Contact-dependent iron uptake from dust revealed by elemental analysis of single Trichodesmium colonies

Aerosol dust deposited on the nutrient-deprived surface ocean can boost phytoplankton growth and oceanic carbon uptake. Low mineral solubility restricts the biological utilization of dust-nutrients, thereby benefiting phytoplankton that actively dissolve dust. The ubiquitous colony-forming, N₂-fixing cyanobacteria Trichodesmium specialize in dust-nutrient utilization, with several dust-dissolution pathways identified in natural populations. Studying active dust dissolution by Trichodesmium, we surveyed the elemental composition (i.e., quotas) of natural colonies from the dust-impacted Red Sea using a benchtop micro-x-ray fluorescence imager. We also examined changes in the colonies' quotas during incubations with dust and nutrients. Accounting for inter-colony variability, we analyzed 106 individual colonies. Since particles often appeared on the surface of colonies, we carefully analyzed all images, removing dust particle signals from colony quotas. Focusing on the colonies' iron (Fe) and phosphorus (P) quotas, we observed contrasting patterns of inter-colony variability and responses to dust, likely reflecting distinct nutrient sources—Fe sourced from dust and P sourced from the dissolved phase. Iron uptake from dust was repeatedly observed, but only upon colony–mineral interactions, indicative of contact-dependent active dissolution. The role of dust in Fe nutrition was also evident from the minor impact of dissolved Fe complexation on Fe quotas. Phosphorus quotas responded rapidly to P addition or removal, but not to dust. Natural colonies collected over a season had heterogeneous Fe quotas but homogeneous P quotas, further supporting their distinct sources. Predictions of Trichodesmium's bloom dynamics in particle-rich and dust-impacted ocean environments should incorporate its ability to dissolve Fe-minerals.