Lateral gene transfer (LGT) is a key driver of evolutionary innovation, underlying protists’ lifestyles and interactions in anaerobic environments. Yet, its significance in free-living protists remains underexplored. Here, we address this gap by presenting the first single-cell transcriptomes of Metopus yantaiensis and genome-wide LGT screens across 36 omics datasets from nine anaerobic APM ciliates (classes Armophorea, Muranotrichea, and Parablepharismea)—a group in soil/sediment environments. Through phylogenetic analyses and validation testing, we identified 63 candidate prokaryotic LGT genes preferentially enriched in APM ciliates. Among these, 19 form interconnected pathways for degrading complex organics (polysaccharides, amino sugars); their high diversity and completeness are rarely seen in reported protist LGTs. A rare fused gene (arcC-OTC) and two novel genes (acs, ME2) were exclusively identified in APM ciliates, with their potential as the first evidence of LGT-mediated carbon metabolite retention and ammonia assimilation in phagotrophic protists inferred. Notably, 27 LGTs (including arcC-OTC, acs, and ME2) trace to candidate phyla radiation (CPR) bacteria or described prokaryotes, marking the first CPR-to-eukaryote LGT documentation. Collectively, these 63 LGTs are predicted to enhance nutrient utilization (complex organics, other carbon metabolites, inorganic elements), bioenergetic efficiency, and stress resistance (heavy metals, oxygen), facilitating soil/sediment adaptation. Overall, our results highlight lateral prokaryotic gene acquisition may be key for free-living anaerobic ciliates’ adaptation to new environments, shedding light on protists’ evolutionary dynamics and ecological roles.
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