Microbial oxidation of manganese in a North Carolina estuary1

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Abstract
Experiments were conducted with 54Mn(II) to determine the kinetics of particulate manganese formation in seawater from the lower Newport River estuary, North Carolina. Dissolved Mn was rapidly converted into particles at constant rates that ranged from 0.36 to 6.2% h−1 (0.62–3.5 nmol liter−1 h−1), yielding turnover times of the dissolved manganese pool of 0.7–11 d. Dissolved Mn turnover rates increased with temperature (Q10 = 2) up to a maximum at 25°–35°C and also increased with the ratio of particulate to dissolved Mn. These two factors explained most of the variation in the observed turnover rates.Sample deoxygenation reduced the formation of particulate 54Mn by 94% and virtually all the particulate 54Mn formed in natural oxygen‐containing samples could be dissolved by 10 µM ascorbic acid, a strong reducing agent. These results indicated that the formation of particulate 54Mn resulted primarily from the oxidation of Mn(II) to manganese oxides (MnOx). The oxidation rates, however, were much too rapid to be accounted for by abiotic mechanisms, and the rate was reduced by 97% following heat sterilization of the seawater. In addition, the rates conformed to the Michaelis‐Menten enzyme kinetics model with Vmax equal to 1.2 × 10−8 mol liter−1 h−1 and Ks equal to 1.9 × 10−7 M. These findings provide strong evidence that oxidation of Mn in the estuarine samples is microbially catalyzed. This catalysis appears to be instrumental in the rapid redox cycling of Mn and in the scavenging of dissolved Mn onto particles in aquatic systems.