Dust- and mineral-iron utilization by the marine dinitrogen-fixer Trichodesmium

Trichodesmium, a filamentous dinitrogen-fixing cyanobacterium, forms extensive blooms in nutrient-poor tropical and subtropical ocean waters. These cyano-bacteria contribute significantly to biological fixation of nitrogen from the atmosphere in these waters, and thereby fuel primary production and influence nutrient flow and the cycling of organic and inorganic matter(1,2). Trichodesmium blooms require large quantities of iron, which is partly supplied by the influx of wind-blown dust(3). However, the processes and mechanisms associated with dust acquisition are poorly understood(3-6). Here, we incubate natural populations and laboratory cultures of Trichodesmium with isotopically labelled iron oxides and desert dust, to determine how these cyanobacteria collect, process and use particulate iron. We show that, like most phytoplankton, Trichodesmium acquires only dissolved iron. However, unlike other studied phytoplankton, Trichodesmium accelerates the rate of iron dissolution from oxides and dust, through as yet unspecified cell-surface processes, and thereby increases cellular iron uptake rates. We show that natural puff (ball-shaped) colonies of Trichodesmium are particularly effective at dissolving dust and oxides, which we attribute to efficient dust trapping in their intricate colony morphology, followed by active shuttling and packaging of the dust within the colony core. We suggest that colony formation in Trichodesmium is an adaptive strategy that enhances iron acquisition from particulate sources such as dust.