Studies on Decolourisation of Synthetic Dyes Using Spent Mushroom Substrates

Abstract

Mushroom cultivation generates two main end products, comprised of the mushroom fruit body and the substrate known as spent mushroom substrate (SMS). Substrate is considered as spent when one full crop of mushroom is harvested and further extension of the period become unremunerative. For each metric tons of fruit bodies of mushrooms produced, at least an equivalent amount of spent mushroom substrate is generated, and this needs to be disposed . This disposal can be costly because it is bulky in nature. During the previous year’s efforts have been made to find out amicable ways to dispose of spent mushroom substrate after crop harvest because if it is left unattended it may cause environmental problems including nuisance. On the other side, wastewater from textile industries poses a threat to the environment in a large part of the world. The use of microorganisms or their enzyme systems are the two different biological approaches widely practiced for treatment of hazardous chemicals and effluents. The present study was planned after giving due considerations to the physico-chemical, microbiological and biochemical properties of spent mushroom substrate, as it may act as a good bioremediative agent. In the present study spent substrate of Pleurotus sajor-caju and Agaricus bisporus had been studied for extracellular ligninolytic enzymes and dyes decolourisation potential. Among SMS of different mushrooms, the SMS from P. sajor-caju exhibited highest population along with diversity of both fungi and bacteria. Five fungi viz., Aspergillus fumigatus,Paecilomyces variotii, Pichia guilliermondii, Schizophyllum commune and Pezizomycotina sp. with potential dye decolourisation potential have been recorded to thrive on SMS of different mushrooms by using 5.8S rRNA gene sequencing and BLASTn techniques. Similarly, by using 16S rRNA gene sequencing and BLASTn tecnhniques, six potential bacteria viz., Bacillus subtilis, B. pumilus, B. licheniformis, Pseudomonas fluorescens, Sphingobacterium multivorum and Rummelibacillus stabekissi have also been identified, out of which B. licheniformis isolated from P. sajor-caju SMS, exhibited highest decolourisation potential of 66.10% against Orange II sodium salt, followed by Bacillus pumilus (57.7%). Out of isolated fungi, Schizophyllum commune from P. sajor-caju SMS has been recorded to exhibit highest decolourisation potential (95%) within 10 days against Chicago sky blue. This fungus also exhibited dye decolourisation potential of 100, 92.50, 81.60, 73.40 and 67.80% against Starch azure, Methyl violet 2B, Rhodamine B, Orange II sodium salt and Methylene blue, respectively. Moreover, the study also evaluated the ligninolytic enzymes activities, dyes decolourisation by spent substrate filtrate and biomass production during dyes decolourisation. In optimization of cultural parameters role of pH, temperature, growing medium, additional carbon sources, heavy metals, initial concentration of dyes, addition of veratryl alcohol and manganese ions, maintaining of agitated/stationary conditions and inoculation with pellet and intact forms of mushroom mycelia were also studied for achieving highest dyes decolourisation with spent substrate of P. sajor-caju. Lower concentration of Methyl violet 2B, Rhodamine B and Azure B showed higher decolourisation efficiency (100%) with spent substrate of P. sajor-caju compared with higher initial concentrations of dyes. Decolourisation of Methyl violet 2B showed that lower concentration of Manganese ions, veratryl alcohol and addition of carbon and starch enhances decolourisation of Methyl violet 2B. Presence of lead, cadmium, cobalt and nickel ions supported higher decolourisation with spent substrate of P.sajor-caju. The study also presents variation in microbial population of Agaricus bisporus, Pleurotus sajor-caju and Volvariella volvacea spent substrates along with ligninolytic enzymes activity and textile effluent decolourisation potential of microorganisms isolated from these SMSs. The effect of temperature, pH, carbon sources and immobilizing agents on effluent decolourisationusing different combinations of these microorganisms has also been studied. Schizophyllum commune and Pezizomycotina sp. from P. sajor-caju SMS, exhibited highest activities of laccase (11.8 and 8.32 UmL-1) and lignin peroxidase (339 and 318 UL-1), while Pseudomonas fluorescens of Manganese peroxidase. Highest decolourisation was in presence of glucose and sucrose at 30 ºC, and microbial consortium comprised of the immobilized forms of S. commune and Pezizomycotina sp. on wheat straw and broth cultures of P. fluorescens, Bacillus licheniformis and Bacillus pumilus. First order kinetics was recorded for the dye decolourisation process by using SMS of P. sajor-caju, A. bisporus and potential fungi isolated from SMS. Bioremediation of synthetic dyes by SMS and their ligninolytic enzymes may be one alternative and effective way for textile effluent treatment and solid waste management of mushroom industry.