Investigation of Siderophore-Promoted and Reductive Dissolution of Dust in Marine Microenvironments Such as Trichodesmium Colonies

Desert dust is a major source of iron (Fe) to phytoplankton in many Fe-poor ocean regions. However, phytoplankton often struggle to obtain dust-bound Fe (dust-Fe) due to its low solubility and short residence time in the euphotic zone. Trichodesmium, a globally important nitrogen-fixing, cyanobacterium, is uniquely adapted for utilizing dust as a source of Fe. Trichodesmium colonies can actively collect and concentrate dust particles within the colony core and enhance dust-Fe dissolution rates via two bio-dissolution mechanisms: reduction and complexation by strong Fe-chelators termed siderophores. Here, mimicking bio-dissolution in Trichodesmium colonies, we studied the kinetics of desert dust dissolution by a siderophore and a reductant in seawater. By concurrent measurements of dissolved Fe, silica (Si), and aluminum (Al) we recognized two major mineral pools that released Fe into seawater over an 8-day period: Fe(hydr)oxides and aluminosilicates. In the presence of the siderophore desferrioxamine-B, we observed two stages of dissolution: a short stage of fast Fe dissolution followed by a lasting stage of slow Fe dissolution that was highly correlated to Al and Si dissolution. In the presence of the reductant, ascorbate, Fe dissolution was not correlated to Al and Si dissolution and was relatively slow. Based on these observations and on dust mineralogy, we constructed a conceptual model for dust-Fe dissolution by a siderophore and a reductant from two major mineral pools: reductive and siderophore-promoted dissolution of Fe(hydr)oxides and slow continuous dissolution of Fe-bearing clays in the presence of a siderophore. Our findings highlight the importance of clays as an Fe source to Trichodesmium and possibly to other marine phytoplankton and can be further used to assess the contribution of dust to the Fe requirements of natural Trichodesmium colonies. Combining our measured bio-dissolution rates with dust concentrations retained within colonies from the Gulf of Aqaba, we calculated the supply of dissolved Fe from dust to single Trichodesmium colonies. Applying published Fe-quotas and growth rates we calculated the Fe requirements of the colonies under Fe-limited and Fe-replete conditions. The calculated dissolved Fe supply from dust retained within colonies can fulfill the Fe requirements of slow growing Fe-limited colonies, but cannot support fast growth and/or higher cellular Fe quotas. We conclude that despite these bio-dissolution mechanisms, dust-Fe availability to Trichodesmium is low and propose that it employs additional mechanisms to actively mine Fe from dust.