Sandeep Bisht

Bioremediation of polyaromatic hydrocarbons (PAHs) using rhizosphere technology

The remediation of polluted sites has become a priority for society because of increase in quality of life standards and the awareness of environmental issues. Over the past few decades there has been avid interest in developing in situ strategies for remediation of environmental contaminants, because of the high economic cost of physicochemical strategies, the biological tools for remediation of these persistent pollutants is the better option. Major foci have been considered on persistent organic chemicals i.e. polyaromatic hydrocarbons (PAHs) due to their ubiquitous occurrence, recalcitrance, bioaccumulation potential and carcinogenic activity. Rhizoremediation, a specific type of phytoremediation that involves both plants and their associated rhizospheric microbes is the creative biotechnological approach that has been explored in this review. Moreover, in this review we showed the significance of rhizoremediation of PAHs from other bioremediation strategies i.e. natural attenuation, bioaugmentation and phytoremediation and also analyze certain environmental factor that may influence the rhizoremediation technique. Numerous bacterial species were reported to degrade variety of PAHs and most of them are isolated from contaminated soil, however few reports are available from non contaminated soil. Pseudomonas aeruginosa , Pseudomons fluoresens , Mycobacterium spp., Haemophilus spp., Rhodococcus spp., Paenibacillus spp. are some of the commonly studied PAH-degrading bacteria. Finally, exploring the molecular communication between plants and microbes, and exploiting this communication to achieve better results in the elimination of contaminants, is a fascinating area of research for future perspective.

Biodegradation of naphthalene and anthracene by chemo-tactically active rhizobacteria of populus deltoides

Several naphthalene and anthracene degrading bacteria were isolated from rhizosphere of Populus deltoides, which were growing in non-contaminated soil. Among these, four isolates, i.e. Kurthia sp., Micrococcus varians, Deinococcus radiodurans and Bacillus circulans utilized chrysene, benzene, toluene and xylene, in addition to anthracene and naphthalene. Kurthia sp and B. circulans showed positive chemotactic response for naphthalene and anthracene. The mean growth rate constant (K) of isolates were found to increase with successive increase in substrate concentration (0.5 to 1.0 mg/50ml). B. circulans SBA12 and Kurthia SBA4 degraded 87.5% and 86.6% of anthracene while, Kurthia sp. SBA4, B. circulans SBA12, and M. varians SBA8 degraded 85.3 %, 95.8 % and 86.8 % of naphthalene respectively after 6 days of incubation as determined by HPLC analysis.

Bioremediation of dyes by fungi isolated from contaminated dye effluent sites for bio-usability

Biodegradation and detoxification of dyes, Malachite green, Nigrosin and Basic fuchsin have been carried out using two fungal isolates Aspergillus niger, and Phanerochaete chrysosporium, isolated from dye effluent soil. Three methods were selected for biodegradation, viz. agar overlay and liquid media methods; stationary and shaking conditions at 25 °C. Aspergillus niger recorded maximum decolorization of the dye Basic fuchsin (81.85%) followed by Nigrosin (77.47%), Malachite green (72.77%) and dye mixture (33.08%) under shaking condition. Whereas, P. chrysosporium recorded decolorization to the maximum with the Nigrosin (90.15%) followed by Basic fuchsin (89.8%), Malachite green (83.25%) and mixture (78.4%). The selected fungal strains performed better under shaking conditions compared to stationary method; moreover the inoculation of fungus also brought the pH of the dye solutions to neutral from acidic. Seed germination bioassay study exhibited that when inoculated dye solutions were used, seed showed germination while uninoculated dyes inhibited germination even after four days of observation. Similarly, microbial growth was also inhibited by uninoculated dyes. The excellent performance of A. niger and P. chrysporium in the biodegradation of textile dyes of different chemical structures suggests and reinforces the potential of these fungi for environmental decontamination.

Consortium of Plant-Growth-Promoting Bacteria: Future Perspective in Agriculture

The term “plant-growth-promoting rhizobacteria” (PGPR) include soil bacteria that colonize the roots of plants following inoculation onto seed and enhance plant growth. The bacteria useful to plants were proposed to be characterized in two general types: bacteria forming a symbiotic relationship with the plant and another the free-living ones found in the soil but are often found near, on, or even within the plant tissues. The PGPR are known to enhance growth by several direct mechanisms—like biofertilizers fix nitrogen, phytostimulators directly promote the growth of plants by the production of hormones, and several other metabolites like siderophore, ACC deaminase, etc., are produced by PGPR strains for plant growth enhancement. Also, biocontrol agents that are able to protect plants from soilborne infection by deleterious microorganisms also offer environment-friendly strategy for pest control. Recently, application of two or more PGPR as consortium is taking gain in field application worldwide. This offers multifarious approach of promoting plant growth and improve yield. In this review, the various strategies for consortium formulation are described. In fact, use of rhizobia with free-living nitrogen fixers or with phosphate solubilizers including VAM fungi has been widely reported. Also, application of biocontrol agents along with direct growth promoters is also observed as holistic approach for sustainable agriculture. Further, tailor-made consortium is sometimes designed to include other benefits like improving soil health.

