Yeala Shaked

The marine cyanobacterium Trichodesmium has the remarkable ability to interact with and utilize air‐borne dust as a nutrient source. However, dust may adversely affect Trichodesmium through buoyancy loss and exposure to toxic metals. Our study explored the effect of desert dust on buoyancy and mortality of natural Red Sea puff‐shaped Trichodesmium thiebautii . Sinking velocities and ability of individual colonies to stay afloat with increasing dust loads were studied in sedimentation chambers. Low dust loads of up to ∼400 ng per colony did not impact initial sinking velocity and colonies remained afloat in the chamber. Above this threshold, sinking velocity increased linearly with the colony dust load at a slope matching prediction based on Stoke's law. The potential toxicity of dust was assessed with regards to metal dissolution kinetics, differentiating between rapidly released metals, which may impact surface blooms, and gradually released metals that may impact dust‐centering colonies. Incubations with increasing dust concentrations revealed colony death, but the observed lethal dose far exceeded dust concentrations measured in coastal and open ocean systems. Removal of toxic particles as a mechanism to reduce toxicity was explored using SEM‐EDX imaging of colonies incubated with Cu‐minerals, yet observations did not support this pathway. Combining our current and former experiments, we suggest that in natural settings the nutritional benefits gained by Trichodesmium via dust collection outweigh the risks of buoyancy loss and toxicity. Our data and concepts feed into the growing recognition of the significance of dust for Trichodesmium 's ecology and subsequently to ocean productivity.

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.