ELSEVIER Marine Chemistry 46 (1994) 133-152 Photoreduction of manganese oxides in seawater William G. Sunda, Susan A. Huntsman Beaufort Laboratory, National Marine Fisheries Service, Beaufort, NC 28516, USA (ReceivedDecember 18, 1992;revisionacceptedJune 24, 1993) Abstract Experiments were conducted on the photoreductive dissolution of 54Mn-labeled synthetic oxides, prepared from MnO 2- oxidation of 54Mn(II), and natural labeled oxides formed in seawater from microbial oxidation of 54Mn(II). Sunlight increased the dissolution rate of synthetic oxides in seawater, an effect that increased with the duration of light exposure. The photodissolution of these oxides was found to result primarily from Mn reduction by H202, produced in seawater from the photoreduction of 02 by dissolved organic matter. This conclusion was based on the previously observed marked stimulation of photodissolution by added hun'tic compounds, the observed reductive dissolution of the oxides by added H202 and on the almost complete reversal of photodissolution by enzymatic (catalase) removal of H202. Sunlight had an even larger stimulatory effect on the reductive dissolution of 54Mn-labeled natural oxides. It increased specific dissolution rates to values of 6-13 % h-l, 6-70 times higher than rates in the dark. In contrast to synthetic oxides, rates for natural oxides did not increase measurably with the duration of light exposure, were not appreciably altered by humic acid addition or by photolytic removal of natural organic matter, and were not substantially reduced by catalase addition. Furthermore, rates for reductive dissolution of natural oxides by H202 were only about 1/6th of those for synthetic oxides. These results indicate that the photoreductive dissolution of natural oxides in seawater is not primarily related to the photoproduction of H202, although such production appears to account for a small portion (ca. 10-20%) of the overall effect. Instead, both the chromophore and the reductant(s) involved in the reaction appear to reside with the bacterial/Mn oxide aggregates themselves. Although several possibilities can be postulated, the exact mechanism of the photochemical reaction remains obscure. 1. Introduction Manganese is the eleventh most abundant element in the Earth's crust (Glasby, 1984) and is an essential micronutrient for all organisms. It exists in natural waters as insoluble Mn(III and IV) oxides and as soluble Mn(II) ions. The relative balance between oxidation of Mn(II) and reduction of Mn oxides largely controls the solubility of man- ganese in seawater, and thereby regulates its removal via particulate scavenging processes. 0304-4203/94/$07.00 SSDI 0304-4203(93)E0062-4 Although Mn(II) is thermodynamically unstable with respect to oxidation by 02, slow oxidation kinetics combined with oxide reduction by organ- ics (Stone and Morgan, 1984) and other reductants permit the majority of Mn in oxygenated seawater to exist as soluble Mn(II). Purely chemical rates of oxidation are exceedingly slow in seawater (Sung and Morgan, 1981), and there is substantial evidence that most, if not virtually all, marine Mn(lI) oxidation is bacterially mediated (Emer- son et al., 1982; Tebo et al., 1984; Tebo and