Mushrooms in the Bio-Remediation of Wastes from Soil

Mushrooms have played a great role in the field of bioremediation. Mushrooms are saprophyte highly specialized group of macro-fungi with a distinctive fruiting body, and have a unique capacity for degradation of certain types of organic pollutants like lignocellulotic wastes and bio-sorption of heavy metals. The degradation of ligenocellulotic wastes are initiated by the release of extracellular enzymes to the environment. The lignocellulaytic enzyme starts to degrade and breakdown the complex lignocellulotic wastes in to smaller and readily available molecules for their utilization. The present reviewed paper describes briefly the concerns regarding the extracellular mushroom enzymes, with having many potential applications in bioremediation of agricultural wastes, heavy metals and toxic organic compounds. Therefore, research is needed to develop understanding of integrated mushroom cultivation that optimizes mushroom utilization in the field of environmental remediation, while supporting other ecosystem services.

Mushroom also, can produce notable nutraceutical products, which have many health benefits. They provide people with an additional vegetable of high quality and enrich the diet with high quality proteins, minerals and vitamins which can be of direct benefit to the human health and fitness. The extractable bio-active compounds from medicinal mushrooms would enhance human's immune systems and improve their quality of life. Edible mushroom are highly nutritious and can be compared with eggs, milk and meat in terms of protein content. The composition of essential amino acids in mushroom is high and close to the need of the human body. Mushroom is easily digestible and it has no cholesterol content (Oei, 2003).
Mushroom is very environmental friendly, capable of converting the lignocellulosic waste materials into food, feed and fertilizers. It can be cultivated in containers like jars, basins, trays, plastic bags and other similar substances by providing artificial controlled conditions (Quimio, 1998). They are relatively fast growing organisms, can be harvested in 3 to 4 weeks after spawning. Thus, mushroom cultivation is short return significant biological decomposer and it plays a crucial role in converting wastes into valuable products. The spent substrate left after harvesting the mushrooms, with innumerable mushroom threads (collectively referred to as mycelia), which have been biochemically modified by the mushroom enzymes into a simpler and more readily digestible form, can be used as animal feed (more palatable), bio-fertilizer for soil fertility enrichment and biogas.
It has been revealed recently that mushroom mycelia can play a significant role in the restoration of damaged environments (myco-restoration) through myco-filtration (using mycelia to filter water or mycelia are used as a filter to remove toxic materials and microorganisms from water in the soil), myco-forestry (using mycelia to restore forests), myco-remediation (using mycelia to eliminate toxic waste, and myco-pesticides (using mycelia to control insect pests). These methods represent the potential to create clean ecosystem, where no damage will be left after fungal implementation (Stamets, 2005).
Besides their ability to degrade and convert lignocellulosic materials into human food, they can also act as effective bio-sorbent of toxic metals (Costa and Leite, 1991). Compared to green plants, mushrooms can build up large concentrations of some heavy metals, such as lead, cadmium and mercury in them as reported by Gast et al. (1988) in his studies on interaction of heavy metals with soil and mushrooms. This would suggest that fungi possess a very effective mechanism that enables them to take up some trace elements from the substrate (Latiff et al., 1996).
Soils contaminated with heavy metals and/or organic pollutants are generally left abandoned for several years because they may not be safe for human health as well as agricultural production. Moreover, to excavate and remove contaminated soil is relatively costly and environmentally unsafe procedure, if use chemical methods. But, mushroom cultivation is cost effective and friendly to environment, a biological process of converting wastes into valuable products. It is therefore, hoped that the advocating for mushroom farming will become a very important activity, which may lead to cleaning contaminated land.
Some of the comprehensive reviewed articles provide basic statues and different perspectives of mushroom cultivation and production viz. research history (Vetayasuporn, 2006) oyster mushroom cultivation on different cellulosic substrates, (Baysal and Peker, 2001) waste paper recycling, (Ashrafuzzaman, 2009) growth and yield of shiitake mushroom on sawdust from different plants, (Imtiaj and Ajijur, 2008) economic viability of mushrooms cultivation Rice straw, wheat straw, sugarcane waste, banana leaves, grass and sawdust. Moreover, it has been reported that mushroom is a bio-sorption of metals; it can absorb and accumulate heavy metals throughout their bodies. More than 90% of studies have been done on the production and cultivation of mushroom from agricultural wastes; few studies have been carried out on Khat (Catha edulis), water hyacinth (EicchorniacrassipesSolms), and industrial sewage, domestic and municipal wastes for the production of mushroom. However, none of them pay attention to integrated mushroom production and cultivation for bioremediation of those wastes especially heavy metals from soil and water. Moreover, these wastes have been discarded along the roads and cause unpleasant odours and unhygienic conditions. So, there is an urgent need to retabulate the real involvement and benefits of this unique fungus in bioremediation/biodegradation of khat, industrial, sewage, domestic and municipal wastes and the mechanism of how to reduce heavy metals.
The presences of extracellular enzymes indicate diverse potential applications of mushroom in decomposition of wastes. Advancement in our knowledge about mushroom biodegradation may facilitate their wide applications for safe and sustainable environmental development and agricultural productivity. This article is aimed to highlight the biodegradation potential of mushroom and provides a moderate review on the progress made in mushroom research related to biodegradation and identifying the critical research needs for developing and implementing successful mushroom bioremediation as a model worldwide. The possible ways to maximize its multiple uses for mitigating the various pollutants were also suggested.
For instance the main reason that white-rot fungi are active to such a wide range of compounds is their release of extracellular lignin-modifying enzymes, with a low substrate specificity, so they can act upon various molecules that are broadly similar to lignin (Adenipekun and Lawal, 2012). The enzymes present in the system employed for degrading lignin include lignin-peroxidase (LiP), manganese peroxidase (MnP), various H2O2 producing enzymes (Kirk and Farrell, 1987) and laccase, although the three types of enzymatic activity are not Genetic engineering in mushroom for enhanced bioremediation Genetic engineering So far there are no transgenic mushroom strains available commercially but several research groups are working towards that direction with good progress. Sequencing the mushroom genome is crucial since mushrooms are now regarded as being very important to the environment. This is in view of the fact that mushrooms help in degradation of agricultural wastes into less harmful substances, removal of heavy metals from waste flows and also play a role in production of biofuels (Thwaites et al., 2007). Genetic engineering in mushrooms may provide opportunities to exploit for maximum benefit in the field of remediation technologies and to manipulate the tolerance, degradation potential of mushrooms against various organic and inorganic pollutants through introduction of desired genes. Thus, the development and application of genetic engineering of the native mushrooms will definitely offer more efficient and enhanced bioremediation of the pollutants viz. heavy metals, organics or co-contaminants, making the bioremediation more viable for environment remediation. Some points have to be put in mind such as biosafety assessment, risk mitigation, and factors of genetic pollution before using the genetically engineered fungi at field level including mushroom.

