The Green Hydra Symbiosis

SUMMARY The Chlorella symbiont of the ‘European’ strain of green hydra has been isolated into pure culture using conventional microbiological techniques. The symbiont grew well on Bold's basal medium, indicating that it had no fastidious nutritional requirements. It grew rapidly on liquid media, but very slowly on agar–which may explain why previous authors have failed to isolate it. The ultra structure of the cultured isolate agrees closely with published descriptions of the symbiont. The cultured isolate could also reinfect aposymbiotic hydra, and a cycle of reisolation and reinfection was followed. The cultured isolate lacked the property of maltose excretion normally shown by symbionts, but this property was restored after reinfection. A hypothesis is advanced to explain the induction of maltose synthesis and release by host animal cells.