Effect of alkali pretreatment time on kitchen waste anaerobic digestion performance enhanced by alkali pretreatment combined with bentonite: performance enhancement, microbial community structure, and functional gene analysis

Kitchen waste was mainly composed of carbohydrates, lipids, and proteins. Anaerobic digestion (AD) of kitchen waste usually occurred acidification and further deteriorated. In our previous study, alkali pretreatment combined with bentonite (AP/Be) treatment was proved to enhance high solid AD of kitchen waste. However, effects of AP time on AP/Be were not yet studied. This study investigated the effects of AP time on AP/Be treatment on enhancing high solid AD. The results showed that compared with the control group, the cumulative methane production rate could be increased by 3.30 times (149.7 mL CH4/g VS) and the volatile solids (VS) reduction rate increased by 63.36%. Microbial community analysis showed that the relative abundance of Methanosarcina and Methanosaeta were increased from 6.49 and 7.83% to 47.14 and 16.39% respectively. Predictive functional analysis showed that AP/Be treatment increased the abundance of energy production and conversion, coenzyme transport, and metabolism. This study revealed the potential mechanism of AP/Be enhanced kitchen waste AD performance and AP/Be was a potential strategy to strengthen AD.

Buffered loofah supported Microalgae-Bacteria symbiotic (MBS) system for enhanced nitrogen removal from rare earth element tailings (REEs) wastewater: Performance and functional gene analysis

Rare earth element tailings (REEs) wastewater, which has the characteristics of high ammonia nitrogen (NH₄⁺-N) and low COD. It can cause eutrophication and biotoxicity in water which is produced in high volumes, requiring treatment before final disposal. Microalgae-Bacteria symbiotic (MBS) system can be applied in REEs wastewater, but its low extent of nitrogen removal and instability limit its application. By adding biodegradable carrier as both carbon source and carrier, the system can be stabilized and the efficiency can be improved. In this work, the extent of NH₄⁺-N removal reached 100% within 24 h in a MBS system after adding loofah under optimal conditions, and the removal rate reached 127.6 mg NH₄⁺-N·L^-1·d^-1. In addition, the carbon release from loofah in 3 d reached 408.7 mg/L, which could be used as a carbon source to support denitrification. During 90 d of operation of the MBS system loaded with loofah, the effluent NH₄⁺-N was less than 15 mg/L. At phylum level, Proteobacteria were dominant which accounted for 78.2%. Functional gene analysis showed that enhancement of microalgae assimilation was the main factor affecting NH₄⁺-N removal. This work expands our understanding of the enhanced role of carbon-based carriers in the denitrification of REEs wastewater.

Site-Directed Mutagenesis of Bifidobacterium Strains

At present, only a limited number of Bifidobacterium species are amenable to genetic manipulation using mutagenesis. This lack of genetic accessibility among the majority of bifidobacterial strains represents a significant roadblock for the study of gene function and expression in these potential probiotics. Genetic tools for generating mutants are difficult to develop for bifidobacteria, as they require workarounds for obstacles such as low transformation efficiencies, and the presence of differing and sometimes multiple restriction modification systems, in different strains. Site-directed mutagenesis is a frequently applied molecular strategy for the generation of targeted mutations, resulting in gene deletion or disruption, or alteration of their expression, thereby revealing information regarding their function. This strategy has been employed as a molecular tool in some Bifidobacterium strains and is typically achieved using a nonreplicating vector, harboring a DNA fragment corresponding to an internal part of the gene to be mutated. This vector is introduced into a bifidobacterial cell of the strain in question by electroporation. Through homologous recombination, this vector is integrated into the genomic DNA of said cell, disrupting the coding region of the targeted gene, thus preventing the expression of a functional protein product. Such mutant versions of Bifidobacterium strains may then be assessed for alterations in their phenotype or gene expression.