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Meeting the photosynthetic demand for inorganic carbon in an alga-invertebrate association: Preferential use of CO by symbionts in the giant clam Tridacna gigas

Proceedings of the Royal Society B: Biological Sciences, ISSN: 1471-2970, Vol: 267, Issue: 1442, Page: 523-529
2000
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Article Description

Unlike most marine invertebrates which excrete respiratory CO, giant clams (Tridacna gigas) must acquire inorganic carbon (C(i)) in order to support their symbiotic population of photosynthetic dinoflagellates. Their capacity to meet this demand will be reflected in the C(i) concentration of their haemolymph during periods of high photosynthesis. The C(i) concentration in haemolymph was found to be inversely proportional to irradiance with a minimum C(i) concentration of 0.75 mM at peak light levels increasing to 1.2 mM in the dark. The photosynthetic rate of isolated zooxanthellae under conditions that prevail in the haemolymph at peak light levels was significantly less than the potential P(max) (maximum photosynthetic rate) indicating that zooxanthellae are carbon limited in hospite. This is consistent with previous studies on the hermatypic coral symbiosis. The P(max) was not affected by pH but there was a dramatic increase in the half-saturation constant for C(i) (k (C(i))) with increasing pH (6.5-9.0) and only a small decrease in k (CO) over the same range. These results indicate that zooxanthellae in giant clams use CO as the primary source of their C(i) in contrast to symbionts in corals, which use bicarbonate. The physiological implications are discussed and comparison is made with the coral symbiosis.

Bibliographic Details

W. Leggat; T. A. V. Rees; D. Yellowlees

The Royal Society

Biochemistry, Genetics and Molecular Biology; Immunology and Microbiology; Environmental Science; Agricultural and Biological Sciences

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