Amino Acid Content of Zooxanthellae Freshly Isolated from Pocillopora damicornis

Abstract

Total amino-N content and glutamine to glutamate ratios (gln: glu) were determined in zooxanthellae freshly isolated from colonies of the coral Pocillopora damicornis (Linnaeus) incubated in ambient seawater or in seawater supplemented with ammonium to give a final concentration of 20 or 50 uM. Addition of ammonium did not change total amino-N content but did increase gln: glu from 0.25 to 0.47-0.48, suggesting that ammonium was directly utilized by the symbiotic zooxanthellae. Gln: glu in zooxanthellae from corals maintained in seawater "stripped" of ammonium fell to 0.18. Sizes of pools of most free amino acids in zooxanthellae from P. damicornis were roughly two to five times those of zooxanthellae from the temperate sea anenome Anemonia viridis, but the latter, which is not believed to be N-limited, exhibited higher gln: glu ratios. These data indicate that gln: glu is a sensitive measure of the response of symbiotic zooxanthellae to exogenous dissolved nitrogen, but despite an increase in gln: glu when seawater is supplemented with ammonium, it cannot be concluded that individual zooxanthellae are normally N-limited.