4.5. Bathy-6 is the dominant subgroup in paddy soils with broad environmental adaptation
Bathy-6 exhibits a widespread presence in terrestrial environments, including soil, freshwater sediments, and mangrove sediments (Pan et al. 2019; Xiang et al. 2017; Zou et al. 2020), although it has also been detected in certain marine sediments (Yu et al. 2018). In numerous prior studies, Bathyarchaeia were primarily observed in anaerobic sediments, characterizing their anaerobic lifestyle. Notably, Lazar et al. identified genes encoding enzymes responsible for responding to oxidative stress inBathy-6 (AD8–1), suggesting that Bathy-6 members possess an ability to adapt to fluctuations in oxygen levels (Lazar et al. 2016). Additionally, Pan et al . reported the presence of oxygen-dependent metabolic pathways within certain Bathy-6genomes, hinting at a microaerophilic lifestyle for Bathy-6 (Pan et al. 2020). In our study, the results underscored that Bathy-6stands as the predominant subgroup within the Bathyarchaeia in all the paddy soils (see Fig. 3a). Flooded paddy soils are characterized by microaerophilic conditions due to the presence of dissolved oxygen in soil porewater and oxygen released by rice roots (Chen et al. 2008; Li et al. 2007). This microoxic nature of paddy soil could elucidate why the anaerobic Bathyarchaeia subgroups are less prevalent, whileBathy-6 becomes dominant in these environments. Collectively, these findings collectively indicate that Bathy-6 possesses wide-ranging environmental adaptability, accommodating both microoxic and anaerobic conditions. This suggests that Bathy-6 may potentially play a distinctive role in the evolutionary transition of life from anaerobic to aerobic environments. Further investigations are necessary to unravel the mechanisms underpinning the high prevalence of Bathy-6 in paddy soils.
Furthermore, beyond adapting to diverse oxygen conditions, certainBathy-6 ASVs exhibit the capability to thrive across a broad pH range from 5 to 8. Bathy-6 demonstrates resilience not only in oligotrophic marine sediments (Yu et al. 2018) but also in eutrophic paddy soils with elevated TC. Our network analysis indicated thatBathy-6 members are subdivided into several groups. These results collectively underscore that Bathy-6 possesses versatile metabolic capabilities, thrives in diverse habitats, and exhibits varied lifestyles, consistent with genomic predictions (Pan et al. 2020).
In addition, genomic analyses have revealed the presence of genes encoding flagella within Bathy-6 (Lazar et al. 2016). All of these attributes associated with Bathy-6 suggest that it may symbolize the transition of Bathyarchaeia from a marine to a terrestrial ecosystem. Moving forward, the comprehensive characterization and potential role of Bathy-6 in paddy soils and even within the global ecosystem warrant further research.