Biotic uptake and release of phosphorus by a wetland sediment

Currently there is considerable controversy about the role of bacteria in t he direct uptake and release of phosphorus by sediments. This paper reports work aimed at testing the hypothesis that microbial uptake and release of phosphorus by sediments is an important process in wetlands and that this p rocess is influenced by the type of organic matter in the sediment. Experim ents were undertaken with a wetland sediment to which was added both a carb on source (glucose or acetate) and orthophosphate (P). The carbon sources w ere chosen to represent a simple available carbon source (glucose) and a fe rmentation product (acetate). Control experiments were also undertaken usin g a sterilised sediment with P added and an unsterilised sediment without a dded P. Under aerobic conditions, 100% of the added orthophosphate was take n up biologically by both the organic carbon-amended. sediments, with 34-45 % of this estimated to be microbial uptake. A smaller amount (ca. 9%) of th e added phosphorus was taken up biotically by sediments that had no added c arbon. When these aerobic uptake experiments were made anaerobic land addit ional organic carbon added), only the acetate-amended sediment released an appreciable amount (ca. 30%) of the initially added phosphorus. We postulat e that the acetate-amended sediment took up P for cell growth and also stor ed some P as polyphosphate (poly-P). This poly-P was then released under an aerobic conditions. The glucose-amended sediment used the added P for cell growth only, and this phosphorus was not released when the sediment was mad e anaerobic for a period of 10 days. These preliminary results suggest an i mportant additional role for sediment bacteria in the cycling of phosphorus . In the presence of bioavailable organic carbon (e.g. glucose) and orthoph osphate, sediment bacteria can grow well under aerobic conditions but do no t seem to release the phosphorus under anaerobic conditions (at least not w ithin the 10 days of the experiments reported here). However, in the presen ce of acetate, it appears the 'luxury uptake' of added phosphorus can occur under aerobic conditions, and that this poly-P is released when the sedime nts are made anaerobic. More work is needed to determine the relative impor tance of these microbial uptake and release mechanisms in different sedimen t environments.