Microbial Degradation of Acetaldehyde in Freshwater, Estuarine and Marine Environments

The oxygenated volatile organic compound, acetaldehyde, plays a fundamental role in atmospheric chemistry by altering the atmosphere’s oxidative status or “self-cleaning capacity”. Acetaldehyde flux from freshwater, estuarine, and marine environments is thought to be under significant microbial control, yet the identity and diversity of microbial acetaldehyde degraders, and the mechanisms of acetaldehyde degradation, are largely unknown. In this thesis, the key acetaldehyde-degrading microorganisms in the Colne Estuary, the River Colne and the River Gipping were identified using metagenetic sequencing and quantitative PCR. Bacteria were primarily responsible for acetaldehyde degradation, with members of the genera Pseudomonas and Arcobacter identified as key acetaldehyde degraders. Seventeen bacterial isolates were cultivated from the Colne Estuary and degraded acetaldehyde at a concentration of 2.27 mM, with Halobacillus strains A4 and A5, Rhodococcus strain A14, and Bacillus strain A17 identified as important acetaldehyde degraders. Fourteen previously identified bacteria, belonging to the Actinobacteria, Bacteroidetes, Firmicutes, Proteobacteria, and Verrucomicrobia, also degraded acetaldehyde, suggesting that the microbial acetaldehyde sink is highly diverse. Using genomics and proteomics, the pathway of acetaldehyde metabolism used by Rhodococcus strain A14 was proposed. Acetaldehyde is oxidised to acetate by aldehyde dehydrogenase enzymes, with acetate converted to acetyl-CoA via acetyl-CoA synthetase. These reactions are thought to be conserved amongst acetaldehyde-degrading bacteria. Under favourable growth conditions, acetaldehyde is dissimilated to CO2 for energy production via the oxidative decarboxylation steps of the tricarboxylic acid cycle. Acetaldehyde-derived carbon is assimilated into biomass by upregulating the glyoxylate shunt and gluconeogenesis pathway, ensuring carbon is conserved for growth. Using 14C-radiolabelling, rates of microbial dissimilation were shown to be significantly higher than assimilation rates in the freshwater rivers, estuaries, and coastal seas of south-west England, with summer rates significantly higher than winter. These findings suggest that microorganisms exert considerable control over acetaldehyde emissions and represent an important component of the global acetaldehyde cycle.