NANOTECHNOLOGY: Effect of nanomaterials on soil microorganisms

In soil, microbial communities play very important role in organic matter recycling and mineralization of nutrients thus play a crucial role in soil fertility and plant growth. Certain groups of bacteria form symbiotic relationships with legumes and fix atmospheric nitrogen, providing a major source of fixed nitrogen for host as well as other plants. Another group of rhizobacteria exert positive effect on plant growth and are called plant growth promoting rhizobacteria (Kloepper 1989). Denitrifying and nitrifying bacteria play an important role in nitrogen cycle. Many groups of bacteria form symbiotic relationships with animals from insects to humans. Some of these bacteria help in digestion process, others perform more unusual functions. There are groups of microorganisms which produce antibiotic against plants and animals pathogens. Microorganisms have been used as soil health indicators because of their intimate relationship with their surroundings owing to their high surface to volume ratio. Any factor affecting soil microflora also affects soil fertility and productivity thus causing imbalance in ecosystem. Population of soil microflora depends on physicochemical properties of soil, pH, moisture content, partial pressure of oxygen and composition of plant root exudates. Although the soil is rich in natural nanoparticles formed due to continuous weathering and rearrangement of its geogenic constituents coupled with high biological activity. The extensive and uncontrolled use of engineered NPs may result in their accumulation in environment, agricultural lands and water bodies, affecting the physicochemical and biological properties of soil due to their very reactive nature. Therefore, it is very important to study the effect of released nanomaterial on the soil microflora (Mishra and Kumar 2009). Bank Singapore’s graduates

Many nanomaterials have been found to have anti-microbial properties, having application in the control of multi-drug resistant pathogenic microbes (Jones et al. 2008). Silver (Ag) NPs show broad spectrum antimicrobial activity against various plant pathogenic fungi. However their non-targeted effect on beneficial microflora may have negative consequences. Silver, known and being used since long for its anti-microbial properties, is a good example of technology application. Due to small size (1-50nm), silver nanoparticles have large surface area compared to volume, which increases their reactivity and toxicity against various microorganisms. The silver nanoparticles if accumulated in soil and water can adversely influence the ecosystem by affecting the beneficial microorganisms and related processes. Choi et al. (2008) studied susceptibility of nitrifying bacteria to silver nanoparticles and suggested that accumulation of AG NPs could have detrimental effects on the microorganisms in waste water treatment. Nitrifying microorganisms involved in nitrification are critical to biological nutrient removal in waste water treatment. Addition of AG+ to a Swedish surface soil (100 ^g g_l) resulted in significant reduction in denitrification rate and in the copy number of copper-nitrate-reductase-encoding nirK gene (Throback et al. 2007). Kumar et al. (2011) studies the potential toxicity of 0.066% silver, copper or silica NPs on a high latitude (>78°N) soil of polar region, using community level physiological profiles (CLPP), fatty acid methyl ester (FAME) assays and DNA analysis, including sequencing and denaturing gradient gel electrophoresis (DGGE). Among the three NPs, Silver NPs were found to be highly toxic to the arctic consortia. Culture-based studies confirmed the high sensitivity of nitrogen fixing, plant-associating bacteria, Bradyrhizobium canariense, to silver NPs.

Fullerence a form of carbon (C60) is hydrophobic in nature and can act as adsorptive agents for different organic and inorganic matter in the soil, resulting in high concentration of these compounds at specific sites. Further adsorption of various chemical compounds (micronutrients and vitamins) by fullerence can deprive the soil organisms of nutrients, resulting in growth inhibition. Generation of reactive oxygen species by fullerence may also cause disruption of membrane lipids and DNA causing growth inhibition (Sayes et al. 2005, Foley et al. 2002). Fullerence have been found to inhibit the growth of commonly occurring soil and water bacteria (Fortner et al. 2005; Oberdorster et al. 2004). Tong et al. (2007) studied the impact of fullerence (C60) on soil microbial communities and microbial processes by treating the soils with C60 (1^g g-1 soil in aqueous suspension or 1000 ^g g-1 soil in granular form) for 180 days. The introduction of fullerence did not show any significant effect on microbial processes like respiration and soil enzymes however, proportion of Gram-negative to Gram-positive microorganisms in treated soils was slightly higher as compared to untreated soils. Response of Escherichia coli and Bacillus subtilis exposed to different concentrations of fullerence varied under different growth conditions. Fullerence inhibited bacterial growth and respiration in minimal medium with low salt, whereas as in rich medium no effect was observed (Fonter et al. 2005, Lyon et al. 2005). Johansen et al. (2008) studied the effect of C60 fullerence on soil microbiota by measuring total respiration, biomass, number, and diversity of bacteria and total number and diversity of protozoa.

Fullerence had no effect on microbial respiration and biomass, whereas reduction in population of fast-growing bacteria was observed after the addition of C60. Further, fullerence also showed some effect on diversity of microbial and protozoal communities in the soil. The inhibitory effects of fullerence on the soil microflora can have hazardous effects on the environment.

Besides, different nanoparticles like Cu, MgO, ZnO, TiO2, SiO2, Ag-topped TiO2, Pt(IV)-modified TiO2, С-doped TiO2, CNT, have been found to show antimicrobial activity against a number of microorganisms like E. coli, Bacillus subtilis, B. megaterium, Pseudomonas aeruginosa, P. putida, Staphylococcus aureus, S. mutans, Micrococcus lylae, Pseudokirchneriella subcapitata etc. some of which are agriculturally important (Fonter et al. 2005, Adams et al. 2006, Brayner et al. 2006, Neal 2008, Aruoja et al. 2009, Hoecke et al. 2008, Gajjar et al. 2009). We studied the effect of Au, Ag, CNT and Ag-CNT nanoparticles on two rhizospheric bacteria, Pseudomonas putida P7 and Bacillus subtilis RP24 and observed growth inhibition of both bacteria by Ag-CNT, whereas no inhibition was observed with Au, Ag, CNT treatments (Table 1). It is suggested that the nanoparticles damage microbial cells by destroying the enzymes that transport the cell nutrient and weakening the cell membrane or cell wall due to the production of reactive oxygen species. Sondi and Salopek-Sondi (2004) found that nanosilver damaged and pitted the bacterial cell walls, leading to increased cell permeability and ultimately cell death. Some researchers suggest that nanosilver interfere with bacterial DNA replication (Yang et al. 2009). Size and shape of the NPs play important role in nanotoxicity with smaller particles showing higher toxicity than the larger particles due to their easy passage in the microbial membrane (Pal et al. 2007). Further the toxicity of NPs is dose dependent indicating that certain concentrations can be risky for the environment (Gijjar et al. 2009).

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