For a given set of environmental conditions, the population of algal symbionts in green hydra remains at a constant level, indicating that the host animal exerts a regulatory influence on the size of its symbiont population. The experiments described here investigated those factors which can cause a reduction in the population size of symbionts: exposure to high light intensities, glycerol, darkness and the photosynthetic inhibitor DCMU. The purpose was to gain further insight into the mechanism of host regulation.
Treatment with high light intensity caused the most rapid reduction in algal numbers, and could result in complete “bleaching” in a few days. Glycerol caused a slower marked reduction, with partial “bleaching”. Darkness reduced algal numbers, but did not eliminate symbionts; the effect of DCMU in the light was similar. Glycerol and DCMU accelerated the effect of high light intensity. Electron microscopy of algal and animal tissues showed that high light intensity and glycerol treatment caused the algae to appear degenerate or even disintegrating, but darkness and DCMU only caused minor morphological changes. It is concluded that the mechanism for symbiont regulation is complex, and involves the following separate components: sloughing off host cells containing symbionts; variation in growth rate of host relative to symbiont; regulation of symbiont growth; ejection of symbionts; digestion of symbionts (rare); detection of photosynthetic incompetence; detection of dead cells.