BIOREMEDIATION OF PESTICIDES
Among all types of xenobiotics, one of the main threats to the environment and human health are pesticides. In the recent years, pesticide use has increased due to the rapid population growth and a greater demand for high-quality food. As indicated by Javorekova et al. (2010), efforts are still being taken to successfully detoxify pesticides in soil by using already existing microflora or supplementing it with a specific microbiological culture. Laffin et al. (2010) show that there are still some lacunae in the knowledge on how microorganisms biodegrade many active substances of pesticides in soil. As reported by Yu et al. (2016), the retention time of such substances is often expressed by the half-life of biologically active ingredients and fillers present in the preparations used. According to Oleszczuk (2007), the scope and pace of all pesticide transformations depend on their chemical structure and concentration, type and number of microorganisms capable of their degradation or transformation, as well as the physicochemical properties of the environment in which the pesticides appear or accumulate. Such investigations are still desirable because pesticides can release more toxic metabolites than primary compounds into the environment. Most research focuses only on active substances and does not take into account the stabilizers, emulsifiers and auxiliaries present in pesticides, which may also adversely affect environmental biocoenosis. There are multidirectional transformations of such compounds in the environment, and identifying all possible intermediates is difficult or actually impossible (Sassolas et al. 2012).
The most frequently isolated microorganisms found in pesticides-contaminated environments include bacteria of the following strains: Acetobacter, Arthrobacter, Pseudomonas, Bacillus, Brevibacterium, Flavobacterium, Methylococcus, Klebsiella, etc. (Table 1) (Badawi et al. 2009, Hussain et al. 2011). Several fungal species have some potential of pesticides’ residues degradation, like Penicillium, Candida, Pleurotus, Trichoderma, Fusarium, Rhodotorula, Phaeneroclmete and Aspergillus (Saikia and Gopal 2004, Senthilkumar et al. 2011).
- 2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most widespread herbicides (McGuinness and Dowling 2009). 2,4-D salts are easily absorbed by the roots of plants and translocated into meristematic tissues of roots and shoots, where the compound acts as a plant hormone, causing their uncontrolled growth. Mobility of 2,4-D in soil often leads to pollution of surface and groundwater. Although the herbicide is biodegradable, it may persist in soil and water for a longer period of time (Germaine et al. 2006). The ability of endophytic bacteria to degrade 2,4-D has been demonstrated in an experiment conducted by Germaine et al. (2006). In these studies, the endophytic Pseudomonas putida VM1450 strain, derived from internal poplar tissues (Populus deltoids), was introduced into peas (Pisurn sativum). The inoculated plants were treated with
- 2,4-D and it was found that strain VM1450 actively colonized internal tissues of the plants. The inoculated plants were characterized by a greater ability to remove 2,4-D from the soil and they did not accumulate this herbicide in their tissues (Germaine et al. 2006).
Badawi et al. (2009) claim that the same bacteria and fungi can break down different groups of pesticides. For example, Arthrobacter globiformis D47, Sphingobium sp. YBL1 and Arthrobacter sp. N2 degrade phenylurea herbicides such as diuron, isoproturon, chlorotoluron and fluometuron (Sun et al. 2011). In turn, among the fungi decomposing isoproturon, chlorotoluron and diuron, there are Rhizoctonia solani, Bjerkandera adusta, Oxysporus sp. and Mortierella sp. Gr4 (Badawi et al. 2009). However, as observed by El-Sebai et al. (2007) and Hussain et al. (2011), Methylopila sp. TES and Sphingomonas sp. SH can mineralize isoproturon and its metabolites, but they are not able to decompose other pesticides related to phenylurea herbicides.
Among technologies of bioremediation in situ, the most used by researchers include biostimulation, bioaugmentation, bioreactors, bioventing, composting and land farming which are implemented to reduce, degrade, eliminate, and/or transform the pesticides in soil (Singh 2008).
BIOREMEDIATION OF HYDROCARBONS
Bioremediation of petroleum pollutants in the environment depends on both abiotic factors (concentration and type of pollution, physicochemical properties of contaminated soil, content of organic and biogenic compounds, e.g. nitrogen and phosphorus, temperature, oxygen content, humidity, reaction), and biotic ones such as the quantitative and qualitative composition of microorganisms in soil. Aromatic hydrocarbons (PAHs) are ubiquitous, and toxic for the environment. Among PAHs, the components of petroleum products are the most burdensome and at the same time dangerous for the environment and living organisms (Chambers et al.
