Pseudomonas sp. AOB-7 utilizes PHA granules as a sustained-release carbon source and biofilm carrier for aerobic denitrification of aquaculture water

Nature-Inspired Strategies for Sustainable Degradation of Synthetic Plastics

Synthetic plastics have become integral to our daily lives, yet their escalating production, limited biodegradability, and inadequate waste management contribute to environmental contamination. Biological plastic degradation is one promising strategy to address this pollution. The inherent chemical and physical properties of synthetic plastics, however, pose challenges for microbial enzymes, hindering the effective degradation and the development of a sustainable biological recycling process. This Perspective explores alternative, nature-inspired strategies designed to overcome some key limitations in currently available plastic-degrading enzymes. Nature's refined degradation pathways for natural polymers, such as cellulose, present a compelling framework for the development of efficient technologies for enzymatic plastic degradation. By drawing insights from nature, we propose a general strategy of employing substrate binding domains to improve targeting and multienzyme scaffolds to overcome enzymatic efficiency limitations. As one potential application, we outline a multienzyme pathway to upcycle polyethylene into alkenes. Employing nature-inspired strategies can present a path toward sustainable solution to the environmental impact of synthetic plastics.

Bacteria for Bioplastics: Progress, Applications, and Challenges

Bioplastics are one of the answers that can point society toward a sustainable future. Under this premise, the synthesis of polymers with competitive properties using low-cost starting materials is a highly desired factor in the industry. Also, tackling environmental issues such as nonbiodegradable waste generation, high carbon footprint, and consumption of nonrenewable resources are some of the current concerns worldwide. The scientific community has been placing efforts into the biosynthesis of polymers using bacteria and other microbes. These microorganisms can be convenient reactors to consume food and agricultural wastes and convert them into biopolymers with inherently attractive properties such as biodegradability, biocompatibility, and appreciable mechanical and chemical properties. Such biopolymers can be applied to several fields such as packing, cosmetics, pharmaceutical, medical, biomedical, and agricultural. Thus, intending to elucidate the science of microbes to produce polymers, this review starts with a brief introduction to bioplastics by describing their importance and the methods for their production. The second section dives into the importance of bacteria regarding the biochemical routes for the synthesis of polymers along with their advantages and disadvantages. The third section covers some of the main parameters that influence biopolymers' production. Some of the main applications of biopolymers along with a comparison between the polymers obtained from microorganisms and the petrochemical-based ones are presented. Finally, some discussion about the future aspects and main challenges in this field is provided to elucidate the main issues that should be tackled for the wide application of microorganisms for the preparation of bioplastics.

Characterization of simultaneous ammonium and nitrate removal and microbial communities in airlift reactor using 3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) as carbon source and biofilm carrier

As a novel trend, solid carbon sources are applied to act as electron donors and biofilm carrier in biological denitrification process. In this study, simultaneous nitrate and ammonium removal process in an airlift sequencing batch reactor using 3-hydroxybutyrate-co-3-hydroxyvalerate (PHBV) as carbon source and biofilm carrier under intermittent aeration conditions was established to treat effluent of synthetic marine recirculating aquaculture system. The results showed that maximum nitrate and ammonia nitrogen removal rates of 0.45 and 0.09 kg m-3 d-1 were achieved. No significant nitrite accumulation was found during 200-day operation, while effluent dissolved organic carbon accumulation and particle size reduction significantly increased. Microbial community analysis and batch tests illuminate that the generated sludge and attached biofilm played important roles in nitrogen removal. This study demonstrates the potential mechanism for the nitrogen removal process mediated by 3-hydroxybutyrate-co-3-hydroxyvalerate and provide a new idea for the alternative solutions of solid carbon sources.

A mechanistic review on aerobic denitrification for nitrogen removal in water treatment

The traditional biological nitrogen removal technology consists of two steps: nitrification by autotrophs in aerobic circumstances and denitrification by heterotrophs in anaerobic situations; however, this technology requires a huge area and stringent environmental conditions. Researchers reached the conclusion that the denitrification process could also be carried out in aerobic circumstances with the discovery of aerobic denitrification. The aerobic denitrification process is carried out by aerobic denitrifying bacteria (ADB), most of which are heterotrophic bacteria that can metabolize various forms of nitrogen compounds under aerobic conditions and directly convert ammonia nitrogen to N₂ for discharge from the system. Despite the fact that there is no universal agreement on the mechanism of aerobic denitrification, this article reviewed four current explanations for the denitrification mechanism of ADB, including the microenvironment theory, theory of enzyme, electron transport bottlenecks theory, and omics study, and summarized the parameters affecting the denitrification efficiency of ADB in terms of carbon source, temperature, dissolved oxygen (DO), and pH. It also discussed the current status of the application of aerobic denitrification in practical processes. Following the review, the difficulties of present aerobic denitrification technology are outlined and future research options are highlighted. This review may help to improve the design of current wastewater treatment facilities by utilizing ADB for effective nitrogen removal and provide the engineers with relevant references.

