Investigation of the seasonal dynamics of grassland soil bacterivores in top- and subsoils

Bacterivorous micro- and macro-organisms are important functional guilds in the soil microbial food web, where their activities influence the community composition. Few studies to date have holistically considered the spatial and seasonal dynamics of such bacterivores in temperate grassland soils; studies of subsoil microbial food webs are particularly scarce. This represents a potential knowledge gap, as environmental conditions (e.g. season and soil depth) may affect prokaryotic and eukaryotic bacterivores differentially. Using a quantitative metatranscriptomic approach, we studied the abundance of SSU rRNA transcripts of bacterivorous bacteria, protists, and nematodes in two German grassland sites from top- and subsoils across seasons. The combined transcript abundance of all bacterivores was significantly reduced with depth. A reduction in the transcript predator-prey ratio was seen for bacterivorous protists, nematodes, and Bdellovibrionota . In contrast, facultative bacterivorous Myxococcota showed a consistently high transcript predator-prey ratio at both depths, suggesting that they dominate the bacterivore functional guild in subsoils. The transcript abundance of all bacterivores and their potential bacterial prey exhibited similar seasonal patterns at both depths. The predator-prey ratio showed bacterivore-specific seasonal and depth-dependent variations. Although this methodology has limitations, as transcript abundance does not directly reflect organism's abundance — our findings suggest a notable role for predatory bacteria as potential main consumers of bacterial biomass in subsoils. In general, increasing soil depth appears to influence the structure of the bacterivorous community, potentially reflected in a shift in the ratio of eukaryotic predators to bacterial prey. • Transcript abundance of eukaryotic bacterivores was lower in subsoils than in topsoils. • Bdellovibrionota transcripts were also less abundant in subsoils than in topsoils. • Myxococcota transcript-to-prey ratios showed no depth-related decrease. • Prey bacteria and bacterivores often had the same seasonal dynamics. • Significance of seasonal shifts was site-, organism-, and depth-dependent.