The green hydra symbiosis and ammonium. I. The role of the host in ammonium assimilation and its possible regulatory significance

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Abstract
Evidence for ammonium assimilation by host and symbiont in algal-invertebrate symbioses is summarized and critically evaluated. The host from all strains of hydra studied possessed glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activities. The host from associations with high maltose releasing algae (E/E, E/3N8) had high GS and low GDH activities, whereas aposymbiotic animals (EALB) and the association with a low maltose releasing alga (E/NC) had sow GS and high GDH activities. The observation that symbiotic animals do not release ammonium in the light, whereas aposymbiotic animals release substantial amounts, may be explicable on the basic of variation in the ability of the host to assimilate ammonium. Thus, the photosynthetic inhibitor DCMU had no effect on ammonium release by symbiotic animals, with the possible exception of E/NC. Methionine sulphoximine (MSO) completely inhibited GS activity from EALB both in vitro and in vivo. In the presence of MSO. ammonium release was enhanced in both EALB and E/E. In continuous darkness, an increase in ammonium released by symbiotic animals (E/E) was correlated with a decrease in host GS activity. It is suggested that the evidence is consistent with host and not symbiont assimilation of ammonium. A model of symbiont regulation is proposed based on regulation of ammonium supply as a means of controlling both perialgal vacuolar pH and symbiont nitrogen status.