COMMUNITY METABOLISM IN MICROBIAL MATS - THE OCCURRENCE OF BIOLOGICALLY-MEDIATED IRON AND MANGANESE REDUCTION

Community metabolism and nutrient, iron (Fe) and manganese (Mn) cyclin g were examined in two intertidal, marine, microbial mat communities d uring short (4-5 days) incubations in closed, flow-through microcosms. Sediment microcosms were incubated under either light (light-dark cyc les) or dark (continuous darkness) conditions to assess the effect(s) of photosynthetic oxygen production and microalgal activity on nutrien t, Fe and Mn cycling. The effects of chemical redox reactions between reduced sulphur (S), Fe and Mn cycling were examined by blocking sulph ate reduction, and reduced S production, with 25 mM molybdate while in cubating under dark conditions. In light-incubated microcosms, negligi ble fluxes of nutrients (nitrogen and phosphorus) and trace metals wer e observed. A substantial sediment-water flux of reduced Fe (Fe2+) and Mn (Mn2+) was observed in microcosms incubated under continuous darkn ess; highest fluxes were observed in molybdate-amended microcosms. At both sites, biologically-mediated redox reactions accounted for a subs tantial (>50%) portion of the Fe2+ and Mn2+ flux. Both microbial mat c ommunities exhibited similar rates of gross photosynthetic oxygen (O-2 ) production, but dramatically different rates of net benthic O-2 flux . Distinct patterns of net O-2 production and trace metal cycling aros e from differences in either trace metal oxide availability or reactiv ity (mineralogy), organic carbon mineralization rates, or sediment cha racteristics (porosity). Variations in the microbial community respons ible for trace metal cycling could have also contributed to the patter n. The present data illustrate that chemically-mediated redox reaction s between metal oxides and reduced S complicate interpretation of Fe a nd Mn fluxes, underscoring the need to separate chemical and biologica l reactions when attempting to determine the role of biological trace metal reduction in organic carbon oxidation. (C) 1996 Academic Press L imited