Ecological tools used in remediation of soil pollutants

The development of environmentally sound, low-cost, low-input technologies with economic benefits is very much essential to tackle remediation of polluted soils. The uses of microbes, plants and organic materials to clean up contaminated soils are cost-effective, environment friendly and equally protective of human health and the environment. Advantages and disadvantages of bioremediation are given in Table 1.

Table 1. Advantages and disadvantages of bioremediation.

Microbes

Soil microorganisms are involved in many biochemical processes in soil that enhance revegetation, thereby increasing the stability of contaminated ecosystems. Microorganisms are the key components of bioremediation enabling both aerobic as well as anaerobic degradation of soil contaminants (Abatenli et al. 2017). Microorganisms survive and sustain in a wide range of environmental conditions, this property of microorganisms makes them most suitable for utilization in treatment of contaminated soils through various techniques. Microorganisms survive and undergo changes by mutation and formation of spores to sustain in unfavorable conditions. This property of microorganisms enables them to survive and sustain in a wide range of environmental conditions and makes them most suitable for utilization in treatment of contaminated soils through various techniques. They are also known as biocatalyst because they can sustain and function in extreme environmental conditions (Tomei and Daugulis 2013). Microorganisms such as bacteria, fungi, algae and actinomycetes can effectively degrade soil contaminants. Microorganisms are an inherent part of the Earth system and play a major role in flow' of nutrients as carbon, nitrogen, phosphorus and sulfur through biogeochemical cycles. Flow' of nutrients is mediated by biogeochemical processes as mineralization, immobilization and redox reactions. These processes are foundational basis of microbial metabolism for utilization of energy source from various substrates. In microbial remediation, microorganisms obtain energy and nutrition from biodegradation or biotrausfonnation of soil pollutants. Polluted soils contain compounds as aliphatic and aromatic hydrocarbons, inorganics such as heavy metals, radioactive elements, biomolecules, and antibiotics mostly in polymeric forms. These compounds primarily contain carbon, nitrogen, phosphorus and sulfur, and trace elements essential for growth and development of microorganisms.

Microbial metabolism and soil pollutants

Microorganisms derive nutrition and primary energy source, i.e., carbon, from the organic and inorganic pollutants present in the soil. Microbial physiology or mode of action determines the transformation rate of toxic soil contaminants into non-toxic and simpler forms and utilize these products of degradation as source of nutrition. The metabolic pathways in all microorganisms are almost similar, involving a series of enzymes functioning in aerobic or anaerobic conditions. According to Suthersan et al. (1999), biotransfomiation or biodegradation by microorganisms may be categorized depending on requirement for carbon as energy source. In the first categoiy, the requirement of carbon is inherently growth-related, microorganisms bio-transform toxic soil contaminants to derive carbon for growth and development, indicating metabolic transformation. Secondly, microorganisms degrade soil contaminants and utilize carbon for sustaining respiration and maintaining cell viability mostly when concentration of carbon is low', such biotransfomiation is known as cometabolic transformation. Metabolism and cometabolism may coexist in both aerobic and anaerobic degradation of soil contaminants by microorganisms (Suthersan and McDonough

1999). For example, aerobic degradation and anaerobic degradation of PAHs and pesticides are accomplished through metabolic as well as cometabolic pathways (Tomei and Dauglulis 2013, Krishna and Philip 2011. Lei et al. 2005, Quintero et al. 2006). Apart from carbon source, nitrogen, sulfur, phosphorus and trace elements as potassium, iron, molybdenum, etc. are equally important for microbial metabolism.

Most common bacteria used in bioremediation of contaminated soils are Pseudomonas, Aeromonas, Flavobacteria, Chlorobacteria, Coiynebacteria, Acinetobacter, Mycobacteria, Streptomyces, Bacillus, Arthrobacter, Aeromonas, Cyanobacteria, Citrobacter, Burkholderiaxenovorans, Desulfovibrio vulgaris, Amycolatopsistucumanensis, etc. (Francis and Nancharaiah 2015, Tomei and Dauglulis 2013, Maui and Kumar 2014, Amoroso and Abate 2010). Fungal strains include Cladosporium resinae, Rhizopus arrhizus, Aspeigillus niger, Pltanarochaelechrysosporium, Bierkanderaadusta, Phlebia sp., Fusarium sp. and Hypocrea sp. (Francis and Nancharaiah 2015, Tomei and Dauglulis 2013, Quintero et al. 2009. Ingle et al. 2014). Among fungal species, most commonly utilized yeast in bioremediation are Candida lipolytica, C. tropicalis, Rhodoturularubra, and Aureobasidion (Trichosporon) pullulans, Rliodotorulaaurantiaca and C. ernobii (Miranda et al. 2007). Algal species that can bioremediate polluted soils are Ascophyllum nodosum, Cldorella vulgaris, Cladophora fascicularis, Fucusvesiculosus, Oscillatoria quadripunctulata, Scenedesmus acutus, Sargassum filipendula, etc. (Yadav et al. 2017).