- 2018). It is related to their resistance to degradation and to their accumulation in terrestrial and aquatic ecosystems (Maliszewska-Kordybach et al. 2008). As noted in the studies by Saleem (2016), such petroleum compounds can be removed using physical, chemical and/or biological methods. Each of these techniques can be employed in situ, i.e. in the place of contamination, or ex situ, in which the soil needs to be transported to the point of its treatment (Thapa et al.
- 2012). Unfortunately, most of the proposed solutions are still at an experimental stage because their use is limited in practice and the knowledge of РАН-degrading bacteria populations in situ is not sufficient.
In soil, petroleum products can come in various forms: as hydrocarbons dissolved in water, those floating on the surface of the soil solution or as pollutants adsorbed on soil particles. Using chemical bonds, hydrocarbons are combined with the organic components of humus and, therefore, PAHs are mainly instanced in the upper levels of soils rich in humus substances (Hajabbasi 2016). Soil resistance to degradation increases with the content of colloids and sorption capacity. The fastest-purifiable soils in biological processes are mainly those made of mineral materials with a small amount of humus. Moreover, Chen et al. (2010) point out that the soils with a high content of humus or clay minerals have a large soil sorption capacity because oil-derivative compounds are absorbed in them and can be more easily decomposed, processed and collected by various microorganisms. To increase the rate of degradation of pollutants, soil tends to be additionally enriched with nutrients necessary for microbial growth.
As reported by Mrozik and Piotrowska-Seget (2010), several approaches have been proposed for PAHs bioremediation in soil, which consist in increasing augmenting microorganisms adapted to heightened contaminant concentrations and/or producing surfactants to enhance bioavailability of PAHs. The most frequently determined microorganisms found in oil- contaminated environments include bacteria of the following genera: Acetobacter, Pseudomonas, Cornynebacterium, and fungi: Aspergillus, Candida, Trichoderma and Mortierella (Liste and Felgentreu 2006).
Noteworthy are Bacillus bacteria because these strains have the ability to produce biosurfactants and degrade oil in soil (Souza et al. 2015). For example, the introduction of B. subtilis Al into contaminated soil decreased the concentration of hydrocarbons in soil by 87% after 7 days (Parthipau et al. 2017). In addition, many Pseudomonas bacteria have enzymes involved in the degradation of aliphatic and aromatic hydrocarbons (Weyens et al. 2009, Pawlik et al. 2017). For this reason, these strains are used as biovaccines that, in assisted phytoremediation, increase the efficiency of the transformation of oil-derived impurities (Sessitsch et al. 2013, Sun et al. 2014).
Biodegradation of petroleum derivatives is a multi-stage process, taking place under both aerobic and anaerobic conditions, involving many different groups of microorganisms, often with a synergistic effect. One of the methods of controlling in situ biodegradation of areas contaminated with petroleum derivatives is to monitor the concentration of so-called key metabolites (Pawlik et al. 2017). Benzoyl-CoA, which does not belong to synthetic compounds, may be a specific marker for the catabolism of toluene, xylene and ethylbenzene, coupled with the sulphate respiration of microorganisms (Gabrielson et al. 2003). In anaerobic conditions, microorganisms can decompose hydrocarbons like benzene, toluene, ethylbenzene, monoaromatic hydrocarbons (BTEX), hexadecane and naphthalene, but their distribution is 4 times slower than in the aerobic environment (Sun et al. 2011).
