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

KURIHARA Haruko, SHIKOTA Tomoaki

doi:10.3755/galaxea.20.1_19

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 Ω_arag 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.

Impact of increased seawater <i>p</i>CO<sub>2</sub> on the host and symbiotic algae of juvenile giant clam <i>Tridacna crocea</i>

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