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Soil is a fundamental component of life on Earth and is the basis of many ecosystems. The minerals, organic matter, and microorganisms in the soil can be considered as one system, with close associations and interactions occurring between them. The interactions of these compounds have an important impact on terrestrial processes, environmental quality, and ecosystem health1. Soil pollution poses serious environmental problems worldwide. The indiscriminate, long-term, and excessive application of recalcitrant and toxic substances, such as pesticides, petroleum products, plastics, and other chemicals, has serious effects on soil ecology and, as a result, can alter soil microbiota. Microbial communities in soils are composed of a wide range of organisms in different physiological states, with the majority being bacteria and fungi. Many of the contaminants in soils have medium- to long-term stability, and their persistence can lead to the development of adaptive mechanisms that allow the microorganisms to utilize recalcitrant substances as nutrients2,3. These microorganisms can, therefore, be considered for bioremediation techniques.
Bioremediation tries to mitigate the effects of pollution by using microorganisms and their enzymes for the degradation or transformation of waste into less toxic or non-toxic compounds. Various species of archaea, bacteria, algae, and fungi possess this bioremediation ability4. As a result of their particular biodegradative actions, fungi are especially promising organisms for bioremediation. They can attack different substrates using their hyphal network, enabling them to penetrate the soil matrix more efficiently than other microorganisms. Additionally, they can reach inaccessible interstices where contaminants are difficult to remove5, and they can also survive low moisture levels6. Moreover, fungi synthesize different cassettes of unspecific enzymes, usually to degrade natural recalcitrant substances such as cellulose, lignin, and humic acids. Those that lack the target substrate can be involved in the degradation of a wide range of recalcitrant pollutants, such as hydrocarbons, plastics, and pesticides7,8,9,10. Therefore, although many fungal species have already been reported as bioremediation agents, there is increasing interest in exploring species that have not yet been studied to select candidates for the bioremediation of recalcitrant contaminating substances. The species already known to have bioremediation properties belong to the phyla Ascomycota11,12,13, Basidiomycota14,15, and Mucoromycota. For example, the genera Penicillium and Aspergillus are well known to be involved in the degradation of aliphatic hydrocarbons13, different plastic polymers16,17,18, heavy metals19, and dyes20. Similarly, studies carried out on basidiomycetes fungi, such as Phanerochaete chrysosporium and Trametes versicolor, have revealed their involvement in the oxidation of recalcitrant materials such as aromatic hydrocarbons13 and plastics21. Another example of fungi involved in the biodegradation processes are the zygomycetes Rhizopus spp., Mucor spp., and Cunninghamella spp.22,23. In particular, Cunninghamella is able to oxidase aromatic hydrocarbons and is considered a model organism for studying the detoxification of products from a wide range of xenobiotics13.
There are several fungal enzymes involved in the major degradative processes of recalcitrant materials24,25, such as esterase, laccase, peroxidase, and protease. Laccases are copper-containing oxidases produced in the cell and subsequently secreted, that allow the oxidation of a variety of phenolic and aromatic compounds. They can degrade ortho and para diphenols, the amino group-containing phenols, lignin, and the aryl group-containing diamines26. Peroxidases use hydrogen peroxide as a mediator to degrade lignin and other aromatic compounds. There are many different peroxidases, but the ones with the greatest potential to degrade toxic substances are lignin peroxidase and manganese peroxidase27.
Esterases and proteases belong to the group of extra- or ecto-cellular enzymes, which act outside their cells of origin but are still bound to them. These enzymes can catalyze the hydrolysis of large recalcitrant molecules into smaller ones. Due to their low substrate specificity, these enzymes can play a key role in the bioremediation of various pollutants, such as textile dyes, effluents released from the pulp and paper industries and leather tanning, petroleum products, plastics, and pesticides28,29,30.
A number of screening methods to select for bioremediative fungal strains have already been published. For example, straw-based agar medium has been used to screen for white-rot fungi with high potential in the polycyclic aromatic hydrocarbons (PAH) degradation31; and small pieces of rotting wood have been placed onto malt extract agar (MEA) to isolate wood-rotting fungi32. However, most of the methods that have already been proposed select very specific fungi for their activity of interest. This research proposes a wider approach for selecting soil fungi with a broader range of action. The method relies on the initial plating of serial dilutions of soil samples onto a medium amended with humic acids or lignocellulose mixed with antibiotics to select fungi with the ability to degrade these natural recalcitrant substances. Humic acids and lignocellulose, in fact, are substances that are extremely resistant to biodegradation since they have very complex molecular structures, and this allows them to be excellent indicators of the degradative ability of the tested fungi33,34. Subsequently, the fungi selected in the first tests are screened to identify those with the potential to degrade specific pollutants such as petrolatum, used engine oil, and plastics. Finally, qualitative enzymatic tests are performed to detect fungal strains able to produce enzymes involved in the biodegradation processes of recalcitrant substances. For this purpose, protease and esterase tests are conducted, while gallic acid and guaiacol are used as indicators of laccase and other ligninolytic enzyme production35,36. These substrates are used because a strong correlation has been found between the ability of fungi to oxidize them to their brown-colored form and the possession of ligninolytic ability37,38,39.
Through these protocols, it is possible to isolate fungal strains with high degradative potential and a broad spectrum of action directly from soil samples. The isolation of these fungal strains could help find new candidates for bioremediation purposes.