This chapter presents the major aspects of bacterial bioenergetics, without the use of chemical formulae and thermodynamic balance equations. It provides a sort of crash course in microbial energy metabolism and physiology to complement the enormous advances in bacterial genomics. Functional traits of energy metabolism constitute fundamental pieces of information to compare and distinguish how bacteria obtain the bioenergy they need for living, and therefore integrate functional information with genomic and phylogenetic data. The purpose is to provide a broad but meaningful view of the functional evolution of bacteria into mitochondria. Sulfate reducing organisms are predominantly distributed among deltaproteobacteria and other taxa of Gram negative bacteria, as well as some Gram positive organisms, in particular Clostridiales. The nitrogen cycle integrates redox modification of various nitrogen compounds carried out by different enzymes, often from bacteria associated in physical or metabolic consortia that utilizes parts of the cycle.