The green hydra symbiosis and ammonium. II. Ammonium assimilation and release by freshly isolated symbionts and cultured algae

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Freshly isolated symbionts from the European strain of green hydra containing native (E/E) or heterologous algae (E/3N8, E/NC), and the Wytham strain of green hydra (W5) assimilated ammonium at pH 7 in light. Both nitrogen-replete and nitrogen-starved cultures of high (3N813A) and low (NC64A) maltose-releasing strains ofChlorellaalso assimilated ammonium at pH 7 in light. However, at pH 4, freshly isolated symbionts from E/E, E/3N8 and W5, and nitrogen-replete cultures of the high maltose-releasing strain 3N813Areleasedammonium, and the rate of release was stimulated in darkness. Freshly isolated symbionts from the association E/NC released ammonium at pH 4 when incubated in darkness but assimilated ammonium in light. Nitrogen-starved cultures of both high and low maltose-releasing strains assimilated ammonium at pH 4 in both light and dark. Ammonium-assimilation characteristics of nitrogen-starved cultures were sufficiently different from those of freshly isolated symbionts to indicate that symbionts are not maintained by the host under nitrogen deficiency at high pH. A detailed model of symbiont regulation is proposed thatsuggeststhat the ammonium compensation point (defined as the pH at which there is no net release or assimilation of ammonium) is important as a homeostatic mechanism for maintaining high rates of maltose release in light and as a mechanism for controlling both symbiont cell division and changes in algal number per digestive cell with changes in environmental conditions. Experimental evidence consistent with the model is presented. Nitrogen-replete cultures of the high maltose-releasing strain 3N813A decreased medium pH during ammonium assimilation and increased medium pH during ammonium release. Furthermore, dark-grown animals of the association E/E released ammonium when transferred to a 12 h light : 12 h dark régime and release was stimulated by the photosynthetic inhibitor 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea.