Beyond O2 – factors influencing microbial terminal oxidases

Habitats having gradients of oxygen (O2) concentrations are widely distributed on Earth, ranging from the human intestine to marine and soil environments. Microorganisms harbor enzymes, called low and high- affinity terminal oxidases (TOs), which allow them to use O2 across these concentration gradients for energy production. It is commonly assumed that the capacity to respire low O2 concentrations is due to the presence of high-affinity TOs, while low-affinity TOs are used at high concentration of O2. Yet, there is growing evidence that utilization of low- and high-affinity TOs does not follow this dogmatic pattern learned from our school textbooks, but that the regulation mechanisms are more complex. We recently found that members of a ubiquitous soil bacterial group, the Acidobacteriota, can use the low-affinity TOs to respire low O2 concentrations contrary to our textbooks. This suggests that other factors beyond O2 govern the expression of these TOs. We will expand upon this observation and explore other physiological and environmental factors that could drive the expression of these TOs, presumably giving the organism more metabolic flexibility to handle stressors. Using a combination of growth-based investigations coupled with gene expression and highly-sensitive O2 measurements, we will characterize the expression of these TOs, under varying conditions such as such as nutrient and carbon concentration, along with different physiological states of the microorganisms. In addition, we will assess how the differential expression of these TOs influence the microorganisms overall physiology by measuring the efficiency by which the microorganism uses carbon for growth. These investigations will initially be explored in the model soil organisms, Acidobacteriota. But we will go beyond these organisms and ascertain the extent of this strategy in other soil bacterial clades. This project seeks to better understand the expression of the low- and high-affinity TOs and the (environmental) conditions (beyond O2 concentration) that regulate their utilization for energy generation in representative soil bacteria. This will be achieved by using a holistic approach of genomics, gene expression, respiratory kinetics and growth-based experiments of environmental relevant model soil microorganisms select strains of the abundant Acidobacteriota and of other representative soil bacteria. This project will be a major contribution to understanding the drivers of the differential use of TOs and can have significant implications in our understanding of microbial physiology. Furthermore, our findings have the potential to extend into eukaryotic organisms, as they too possess multiple TOs. The project will be led by Stephanie A. Eichorst, in collaboration with Dagmar Woekben, Anna Lopatina at the University of Vienna and Emilio G. Garcia-Robledo at the University of Cadiz, Spain.