Nitrate removal performance and diversity of active denitrifying bacteria in denitrification reactors using poly(L-lactic acid) with enhanced chemical hydrolyzability

Insight into universality and characteristics of nitrate reduction coupled with arsenic oxidation in different paddy soils

Nitrate reduction coupled with arsenic (As) oxidation strongly influences the bioavailability and toxicity of As in anaerobic environments. In the present study, five representative paddy soils developed from different parent materials were used to investigate the universality and characteristics of nitrate reduction coupled with As oxidation in paddy soils. Experimental results indicated that 99.8 % of highly toxic aqueous As(III) was transformed to dissolved As(V) and Fe-bound As(V) in the presence of nitrate within 2-8 d, suggesting that As was apt to be reserved in its low-toxic and nonlabile form after nitrate treatment. Furthermore, nitrate additions also significantly induced the higher abundance of 16S rRNA and As(III) oxidase (aioA) genes in the five paddy soils, especially in the soils developed from purple sand-earth rock and quaternary red clay, which increased by 10 and 3-5 times, respectively, after nitrate was added. Moreover, a variety of putative novel nitrate-dependent As(III)-oxidizing bacteria were identified based on metagenomic analysis, mainly including Aromatoleum, Paenibacillus, Microvirga, Herbaspirillum, Bradyrhizobium, Azospirillum. Overall, all these findings indicate that nitrate reduction coupled with As(III) oxidation is an important nitrogen-As coupling process prevalent in paddy environments and emphasize the significance of developing and popularizing nitrate-based biotechnology to control As pollution in paddy soils and reduce the risk of As compromising food security.

Characterization of Enrichment Cultures of Anammox, Nitrifying and Denitrifying Bacteria Obtained from a Cold, Heavily Nitrogen-Polluted Aquifer

Anammox bacteria related to Candidatus Scalindua were recently discovered in a cold (7.5 °C) aquifer near sludge repositories containing solid wastes of uranium and processed polymetallic concentrate. Groundwater has a very high level of nitrate and ammonia pollution (up to 10 and 0.5 g/L, respectively) and a very low content of organic carbon (2.5 mg/L). To assess the potential for bioremediation of polluted groundwater in situ, enrichment cultures of anammox, nitrifying, and denitrifying bacteria were obtained and analyzed. Fed-batch enrichment of anammox bacteria was not successful. Stable removal of ammonium and nitrite (up to 100%) was achieved in a continuous-flow reactor packed with a nonwoven fabric at 15 °C, and enrichment in anammox bacteria was confirmed by FISH and qPCR assays. The relatively low total N removal efficiency (up to 55%) was due to nonstoichiometric nitrate buildup. This phenomenon can be explained by a shift in the metabolism of anammox bacteria towards the production of more nitrates and less N₂ at low temperatures compared to the canonical stoichiometry. In addition, the too high an estimate of specific anammox activity suggests that N cycle microbial groups other than anammox bacteria may have contributed significantly to N removal. Stable nitrite production was observed in the denitrifying enrichment culture, while no "conventional" nitrifiers were found in the corresponding enrichment cultures. Xanthomonadaceae was a common taxon for all microbial communities, indicating its exclusive role in this ecosystem. This study opens up new knowledge about the metabolic capabilities of N cycle bacteria and potential approaches for sustainable bioremediation of heavily N-polluted cold ecosystems.

Biochar-Polylactic Acid Composite Accelerated Reductive Dechlorination of Hexachlorobenzene in Paddy Soils under Neutral pH Condition

In order to clarify the effect of biochar-polylactic acid (PLA) composite on reductive dechlorination of HCB in paddy soils, an anaerobic incubation experiment was conducted with four treatments of Sterile control, Control, Biochar, and Biochar-PLA in Hydragric Acrisols (Ac) and Gleyi-Stagnic Anthrosols (An). The results showed that in Ac, biochar addition significantly promoted HCB degradation during the whole incubation period, but biochar-PLA composite inhibited HCB dechlorination due to the low soil pH in the early period and then accelerated HCB degradation while soil pH climbed to nearly neutral. The dechlorination rate of HCB in An was: Biochar-PLA > Biochar > Control > Sterilization control. The degradation rate of HCB in An was faster than in Ac, due to the higher iron content and neutral pH condition in An. The results indicated that biochar-PLA composite promoted the reductive dechlorination of HCB efficiently in paddy soil under nearly neutral pH condition.