a dominated the nearshore G. fascicularis and C1 dominated offshore corals. In paper IV there was a clear inshore-offshore pattern of D1a along the whole coast of Vietnam, where D1a dominated inshore. In contrast, the five symbionts belonging to group C displayed a strong latitudinal gradient, with diversity increasing from north to south. The coral host showed higher diversity offshore than inshore.

The thesis emphasizes the importance of improving water quality (paper I and II) and protecting marginal areas since tolerant coral hosts and symbionts can be found there (paper III and IV), as well as safeguarding areas with high symbiont diversity (paper IV) to increase the ability of corals to withstand future environmental changes.