Soil microbial communities-mediated bioattenuation in simulated aquifer storage and recovery (ASR) condition: Long-term study

Comprehensive assessment of chlorination disinfection on microplastic-associated biofilms

Chlorination on microplastic (MP) biofilms was comprehensively investigated with respect to disinfection efficiency, morphology, and core microbiome. The experiments were performed under various conditions: i) MP particles; polypropylene (PP) and polystyrene (PS), ii) MP biofilms; Escherichia coli for single-species and river water microorganisms for multiple-species, iii) different chlorine concentrations, and iv) different chlorine exposure periods. As a result, chlorination effectively inactivated the MP biofilm microorganisms. The disinfection efficiency increased with increasing the free chlorination concentration and exposure periods for both single- and multiple-species MP biofilms. The multiple-species MP biofilms were inactivated 1.3-6.0 times less than single-species MP biofilms. In addition, the PP-MP biofilms were more vulnerable to chlorination than the PS-MP biofilms. Morphology analysis verified that chlorination detached most MP biofilms, while a small part still remained. Interestingly, chlorination strongly changed the biofilm microbiome on MPs; the relative abundance of some microbes increased after the chlorination, suggesting they could be regarded as chlorine-resistant bacteria. Some potential pathogens were also remained on the MP particles after the chlorination. Notably, chlorination was effective in inactivating the MP biofilms. Further research should be performed to evaluate the impacts of residual MP biofilms on the environment.

Changes of characteristics and disinfection by-products formation potential of intracellular organic matter with different molecular weight in metalimnetic oxygen minimum

Metalimnetic oxygen minimum (MOM) occurs in reservoirs or lakes due to stratification and algal blooms, which has low dissolved oxygen (DO) levels and leads to the deterioration of water quality. The transformation mechanism and the impact on the water quality of intracellular organic matter (IOM) derived from algae are poorly understood under MOM conditions. In this study, IOM extracted by Microcystis aeruginosa was divided into five components according to molecular weight (MW), and the changes of characteristics and correlated disinfection by-products formation potential (DBPFP) were analyzed and compared under MOM conditions. The removal efficiency of dissolved organic carbon (DOC) in the <5 kDa fraction (66.6%) was higher than that in the >100 kDa fraction (41.8%) after a 14-day incubation under MOM conditions. The same tendency also occurred in Fmax and DBPFP. The decrease in Fmax was mainly due to the decline in tryptophan-like and tyrosine-like for all IOM fractions. The diversity of microorganisms degrading the MW > 100 kDa fraction was lower than others. Besides low MW fractions, these findings indicated that more attention should be paid to high MW fractions which were resistant to biodegradation under MOM conditions during water treatment.

Synergetic effect of nitrate on dissolved organic carbon attenuation through dissimilatory iron reduction during aquifer storage and recovery

Aquifer storage and recovery (ASR) is a promising water management technique in terms of quantity and quality. During ASR, iron (Fe) (hydr)oxides contained in the aquifer play a crucial role as electron acceptors in attenuating dissolved organic carbon (DOC) in recharging water through dissimilatory iron reduction (DIR). Considering the preference of electron acceptors, nitrate (NO3⁻), possibly coexisting with DOC as the prior electron acceptor to Fe (hydr)oxides, might influence DIR by interrupting electron transfer. However, this phenomenon is yet to be clarified. In this study, we systematically investigated the potential effect of NO3⁻ on DOC attenuation during ASR using a series of sediment columns representing typical aquifer conditions. The results suggest that DOC attenuation could be enhanced by the presence of NO3⁻. Specifically, total DOC attenuation was notably higher than that from the stoichiometric calculation simply employing NO3⁻ as the additional electron acceptor to Fe (hydr)oxides, implying a synergetic effect of NO3⁻ in the overall reactions. X-ray photoelectron spectroscopy analyzes revealed that the Fe(II) ions released from DIR transformed the Fe (hydr)oxides into a less bioavailable form, inhibiting further DIR. In the presence of NO3⁻, however, no aqueous Fe(II) was detected, and another form of Fe (hydr)oxide appeared on the sediment surface. This may be attributed to nitrate-dependent Fe(II) oxidation (NDFO), in which Fe(II) is (re)oxidized into Fe (hydr)oxide, which is available for the subsequent DOC attenuation. These mechanisms were supported by the dominance of DIR-relevant bacteria and the growth of NDFO-related bacteria in the presence of NO3⁻.

