EFFECT OF SUBLETHAL CONCENTRATIONS OF ZINC, CADMIUM AND MERCURY ON THE PHOTOSYNTHETIC CARBON-REDUCTION CYCLE OF EUGLENA

In Euglena gracilis Z the heavy metals Zn, Cd and Hg decreased the activities of four enzymes involved in the fixation Of CO2 (carbon reduction cycle) for at least the first two days of exponential growth, but did not significantly affect two other enzymes tested. The enzymes which required energy from ATP appeared to be the ones more adversely affected, and all enzymes measured recovered some of their lost activity compared to controls as the cultures aged. This recovery was consistent with initial large decreases in photosynthetic rates followed by some recovery. Heavy metals caused a significant lowering of two key metabolites related to the early products Of CO2 fixation while fructose-1,6-bisphosphate was not affected. However later products, as well as glycolate were increased with heavy metals from the beginning. The immediate products of the enzymes more strongly inhibited by metals were also present at their lowest levels, and as cells aged there was also a recovery of metabolite concentrations. These observations are interpreted in terms of the action of heavy metals directly on enzymes as well as on levels of high energy phosphate.