The active subset of grassland soil microbiomes changes with soil depth, water availability and prominently features predatory bacteria and episymbionts

Mediterranean grasslands, vital natural and agricultural ecosystems, experience seasonal variation in water content that likely affect microbial activity. We used metagenomics-informed stable isotope probing to investigate how the activities of microorganisms in Angelo Reserve (2160 mm rainfall) and Hopland (956 mm rainfall) soil change over depth and the seasons. At both sites, we find that the relative abundances of organisms in shallow soil changes relatively little but the most abundant organisms vary greatly with soil depth. Notably the highest levels of isotope incorporation, indicative of growth, occurs in deep soils. The active part of the 0-10 cm soil community varies over time, especially in Hopland soils during the fall rewetting. We defined a large, novel clade of Actinomycetota with notable capacity for thiosulfate oxidation whose representatives are prevalent and active in deep soils (>20 cm) across both ecosystems. Active Saccharibacteria unexpectedly encode nucleotide synthesis genes that enabled isotope incorporation while growing in shallow Angelo soils over all time periods. In contrast to predicted episymbiotic lifestyles of Saccharibacteria, other highly active bacteria are predicted predators. Obligately predatory Pseudobdellovibrio are active in intermediate depth Hopland soils whereas bacteria of the order Haliangiales are active in shallow Angelo soils. Supporting predatory lifestyles of Haliangiales, we used in silico structure prediction to assemble a large protein complex that we identify as a contractile injection system. Overall, the results indicate the potential for active carbon turnover in deep grassland soil and strong seasonal changes in the active members of microbial communities, despite relatively minor shifts in community composition.