Irons differently modulate bacterial guilds for leading to varied efficiencies in simultaneous nitrification and denitrification (SND) within four aerobic bioreactors.

As a novel biological wastewater nitrogen removal technology, simultaneous nitrification and denitrification (SND) has gained increasing attention. Iron, serving as a viable material, has been shown to influence nitrogen removal. However, the precise impact of iron on the SND process and microbiome remains unclear. In this study, bioreactors amended with iron of varying valences were evaluated for total nitrogen (TN) removal efficiencies under aerobic conditions. The acclimated control reactor without iron addition (NCR) exhibited high ammonia nitrogen (AN) removal efficiency (98.9%), but relatively low TN removal (78.6%) due to limited denitrification. The reactor containing zero-valent iron (Fe0R) demonstrated the highest SND rate of 92.3% with enhanced aerobic denitrification, albeit with lower AN removal (84.1%). Significantly lower SND efficiencies were observed in reactors with ferrous (Fe2R, 66.3%) and ferric (Fe3R, 58.2%) iron. Distinct bacterial communities involved in nitrogen metabolisms were detected in these bioreactors. The presence of complete ammonium oxidation (comammox) genus Nitrospira and anammox bacteria Candidatus Brocadia characterized efficient AN removal in NCR. The relatively low abundance of aerobic denitrifiers in NCR hindered denitrification. Fe0R exhibited highly abundant but low-efficiency methanotrophic ammonium oxidizers, Methylomonas and Methyloparacoccus, along with diverse aerobic denitrifiers, resulting in lower AN removal but an efficient SND process. Conversely, the presence of Fe2+/Fe3+ constrained the denitrifying community, contributing to lower TN removal efficiency via inefficient denitrification. Therefore, different valent irons modulated the strength of nitrification and denitrification through the assembly of key microbial communities, providing insight for microbiome modulation in nitrogen-rich wastewater treatment.

Cross-Feeding between Members of Thauera spp. and Rhodococcus spp. Drives Quinoline-Denitrifying Degradation in a Hypoxic Bioreactor.

The complex bacterial community in a quinoline-degrading denitrifying bioreactor is predominated by several taxa, such as Thauera and Rhodococcus However, it remains unclear how the interactions between the different bacteria mediate quinoline metabolism under denitrifying conditions. In this study, we designed a sequence-specific amplification strategy to isolate the most predominant bacteria and obtained four strains of Thauera aminoaromatica, a representative of a key member in the bioreactor. Tests on these isolates demonstrated that all were unable to degrade quinoline but efficiently degraded 2-hydroxyquinoline, the hypothesized primary intermediate of quinoline catabolism, under nitrate-reducing conditions. However, another isolate, Rhodococcus sp. YF3, corresponding to the second most abundant taxon in the same bioreactor, was found to degrade quinoline via 2-hydroxyquinoline. The end products and removal rate of quinoline by isolate YF3 largely varied according to the quantity of available oxygen. Specifically, quinoline could be converted only to 2-hydroxyquinoline without further transformation under insufficient oxygen conditions, e.g., less than 0.5% initial oxygen in the vials. However, resting YF3 cells aerobically precultured in medium with quinoline could anaerobically convert quinoline to 2-hydroxyquinoline. A two-strain consortium constructed with isolates from Thauera (R2) and Rhodococcus (YF3) demonstrated efficient denitrifying degradation of quinoline. Thus, we experimentally verified that the metabolic interaction based on 2-hydroxyquinoline cross-feeding between two predominant bacteria constitutes the main quinoline degradation mechanism. This work uncovers the mechanism of quinoline removal by two cooperative bacterial species existing in denitrifying bioreactors.IMPORTANCE We experimentally verified that the second most abundant taxon, Rhodococcus, played a role in degrading quinoline to 2-hydroxyquinoline, while the most abundant taxon, Thauera, degraded 2-hydroxyquinoline. Metabolites from Thauera further served to provide metabolites for Rhodococcus Hence, an ecological guild composed of two isolates was assembled, revealing the different roles that keystone organisms play in the microbial community. This report, to the best of our knowledge, is the first on cross-feeding between the initial quinoline degrader and a second bacterium. Specifically, the quinoline degrader (Rhodococcus) did not benefit metabolically from quinoline degradation to 2-hydroxyquinoline but instead benefited from the metabolites produced by the second bacterium (Thauera) when Thauera degraded the 2-hydroxyquinoline. These results could be a significant step forward in the elucidation of the microbial mechanism underlying quinoline-denitrifying degradation.

Non-synchronous Structural and Functional Dynamics During the Coalescence of Two Distinct Soil Bacterial Communities.

Soil is a unique environment in which the microbiota is frequently subjected to community coalescence. Additions of organic fertilizer and precipitation of dust induce coalescent events in soil. However, the fates of these communities after coalescence remain uncharted. Thus, to explore the effects of microbiota coalescence, we performed reciprocal inoculation and incubation experiments in microcosms using two distinct soils. The soils were, respectively, collected from a cropland and an industrial site, and the reciprocal inoculation was performed as models for the incursion of highly exotic microbiota into the soil. After incubation under either aerobic or anaerobic conditions for two months, the soils were assayed for their bacterial community structure and denitrification function. According to the 16S rRNA gene sequencing results, the inoculated soil showed a significant shift in bacterial community structure after incubation-particularly in the industrial soil. The structures of the bacterial communities changed following the coalescence but were predicted to have the same functional potential, e.g., nitrogen metabolism, as determined by the quantification of denitrifying genes and nitrogen gas production in the inoculated soil samples, which showed values equivalent those in the original recipient soil samples regardless of inoculum used. The functional prediction based on the known genomes of the taxa that shifted in the incubated sample communities indicates that the high functional overlap and redundancy across bacteria acted as a mechanism that preserved all the metabolic functions in the soil. These findings hint at the mechanisms underlying soil biodiversity maintenance and ecosystem function.