Researchers pay special attention to endophytic bacteria which can promote the growth and development of plants, consequently increasing their biomass in soil contaminated with petroleum substances. They can also affect the bioavailability of organic pollutants in soil (Weyens et al. 2009). Many endophytic bacteria are able to grow in the presence of PAHs, and some can degrade these compounds, using them as a source of carbon and energy. Examples comprise endophytic Pseudomonas putida PD1 and Pseudomonas sp. Ph6 degrading phenanthrene (Hajabbasi 2016) or strains Stenotrophomonas sp. PI and Pseudomonas sp. P3 isolated from Canadian canon (Conyza canadensis) and cloverleaf (Trifolium pratense L.) capable of degrading naphthalene, phenanthrene, fluorene, pyrene and benzo(a)pyrene. Zhang et al. (2014) show that the inoculation of plants and soil with a bacterial consortium containing Bacillus subtilis strain J4 AJ (capable of degradation of diesel fuel) and strain Pseudomonas sp. U-3 (producing a bio-surfactant effectively reducing surface tension) promote the removal of diesel oil from the environment. In their studies, scholars observed that biosurfactants secreted by bacteria may also be carriers of microbial material, thanks to which the bioremediation process also takes place inside the sorbent. The use of metabolic cycles of microorganisms can therefore lead to final transformation products, i.e. C02 and H20 (Aidberger et al. 2005, Zhang et al. 2014). Moreover, degradation of phenanthrene by Acinetobacter sp., Flavobacterium sp., Pseudomonas sp., Rhodococcus sp. and Stretomyces flavovirens produces 2,4-hydrosyphenantrene, 1,2-dihydroxynaphthalene and plithalates, while 3,4-dihydroxyfluoren, 3,4 -dihydroxycoumarin, 1-indanone and salicylate were identified during biotransformation of fluorene (Zhang et al. 2014).
The literature provides a number of studies on the biodegradation processes of BTEX and PAHs in soil using fungi. Sasek et al. (2003) conducted research on the use of the fungus
Cladophialophoria sp. for the mineralization of BTEX, which showed that such degradation proceeds with higher dynamics for toluene, ethylbenzene and m-xylene than for benzene. Only the introduction of bacteria of the genus Rhodococcus sp. significantly accelerated the process of benzene mineralization. Metabolic profiles and the inhibitory nature of substrate interaction indicate that ethylbenzene, toluene, and m-xylene are degraded in the side chain by the same monooxygenase enzyme (Wiesche et al. 2003).
Wiesche et al. (2003) and Sasek et al. (2003) conducted PAH biodegradation processes with biopreparation based on indigenous microorganisms enriched with fungi of the genus Pleurotus ostreatus and Dichomitus sqitalens. Research with the above fungi showed that they have the ability to produce ligninoline enzymes responsible for biodegradation of PAHs with a higher number of rings (5-6) in soil (Ahn et al. 2010). Enrichment of microorganisms with fungi of the genus Pleurotus ostreatus increases the biodegradation efficiency of pentacyclic PAHs (benzo(a)pyrene (BaP), benzo(a)anthracene (BaA) and dibenzo(a, h)anthracene (DaA)) as compared to the action of a biopreparation only based on the bacteria themselves (Wiesche et al. 2003). In addition, by using bioaugmentation with a biopreparation containing bacterial cultures Pseudomonas putida enriched with fungi of the genus Pleurotus ostreatus and Irpex lacteus, the content of individual PAHs was reduced by 66% over a period of 10 weeks (Li et al. 2007).
The interaction between fungi and bacteria is beneficial for the process of mineralization of petroleum hydrocarbons. The above mentioned mushrooms have a metabolic capacity of biodegradation of BTEX and PAH, similar in many respects to that of bacteria. Therefore, fungi should not be ignored in the development of effective bioremediation strategies (Wiesche et al. 2003).
Moreover, research is being conducted using biofilters technology where both bacteria and fungi strains are applied. However, some strains of fungi are able to stay longer on the biofilter as compared to bacteria (Husaini et al. 2008). For example, Exophiala oligospermum placed into the perlite deposit biodegraded toluene in a stable way for several months (Esteves et al. 2004), while spores of Scedosporium apiospermum introduced into the vermiculite deposit increased greatly the speed of toluene purification at high loads for 2 months of continuous operation of the deposit (Kennes and Veiga 2004). Therefore, one should further explore the possibility of inoculation of biofilters with strains of fungi in bioremediation technology. The focus should not be placed only on bacteria, as usually is the case, but rather on filamentous mycelium fungi that offer greater prospects than yeast fungi. Therefore, fungi should not be ignored in the development of effective bioremediation strategies (Wiesche et al. 2003).
