How many microbial species are on Earth?

Biologists have long pondered the number of plant and animal species on our planet. Until recently, microbes were absent from estimates of global biodiversity despite being the most abundant group of organisms on Earth with an estimated abundance of 10³⁰ individuals. Using some of the largest community data sets to date, we estimate that Earth may be home to one trillion (10¹²) “species”. We explore the feasibility of this estimate and the upper limits of life through deep time using a macroevolutionary perspective.