The green hydra symbiosis. VIII. Mechanisms in symbiont regulation

Abstract

The relative amount of symbiotic algae and animal tissue in the European strain of green hydra was altered by changes in illumination and feeding regimes. This indicates that the host can regulate the algal population to different sizes depending on external conditions. For animals maintained in continuous illumination, 12 h light: 12 h dark, and continous darkness, each with thrice-weekly feeding, a highly significant regression of algal volume per digestive cell on digestive cell volume was demonstrated, suggesting that the space available for the algae may be one factor that determines the population size of the algal symbionts. Seven strains ofChlorellaoriginally symbiotic with other invertebrates formed stable associations with the European strain of green hydra; this included one strain (NC64A) which released very little maltose at pH 4-5. Althought the relative amounts of algal and host biomass of these experimental associations were very similar under standard culture conditions, large numbers of cells of strain NC64A were regularly expelled from the host. This suggests that the ability of the host to control the growth rate of its symbionts is related to the alga’s capacity for maltose release. The latter characteristic is also correlated with a sensitivity of growth to acid conditions. Of the five cultured strains of symbioticChlorellaexamined, only the two strains that released substantial amounts of maltose at pH 4-5 failed to grow at pH 4.0 and pH 4.5. It is proposed that the regulation of algal cell division in the natural symbiosis is principally mediated through relatively small and temporary changes in the pH of the perialgal vacuole. At more acid values, photosynthetically fixed carbon is primarily directed towards maltose release and little or no algal growth occurs. At higher pH values, maltose release declines sharply and the carbon becomes primarily directed towards symbiont growth. Such a relatively simple hypothetical model, involving stimulation of symbiont growth by temporary alkalinization of the perialgal vacuole, can explain the observed responses to change in environmental conditions, as well as the relation between the timing of symbiont and host cell division.