Making waves: The expanding role of polyphosphate-accumulating organisms in internal phosphorus cycling in freshwater...

Internal phosphorus (P) loading in freshwater sediments, driven by redox fluctuations and microbial activities, acts as persistent P reservoir that undermines eutrophication mitigation strategies focused solely on external inputs. Besides well-characterized iron-/aluminum-bound P, a substantial fraction of internal P pool exists as polyphosphate (polyP) (up to 18% of total P), which is dynamically synthesized and decomposed by polyphosphate-accumulating organisms (PAOs). To date, our understanding of sedimentary PAOs remains limited, yet there is preliminary evidence of their activity outside wastewater treatment plants (WWTPs). This study aims to illustrate the important yet underestimated ecological roles of PAOs and polyP in internal P cycling within freshwater sediments. We extend the well-known PAOs in WWTPs across the landscape to shed light on PAO-involved P cycling in natural sediments and identify critical knowledge gaps in microbial P forms and transformations. Beyond canonical PAOs, we also highlight the polyP storage capacity and ecological significance of magnetotactic bacteria and sulfur-oxidizing bacteria, which likely represent only a fraction of sedimentary polyP reservoir. We propose that systematic identification and characterization of these polyP-storing communities through integrated analytical approaches are crucial for constructing a holistic model of sedimentary P cycling, with important implications for effective freshwater eutrophication management.