Linking trace metal dynamics to bacterial community and predicted metabolism in estuarine transition zones

Trace metals regulate microbially mediated biogeochemical processes, yet field evidence remains limited on whether they act as micronutrients or metabolic inhibitors in estuaries. Here, we investigated the coupling between trace metal dynamics and bacterial communities along the salinity gradient of Pearl River Estuary (PRE) by integrating dissolved and particulate metal analysis, 16 S rRNA gene sequencing, functional prediction, and multivariate modeling. Most dissolved metals showed positive deviations from the conservative mixing line, whereas particulate metals were depleted particularly in the low salinity zone (LS, 0-13.67‰). Metal partitioning coefficients (K_metal) were primarily controlled by salinity and suspended particulate matter. Bacterial communities differed markedly between LS and high salinity zones (HS, >13.67‰), and physicochemical properties, K_metal, and nutrients jointly explained 73.7% of the total community variation, with salinity accounting for 19.36%, followed by K_Cd (12.15%) and K_Fe (10.23%). Co-occurrence network revealed a densely connected LS community structure, where keystone taxa were associated with Cu availability. Functional predictions showed significant enrichment of carbon metabolism in the LS zone. Partial least squares path modeling indicated positive associations between essential metal partitioning (Fe, Cu, Co, and V) and central carbon metabolic pathways, whereas non-essential metals (Cd, Pb, and As) exerted negative effects. These associations were further supported by enriched metal-dependent enzymes within each pathway. Comparison with other estuaries suggests that elevated essential metal inventories of PRE are characteristics of particle-rich and river-dominated systems with chemically buffered metal availability. Overall, our findings highlight essential metals as overlooked micronutrients linked to bacterial carbon processing in dynamic estuarine environments.