Many ecosystems are wholly microbial and the activities of microorganisms provide the biochemical foundation for plant and animal life. ‘Microbial ecology and evolution’ describes how plants depend upon the complex redox reactions of microbes that fertilize the soil by fixing nitrogen, converting nitrites to nitrates, enhancing the availability of phosphorus and trace elements, and recycling organic matter. Eukaryotic microorganisms are similarly plentiful and essential for the sustenance of plants and animals. Bacteria, archaea, and single-celled eukaryotes are the masters of the marine environment, harnessing the energy that supports complex ecological interactions between aquatic animals. Bacteria and archaea form 90% of the ocean biomass and surface waters are filled with eukaryotic algae.