Soil Thermophiles and Their Extracellular Enzymes: A Set of Capabilities Able to Provide Significant Services and Risks

During this century, a number of reports have described the potential roles of thermophiles in the upper soil layers during high-temperature periods. This study evaluates the capabilities of these microorganisms and proposes some potential consequences and risks associated with the activity of soil thermophiles. They are active in organic matter mineralization, releasing inorganic nutrients (C, S, N, P) that otherwise remain trapped in the organic complexity of soil. To process complex organic compounds in soils, these thermophiles require extracellular enzymes to break down large polymers into simple compounds, which can be incorporated into the cells and processed. Soil thermophiles are able to adapt their extracellular enzyme activities to environmental conditions. These enzymes can present optimum activity under high temperatures and reduced water content. Consequently, these microorganisms have been shown to actively process and decompose substances (including pollutants) under extreme conditions (i.e., desiccation and heat) in soils. While nutrient cycling is a highly beneficial process to maintain soil service quality, progressive warming can lead to excessive activity of soil thermophiles and their extracellular enzymes. If this activity is too high, it may lead to reduction in soil organic matter, nutrient impoverishment and to an increased risk of aridity. This is a clear example of a potential effect of future predicted climate warming directly caused by soil microorganisms with major consequences for our understanding of ecosystem functioning, soil health and the risk of soil aridity.