Abstract
Desert dust is a major source of iron (Fe) to Fe-limited ocean regions. However, Fe-limited 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 an Fe-source. Trichodesmium can actively collect and concentrate dust particles within the core of its colonies and enhance dust-Fe dissolution rates via two bio-dissolution mechanisms: reduction and complexation by a siderophore. 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 and aluminum 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 aluminum and silica dissolution. In the presence of the reductant, ascorbate, Fe dissolution was not correlated to aluminum and silica dissolution and was relatively slow. Based on these observations and on dust minerology, 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 marine phytoplankton in general and can be further used to assess the contribution of dust-Fe to the Fe requirements of natural Trichodesmium colonies. From our measured bio-dissolution rates and concentrations of dust retained within colonies from the Gulf of Aqaba, we calculated the supply of dissolved Fe from dust to Trichodesmium. Based on published Fe-quotas and growth rates we calculated Trichodesmium’s Fe requirements under Fe-limited and Fe-replete conditions. The calculated dissolved Fe supply from dust, retained by a colony, can fulfil the minimal Fe requirements of slow growing, Fe-limited colonies, but cannot support fast growth and/or higher cellular Fe quotas.
Highlights
Iron (Fe) is an essential trace nutrient, required for a wide range of enzymatic processes, including photosynthesis and nitrogen fixation (Morel and Price, 2003)
Comparing the supply rates to the Fe requirements of Trichodesmium, we find that these calculated rates can fulfill the Fe requirements of slow-growing Felimited colonies, but cannot support fast growth and/or higher cellular Fe quotas
Desert-dust collected at the northern edge of the Gulf of Aqaba was analyzed by several methods to obtain a broad characterization
Summary
Iron (Fe) is an essential trace nutrient, required for a wide range of enzymatic processes, including photosynthesis and nitrogen fixation (Morel and Price, 2003). The cyanobacterium Trichodesmium spp. is an important ecosystem player in oligotrophic ocean regions, which contributes up to ∼50% of marine nitrogen fixation and forms extensive surface blooms visible even from space (Carpenter et al, 1992; Westberry and Siegel, 2006). Reductive Dissolution Fe dissolution and accumulation in solution were promoted by ascorbate, at concentrations of 30 μM and above (Figure 3A). As these experiments were conducted at ambient oxygen levels, Fe(III) reduction and Fe(II) oxidation by oxygen occurred simultaneously. The changing balance between Fe reduction and oxidation eventually shifted the system from netdissolution to net-precipitation This shift is manifested as an overturn in slope after 4 days, at 30 and 100 μM of ascorbate.
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