Assessing the role of soil chemoautotrophs in carbon cycling: An investigation into isotopically labelled soil microorganisms

Abstract

Recently observed increases in atmospheric CO2 have created great interest in carbon capture technologies and natural sinks of this major component of the carbon cycle. Humic substances are a large, operationally defined fraction of soil organic matter. It was thought that humic substances consist of cross-linked macromolecular structures forming a distinct class of compounds. However, it was recently concluded by members of my research group that the vast majority of humic material in soils, are a complex mixture of microbial/plant biopolymers and degradation products, and not a distinct chemical category. The postulation that microbial inputs to soil carbon are greatly underestimated was put forward by my research group in 2007. Therefore, I have attempted to demonstrate the inputs made by soil chemoautotrophic bacteria. A method was developed where soil samples were measured for chemoautotrophic activity by subjecting them to a suite of scientific techniques. A growth chamber was used to propagate extant soil chemoautotrophic bacteria from different soils and subjected to an array of chemical and biological analyses. The growth chamber was used to measure CO2 concentrations and introduce stable isotopic 13CO2. Estimations of CO2 sequestration were made using direct measurements for Irish soils and one Eurasian soil. Isotope labelled DNA was isolated using cesium chloride gradient ultracentrifugation. The dominant chemoautotrophic bacteria uncovered were Thiobacillus denitrificans and Thiobacillus thioparus. Labelled biomass was isolated and described using GCMS-IRMS and NMR, where an array of PLFAs, protein/peptide, carbohydrates and aliphatics were observed. Finally, an attempt to mimic common agricultural practice was performed to measure soil chemoautotrophic activity. This demonstrated the capability of this approach to benefit carbon flux estimations and hopefully in the future help to elucidate carbon flow into soils for the greater environment