Co‐acquisition of mineral‐bound iron and phosphorus by natural Trichodesmium colonies

Abstract

AbstractLow iron (Fe) and phosphorus (P) ocean regions are often home to the globally important N2‐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 33P‐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 33P‐labeled ferrihydrite, and elevated 33P 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 33P or 55Fe, 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 33P 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.