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A time-resolved 16S rRNA stable isotope probing experiment was conducted to resolve the microbiome members that were involved in the aerobic and anaerobic degradation of chitin, a major component of the particulate organic matter in agricultural soils. [13C]-chitin was largely mineralized within 20 days under oxic conditions. Cellvibrio, Massilia, and several Bacteriodetes families were identified as initially active chitin degraders. Subsequently, Planctomycetes and Verrucomicrobia were labeled by assimilation of 13C carbon either from [13C]-chitin or from 13C-enriched components of primary chitin degraders. Bacterial predators (e.g., Bdellovoibrio and Bacteriovorax) were labeled and non-labeled microeukaryotic predators (Alveolata) increased in relative abundance towards the end of the experiment (70 days), indicating that chitin degraders were subject to predation. Microbial trophic interactions differed substantially under anoxic compared to oxic conditions. Various fermentation types occurred along with iron respiration. While Acidobacteria and Chloroflexi were initially labeled, Firmicutes and uncultured Bacteroidetes were predominantly labeled, suggesting that the latter two bacterial groups were mainly responsible for the degradation of chitin, and also provided substrates for iron reducers. Eventually, our study revealed that (a) hitherto unrecognized Bacteria were involved in a chitin-degrading microbial food web of an agricultural soil, (b) trophic interactions were substantially shaped by the oxygen availability, and (c) predation was restricted to oxic conditions. The gained novel insights into trophic interactions of a chitin-degrading microbiome of an agricultural soil improve the understanding of turnover of chitin, which is crucial for an improved understanding of soil carbon dynamics. |