Bioavailable Dissolved Organic Carbon Serves as a Key Regulator of Phosphorus Dynamics in Stream Biofilms

Phosphorus (P) dynamics at the sediment–water interface of aquatic ecosystems are receiving increasing attention due to their implications for water quality. P uptake by microbial biofilms can serve as a mechanism to control and mitigate the risk of eutrophication. Microbial biofilms capture P both intracellularly and extracellularly. While the significance of extracellular P entrapment in biofilms in engineered systems has recently been established, little is known about its dynamics in aquatic ecosystems. Current research on eutrophication control predominantly emphasises nitrogen, phosphorus or nitrogen-phosphorus ratio-based approaches, often overlooking the potential indirect influence of bioavailable dissolved organic carbon (DOC) on P uptake by heterotrophic microorganisms. In this study, we tested the effect of bioavailable DOC on P entrapment patterns in biofilms and in biofilm P-regulation mechanisms such as polyphosphate accumulation and alkaline phosphatase activity in semi-natural flow-through experimental flumes. Our results show that intracellular P entrapment is limited by bioavailable DOC, while extracellular P entrapment is independent of bioavailable DOC and has the potential to offset intracellular P saturation. We further demonstrate that DOC bioavailability influences benthic P cycling and that its implications extend into critical areas of ecosystem functioning such as river self-purification, competitive resource utilisation and organic P cycling.