Paper Reviewed
Leung, J. Y.S., Doubleday, Z.A., Nagelkerken, I., Chen, Y., Xie, Z. and Connell, S.D. 2019. How calorie-rich food could help marine calcifiers in a CO2-rich future. Proceedings of the Royal Society B 286: 20190757.

Introducing their work, Leung et al. (2019) write that the ability of calcifying organisms "to build calcareous shells or skeletons is predicted to be impaired by ocean acidification." Yet they add that "this classic paradigm of calcification is, however, increasingly challenged by recent evidence showing that some calcifiers are able to maintain or even enhance shell growth and shell strength under ocean acidification." Thus, they contend that more research is required, especially research conducted in natural environments (as opposed to controlled-laboratory conditions) so that trophic interactions between organisms can be examined before the full extent of the future impacts of ocean acidification can be ascertained.

In this regard, the team of six scientists studied the impacts of ocean acidification on an herbivorous calcifying gastropod (Eatoniella mortoni) in its natural environment. Species were collected from a rocky reef habitat at Te Puia o Whakaari (White Island), Bay of Plenty, New Zealand at control sites (pH of 8.1) and naturally acidified sites (pH 7.8) approximately 25 m away that experienced lower pH values due to CO2 plumes emitted from volcanic vents. There was no other differences between the sites other than pCO2 (i.e., seawater pH); seawater chemistry reflected consistent nutrient and mineral concentrations. At each of the sites (two control and two reduced pH) Leung et al. sampled gastropod shell quality (thickness, crystallinity, mechanical resilience and organic matter content) and the nutritional quality (protein, carbohydrate, lipid and energy content) of the turf algae they fed upon. And what did those analyses reveal?

Describing their findings, Leung et al. write that "the energy content of the algae was boosted by CO2 enrichment" such that the algae near the CO2 vents had greater protein and carbohydrate contents, as well as a reduced carbon:nitrogen ratio, which combinations of factors increased the overall nutritional quality of the algae. With respect to shell properties, the authors report that gastropods living in the reduced pH seawater "showed their capacity to adjust shell building to acidified conditions (i.e. production of thicker, more crystalline and more mechanically resilient shells." Continuing, they say that "such mechanically robust shells have higher resilience to breakage and thus help reduce shell damage and even mortality (e.g. due to predators and waves)."

In commenting on their findings, Leung et al. say the CO2-induced enhancement of the nutritional quality of the food consumed by the gastropods compensated for the energetic burden of ocean acidification, enabling the calcifiers to "build energetically costly shells that are robust to acidified conditions." Consequently, they conclude that the results of this natural ocean water experiment "unlock a possible mechanism underlying the persistence of calcifiers in acidifying oceans."