Ammonium Metabolism in the Green Hydra Symbiosis

Inhibitors of enzymes of ammonium assimilation were used to test if assimilation of ammonium in the green hydra-Chlorella symbiosis was due to host or symbionts. Both methionine sulphoximine (MSX, an inhibitor of glutamine synthetase, found in both host and symbionts) and azaserine (AZS, an inhibitor of 2-oxoglutarate amido transferase, not found in the host) inhibited ammonium uptake by the intact symbiosis. MSX was taken up and caused predictable changes in pools of glutamate and glutamine in both freshly isolated symbionts and cultured ex-symbiotic Chlorella. However, after treatment of the intact symbiosis with MSX, no MSX was found in the symbiotic Chlorella, and glutamine and glutamate pools of both host and symbionts were unaffected. Although both MSX and AZS inhibited ammonium uptake by Chlorella, MSX caused seven times as much ammonium release from the intact symbiosis as did AZS. AZS treatment of the intact symbiosis caused an increase in glutamine pools in both host and symbionts, and AZS also competitively inhibited glutamine uptake by Chlorella. Further, ammonium treatment of intact hydra did not affect the nitrogen status of the algal symbionts, although it did cause a small increase in the number of algae in each digestive cell of the host. It is suggested that primary ammonium assimilation in the green hydra symbiosis occurs by means of animal glutamine synthetase, and that the resulting glutamine may be taken up and further processed by the symbiotic algae. Freshly isolated symbionts were able to process glutamine into glutamate even when incubated at low pH, which causes them to release a substantial proportion of fixed carbon as maltose.