Identification of microbial species present in a pesticide dissipation process in biobed systems using typical substrates from southeastern Mexico as a biomixture at a laboratory scale
Graphical abstract
Introduction
Biological beds or biobeds are a technology whose purpose is to contain the drip and effluents contaminated with pesticides, thus providing maximum adsorption and optimal conditions for the degradation of pesticides by the microbial activity present in the biomixture (Torstensson and Castillo, 1997). In biobeds, a biomixture is the most important component, and its correct composition is a prerequisite for the successful degradation of pesticides in contaminated water flows. The efficacy of biobeds is based on their ability to retain and degrade pesticides via the microbial activity (Karanasios et al., 2010a, Karanasios et al., 2010b).
For example, the use of bacteria for degradation and detoxification of numerous toxic chemicals is an important approach to consider for decontamination of polluted sites (Mervat, 2009). Nevertheless, it is known that degradation processes in biobed systems respond to the microbial activity present, as is the case for often-mentioned white-rot fungi during degradation of pesticides such as atrazine (Castillo et al., 2008). Some studies on biobeds or related research on microbial diversity normally are conducted using a straw-based biomixture as in the original biobed. Coppola et al. (2011) reported no significant changes in the composition of the microbial community at the end of a pesticide degradation process. Tortella et al. (2014) established that the microbial community structure (bacteria and fungi) remained relatively stable over time when high diazinon doses were applied to the biomixture being tested. These findings were taken into account to conduct the present study; additionally, it is not known for certain what other types of degrading microorganisms are present in the systems in operation. Several microorganisms able to utilize pesticides as a source of energy have been isolated. Some fungi such as Trametes hirsutus, Phanerochaete chrysosporium, Phanerochaete sordida, and Cyathus bulleri can degrade lindane and other pesticides (Singh and Kuhad, 1999, Singh and Kuhad, 2000; Singh et al., 1999); white-rote fungi belonging to the genus Phlebia (P. acanthocystis, P. brevispora, and P. aurea) participating in aldrin and dieldrin degradation have been reported (Xiao et al., 2011). Nonetheless, most evidence suggests that soil bacteria are the principal components responsible for enhanced biodegradation (Walker and Roberts, 1993). Bacteria such as Enterobacter cloacae, Bacillus cereus, Bacillus anthracis, Pseudomonas aeruginosa, Pseudomonas balearica, Pseudomonas indica, Pseudomonas otitidis, Ochrobactrum intermedium, and Providencia vermicola have proven to be capable of degrading atrazine in soil (El-Bestawy et al., 2013). Other bacteria such as Achromobacter xylosoxidans and Ochrobactrum sp. have been shown to degrade chlorpyrifos (Akbar and Sultan, 2016), and Streptomyces sp. was reported to degrade diazinon (Briceño et al., 2016).
Although bacterial activity is important for pesticide degradation, materials where microorganisms are deposited and growing are crucial too. Correct materials favor the immobilization process to carry out the biodegradation of pesticides. Materials such as chitosan, sawdust, straw, charcoal, plant fibers, corncob, bagasse, rice, husks of sunflower seeds, diatomite, and mycelium have proven to be good support materials in a process of biodegradation of pesticides (Dzionek et al., 2016).
Before we conducted the present research, Góngora-Echeverría et al. (2017) showed high pesticide dissipation rates and that a biomixture composed of soil–corn stover in the ratio 1:1 (V/V) is the most effective; additionally, it was proved that the biomixture and its physicochemical characteristics (lignin, pH, carbon‑nitrogen ratio [C/N], and water-holding capacity [WHC]) have significant effects on pesticide dissipation during testing of 11 soil-based biomixtures and soil; however, microbial diversity was not considered, and its relation with residual amounts of pesticides and with biomixtures was not studied.
Thus, the aim of this study was to identify the microbial diversity (archaea, bacteria, and fungi) during dissipation of five pesticides (2,4-D, atrazine, carbofuran, diazinon, and glyphosate) in miniaturized biobed systems at a laboratory scale involving local materials (agricultural soil, compost, sisal, corn stover, and seaweed) from southeastern Mexico as biomixture components. This is because they are available in different agricultural areas in Yucatán State. Accordingly, the relations of the microbial taxa with a biomixture and its physicochemical parameters were studied.
Section snippets
Substrates and biobed implementation
Previously, for microorganism identification in biobeds after a pesticide degradation process, these systems were implemented at a laboratory scale. Substrates used in biobeds were agricultural soil, sisal pulp, and vegetable compost, corn stover and seaweed in different proportions (soil always constituted 50% in all biobeds). According to the study (Góngora-Echeverría et al., 2017) where these substrates were tested, the main physicochemical characteristics were as follows: organic matter (OM)
Residual amounts of pesticides in biomixtures
Fig. 1 shows the residual amounts at 20 and 41 days for all pesticides in all biomixtures. It can be observed that 20 days was enough to dissipate >98% of all pesticides, with 2,4-D and atrazine being the most dissipated; biomixture M5 was the most effective in dissipating most pesticides, and soil M1 the least effective.
According to pesticide residual amounts, very fast dissipation was observed for all pesticides, with glyphosate being the only pesticide dissipated completely at 41 days in all
Conclusions
Microbial diversity and the relation with physicochemical parameters after a pesticide dissipation process in biobed systems were studied. High dissipation rates of five pesticides under study were observed in two cases, with glyphosate being the only one completely dissipated in all biomixtures at 41 days. Judging by DNA and sequencing analyses, bacteria were the microorganisms with the highest diversity of species. The presence of white-rot fungi (normally associated with pesticide dissipation
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