THE ROLE OF MICROORGANISMS IN MOBILIZATION AND FIXATION OF PHOSPHORUS IN SEDIMENTS

Cycling of phosphorus (P) at the sediment/water interface is generally considered to be an abiotic process. Sediment bacteria are assumed to play only an indirect role by accelerating the transfer of electron from electron donors to electron acceptors, thus providing the necessary conditions for redox- and pH-dependent, abiotic sorption/desorption or precipitation/dissolution reactions. Results summarized in this review suggest that (1) in eutrophic lakes, sediment bacteria contain as much P as settles with organic detritus during one year, (2) in oligotrophic lakes, P incorporated in benthic bacterial biomass may exceed the yearly deposition of bioavailable P several times, (3) storage and release of P by sediment bacteria are redox-dependent processes, (4) an appreciable amount of P buried in the sediment is associated with the organic fraction, (5) sediment bacteria not only regenerate PO4, they also contribute to the production of refractory, organic P compounds, and (6) in oligotrophic lakes, a larger fraction of the P settled with.organic detritus is converted to refractory organic compounds by benthic microorganisms than in eutrophic lakes. From this we conclude that benthic bacteria do more than just mineralize organic P compounds. Especially in oligotrophic lakes, they also may regulate the flux of P across the sediment/water interface and contribute to its terminal burial by the production of refractory organic P compounds.