Subhajit Basu

Low iron (Fe) and phosphorus (P) ocean regions are often home to the globally important N 2 ‐fixing cyanobacterium Trichodesmium spp., which are physiologically adapted to Fe/P co‐limitation. Given Trichodesmium 's eminent ability to capture particles and the common associations between Fe and P in sediments and aerosols, we hypothesized that mineral bio‐dissolution by Trichodesmium spp. may enable them to co‐acquire Fe and P. We present a new sensitive assay to determine P uptake from particles, utilizing 33 P‐labeled ferrihydrite. To validate the method, we examined single natural Trichodesmium thiebautii colonies in a high‐resolution radiotracer ß‐imager, identifying strong colony‐mineral interactions, efficient removal of external 33 P‐labeled ferrihydrite, and elevated 33 P uptake in the colony core. Next, we determined bulk P uptake rates, comparing natural Red Sea colonies and P‐limited Trichodesmium erythraeum cultures. Uptake rates by natural and cultured Trichodesmium were similar to P release rates from the mineral, suggesting tight coupling between dissolution and uptake. Finally, synthesizing P‐ferrihydrite labeled with either 33 P or 55 Fe, we probed for Fe/P co‐extraction by common microbial mineral solubilization pathways. Dissolution rates of ferrihydrite were accelerated by exogenous superoxide and strong Fe‐chelator and subsequently enhanced 33 P release and uptake by Trichodesmium . Our method and findings can facilitate further Fe/P co‐acquisition studies and highlight the importance of biological mechanisms and microenvironments in controlling bioavailability and nutrient fluxes from particles.