Spatial and temporal changes in phosphorus (P) distribution, partitioning and mobility in the benthic cyanobacterial mat (CBM) were evaluated using sequential chemical fractionation. Total P (TP) content was extremely low, ranging from 0.025 to 0.1 mg g−1 DW. Exchangeable and loosely bound P, which we consider to be mainly associated with extracellular polymeric substances (EPS), constituted the most significant proportion of TP (up to 52%, 55 μg g−1 DW), followed by P associated with the authigenic apatites (up to 35% of TP or 18 μg g−1 DW). While we found virtually no exchange of P with the ambient environment, our results show that the partitioning of P forms within CBM is dependent on spatial and temporal fluctuations of physico-chemical parameters, mainly pH and dissolved oxygen. A conspicuous diurnal increase in the reactive, exchangeable and loosely bound P in the top CBM layers was observed. This observation has important ecological implications, as CBM microorganisms therefore have an increased possibility for P “luxury” uptake during the night. This hypothesis is further supported by the fact that P in the organic fraction rises by as much as 53% in the upper layers during the night, indicating some form of cellular uptake. The P-binding potential of EPS also has ecological or biogeochemical consequences and should be considered in stoichiometrical studies where it represents potential danger for great overestimates of cellular P values or the nutritional status of cells.