Rhizoremediation: A Promising Rhizosphere Technology

An increasingly urban population and industrialized global economy over the last century have serious consequences on the environment. Understanding the sources, pathways and contaminants in the urban environment is essential for making informed management decisions. Urban areas are major concentrators, repositories and emitters of a myriad of chemicals because of the wide range and intensity of human and anthropogenic activities. Common contaminants include petroleum hydrocarbons (PHCs), polycyclic aromatic hydro‐ carbons (PAHs), halogenated hydrocarbons, pesticides, solvents, metals, salt and the resulting stresses on human and ecosystem health are well documented [1]. Polycyclic aromatic hydrocarbons are a class of complex organic chemicals consisting of over hundred different organic compounds. PAHs are unique contaminants in the environment because they are generated continuously by incomplete combustion of organic matter, for instance in forest fires, home heating, traffic, and waste incineration [2]. PAHs are hydrophobic compounds and their persistence in the environment is chiefly due to their low water solubility [3]. Generally, solubility of PAHs decreases and hydrophobicity increases with an increase in number of fused benzene rings. In addition, volatility decreases with an increasing number of fused rings [4]. The major source of PAHs is from the combustion of organic material [5]. PAHs are formed naturally during thermal geologic production and during burning of vegetation in forest and bush fires [6]. PAHs and their alkyl homologous may also be derived from biogenic precursors during early diagnosis [7]. However, anthropogenic sources, particularly from fuel combus‐ tion, pyrolytic processes, spillage of petroleum products, waste incinerators and domestic heaters [8] are significant sources of PAHs in the environment. At depth 90-135 cm, only

Interaction Among Rhizospheric Microbes, Soil, and Plant Roots: Influence on Micronutrient Uptake and Bioavailability

Soils resulting in micronutrient deficiency in agricultural land and pastureland are increasing globally. Such micronutrient deficiency is due to lower nutrient availability, lower nutrient mobility, and lower capacity of plants to take up nutrients from the rhizosphere. The rhizosphere extends up to a few millimeters from the root surface into the surrounding soil and is rich in microbial activity and diversity. The activity and types of microbes and the soil characteristics influence the uptake and transport of micronutrients in the roots. From the root zone, mobilization of micronutrients in the edible part of plants and their bioavailability is another question. The availability and uptake of various micronutrients in the rhizosphere is again influenced by soil properties and plant root exudates, and depends on microbial interactions with plant roots. The micronutrient transfer dynamics from the microbial cell to the plant cell is also influenced by the physiology of plant–microbe interactions. For diffusion-supplied micronutrients, if a large diffusion gradient exists between the root surfaces and the soil, a large amount could be shipped toward the roots. Conversely, when the capacity of root cells to take up micronutrients exceeds the rate of nutrient replenishment in the root zone, the uptake rate is regulated by nutrient availability rather than the capacity of plant roots to absorb nutrients. Plants exude a wide range of organic compounds and inorganic ions into the rhizosphere, changing the micro-chemical and biological zone of the rhizosphere and enhancing acclimatization or modification toward a particular biotic and abiotic environment. Absolute understanding of the multifaceted and intricate interactions dominating the relationship among plants, microbes, and soil that influence the composition of root exudates is still far off. Understanding of the plant–microbe–soil interaction mechanism for the uptake and mobilization of micronutrients and their bioavailability in the edible part of plants will open an avenue in biological science which could help solve the problem of micronutrient deficiency in consumers.

Biotrophic Plant-Microbe Interactions for Land Reclamation and Sustainable Agriculture Development

Anthropogenic undesired actions intended at agricultural and technological advancement have led to the non-judicious creation and usages of various chemicals. Contamination of soil and formation of barren lands are a worldwide crisis, and reclamation of this using chemical or physical means is not a solution. The negative aspects of pollutants in the soil and environment lead to diverse impact on human beings, flora and fauna also. This undesirable facet relies on the pollution type, its severity and nature. The hunt for alternative methods for digging and incineration to clean contaminated sites resulted in the application of bioremediation techniques, but this is not cost-effective. The cost-effective and viable mode could be efficient utilization of plant-microbe interaction (PMI) pair in agricultural land reclamation. In the process of active rhizosphere functioning, root exudates of plant lead to proliferation, survival, and working of microorganisms, which subsequently results in a more efficient degradation of contaminants. The plant root system actually helps to spread microbes in the soil and assists in penetrating otherwise hard soil layers and surfaces. The inoculation of pollutant-degrading bacteria on plant seed can be an important additive to improve the efficacy of bioremediation or plant bioaugmentation. Biotrophic PMI is promising, a relatively novel technique employed in reclamation of the contaminated or degraded agricultural soils. It may be defined as the exploitation of efficient microbes along with their host plants to utilize or remove, obliterate, or impound hazardous chemicals at a particular site. This technology has so far been used experimentally to take away toxic heavy metals and other pollutants from contaminated soil; expansion of its capacity for applications to remove and degrade organic pollutants in the environment is the next phase. This chapter presents an overview of present aspects of microbes-plant relations in reclamation for feasible and viable augmentation of agriculture land biodiversity.