Microbes are the most abundant living entities on earth and are key engines that drive earth's biogeochemical cycles. I will first briefly illustrate some of our research activities for better harnessing microbial metabolic abilities in environmental technology processes such as wastewater treatment plants and anaerobic digesters. Then, I will more specifically focus on how the observation of microbial community assembly patterns in these engineered ecosystems has led us to propose the Microbial Transition State (MTS) theory of growth. The MTS approach aims at linking the chemical energy available in a given environment to the growth rate of microbes, on the basis of generic principles borrowed from statistical physics.