Stress Resistance in an Extreme Environment: Lessons Learnt from a Temperate Symbiotic Sea Anemone
Coral bleaching, the loss of symbiotic dinoflagellates (zooxanthellae) or their photosynthetic pigments in response to environmental stress, is of huge global concern. In contrast to tropical corals, which are highly sensitive to fluctuations in environmental parameters such as temperature, light and salinity, zooxanthellate invertebrates in temperate waters rarely bleach despite highly variable conditions. In this study, we tested the effects of salinity with combined effects of light and temperature stress on the photophysiology and stability of the temperate symbiotic sea anemone, Anthopleura aureoradiata, through chlorophyll fluorescence. In the field it was demonstrated that A. aureoradiata was resilient to abiotic fluctuations of considerable magnitude in the intertidal zone. Salinity was revealed to range naturally between a winter low of 30 and summer high of 40 ppt in an elevated tide pool with no measurable effects on the photophysiology of A. aureoradiata residing within. In a controlled environment, only extreme high and low salinities had an effect on the zooxanthellar photosystem, with a wide range of tolerance between 15-50 ppt dependent on the levels of temperature and light. Both high and low light, and temperature, also impacted upon photophysiology. Moreover, each of these variables independently, as well as combined, exacerbated the impact of salinity stress. In addition, the duration of exposure played an important role in the survival of this symbiosis, with only 48-96 h exposure to the extreme salinities of 5, 10, 55 and 60 ppt inducing irreversible photosynthetic failure, bleaching and death. Thus, the data supports the idea that this anemone-zooxanthellar symbiosis is highly resilient to considerable amounts of abiotic stress, a likely a function of the robust photophysiology of its zooxanthellae. This resilience to bleaching suggests that A. aureoradiata and its zooxanthallae have evolved a combination of powerful defensive mechanisms to help aid against the heterogenous environment from which they come. I will present an overview of these osmoregulatory mechanisms, photoacclimatory strategies and behaviours that this symbiosis likely deploys in order to combat environmentally realistic ranges in abiotic factors. Further studies would be necessary to deduce whether it is the host or zooxanthellae which are responsible for the breakdown of this symbiosis.