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ABSTRACT Much of life on Earth is at the mercy of currents and flow. Residence time (τ) estimates how long organisms and resources remain in a system based on the ratio of volume (V) to flow rate (Q). Shortτshould promote immigration but limit species establishment, while longτshould favour species that survive on limited resources. Theory suggests these opposing forces shape the abundance, diversity and function of flowing systems. We experimentally tested how residence time affects a lake microbial community by exposing chemostats to aτgradient spanning seven orders of magnitude. Microbial abundance, richness and evenness increased non‐linearly withτ, while functions like productivity and resource consumption declined. Taxa formed distinct clusters of short‐ and long‐τspecialists consistent with niche partitioning. Our findings demonstrate that residence time drives biodiversity and community function in flowing habitats that are commonly found in environmental, engineered and host‐associated ecosystems. 

Abstract Biologists have long sought to quantify the number of species on Earth. Often missing from these efforts is the contribution of microorganisms, the smallest but most abundant form of life on the planet. Despite recent large‐scale sampling efforts, estimates of global microbial diversity span many orders of magnitude. It is important to consider how speciation and extinction over the last 4 billion years constrain inventories of biodiversity. We parameterized macroevolutionary models based on birth–death processes that assume constant and universal speciation and extinction rates. The models reveal that richness beyond 10¹²species is feasible and in agreement with empirical predictions. Additional simulations suggest that mass extinction events do not place hard limits on modern‐day microbial diversity. Together, our study provides independent support for a massive global‐scale microbiome while shedding light on the upper limits of life on Earth.