Microbial iron acquisition: marine and terrestrial siderophores.

Abstract

The vast majority of bacteria require iron for growth.1,2 Iron is an essential element required for key biological processes including amino acid synthesis, oxygen transport, respiration, nitrogen fixation, methanogenesis, the citric acid cycle, photosynthesis and DNA biosynthesis. However, obtaining iron presents challenges for the majority of microorganisms. While iron is the fourth most abundant transition metal in the Earth's crust, the insolubility of iron(III) [Ksp of Fe(OH)3 = 10-39] at physiological pH in aerobic environments severely limits the availability of this essential nutrient. Pathogenic and marine bacteria face similar challenges for obtaining iron because both live in very low iron environments. Bacteria typically require micromolar levels of total iron for growth, yet the iron concentration in the surface waters of the oceans is only 0.01-2 nM.3-7 In humans cellular iron is also very low and is sequestered by lactoferrin, transferrin, and ferritin as a primary defense mechanism at the onset of infection.8 Given its cellular importance, it is not surprising that microbes have evolved multiple pathways designed to extract iron from their surrounding environments, tailored to the molecular constraints of the iron pool (Figure 1). Figure 1 Microbial (Gram negative) iron uptake pathways. In this review the general pathways by which bacteria acquire iron are considered first as an overview to illustrate the singular importance of iron for microbial growth. The focus of this review is on siderophore-mediated iron uptake, particularly structural characteristics of marine siderophores and the reactivity that these characteristics confer. Relatively little is known about marine microbial iron transport compared to that for terrestrial and pathogenic microbes, yet comparison of the structures and reactivity may hint at the biological advantage that these structural traits confer to marine microbes and very possibly provide insights to siderophore-mediated iron uptake in some pathogens.