Free-living bacteria and particle-attached bacteria in the Bohai Sea water environment: community composition and roles in nitrogen cycling

Abstract:

[Background] Free-living bacteria (FLB) and particle-attached bacteria (PAB) are two types of bacteria with different lifestyles in the ocean. Due to their different ecological niches, it is hypothesized that they exhibit significantly different community structures and roles in nitrogen cycling. However, the research on the relationship between bacterial lifestyles and roles in nitrogen cycling remains limited. [Objective] To reveal the community composition of FLB and PAB in the Bohai Sea water environment and compare the functional genes related to nitrogen cycling mediated by the two bacterial communities. [Methods] FLB and PAB samples from different water layers of the Bohai Sea were obtained by graded filtration. The species composition and roles in nitrogen cycling of the two communities were analyzed by high-throughput sequencing and fluorescence quantitative PCR. [Results] The FLB community was dominated by Alphaproteobacteria, while the PAB community was primarily composed of Gammaproteobacteria and Flavobacteriia. Functional predictions indicated that the roles of FLB and PAB in nitrogen cycling were consistent across surface and bottom layers, with both communities primarily involved in nitrate reduction and aerobic ammonia oxidation. However, they played distinct roles. Specifically, nitrate reduction was enriched in the FLB community, whereas nitrification and denitrification were enriched in the PAB community. Furthermore, RT-qPCR was employed for quantitative analysis of differentially expressed functional genes. Quantitative analysis and functional prediction results showed consistency, indicating that the abundance of nitrogen cycling-related genes in the PAB community was significantly higher than that in the FLB community, and the difference was more significant in the bottom layer than in the surface layer. Functional genes in the FLB community were significantly correlated with NO₃⁻ and NH₄⁺, whereas those in the PAB community showed significant associations with depth and alpha diversity.[Conclusion] The PAB community may play a more significant role than the FLB community in the nitrogen cycling of the Bohai Sea. This article provides new insights into the mechanism of nitrogen cycling mediated by marine microorganisms.