The green hydra symbiosis. III. The biotrophic transport of carbohydrate from alga to animal

Abstract

Pulse and pulse-chase experiments were used to study the movement of photosynthetically fixed14C from alga to animal tissue in green hydra. The pool of mobile carbohydrate (maltose) was small, turned over rapidly and was preferentially fed with newly fixed carbon. Transport between the symbionts could also occur in darkness, and in this case the mobile maltose pool was fed from algal polysaccharide reserves. The specific activity of the maltose released by freshly isolated algae varied considerably according to experimental conditions. Cultured symbionts from European hydra do not release maltose and homogenates of animal tissue were ineffective in restoring this property. Contaminating host tissue could not be completely removed from freshly isolated symbionts during their isolation, and the possible effects that this may have on maltose release are considered. [14C]maltose received from the alga is rapidly metabolized in animal tissue. All the major animal fractions incorporate14C, with most accumu­lating in glycogen. Under standard conditions, green hydra have higher glycogen: protein ratios than aposymbionts and starvation causes an immediate utilization of glycogen in both forms. There was no good evidence that the animal can regulate carbon trans­port according to its requirements. Preliminary experiments suggest that the degree of transport is determined primarily by the amount of surplus photosynthate available to algal symbionts and not by any capacity of the animal to regulate algal release.