Ejection of algae in the green hydra hydra viridissima symbiosis
Journal of Experimental Zoology 217(1): 23-32
Green hydra appears to regulate strictly the symbiotic algae within its digestive cells. Numbers of algae in the Frome and Jubilee strains were not affected by either sulfate enrichment of the medium or starvation, although numbers of algae fell when photosynthesis was inhibited by darkness and/or DCMU [3-(3,4-dichlorophenyl-1)-1-dimethyl urea] due to cessation of algal growth. Treatment with glycerol or the antibiotic trimethoprim caused ejection of algae; trimethoprim, but not glycerol, also inhibited algal growth. Glycerol and trimethoprim caused an increase in numbers of algae at the apical ends of digestive cells, and appearance of cohesive algal pellets in treated hydras' coelenterons and in the culture dish; in control hydra and those treated with darkness or DCMU, fewer algae were at the apexes of digestive cells, and no algal pellets were produced. Ejection of algae is not the normal regulatory mechanism in the green hydra symbiosis. The fact that certain treatments may cause algal ejection should be taken into account when algal proliferation is measured in green hydra.