Contribution of bacteria to release and fixation of phosphorus in lake sediments

Cycling of phosphorus at the sediment‐water interface is traditionally considered to be controlled by pH‐ and redox‐dependent, abiotic processes, such as formation and dissolution of FeOOH‐PO 4 complexes. In this study, however, a large part of total P in sediments of Lake Sempach, an 87‐m deep eutrophic lake, was estimated to be incorporated in bacterial biomass. Laboratory experiments indicated that sediment microorganisms can rapidly take up and release soluble reactive P (SRP), depending on redox conditions, and that sterilization of oxic sediments decreased their SRP sorption capacity. In an in situ experiment conducted in the lake, bacteria also contributed considerably to SRP fixation when water enclosed within a sediment flux chamber was reoxygenated following anoxia. Moreover, in that experiment and in data sets from several other lakes, anoxic releases of Fe and P from sediments appeared to be partly uncoupled. As part of an ongoing revision of the classical model for P exchange across the sediment‐water interface, these results provide direct evidence that fixation and release of SRP may be controlled partly by redox‐dependent changes in microbial physiology, as well as by production and decomposition of microbial biomass.