Isoprene Degradation in the Terrestrial Environment

Isoprene production comprises of one third of the total global hydrocarbon release, and influences atmospheric chemistry, including increasing global temperatures and raising ozone concentrations. Although isoprene production, and the chemical attenuation of isoprene have been well studied, microbial degradation of isoprene has had little attention. In this thesis, seasonal isoprene degradation rates of soil were measured, demonstrating an average isoprene consumption rate of 4.77 nmol m-2 h-1, which indicates that the effect of soil microbiota acting as a sink for atmospheric isoprene is lower than previously presumed. Neither the season, nor proximity to isoprene-producing trees affected the rate of isoprene degradation in soil, suggesting other isoprene sources in the soil. A series of isoprene enrichment experiments were performed with soil, using the sensitivity of high isoprene concentrations, the evidence of direct isoprene degradation using stable isotope probing, and the relatability to the natural environment using low isoprene concentrations. Different bacteria were enriched at different levels of isoprene, with genera such as Rhodococcus being enriched at high concentrations, and Methylobacterium being enriched at low concentrations, suggesting specialisation for different isoprene concentrations. Additionally, the clade TM7 was shown to be highly enriched in some isoprene enrichments, although did not incorporate isoprene into its DNA. Changes in bacterial community structure analysis through isoprene enrichment suggested a replicable isoprene-degrading community. An extensive collection of isoprene-degrading isolates was created. Isoprene-degrading genes were investigated at the amplicon and genomic levels. The isoprene-degrading operon displayed variable chromosomal positioning, appearing in either plasmids or on the main chromosome of closely related species. IsoA gene sequence diversity was larger than in previous studies, and the isoA gene sequences were not correlated to the 16S rRNA gene sequences, which could indicate horizontal gene transfer. Isoprene-degrading bacteria were present on a variety of leaves; and communities of bacteria derived from leaves degraded isoprene.