Effects of host feeding and dissolved ammonium on cell division and nitrogen status of zooxanthellae in the hydroid Myrionema amboinense

There is a relationship between host feeding, nitrogen status and mitotic activity of zooxanthellae symbiotic with the marine hydroid Myrionema amboinense. Decreases in the mitotic index of zooxanthellae in starved M. amboinense, and in internal pool sizes of glutamine and glutamate, amino acids involved in ammonium assimilation via the glutamine synthetase-glutamate synthase (GS/GOGAT) pathway, were partially restored by addition of ammonium chloride to seawater in which hydroids were incubated. Levels of glutamine were more sensitive to host starvation than levels of glutamate, resulting in a decrease in the glutamine: glutamate molar ratio to that found in zooxanthellae cultured on nitrate. Hydroids starved for 5 d and then incubated in different concentrations of ammonium chloride showed a positive correlation between ammonium concentration and mitotic index of their symbiotic zooxanthellae. Host starvation caused a decrease in perturbation of levels of glutamine and glutamate during ammonium assimilation, as well as decreases in rates of assimilation of ¹⁴C]-leucine into TCA-insoluble protein, and in photosynthetic incorporation of ¹⁴C]-bicarbonate. These observations suggest that host starvation reduces nitrogen supply to the zooxanthellae, causing nitrogen stress to the symbionts and reduction in metabolic processes associated with nitrogen assimilation and photosynthesis as well as with cell division.