Nimble vs. Torpid Responders: differential adaptations to hydration pulse duration among soil microbes

Most environmental parameters are characterized by some level of variability, rather than constancy. This is important in environments like soils, where microbial activity follows pulses of water availability driven by precipitation inputs. We hypothesized that arid soil microorganisms have adaptively diversified in response to specific pulse regimes differing in frequency and duration. To test this, we incubated natural Chihuahuan Desert soil microbiomes for 60 hours of hydration (activity) under separate treatments in which this was reached with pulses of different pulse duration (PD), punctuated by intervening periods of desiccation. Using 16S rRNA gene amplicon data, we measured treatment effects on microbiome growth, diversity, and species composition, tracking the individual fate of 370 common phylotypes. Consistent with theoretical predictions, microbial diversity was a direct, saturating function of PD. Increasingly larger community composition shifts were detected with decreasing PD, as differently adapted phylotypes became more prominent. Some twenty percent of phylotypes responded consistently to PD, some preferring short pulses (nimble responders; NIRs) and some longer pulses (torpid responders; TORs). Contrary to theoretical predictions, biomass yield was an inverse function of PD. PD in pulsed environments like soils constitutes a major driver of microbial community composition and diversification, but its functional consequences remain mystifyingly complex.