Anaerobic microbial metabolism of isoprene

Abstract

Isoprene (IUPAC: 2-methyl-1,3-butadiene, CH2 = C(CH3)-CH = CH2) represents the most abundant biogenic volatile organic compound (BVOC) on Earth which is similar in magnitude to methane sources. It comprises one third of the total global BVOC emission, and influences atmospheric chemistry, leading to increasing global temperatures and raising ozone concentrations. The conjugated diene system of isoprene reacts preferentially with hydroxyl radicals (•OH) in the atmosphere resulting in complex secondary organic aerosols that effect climate and human health. The impacts of isoprene on the climate and atmosphere are well studied however its biological and biogeochemical role remains unclear. Little is known about microbiological processes serving as terrestrial sinks for isoprene. Whilst aerobic isoprene degrading bacteria have been identified, there is nothing known about anaerobic, isoprene-metabolizing organisms. Given the environmental abundance and ubiquity of isoprene, it was hypothesized that it is available to anaerobic microorganisms. In this thesis various inocula were examined for anaerobic microbial depletion of isoprene. Under anaerobic conditions isoprene was reduced stoichiometrically to methylbutene isomers (i.e. 2-methyl-1-butene (>97%), 3-methyl-1-butene (≤2%), 2-methyl-2-butene (≤1%)). The reduction was attributed to a novel hydrogenotrophic Acetobacterium wieringae strain that used isoprene as a terminal electron acceptor simultaneously with HCO3−. In the presence of isoprene, the strain generated 40% less acetate relative to isoprene free controls but achieved a similar biomass yield. Taken together these findings suggest that isoprene reduction was coupled to energy conservation. Genomic and proteomic analysis identified a five gene operon in the genome of A. wieringae strain ISORED-2 that is upregulated in the presence of isoprene. One gene in this operon encodes a nickel-dependent enzyme that contains a binding site for NADH, FAD and 4Fe-4S ferredoxin and is speculated to be an isoprene reductase. Phylogenetic analysis of the putative isoprene reductase revealed that its homologs are mostly spread among Firmicutes, but could also be found in Spirochaetes, Tenericutes, Actinobacteria, Chloroflexi, Bacteroidetes and some Proteobacteria. Methylbutene isomers were measured in the headspace of wetland samples collected from three different sample sites and results imply that isoprene reduction could be at least one of the responsible metabolisms resulting in methylbutene formation. Since isoprene reduction was found in natural environments like wetlands, it suggests its presence in other anaerobic environments. Furthermore, isoprene had an inhibitory effect on microbial methanogenesis indicating an interconnection between isoprene emission and methane biosynthesis.