Improved regression model for anaerobic ammonium oxidation by repeated and prolonged batch assay under stressful salinity and pH conditions

The aim of this study was to propose repeated and prolonged batch (RPB) assay as a promising specific anammox activity (SAA) methodology assessing the anammox activity under stressed salinity and pH conditions. Response surface analysis (RSA) was used as a regression tool to evaluate statistical significance. The feasibility of RPB was investigated at 0 to 15 g-NaCl/L of salinity and pH 6 to 8 with reflecting the results of preliminary SAA. As a result, conventional SAA was statistically insignificant. In addition, the RSA results obtained from repeated batch did not meet the statistical significance despite ten times iterative reaction. Interestingly, the RPB assay (i.e., applied both repeated and prolonged reaction) was effective to obtain the reliable results. Candidadus Brocadia and Candidadus Jettenia were functional anammox microbiome during RPB. Outcomes of this study suggest that RPB assay can be applied to accurately determine the anammox activity under various stressful conditions.

Elucidating prioritized factor for mainstream partial nitritation between C/N ratio and dissolved oxygen: Response surface methodology and microbial community shifts

Recently, C/N ratio is suggested as a promising control factor with dissolved oxygen (DO) achieving mainstream partial nitritation (PN); however, their combined effects on mainstream PN are still limited. This study evaluated the mainstream PN with respect to the combined factors, and investigated the prioritized factor affecting the community of aerobic functional microbes competing with NOB. Response surface methodology was performed to assess the combined effects of C/N ratio and DO on the activity of functional microbes. Aerobic heterotrophic bacteria (AHB) played the greatest role in oxygen competition among functional microbes, which resulted in relative inhibition of nitrite-oxidizing bacteria (NOB). The combination of high C/N ratio and low DO had a positive role in the relative inhibition of NOB. In bioreactor operation, the PN was successfully achieved at ≥ 1.5 of C/N ratio for 0.5-2.0 mg/L DO conditions. Interestingly, aerobic functional microbes outcompeting NOB were shifted with C/N ratio rather than DO, suggesting C/N ratio is more prioritized factor achieving mainstream PN. These findings will provide insights into how combined aerobic conditions contribute to achieve mainstream PN.

Denitrification dynamics in unsaturated soils with different porous structures and water saturation degrees: A focus on the shift in microbial community structures

Despite its environmental significance, little is known about denitrification in vadose zones owing to the complexity of such environments. Here, we investigated denitrification in unsaturated soils with different pore distributions. To this end, we performed batch-type denitrification experiments and analyzed microbial community shifts before and after possible reactions with nitrates to clarify the relevant denitrifying mechanism in the microcosms. For quantitative comparison, pore distribution in the test soil samples was characterized based on the uniformity coefficient (C_u) and water saturation degree (SD). Micro-CT analysis of the soil pore distribution confirmed that the proportion of bigger-sized pores increased with decreasing C_u. However, oxygen diffusion into the system was controlled by SD rather than C_u. Within a certain SD range (51-67%), the pore condition changed abruptly from an oxic to an anoxic state. Consequently, denitrification occurred even under unsaturated soil conditions when the SD increased beyond 51-67%. High throughput sequencing revealed that the same microbial species were potentially responsible for denitrification under both partially (SD 67%), and fully saturated (SD of 100%) conditions, implying that the mechanism of denitrification in a vadose zone, if it exists, might be possibly similar under varying conditions.

