Impact of increased seawater pCO2 on the host and symbiotic algae of juvenile giant clam Tridacna crocea

Increases in atmospheric CO2 cause decreases in calcium carbonate saturation, which is predicted to affect the calcification process of most marine calcifiers. At the same time, the increase of seawater pCO2 is also known to increase the productivity of primary producers. Giant clams host symbiotic dinoflagellates (‘zooxanthellae’: Symbiodinium spp.) that provide nutrition and use CO2 as their primary source for photosynthesis. This leads to the hypothesis that increased seawater pCO2 rise could positively affect the production of giant clam zooxanthellae, and dampen effects of CO2 on host giant clams. To test this hypothesis, we measured the shell growth rate, photosynthesis rate, respiration rate and zooxanthellae density of the juvenile Tridacna crocea reared under three different pCO2 conditions. Results revealed that negative shell growth of juvenile Tridacna crocea was observed once seawater Ωarag reached less than 2.33. Additionally, although zooxanthellae density in T. crocea increased with seawater pCO2 rise, zooxanthellae productivity did not change, suggesting that the productivity per zooxanthella decreased in high pCO2 seawater. Our findings suggest future seawater pCO2 rise will not increase productivity of zooxanthellae, thus giant clam will be negatively impacted in the coming centuries.

Kurihara H. & Shikota T., 2018. Impact of increased seawater pCO2 on the host and symbiotic algae of juvenile giant clam Tridacna crocea. Galaxea, Journal of Coral Reef Studies 20(1): 19-28. Article.

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