Dissolved organic phosphorus utilization and composition under different water regimes of a river-dominated estuary

Dissolved organic phosphorus (DOP) has been closely linked to microbial alkaline phosphatases (AP) whose affiliation and diversity is largely unknown in coastal waters. Here we assessed genetic diversity and abundance of bacterial alkaline phosphatases phoD and phosphate transporter phoD and explored how AP activity interacting with them along the salinity gradient of Pearl River Estuary (PRE), which was under heavy anthropogenic pressures. Partial least squares path modeling (PLS-PM) revealed the pathway from environmental variables (pH and salinity) to phoD-harboring bacterial taxa in particle-attached fraction and then to phoD gene copies was the determinant process for AP activity; while AP activity in free-living fraction was mainly controlled by the pathway from dissolved inorganic phosphorus (DIP) to phoD encoding community structure and its gene abundance. Our study highlighted the importance of diverse phoD phosphorus mineralizers, such as some members from Actinobacteria (Actinomadura), Alphaproteobacteria (unclassified Rhodobacteraceae, Roseovarius, Mesorhizobium) and Betaproteobacteria (Ralstonia), while pstS-harboring community was composed of picocyanobacteria. In the outer estuary with the lowest DIP concentration, AP level was activated substantially herein, which corresponded well with the spatial distribution of phoD and pstS gene abundance. High number of phosphatase and transporter genes potentially implied effective hydrolyzation rate of DOP to supplement inorganic phosphorus in the estuary. Researches on the characterization and transformation of DOP are insufficient owing to their complex composition and extraction difficulty. In the recent study, we applied Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to present a preliminary feature of DOP molecular composition in the PRE. The phosphoesters represented more than 95 % of total DOP, and CHOP compounds (mainly lipids) was a potential substrate for bacterial AP. Our study unveils the key biogeochemical role of AP for mineralizing specific DOP to support more phytoplankton biomass and emphasizes the supervision and management of both DOP and DIP entering the estuaries from land-based sources.