Sediments underlying the major costal upwelling systems of the world oceans are hot-spots of modern formation of hydroxyapatites, often associated with benthic communities of large, nitrate-accumulating sulfur bacteria. We studied the association between phosphate release, organic phosphorus mineralization, and occurrence of dense communities of the filamentous sulfur bacteria, Thioploca spp., on the continental shelf off central Chile during the austral summer when high phytoplankton productivity and anoxic bottom water prevailed. Freshly deposited phytodetritus stimulated extremely high sulfate reduction rates, which supported a large Thioploca community of up to 100 g biomass per m2. Effective bacterial sulfide uptake kept the sulfide concentration low, which enabled the accumulation of free iron, thus demonstrating intensive iron reduction concurrent with sulfate reduction. Phosphate released to the pore water reached 100-300 μM peak concentrations within the uppermost 0-5 cm and phosphate was lost to the overlying anoxic water column. The large phosphate release was not directly related to the presence of Thioploca but was rather the result of a high deposition and mineralization rate of fresh organic detritus. Although the pore water was super-saturated with respect to hydroxyapatite, this mineral was only a minor P-component in the sediment. Most solid-phase phosphate was bound to iron.