Biogenic origin and clay-assisted sink of ‘visible phosphorus’ in a shallow, calcareous lake

Abstract Phosphorus (P) released from calcareous lake sediments causes harmful algal blooms but neither the mineralogical speciation of P, nor the mechanisms of its mobilization are well understood. Lake Balaton is a model for a shallow, large lake with a silty sediment dominated by carbonate minerals. To study particulate P speciation, we used scanning transmission electron microscopy (STEM), targeting ‘visible P’, i.e., particles in which the concentration of P exceeded the detection limit (∼0.2 wt% P) of energy-dispersive X-ray spectroscopy (EDS). We used electron diffraction and high-resolution imaging to identify various crystalline phosphate phases. Three main types of P-rich nanoparticles were identified in the lake, including (1) intracellular, amorphous polyphosphate (polyP) particles in algal cells; (2) extracellular amorphous nanoparticles near cells; and (3) amorphous and crystalline calcium (Ca) phosphates (tricalcium phosphate and apatite) in the sediment. ‘Visible P’ particles in the sediment adhered to the clay mineral smectite. Key aspects of the hypothesized transformations of P-rich particles in the lake were simulated and confirmed experimentally, outlining a pathway of P-rich particles: intracellular polyP is released upon cell lysis, then partly hydrolyzed, partly stabilized on clays. Thus, whereas part of the polyP pool is recycled by microorganisms in the water column or in the sediment, the association of apatite with the clay mineral smectite probably represents a long-term P sink. This biogenic-inorganic link may be a key but overlooked aspect of P cycling in lakes.