Maltose excretion by the symbiotic algae of Hydra viridis

The sym biotic algae (zoochlorellae) of Hydra viridis live inside the gastrodermal cells. When isolated into pure suspension free of animal tissue, zoochlorellae liberate maltose to the medium during photosynthesis. Maltose synthesis and excretion are very sensitive to external pH. At pH 4.0, about 40 to 50 % of the carbon fixed in photosynthesis may be released from the cells as maltose, and a further 4 to 6 % as other compounds (including alanine, glycollie acid, glucose, and an oligosaccharide provisionally identified as maltotriose). As the pH rises, excretion progressively diminishes, and at pH 7.0, only about 1% of the photo synthetically fixed carbon is excreted, about half as maltose. Only traces of maltose are ever found within the cells, and sucrose is always the predominant intracellular soluble sugar. When cells previously labelled with 14 C at pH 7.0 are transferred to non-radioactive media in the dark at pH 4.0, they immediately begin to synthesize and excrete [ 14 C ]maltose; the increase of [ 14 C]maltose is closely correlated with a decrease of 14 C-labelled hexose monophosphates and is not accompanied by any loss of 14 C from the insoluble fraction. This suggests that maltose is synthesized from hexose monophosphates by a process which is not directly light dependent. In short-term photosynthesis experiments at pH 4.0, fixed 14 C appears in sucrose within 20s, but none appears in maltose until 60 s. This, together with the near absence of intracellular maltose and the marked sensitivity of maltose synthesis to external pH, suggests that the mechanism of synthesis is at or near the cell surface. The experimental results were consistent with the hypothesis that maltose synthesis is UDPG -dependent, but direct proof of this was n o t obtained. Although excretion of photosynthetically fixed 14 C at pH 4.0 diminishes in the presence of external maltose, it could still continue at an appreciable rate when the external maltose concentration was as high as 10% (w/v). In 10% maltose media, some of the excreted 14 C was still in maltose, but most was in compounds provisionally identified as maltotriose and maltotetrose, suggesting that a mechanism for transglycosylation may exist on the surface of the cells. Unlike symbiotic zooxanthellae and lichen algae, Hydra zoochlorellae show no signs of losing their ability of excreting carbohydrate during the first 24 h after isolation from the symbiosis. In the case of Hydra , it is suggested that the host might be able to control maltose excretion from its zoochlorellae by variations in the intracellular pH of the gastrodermal cells, but evidence for such changes is still lacking.