Other research needs related to mushrooms
It is well known that mushrooms are saprophytes and globally important in oxidizing the potent wastes. There is an urgent need to optimize the effect of different wastes for mushroom cultivation and production. The population dynamic and diversity of the mushrooms need to be studied with respect to edaphic and climatic conditions of environment. Moreover, integrated mushroom production and cultivation for bioremediation of wastes especially heavy metals from soil and water for sustainable agricultural production should be studied properly. Some of the commercial edible mushrooms and their origin are presented in table 2.

Common cultivated mushrooms
Although there are well over 300 genera of mushrooms and related fleshy basidiomycetes, only a few species of these fungi are cultivated commercially. This may be due to the fact that many of them are mycorhizal and may not sporulate in the absence of the host. But many saprophytic species have been amenable to cultivation. Some of the more common cultivated species listed here ( Table 2). The most common cultivated mushroom were oyster mushroom (Pleurotus ostreatus), button mushroom (Agaricus bisporus), Shiitake mushroom (Lentinus edodes), velvet stem mushroom (Flammulina velutipes), paddy straw mushroom (Volvariella volvacaea), ear fungus (Auricularia auricula). However, other cultivated mushrooms serve as medicinal and flavoring agent are the Reishi mushroom (Ganoderma lucidum) which is used as an alternative medicine and also as flavouring agent in Japan; the Nameko (Pholiota nameko) grown in the orient and Tremelia fuciformis or white jelly fungi that is grown for use as food supplements in Taiwan. Varieties of A. bisporus that are grown commercially include the crimini and portabello. Truffles (Tuber species) live in close mycorhizal association with roots of specific trees. They are considered a food delicacy and rated as one of the most expensive natural food in the world (Trappe et al., 2007). Some of the edible mushrooms involved in the decomposition of various agricultural wastes are as presented in Table 3.

Conclusion
Mushrooms were discovered over a century ago; however, mushrooms have not been explored well. The research related to bioremediation potential of mushrooms is still in infancy stage. The production of new mushroom cultivars with novel and improved traits will provide the industry with options for solving food problems and increase the production efficiency. Improvement of tools available to the breeder, decoding mushroom genome and commercial pressure facing the industry can propel efforts for new strain development in the future. For better harnessing of mushroom in industrial application and bioremediation, a number of limitations need to be worked out such as integrated mushroom production and cultivation for bioremediation of wastes especially heavy metals from soil and water for sustainable agricultural production, metal uptake mechanism, antagonistic and synergetic effects of metals in uptake and the characters of these accumulates in molecular level. The metal uptake potential of mushrooms should be critically analyzed for health risk as 46 mushrooms have become a popular delicacy of modern world. With the combination of biotechnology and genetic engineering, mushrooms can be exploited for in situ bioremediation of a wide range of inorganic and organic pollutants.