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 act ivities of four enzymes involved in the fixation Of CO2 (carbon reduct ion cycle) for at least the first two days of exponential growth, but did not significantly affect two other enzymes tested. The enzymes whi ch required energy from ATP appeared to be the ones more adversely aff ected, and all enzymes measured recovered some of their lost activity compared to controls as the cultures aged. This recovery was consisten t with initial large decreases in photosynthetic rates followed by som e recovery. Heavy metals caused a significant lowering of two key meta bolites related to the early products Of CO2 fixation while fructose-1 ,6-bisphosphate was not affected. However later products, as well as g lycolate were increased with heavy metals from the beginning. The imme diate products of the enzymes more strongly inhibited by metals were a lso present at their lowest levels, and as cells aged there was also a recovery of metabolite concentrations. These observations are interpr eted in terms of the action of heavy metals directly on enzymes as wel l as on levels of high energy phosphate.