Nutrient limitation in the giant clam-zooxanthellae symbiosis: effects of nutrient supplements on growth of the symbiotic partners

Belda, C.A., Lucas, J.S., and Yellowlees, D. (1993) Nutrient limitation in the giant clam-zooxanthellae symbiosis: effects of nutrient supplements on growth of the symbiotic partners. Marine Biology, 117 (4). pp. 655-664.

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The effect of ammonium (5, 10 μM N) and phosphate (2, 5, 10 μM P) on the growth of the giant clam Tridacna gigas and its symbiotic dinoflagellate Symbiodinium sp. was examined. A 3 mo exposure to these nutrients significantly increased the N or P composition of the soft tissues, as reflected in a corresponding change in C:N:P ratio. Furthermore, exposure to N or N + P markedly increased the amount of soft tissue, but P alone did not, demonstrating that increased availability of inorganic nitrogen enhances tissue growth of the clam host. With addition of N, or N + P, there was a significant increase in the total number of zooxanthellae per clam, with a corresponding decrease in chlorophyll a (chl a) content per zooxanthella. However, only with N + P was there an increase in the zooxanthellae mitotic index. The inverse relationship between zooxanthellae number and chl a per zooxanthella is consistent with phytoplankton studies indicating conditions of nutrient-limitation. Furthermore, the unaffected C:N:P composition of the zooxanthellae and their relatively low specific-growth rates (4 to 10%) also suggest that they are nutrient-limited in vivo. In particular, their high mean C:N:P ratio of 303:52:1 indicates that, relative to C, they are much more depleted in P and less in N than are free-living phytoplankton. Furthermore, polyphosphates (phosphate reserves) were undetectable, and the activity levels of acid phosphatase in the zooxanthellae were relatively high and not influenced by the host's exposure to increased P concentrations in the sea water, implicating the clam host in active regulation of P availability to its symbiotic algae. This is strong evidence that N-limitation of clam zooxanthellae is a function of the availability of ammonium to the symbiosis while, irrespective of nutrient levels in sea water, clam zooxanthellae still show characteristics of P-limitation.

Item ID: 33822
Item Type: Article (Research - C1)
ISSN: 1432-1793
Funders: ACIAR, ARC, URG
Date Deposited: 24 Jun 2014 04:37
FoR Codes: 06 BIOLOGICAL SCIENCES > 0608 Zoology > 060807 Animal Structure and Function @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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