Investigation of the Persistence, Toxicological Effects, and Ecological Issues of S-Triazine Herbicides and Their Biodegradation Using Emerging Technologies: A Review

S-triazines are a group of herbicides that are extensively applied to control broadleaf weeds and grasses in agricultural production. They are mainly taken up through plant roots and are transformed by xylem tissues throughout the plant system. They are highly persistent and have a long half-life in the environment. Due to imprudent use, their toxic residues have enormously increased in the last few years and are frequently detected in food commodities, which causes chronic diseases in humans and mammals. However, for the safety of the environment and the diversity of living organisms, the removal of s-triazine herbicides has received widespread attention. In this review, the degradation of s-triazine herbicides and their intermediates by indigenous microbial species, genes, enzymes, plants, and nanoparticles are systematically investigated. The hydrolytic degradation of substituents on the s-triazine ring is catalyzed by enzymes from the amidohydrolase superfamily and yields cyanuric acid as an intermediate. Cyanuric acid is further metabolized into ammonia and carbon dioxide. Microbial-free cells efficiently degrade s-triazine herbicides in laboratory as well as field trials. Additionally, the combinatorial approach of nanomaterials with indigenous microbes has vast potential and considered sustainable for removing toxic residues in the agroecosystem. Due to their smaller size and unique properties, they are equally distributed in sediments, soil, water bodies, and even small crevices. Finally, this paper highlights the implementation of bioinformatics and molecular tools, which provide a myriad of new methods to monitor the biodegradation of s-triazine herbicides and help to identify the diverse number of microbial communities that actively participate in the biodegradation process.

Melamine degradation to bioregenerate granular activated carbon

The industrial chemical melamine is often detected in surface water used for drinking water production, due to its wide application and insufficient removal in conventional wastewater treatment plants. Melamine can be removed from water by adsorption onto granular activated carbon (GAC), nevertheless, GAC needs periodic reactivation in costly and energy intense processes. As an alternative method, GAC can also be regenerated using biomass capable of degrading melamine in a process called bioregeneration. We assessed melamine biodegradation in batch experiments in fully oxic and anoxic, as well as in alternating oxic and anoxic conditions. Additionally, we studied the effect of an additional carbon source on the biodegradation. The most favourable conditions for melamine biodegradation were applied to bioregenerate GAC loaded with melamine. We demonstrate that melamine can be biodegraded in either oxic or anoxic conditions and that melamine degrading biomass can restore at least 28% of the original GAC adsorption capacity. Furthermore, our results indicate that bioregeneration occurs mainly in the largest pore fraction of GAC, impacting adsorption kinetics. Overall, we show that bioregeneration has a large potential for restoring GAC adsorption capacity in industrial wastewater.

Bacterial biodegradation of melamine-contaminated aged soil: influence of different pre-culture media or addition of activation material

This study aimed to investigate the biodegrading potential of Arthrobacter sp. MCO, Arthrobacter sp. CSP, and Nocardioides sp. ATD6 in melamine-contaminated upland soil (melamine: approx. 10.5 mg/kg dry weight) after 30 days of incubation. The soil sample used in this study had undergone annual treatment of lime nitrogen, which included melamine; it was aged for more than 10 years in field. When R2A broth was used as the pre-culture medium, Arthrobacter sp. MCO could degrade 55 % of melamine after 30 days of incubation, but the other strains could hardly degrade melamine (approximately 25 %). The addition of trimethylglycine (betaine) in soil as an activation material enhanced the degradation rate of melamine by each strain; more than 50 % of melamine was degraded by all strains after 30 days of incubation. In particular, strain MCO could degrade 72 % of melamine. When the strains were pre-cultured in R2A broth containing melamine, the degradation rate of melamine in soil increased remarkably. The highest (72 %) melamine degradation rate was noted when strain MCO was used with betaine addition.

Melaminivora alkalimesophila gen. nov., sp. nov., a melamine-degrading betaproteobacterium isolated from a melamine-producing factory

A taxonomic study was carried out on strain CY1T, which is a novel bacterium isolated from wastewater sludge of a melamine-producing factory in Sanming city, Fujian, China. Strain CY1T was shown to rapidly and completely degrade melamine to NH3 and CO2 under aerobic conditions. The isolate was Gram-stain-negative, short-rod-shaped and motile by one unipolar flagellum. Growth was observed at salinities from 0 to 7 % NaCl (optimum, 0.1 %), at temperatures from 15 to 50 °C (optimum, 40–45 °C) and at pH 7–9.5 (optimum pH 9.5). Quinone-8 was detected as the major respiratory quinone. 16S rRNA gene sequence comparisons showed that strain CY1T was affiliated to the family Comamonadaceae in the class Betaproteobacteria . It was most closely related to members of the genera Alicycliphilus (95.5 %), Diaphorobacter (94.6–95.1 %), Acidovorax (92.9–95.4 %), Delftia (93.0–93.6 %) and Comamonas (92.6–93.9 %). The average nucleotide identity (ANI) values between strain CY1T and those representing related genera ranged from 84.0 to 86.1 % using Mummer, and from 74.9 to 81.1 % using blast. The dominant fatty acids were C16 : 1ω7c and/or C16 : 1ω6c, C16 : 0, C10 : 0 3-OH and C18 : 1ω7c and/or C18 : 1ω6c, and the major polar lipids consisted of phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, one unidentified phospholipid and one unidentified aminophospholipid. The G+C content of the chromosomal DNA was 69.5 mol%. On the basis of the phenotypic and phylogenetic data, strain CY1T represents a novel species of a new genus, for which the name Melaminivora alkalimesophila gen. nov., sp. nov. is proposed. The type strain of Melaminivora alkalimesophila is CY1T ( = CCTCC AB 2012024T = DSM 26006T).

Characterization of pure cultures isolated from sulfamethoxazole-acclimated activated sludge with respect to taxonomic identification and sulfamethoxazole biodegradation potential

Background

Sulfamethoxazole (SMX, sulfonamide antibiotic) biodegradation by activated sludge communities (ASC) is still only partly understood. The present work is focusing on nine different bacteria species capable of SMX biodegradation that were isolated from SMX-acclimated ASC.

Results

Initially 110 pure cultures, isolated from activated sludge, were screened by UV-absorbance measurements (UV-AM) for their SMX biodegradation potential. Identification via almost complete 16S rRNA gene sequencing revealed five Pseudomonas spp., one Brevundimonas sp., one Variovorax sp. and two Microbacterium spp.. Thus seven species belonged to the phylum Proteobacteria and two to Actinobacteria. These cultures were subsequently incubated in media containing 10 mg L^-1 SMX and different concentrations of carbon (sodium-acetate) and nitrogen (ammonium-nitrate). Different biodegradation patterns were revealed with respect to media composition and bacterial species. Biodegradation, validated by LC-UV measurements to verify UV-AM, occurred very fast with 2.5 mg L^-1 d^-1 SMX being biodegraded in all pure cultures in, for UV-AM modified, R2A-UV medium under aerobic conditions and room temperature. However, reduced and different biodegradation rates were observed for setups with SMX provided as co-substrate together with a carbon/nitrogen source at a ratio of DOC:N – 33:1 with rates ranging from 1.25 to 2.5 mg L^-1 d^-1.

Conclusions

Media containing only SMX as carbon and nitrogen source proved the organisms’ ability to use SMX as sole nutrient source where biodegradation rates decreased to 1.0 – 1.7 mg L^-1 d^-1. The different taxonomically identified species showed specific biodegradation rates and behaviours at various nutrient conditions. Readily degradable energy sources seem to be crucial for efficient SMX biodegradation.