Shifting from mutualism to parasitism under environmental stress: Physiological evidence in symbiotic algae

Background/Question/Methods

Coral reef ecosystems rely on the symbiotic relationship existing between the coral animal and algae from the diverse genus Symbiodinium.  This mutualism enables the structural framework for the entire ecosystem by the translocation of algal photosynthate to the host, providing the majority of the host nutrition and increasing reef-building calcification rates. In the past two decades reefs world-wide have experienced periods of higher than average summer temperatures, leading to significant coral die-offs.  This is largely due to the phenomenon of coral bleaching, where the symbiosis has broken down due to the disruption of photosynthesis. Researchers have found that hosting different symbiont types correlates with variations in coral stress tolerance, and these differences are attributed to variations in algal physiology. To further understand the mechanisms behind these differences, cultures of 5 different types of Symbiodinium (Types A1, B1, B2, E1 and F2) were exposed to elevated temperatures, and changes in algal and oxygen consumption rates were measured.  Growth rates at 26°, 30°, and 32°C were compared among species.  Oxygen consumption rates were measured during acute exposure to temperatures ranging from 25° to 37°C, and compared among species.

Results/Conclusions

Growth rates for all five algal symbiont types were the same at 26°C, one type out of the five showed a decrease in growth rate at 30°C, and three types showed decreases in growth rates at 32°C.  Oxygen consumption rates followed a similar trend, and differences were observed in the temperature at which maximum oxygen consumption rates were reached. Oxygen consumption continued beyond temperatures where beneficial symbiosis is sustained (>32°C), indicating that algal cells were still alive but are likely unable to produce enough photosynthate to support a host. These changes in algal physiology may cause a shift in the relationship between host and symbiont from a mutualism to one more characteristic of a parasitism. These findings will be applied to elucidate the physiological responses of Symbiodinium to stressors associated with climate change and address their role in coral decline.