MICROORGANISMS AND MANGANESE CYCLING IN A SEASONALLY STRATIFIED FRESH-WATER DAM

This work describes investigations into the role that biotic and abiot ic mechanisms play in the manganese redox cycle in a freshwater dam ov er a twelve month period. Enzymatic control of manganese oxidation was taking place with a temperature optimum of approx. 30 degrees C. Mang anese oxidation was only significant above about 19 degrees C. The tem perature and season play vital roles in determining the extent to whic h abiotic and microbial mechanisms contribute to manganese oxidation. Results showed that microbial catalysis is overwhelmingly responsible for manganese oxidation in the lower epilimnion from November to May. Significant abiotic catalysis (up to 25%) can occur in late summer/aut umn when the water temperature is greatest. Mn(II) oxidation, pseudo-f irst order rate constants to 1.12 x 10(22) M(-4). d(-1) were measured while poisoned sample experiments confirmed the role of biological med iation. In winter, biological control could not occur because of the l ower temperature of the water column. The measurement of ''x'' in MnOx showed that higher manganese oxidation states were expected when the manganese oxidation rate was at a maximum and therefore when microbial activity was greatest. Direct microbial reduction of MnOx in the wate r column was of much less significance. However, indirect reduction ma y have taken place through the reaction of MnOx with sulfide. The resu lts of this work have important implications for the design and operat ion of artificial destratification units for the control of manganese speciation.