Inorganic nitrogen and phosphorus nutrition in tridacnid clams and their algal symbionts

Belda, Carmelita A. (1994) Inorganic nitrogen and phosphorus nutrition in tridacnid clams and their algal symbionts. PhD thesis, James Cook University of North Queensland.

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This study examined inorganic nutrition in tridacnids to address the question of nutrient limitation of their zooxanthellae, and the influence of elevated nutrient concentrations on calcification. Experiments consisted of three-month-long exposures of cultured Tridacna gigas in outdoor tanks and indigenous T. maxima in situ to increased levels of ammonia (N) and phosphate (P), alone or in combination (N+P). Following the incubation experiments, biomass and nutrient-depletion of the symbiotic partners were measured. In addition, a more detailed study was carried out on the effect of elevated P on the zooxanthellae, both in situ and in culture.

Major findings from the outdoor experiments were confirmed in the field study, while laboratory experiments elucidated host involvement in nutrient limitation of the zooxanthellae. In particular, zooxanthellae are N-limited in vivo, as demonstrated by the increased zooxanthellae density in N-supplemented clams. The decrease in ammonium-depletion by zooxanthellae isolated from these clams demonstrates that zooxanthellae have access to increased N in sea water. N-limitation of zooxanthellae, therefore, is a function of sea water concentrations of inorganic N, particularly ammonia, as this nutrient can diffuse across cell membranes depending on its concentration gradient.

Also, zooxanthellae are P-limited in vivo. Regardless of ambient phosphate concentrations in sea water, zooxanthellae inside the animal host were not affected, and they exhibited N:P ratios (>30:1) and P-depletion rates similar to those of P-starved cultured zooxanthellae. Host involvement in P availability to the zooxanthellae is strongly implicated by: (1) the unaffected acid phosphatase activity (P-liberating enzymes) and undetectable polyphosphates (P reserves) in zooxanthellae, regardless of the clams' P environment; and (2) rapid growth, decreased N:P ratio, and decreased P-depletion by zooxanthellae grown under P-sufficient culture conditions. Host influence on the algae's P environment in vivo is further substantiated by the consistently low levels of inorganic phosphate (< 0.1 pM)) in the haemolymph surrounding the zooxanthellal tubes, despite the clams' uptake and assimilation of P from sea water (i.e., decreased N:P ratio). P-limitation of zooxanthellae, therefore, is independent of ambient sea water concentrations of phosphate, which cannot passively cross cell membranes. The host's role in P availability to its zooxanthellae is either: (1) a natural consequence of the morphological and spatial relationships between the symbiotic partners, with the host retaining phosphate for its own use before the nutrient can reach the zooxanthellae; or (2) through active P restriction by the host to control its algal population.

Investigation of calcification demonstrated that increased ambient levels of N and P modified shell formation in T. gigas as follows: (1) enhanced shell-extension rates, but reduced shell weights at equivalent size; (2) changes in crystal lattice parameters based on X-ray diffractometry on the shells; and (3) structural alterations in the outer shell layer, with misshapen aragonite crystals, irregular crossed-lamellar orientation, and increased porosity. Such skeletal weakening has adverse implications for calcifying organisms in eutrophic reef waters.

Item ID: 33762
Item Type: Thesis (PhD)
Keywords: calcification; giant clams; growth; nutrition; symbionts; symbiosis; Tridacna gigas; tridacnids; zooxanthellae
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Belda, C.A., Cuff, C., and Yellowlees, David (1993) Modification of shell formation in the giant clam Tridacna gigas at elevated nutrient levels in sea water. Marine Biology, 117 (2). pp. 251-257.

Belda, C.A., Lucas, J.S., and Yellowlees, David (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.

Date Deposited: 08 Jul 2014 00:48
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 30%
06 BIOLOGICAL SCIENCES > 0608 Zoology > 060806 Animal Physiological Ecology @ 70%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 50%
96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
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