Evaluation of NOx removal from flue gas and Fe(II)EDTA regeneration using a novel BTF-ABR integrated system

A promising process is under development for the removal of NOx and regeneration of Fe(II)EDTA in a novel biotrickling filter-anaerobic baffled reactor (BTF-ABR) integrated system at 50 ± 0.5 ℃. In this work, we investigated the NOx removal capacity of a BTF under different O₂ concentrations (7.0 vol%, 5.25 vol% and 3.5 vol%), and tested the effect of an ABR on NOx removal and regeneration of Fe(II)EDTA. The results showed that the NOx removal capacity was significantly increased with the O₂ concentration reduced from 7.0% to 3.5%. The microoxygen environment produced by the BTF-ABR integrated system was more conducive to the removal of NOx and regeneration of Fe(II)EDTA compared with that in the BTF. Real-time polymerase chain reaction (PCR) analysis showed that the coordinated expression of denitrification genes was the major reason for no N₂O emission, along with no nitrate and nitrite accumulation. The 16S rRNA gene amplicon sequencing analysis showed that the cooperation of denitrifying bacteria (Klebsiella, Petrimonas, Rhodococcus and Ochrobactium) and iron-reducing bacteria (Klebsiella, Geobacter and Petrimonas) in the system was the key to the stable and efficient removal of NOx and the regeneration of Fe(II)EDTA simultaneously.

Ammonia removal through combined methane oxidation and nitrification-denitrification and the interactions among functional microorganisms

It would be highly beneficial to use the methane produced by anaerobic digestion, which is low cost and accessible, as the carbon source in the removal of nitrogenous contaminants in wastewater. However, there is a knowledge gap regarding coupling systems that entail methane oxidation, nitrification, and denitrification, which restricts their industrial application. In this study, we acclimated a mixed culture to deal with simultaneous nitrification-denitrification coupled to methane oxidation in a laboratory-scale hollow-fiber membrane biofilm reactor, which achieved a steady ammonia removal rate of 38.09 mg N/(L•d). Furthermore, a series of batch experiments were conducted to test methane oxidation coupled to nitrate denitrification (AME-D3), nitrite denitrification (AME-D2), and simultaneous nitrification and denitrification (ME-SND). The molar ratio between methane consumed and nitrate reduced (C/N) equals 10 and 5 mol CH₄C mol^-1 NO₃N in AME-D3 and AME-D2, averagely and respectively. Without methane injection, the removal of nitrates and nitrites was very low, indicating that the coupling of nitrate/nitrite denitrification and methane oxidation was beneficial. The average ammonia removal rates in the 20% O₂ and 25% O₂ groups were 20.06 and 22.03 mg N/(L•d) in the ME-SND system, respectively. Without methane, the ammonia oxidation rate declined, and large amounts of nitrite accumulated. As traditional ammonia and nitrite oxidation approaches are autotrophic, we proposed the possibility of heterotrophic nitrification-aerobic denitrification (HN-AD). To study the coupling systems, the microbial communities and functional bacteria were analyzed. The results indicated that the system contained a guild of methanotrophs (mainly Methylobacter) and HN-AD bacteria (mainly Chrysobacterium and Comamonas).

A critical review of aerobic denitrification: Insights into the intracellular electron transfer

Aerobic denitrification is a novel biological nitrogen removal technology, which has been widely investigated as an alternative to the conventional denitrification and for its unique advantages. To fully comprehend aerobic denitrification, it is essential to clarify the regulatory mechanisms of intracellular electron transfer during aerobic denitrification. However, reports on intracellular electron transfer during aerobic denitrification are rather limited. Thus, the purpose of this review is to discuss the molecular mechanism of aerobic denitrification from the perspective of electron transfer, by summarizing the advancements in current research on electron transfer based on conventional denitrification. Firstly, the implication of aerobic denitrification is briefly discussed, and the status of current research on aerobic denitrification is summarized. Then, the occurring foundation and significance of aerobic denitrification are discussed based on a brief review of the key components involved in the electron transfer of denitrifying enzymes. Moreover, a strategy for enhancing the efficiency of aerobic denitrification is proposed on the basis of the regulatory mechanisms of denitrification enzymes. Finally, scientific outlooks are given for further investigation on aerobic denitrification in the future. This review could help clarify the mechanism of aerobic denitrification from the perspective of electron transfer.