Hydrogen stable isotope probing of lipids demonstrates slow rates of microbial growth in soil

The rate at which microorganisms grow and reproduce is fundamental to our understanding of microbial physiology and ecology. While soil microbiologists routinely quantify soil microbial biomass levels and the growth rates of individual taxa in culture, there is a limited understanding of how quickly microbes actually grow in soil. For this work, we posed the simple question: what are the growth rates of soil microorganisms? In this study, we measure these rates in three distinct soil environments using hydrogen stable isotope probing of lipids with 2H-enriched water. This technique provides a taxa-agnostic quantification of in situ microbial growth from the degree of 2H enrichment of intact polar lipid compounds ascribed to bacteria and fungi. We find that average apparent generation times in soil are quite slow (20 to 64 days) but also highly variable at the compound-specific level (6 to 1137 days), suggesting differential growth rates between community subsets. We observe that low-biomass communities can exhibit more rapid growth rates than high-biomass communities, highlighting that biomass quantity alone does not predict microbial productivity in soil. Furthermore, within a given soil, the rates at which specific lipids are being synthesized do not relate to their quantity, suggesting a general decoupling of microbial abundance and growth in soil microbiomes. More generally, we demonstrate the utility of lipid stable isotope probing for measuring microbial growth rates in soil and highlight the importance of measuring growth rates to complement more standard analyses of soil microbial communities. SignificanceGeneration times, how quickly organisms grow and reproduce, are a key feature of biology. However, there are few measurements of microbial generation times in soil, despite the crucial importance of soil microbes to terrestrial ecosystems. By measuring the rate at which isotopically labeled water is incorporated into microbial membranes, we find that the generation times of soil microorganisms are far longer than those typically observed in culture. Surprisingly, we observe that lower-biomass soils exhibited faster growth rates than high-biomass soils. More abundant microorganisms are not necessarily the fastest growing and most soil microorganisms are slow growers. Our results underscore the importance of considering slow and variable growth rates when studying microbial communities and their contributions to ecosystem processes.