Carrier type affects anammox community assembly, species interactions and nitrogen conversion

The impacts of carrier type on anammox community assembly, species interactions and nitrogen conversion were studied in this work. It was found that in addition to shared species with higher abundance, different carrier types recruited rare species by imposing selection pressure. Results from co-occurrence networks revealed that carrier type strongly influenced interactions between keystone species inhabiting within anammox biofilm through potentially inducing niche differences. Overall, elastic cubic sponges would lead to closer cooperation between different populations, whereas plastic hollow cylinders would trigger fiercer competition. Meanwhile, the results based on metagenomics sequencing showed carrier type significantly affected nitrogen conversion related genes abundances, and higher reads number was detected on the elastic cubic sponges. The information obtained in this work could provide some valuable information for the selection and optimization of carrier type in the anammox process.

Microbial community regulation and performance enhancement in gas biofilters by interrupting bacterial communication

BACKGROUND

Controlling excess biomass accumulation and clogging is important for maintaining the performance of gas biofilters and reducing energy consumption. Interruption of bacterial communication (quorum quenching) can modulate gene expression and alter biofilm properties. However, whether the problem of excess biomass accumulation in gas biofilters can be addressed by interrupting bacterial communication remains unknown.

RESULTS

In this study, parallel laboratory-scale gas biofilters were operated with Rhodococcus sp. BH4 (QQBF) and without Rhodococcus sp. BH4 (BF) to explore the effects of quorum quenching (QQ) bacteria on biomass accumulation and clogging. QQBF showed lower biomass accumulation (109 kg/m³) and superior operational stability (85-96%) than BF (170 kg/m³; 63-92%) at the end of the operation. Compared to BF, the QQBF biofilm had lower adhesion strength and decreased extracellular polymeric substance production, leading to easier detachment of biomass from filler surface into the leachate. Meanwhile, the relative abundance of quorum sensing (QS)-related species was found to decrease from 67 (BF) to 56% (QQBF). The QS function genes were also found a lower relative abundance in QQBF, compared with BF. Moreover, although both biofilters presented aromatic compounds removal performance, the keystone species in QQBF played an important role in maintaining biofilm stability, while the keystone species in BF exhibited great potential for biofilm formation. Finally, the possible influencing mechanism of Rhodococcus sp. BH4 on biofilm adhesion was demonstrated. Overall, the results of this study achieved excess biomass control while maintaining stable biofiltration performance (without interrupting operation) and greatly promoted the use of QQ technology in bioreactors. Video Abstract.

Low nitrous oxide concentration and spatial microbial community transition across an urban river affected by treated sewage

Urban rivers receive used water derived from anthropogenic activities and are a crucial source of the potent greenhouse gas nitrous oxide (N₂O). However, considerable uncertainties still exist regarding the variation and mechanisms of N₂O production in response to the discharge of treated sewage from municipal wastewater treatment plants (WWTPs). This study investigated N₂O concentrations and microbial processes responsible for nitrogen conversion upstream and downstream of WWTPs along the Tama River flowing through Tokyo, Japan. We evaluated the effect of treated sewage on dissolved N₂O concentrations and inherent N₂O consumption activities in the river sediments. In summer and winter, the mean dissolved N₂O concentrations were 0.67 µg-N L^-1 and 0.82 µg-N L^-1, respectively. Although the dissolved N₂O was supersaturated (mean 288.7% in summer, mean 240.7% in winter) in the river, the N₂O emission factors (EF_5r, 0.013%-0.025%) were significantly lower than those in other urban rivers and the Intergovernmental Panel on Climate Change default value (0.25%). The nitrate (NO₃^-) concentration in the Tama River increased downstream of the WWTPs discharge sites, and it was the main nitrogen constituent. An increasing trend of NO₃^- concentration was observed from upstream to downstream, along with an increase in the N₂O consumption potential of the river sediment. A multiple regression model showed that NO₃^- is the crucial factor influencing N₂O saturation. The diversity in the upstream microbial communities was greater than that in the downstream ones, indicating the involvement of treated sewage discharge in shaping the microbial communities. Functional gene quantification for N₂O production and consumption suggested that nirK-type denitrifiers likely contributed to N₂O production. Structural equation models (SEMs) revealed that treated sewage discharged from WWTPs increased the NO₃^- loading from upstream to downstream in the river, inducing changes in the microbial communities and enhancing the N₂O consumption activities. Collectively, aerobic conditions limited denitrification and in turn facilitated nitrification, leading to low N₂O emissions even despite high NO₃^- loadings in the Tama River. Our findings unravel an overestimation of the N₂O emission potential in an urban oxygen-rich river affected by treated sewage discharge.

Inhibited effect of biochar application on N2O emissions is amount and time-dependent by regulating denitrification in a wheat-maize rotation system in North China

Biochar application is considered an effective method of reducing nitrous oxide (N₂O) emissions in soil. However, the mechanism and temporal effect of different doses of biochar on N₂O emissions is still obscure. Here, we conducted a two-year field experiment to test the effects of different input amounts and frequencies of biochar on soil N₂O emissions in North China. Biochar was applied in six different treatments in a winter wheat and summer maize rotation system: applications of 0 t/ha biochar (C0), 2.25 t/ha biochar (C1), 4.5 t/ha biochar (C2), 9 t/ha biochar (C3), and 13.5 t/ha biochar (C4) each year, and a single application of 13.5 t/ha biochar (CS) in the first year. The results showed that biochar could inhibit N₂O emissions, reaching 20.6% to 60.1% in the wheat season and 18.1% to 39.4% in the maize season. The inhibitory effect of biochar on soil N₂O emissions was dependent on amount and time. C3 had the best results in the wheat season, although its inhibitory effect in the maize season was not as good relative to C4 due to the lower biochar application. In addition, CS significantly reduced (27.7%) the cumulative N₂O emissions in the first year, although the inhibitory effect disappeared in the second year. Biochar increased the nosZ gene copy numbers and promoted a reduction of N₂O in the soil via the denitrification process. In conclusion, the inhibition of N₂O emissions during denitrification is an important factor for reducing soil N₂O emissions by biochar, and the inhibition of biochar is influenced by the input amount and time.

Functional gene expression of oil-degrading bacteria resistant to hexadecane toxicity

Contamination with oil poses a threat to the environment and to human health worldwide. Biological methodologies have proved to be economical, versatile and efficient for the remediation of pollutants. In this paper, a highly efficient oil-degrading bacterial strain USTB-2 was isolated from an oil production well of Dagang oil field in Tianjin, China. The 16S rRNA sequence of USTB-2 showed 100% similarity with that of Bacillus subtilis BSn5. Hexadecane is one of the most important components in petroleum. The half inhibitory ratio (IC₅₀) of hexadecane inhibited organisms, determined by microcalorimetry, was lower in USTB-2 than in B. BSn5. The results indicate that the strain USTB-2 degrades hexadecane to make it less toxic compared with the normal strain. RT-PCR was used to evaluate the expression of oil-degrading enzymes, specifically 4-hydroxyphenylacetate 3-monooxygenase genes (HPMO). A sharp increase in the expression of HPMO genes was observed for USTB-2, while the expression of HPMO genes in reference strain B. BSn5 remained relatively stable. These methods can be used to study the metabolic potential of microorganisms for in situ oil decontamination.