Differences in FTIR spectra measured in olivines derived from depleted and metasomatised sections of the earth's mantle

Background
Microorganisms inhabiting selenium contaminated sediments are able to reduce dissolved selenate oxyanions to sparingly soluble elemental selenium. The reductive precipitation of selenium by Se-reducing bacteria has been proposed as a potential bioremediation strategy. However, in order to employ Se-reducing bacteria for practical bioremediation purposes, reliable models must be developed to predict microbial behavior, including when they are active, what mechanisms are involved, and under what conditions the mechanisms function. In this study, we demonstrate that
anaerobic electron carriers play an essential role in governing the activity of a Se-reducing bacterium.

Methods
Transposon mutagenesis was used to generate mutant strains of Enterobacter cloacae SLD1a-1 that have lost the ability to reduce selenate. We characterized a mutant strain designated as 4E6 that was defective in selenate reduction activity. Genomic libraries of SLD1a-1 and 4E6 were constructed to clone the mutated operon that resulted in the loss of selenate reduction activity. Sequencing of the operon containing the transposon was obtained by restriction mapping and primer walking. Selenate reduction activity of wild-type and mutant strains were quantified using batch kinetic
experiments. Finally, the ability of menaquinone analogues and precursor compounds to rescue the selenate reduction activity was tested.

Results and Discussion
Mutagenesis experiments showed that mutation of the menD gene in the menaquinone biosynthesis operon produces derivative strains that are deficient in selenate reduction activity. Complementation by the wild-type sequence and menaquinone analogues restored the ability of mutant strains to reduce selenate. Menaquinones are small lipid-soluble molecules that act as electron shuttles in the bacterial electron transport chain. The biosynthesis of menaquinone occurs
under oxygen-limiting conditions, and is known to mediate anaerobic respiration of alternate electron acceptors. The results of this study indicate that anaerobic electron carriers are required for microbial selenate reduction.