Microbial siderophores as molecular shuttles for metal cations: sources, sinks and application perspectives

Abstract

Iron is one of the highly abundant elements on the earth's crust, an essential micronutrient for a majority of life forms, and exists in two frequent oxidation states such as ferrous (Fe²⁺) and ferric (Fe³⁺). These two oxidation states are interconvertible by redox reactions and form complexes with a wide range of siderophores. At neutral pH in soil, Fe²⁺ is highly soluble upto 100 mM but have less biological value, whereas Fe³⁺ is less soluble upto 10^-9 M. This reduced bioavailability of Fe³⁺ induces competition among microorganisms. As many microorganisms need at least 10^-6 M of Fe³⁺ form of iron for their growth, siderophores from these microbes readily withdraw Fe³⁺ iron from a variety of habitats for their survival. In this review, we bring into light the several recent investigations related to diverse chemistry of microbial siderophores, mechanisms of siderophore uptake, biosynthetic gene clusters in microbial genomes, various sources of heavy metal cations in soil, siderophore-binding protein receptors and commercialisation perspectives of siderophores. Besides, this review unearths the recent advancements in the characterisation of novel siderophores and its heavy metal complexes alongside the interaction kinetics with receptors.

Keywords: Biosensors; Biosynthetic genes; Microbial siderophores; Organometallics; Siderophore-binding proteins.