Paper Reviewed
Ventura, P., Jarrold, M.D., Merle, P.-L., Barnay-Verdier, S., Zamoum, T., Rodolfo-Metalpa, R., Calosi, P. and Furla, P. 2016. Resilience to ocean acidification: decreased carbonic anhydrase activity in sea anemones under high pCO2 conditions. Marine Ecology Progress Series 559: 257-263.

Writing as background for their work, Ventura et al. (2016) say that "non-calcifying photosynthetic anthozoans have emerged as a group that may thrive under high carbon dioxide partial pressure (pCO2) conditions via increased productivity," yet they add that "the physiological mechanisms underlying this potential success are unclear."

Hoping to therefore provide some clarity on the subject, the international team of eight scientists set out to investigate the impact of high pCO2 (i.e., ocean acidification) on the dissolved inorganic carbon (DIC) use in a sea anemone (Anemonia viridis). It was their hypothesis that the increase in seawater pCO2 would cause a decrease in carbonic anhydrase (an enzyme involved in the energy-demanding process of DIC absorption) activity, as "anemones would need to rely less on carbonate ion concentration as their primary DIC source" in CO2-enriched waters.

To accomplish their objective, Ventura et al. examined rates of net photosynthesis, chlorophyll a (chl a) content, Symbiodinium density and carbonic anhydrase (CA) activity in A. viridis specimens living along a CO2 gradient near natural volcanic vents around the island of Vulcano, Italy. In addition, they conducted a short-term laboratory experiment in which they exposed A. virdis organisms to three weeks of either control (pH of 8.01) or high pCO2 (pH of 7.59) conditions and measured the same parameters.

Results of the field experiment revealed that anemones growing in normal (pH of 8.13) vs high pCO2 waters (pH of 7.71) maintained a similar chl a content and Symbiodinium density. CA activity, however, was "approximately 30% lower in anemones exposed to higher pCO2 conditions." In the laboratory experiment, chl a and net photosynthetic rates were not affected, but Symbiodinium density and CA activity were both reduced, the latter by 78 percent by the end of the 21-day experiment.

The finding that CA activity was decreased at high pCO2 in both the field and laboratory environment supports the authors' initial hypothesis that, in a high pCO2 environment, CO2 replaces carbonate ion concentration (HCO3^-) as the main carbon source for photosynthesis, which leads to a decrease in energy investment in the organisms' CO2-concentrating mechanisms. This situation allows for increased energy to become available for other functions such as organism growth and reproduction, allowing A. viridis to survive and even thrive under future projections of ocean acidification. Indeed, as concluded by Ventura et al., "this study showed that being able to shift the utilization of different inorganic carbon species toward energetically most favorable ones (i.e., from HCO3^- to CO2) is a key feature enabling non-calcifying photosynthetic anthozoans to tolerate and thrive under long-term exposure to high pCO2 levels, but also to allow rapid (3 wk, from this study) acclimation to future predicted CO2 conditions."