Microbial cycling of isoprene, the most abundantly produced biological volatile organic compound on Earth

Terry J Mcgenity,Andrew T Crombie,J Colin Murrell

doi:10.1038/s41396-018-0072-6

Abstract

Isoprene (2-methyl-1,3-butadiene), the most abundantly produced biogenic volatile organic compound (BVOC) on Earth, is highly reactive and can have diverse and often detrimental atmospheric effects, which impact on climate and health. Most isoprene is produced by terrestrial plants, but (micro)algal production is important in aquatic environments, and the relative bacterial contribution remains unknown. Soils are a sink for isoprene, and bacteria that can use isoprene as a carbon and energy source have been cultivated and also identified using cultivation-independent methods from soils, leaves and coastal/marine environments. Bacteria belonging to the Actinobacteria are most frequently isolated and identified, and Proteobacteria have also been shown to degrade isoprene. In the freshwater-sediment isolate, Rhodococcus strain AD45, initial oxidation of isoprene to 1,2-epoxy-isoprene is catalyzed by a multicomponent isoprene monooxygenase encoded by the genes isoABCDEF. The resultant epoxide is converted to a glutathione conjugate by a glutathione S-transferase encoded by isoI, and further degraded by enzymes encoded by isoGHJ. Genome sequence analysis of actinobacterial isolates belonging to the genera Rhodococcus, Mycobacterium and Gordonia has revealed that isoABCDEF and isoGHIJ are linked in an operon, either on a plasmid or the chromosome. In Rhodococcus strain AD45 both isoprene and epoxy-isoprene induce a high level of transcription of 22 contiguous genes, including isoABCDEF and isoGHIJ. Sequence analysis of the isoA gene, encoding the large subunit of the oxygenase component of isoprene monooxygenase, from isolates has facilitated the development of PCR primers that are proving valuable in investigating the ecology of uncultivated isoprene-degrading bacteria.

Highlights

Background and isoprene productionIsoprene’s properties, abundance, and climate impactIsoprene (2-methyl-1,3-butadiene; CH2=C(CH3)CH=CH2) is an abundant biogenic volatile organic compound (BVOC), with atmospheric emissions of around 500 Tg C year−1, which is approximately equal to that of methane and of the same magnitude as emissions of all other BVOCs [1]
The identity and availability of genes encoding key enzymes of isoprene metabolism by bacteria have paved the way for cultivation-independent studies to determine the distribution, diversity and activity of isoprene degraders in the environment
The picture is emerging of isoprene metabolism being widespread across many genera of diverse bacteria [42, 60, 61, 71], and so development of 16S rRNA gene primers or probes to detect isoprene degraders directly from the environment without coupling these with other techniques such as stable isotope probing (SIP) or Raman microspectroscopy will not be fruitful

Summary

Introduction

Background and isoprene productionIsoprene’s properties, abundance, and climate impactIsoprene (2-methyl-1,3-butadiene; CH2=C(CH3)CH=CH2) is an abundant BVOC, with atmospheric emissions of around 500 Tg C year−1, which is approximately equal to that of methane and of the same magnitude as emissions of all other BVOCs [1]. A number of isoprene-degrading Rhodococcus species from soils and leaves of isoprene-producing trees such as Poplar and Willow have been isolated and are currently being characterized ([42]; Murrell, McGenity, Crombie, Murphy et al unpublished), suggesting that these metabolically versatile Bacteria may be major players in the biological isoprene cycle.

Results

Conclusion