Vesicle self-assembly of amphiphilic siderophores produced by bacterial isolates from Soap Lake, Washington

Abstract

Soap Lake, located in Washington State, is a meromictic soda lake which was the subject of a prior NSF funded Microbial Observatory. Several organisms inhabiting this lake have been identified as producers of siderophores that are unique in structure. Two isolates found to be of the species Halomonas, SL01 and SL28 were found to produce suites of amphiphilic siderophores consisting of a peptidic head-group, which binds iron appended to fatty acid moieties of various lengths. The ability for siderophores to self-assemble into vesicles was determined for three suites of amphiphilic siderophores of unique structure (two from SL01 and one from SL28). These siderophores resemble the amphiphilic aquachelin siderophores produced by Halomonas aquamarina strain DS40M3, a marine bacterium. Vesicle self-assembly studies were performed by dynamic light scattering (DLS) and epifluorescence microscopy. The addition of ferric iron (Fe +3 ) at different equivalents, where an equivalence of iron is defined as equal to the molarity of the siderophore, demonstrated vesicle formation. This was suggested by both DLS and epifluorescence microscopy. Bacteria thriving under saline and alkaline conditions are capable of producing unique siderophores that self-assemble in micelles and vesicles due to ferric iron chelation. (186 words) Iron is an important mineral for nearly all living organisms, and bacteria have developed siderophores for its uptake. Siderophores are low molecular weight compounds that specifically coordinate ferric iron. In addition to siderophores, metal chelation can occur through different molecules, like cardiolipins and phosphatidylserine which bind ferric iron and manganese. 1 While specific for the coordination of iron, some siderophores can form stable complexes with other metal ions. Examples include the chelation of uranium (VI) by deferrioxamine B and pyochelins. 2, 3 Other researchers have demonstrated neptunium (V) and boron chelation. 4, 5 Some siderophores are amphiphilic, which means the molecule has both hydrophilic and hydrophobic properties. Amphiphilic siderophores form micelles above a threshold concentration and are shown to self-assemble into vesicles when excess ferric iron is chelated. 6-8 The self-assembly in amphiphilic siderophores was first described by Martinez and co-workers (2000). 8 Those physicochemical properties confer upon siderophores the potential to form micelles in their deferrated form, when over their critical micelle concentration (cmc), which in marinobactins and aquachelins is about 25 to 150 µM. Trends of higher cmc are expected when siderophores are ferrated. Micelle-to-vesicle transitions for marinobactin E, yielding vesicles of approximately 140 to 180 nm in size was shown via dynamic light scattering (DLS). 8 Because the diameter of micelles formed by siderophores is near the limit of detection of DLS, other methods are typically used to determine their size. For example, small-angle neutron scattering (SANS) in combination with DLS has been used to account for micelle ...

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“…Compared to other siderophore-producing species of the genus Halomonas , which synthesized 40 μM sodachelin ( Serrano Figueroa et al, 2015 ) or 25.4 μM halochelin ( Matsui and Nishino, 2016 ), respectively, significantly higher concentrations of 85 μM were obtained with strain ATCH28 T in IM1. This concentration could be substantially increased to more than 1 mM or roughly 600 mg/L, by utilizing M9 with adjustments in NaCl content and pH-value as well as the addition of the DFOE precursor Lysine.…”

“…As the genus is large and their natural habitats often iron-limited, various different types of siderophores have been reported to be produced by members of Halomonas . Those siderophores are aquachelins ( Martinez et al, 2000 ), loihichelins ( Homann et al, 2009 ), potashchelins ( Li et al, 2020 ), halochelins ( Figueroa et al, 2015 ), as well as sodachelins ( Serrano Figueroa et al, 2015 ), all of which are of amphiphilic character.…”

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