Mineral iron utilization by natural and cultured Trichodesmium and associated bacteria

The high iron (Fe) demands of Trichodesmium, a keystone nitrogen-fixing cyanobacterium, are often met by dust deposition at the ocean surface. Following up on our findings of unique dust capturing and processing by Trichodesmium, we explored the ability of natural Trichodesmium colonies from the Gulf of Aqaba and Fe-limited laboratory culture (IMS101) to obtain Fe from the mineral ferrihydrite and compete with their epibiotic bacteria for this Fe source. To study this complex system, we carefully optimized a radiotracer method ensuring complete removal of external ferrihydrite and efficient separation of bacteria from the colonies. Trichodesmium-only uptake rates of natural colonies were 5–50 times faster than those of laboratory culture, suggesting that natural colonies acquire ferrihydrite at a greater efficiency. In some days, total uptake rates of natural colonies exceeded dissolved Fe release from ferrihydrite, indicating that the colonies enhance the mineral dissolution rate. Furthermore, uptake rates of bacteria associated with natural colonies were faster than those of bacteria associated with the culture, implying that the bacteria benefit from the Trichodesmium-enhanced mineral dissolution. At the cellular level, surface area normalized uptake rates of bacteria always exceeded those of cultured Trichodesmium, but in natural populations, the dominance shifted between Trichodesmium and bacteria on different days. At the community level, when accounting for the total bacteria and Trichodesmium cells, Trichodesmium dominated Fe uptake in both cultured and natural colonies. Overall, our findings suggest that natural Trichodesmium colonies are exceptionally adapted for accessing mineral Fe and maintaining a sustainable relationship with their associated bacteria.