Selective collection of iron-rich dust particles by natural Trichodesmium colonies

Abstract

Dust is an important iron (Fe) source to the ocean, but its utilization by phytoplankton is constrained by rapid sinking and slow dissolution dust-bound iron (dust-Fe). Colonies of the globally important cyanobacterium, Trichodesmium, overcome these constraints by efficient dust capturing and active dust-Fe dissolution. In this study we examined the ability of Trichodesmium colonies to maximize their Fe supply from dust by selectively collecting Fe-rich particles. Testing for selectivity in particle collection, we supplied ~600 individual colonies, collected on multiple days from the Gulf of Aqaba, with natural dust and silica minerals that were either cleaned of or coated with Fe. Using a stereoscope, we counted the number of particles retained by each colony shortly after addition and following 24 h incubation with particles, and documented translocation of particles to the colony core. We observed a strong preference for Fe-rich particles over Fe-free particles in all tested parameters. Moreover, some colonies discarded the Fe-free particles they initially collected. The preferred collection of Fe-rich particles and disposal of Fe-free particles suggest that Trichodesmium can sense Fe and selectively choose Fe-rich dust particles. This ability assists Trichodesmium obtain Fe from dust and facilitate its growth and subsequent contribution to nutrient cycling and productivity in the ocean.

Iron Uptake Mechanisms in Marine Phytoplankton

“…Atmospheric dust is also a significant source of iron in iron-limited regions of the ocean, but this dust rapidly sinks below the surface and the low solubility of iron from dust restricts its acquisition by phytoplankton. Trichodesmium colonies can capture dust particles in their core, and Kessler et al (2020) have shown that these colonies display a preference for iron-rich particles over iron-free particles. The preferred collection of iron-rich particles and rejection of iron-free particles suggest the ability to sense iron and selectively utilize ironrich dust particles (Kessler et al, 2020).…”

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

“…This site receives high loads of dust originating from the Sahara and the Arabian Peninsula (Chen et al, 2007(Chen et al, , 2008Torfstein et al, 2017). This dust is the major source of mineral particles to the surface water of the Gulf (Torfstein et al, 2017) and probably to Trichodesmium colonies that prevail in these waters in spring and autumn (Rubin et al, 2011;Basu and Shaked, 2018;Basu et al, 2019;Kessler et al, 2019).…”

“…Colonies were collected at the IUI pier with a 200 µM plankton net subjected to ambient currents (see details in Kessler et al, 2019). Freshly collected colonies were fixed in filtered seawater with 2% glutaraldehyde, air dried on a 0.2 µM Supor membrane filter and coated with 15-20 nM of gold, using a Hummer sputter coater (Technics).…”

“…Trichodesmium may also optimize its Fe supply from particles by selective collection of particles that contain Fe. We recently demonstrated that natural Trichodesmium colonies can distinguish between Fe-rich and Fe-free particles and are capable of selective collection and retention of Fe-rich particles in their cores (Kessler et al, 2019). Further research is required to unfold the pathways employed by Trichodesmium to mine valuable nutrients such as Fe and probably P from dust.…”

“…Maintaining the enzymatic machinery required for nitrogen fixation raises Trichodesmium's Fe requirements, resulting in cellular Fe quotas that exceed other oceanic phytoplankton by ∼10-fold (Berman-Frank et al, 2001;Kustka et al, 2003;Nuester et al, 2012). Accumulating research indicates that Trichodesmium is especially adapted to utilize dust as a source of Fe (Rueter et al, 1992;Tovar-Sanchez et al, 2006;Rubin et al, 2011;Langlois et al, 2012;Basu and Shaked, 2018;Polyviou et al, 2018;Basu et al, 2019;Eichner et al, 2019a,b;Gledhill et al, 2019;Kessler et al, 2019). Trichodesmium employs different strategies to overcome the physical constraints on dust-Fe acquisition: dust sinking to depth and loss of attached particles.…”