Physiological plasticity preserves the metabolic relationship of the intertidal non-calcifying anthozoan-Symbiodinium symbiosis under ocean acidification

- The effects of OA on an intertidal anthozoan-Symbiodinium were investigated.
- Physiological plasticity maintained the metabolic relationship at pH 7.7 and 7.4.
- At pH 7.4 Symbiodinium density decreased and Symbiodinium productivity increased.
- At pH 7.4 anemone gross photosynthetic and respiration rates were unaffected.
- At pH 6.8 the symbiosis broke down due to death of the anemones.

Ocean acidification (OA) is predicted to have profound effects on the physiological performance of marine organisms. Intertidal organisms may be better able to cope with future OA scenarios, as they tend to exhibit high levels of physiological plasticity and already experience pH levels predicted to occur with future OA. Here we investigated the physiological plastic responses and performance under OA conditions (pH 7.7, 7.4 and 6.8) of an intertidal non-calcifying anthozoan-Symbiodinium symbiosis, using the snakelocks sea anemone Anemonia viridis. We focussed on Symbiodinium productivity as this is the primary physiological feature of the symbiosis in terms of the shared metabolic relationship. Following a reduction in sea water pH to 7.4, Symbiodinium density decreased. However, Symbiodiniumproductivity significantly increased, which may have triggered their expulsion by the anemone. There was also a significant increase in Symbiodinium chlorophyll content which maintained normal anemone chlorophyll levels. As a result, the photosynthetic capacity of the symbiosis was preserved. This along with the unaffected anemone respiration rates indicated that the metabolic relationship was also preserved. Our work has shown that the physiological performance of an intertidal non-calcifying anthozoan-Symbiodinium symbiosis was maintained under OA conditions, due to the fact that both symbionts exhibited high levels of physiological plasticity. It is suggested that physiological plasticity could be an important mechanism enabling sea anemones to be successful in a future high CO2 world.