Iron storage proteins in the marine environment

Abstract

Iron serves as a cofactor in a multitude of proteins required for many life-sustaining processes and is an essential element for most living organisms. With the advent of an oxygen-rich atmosphere the predominant form of iron switched from the relatively soluble ferrous state to the extremely insoluble ferric form. The low solubility of iron in oxygenated waters limits its bioavailability to aquatic organisms. Due to the low solubility and poor availability of ferric iron in ocean ecosystems, iron deficiency is a recurrent nutrient stress condition for microorganisms in general.

Iron storage proteins are used by organisms to sequester and store intracellular iron in a safe manner and provide a source of iron that can be drawn on when external iron supplies are limited (5, 48). Ferritins constitute a family of iron storage proteins found in aerobic and anaerobic organisms. Mammalian-type ferritins are present in plants, animals, and lower eukaryotes, whereas the heme-containing bacterioferritins and Dps (DNA-binding protein from starved cells) proteins are found only in bacteria. However, no iron storage protein has been observed in any marine microorganism.

This thesis focuses on investigating the role that iron storage proteins play in the marine environment. Through genomic analysis of Trichodesmium erythraeum a putative Dps protein was identified (Dps_tery). Upon expression and purification, we show that the Dps_tery, like other Dps, is able to bind Fe and DNA and to protect DNA from degradation by DNase. Through experiments with Dps_tery and horse spleen ferritin, we demonstrate that these proteins are a viable Fe source to a variety of marine phytoplankton. Both eukaryotic and prokaryotic species were able to grow rapidly on horse spleen ferritin or Dps_tery as a sole Fe source in the medium. The availability of the iron in these storage proteins results from a spontaneous release of Fe(III) to solution with an apparent first-order rate of release constant ∼0.15 d^-1. The spontaneous release of Fe from ferritin brings forth the hypothesis that intracellular iron might be controlled dynamically though continuous dissolution and re-precipitation of the Fe cores in iron storage proteins.