Microbial community niches on microplastics and prioritized environmental factors under various urban riverine conditions

Microplastics (MPs) provide habitats to microorganisms in aquatic environments; distinct microbial niches have recently been elucidated. However, there is little known about the microbial communities on MPs under urban riverine conditions, in which environmental factors fluctuate. Therefore, this study investigated MP biofilm communities under various urban riverine conditions (i.e., organic content, salinity, and dissolved oxygen (DO) concentration) and evaluated the prioritized factors affecting plastisphere communities. Nine biofilm-forming reactors were operated under various environmental conditions. Under all testing conditions, biofilms grew on MPs with decreasing bacterial diversity. Interestingly, biofilm morphology and bacterial populations were driven by the environmental parameters. We found that plastisphere community structures were grouped according to the environmental conditions; organic content in the water was the most significant factor determining MP biofilm communities, followed by salinity and DO concentration. The principal plastisphere communities were Proteobacteria, Bacteroidetes, Cyanobacteria, and Firmicutes phyla. In-depth analyses of plastisphere communities revealed that biofilm-forming and plastic-degrading bacteria were the predominant microbes. In addition, potential pathogens were majorly discovered in the riverine waters with high organic content. Our results suggest that distinct plastisphere communities coexist with MP particles under certain riverine water conditions, implying that the varied MP biofilm communities may affect urban riverine ecology in a variety of ways.

Effect of aerobic microbes' competition for oxygen on nitrogen removal in mainstream nitritation-anammox systems

The effects of C/N ratio in mainstream partial nitritation (PN)-anaerobic ammonia oxidation (ANAMMOX) considering competitive relationship of aerobic microbes competing for oxygen were investigated. Thy system was operated for 501 d with various C/N ratio. Competitive growth of aerobic heterotrophic bacteria (AHB) at ≥ 1 of C/N ratio acted effectively on the selective inhibition of nitrite-oxidizing bacteria (NOB) while contributing to stable PN-A. In-depth kinetic analysis indicated oxygen affinity of aerobic microbes was in the order of AHB > ammonia-oxidizing bacteria (AOB) > NOB. In addition, potential of denitritation by AHB could contributed to improving nitrogen removal up to 87.5 ± 4.3%. AHB was comparatively clustered into two groups with a C/N ratio of 1. Nitrosomonas sp. PY1 became predominant while Nitrospira spp. were the major NOB. The potential of AHB in establishing selective inhibition of NOB was identified, which could be a novel approach to stabilze the mainstream PN-A.

The Growth of Vallisneria natans and Its Epiphytic Biofilm in Simulated Nutrient-Rich Flowing Water

This paper investigates the effects of water flow on the growth and physiological indicators of the submerged macrophyte, Vallisneria natans, and the bacteria and algae community composition on its epiphytic biofilm-covered leaves. The authors set up a simulated flowing water laboratory experiment testing high nitrogen (N) and phosphorus (P) concentrations. Total chlorophyll and dissolved oxygen (DO) was significantly enhanced, and turbidity was reduced, thereby accelerating the growth of V. natans. These experiments were compared to another set of observations on a static group. The accumulation of malonaldehyde (MDA) in the dynamic groups was significantly higher than that in the static group. As an antioxidant stress response, the total superoxide dismutase (T-SOD) was also induced in plants exposed to nutrient-rich flowing water. The results of 16S rRNA high-throughput sequencing analyses showed that the water flow increased the bacteria community diversity of biofilm-producing bacteria with N and P removing bacteria, carbon cycle bacteria, and plant growth-promoting rhizobacteria on the epiphytic biofilm. This research determined that water flow alleviates the adverse effects of eutrophication when V. natans grows in water containing high N and P concentrations. Water flow also inhibits the growth of cyanobacteria (also referred to as blue-green algae) in epiphytic biofilm. The ecological factor of water flow, such as water disturbance and aeration measures, could alleviate the adverse effect of eutrophic water by providing a new way to restore submerged macrophytes, such as V. natans, in eutrophic water.