Halophilic Microbes for Bio-Remediation of Salt Affected Soils
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Vol. 8(33), pp. 3070-3078, 13 August, 2014 DOI: 10.5897/AJMR2014.6960 Article Number: 16166C346757 ISSN 1996-0808 African Journal of Microbiology Research Copyright © 2014 Author(s) retain the copyright of this article http://www.academicjournals.org/AJMR Review Halophilic microbes for bio-remediation of salt affected soils Sanjay Arora1, Meghna J. Vanza2*, Riddhi Mehta2, Chirag Bhuva2 and Purvi N. Patel1 1Central Soil Salinity Research Institute, Regional Research Station, Bharuch 392012, Gujarat, India. 2V.N. South Gujarat University, Surat, Gujarat, India. Received 12 June, 2014; Accepted 21 July, 2014 Bacteria inhabiting soil play a role in conservation and restoration biology of higher organisms. The salt affected soils are dominated by many types of halophilic and halotolerant microorganisms, spread over a large number of phylogenetic groups. These microbes have potential for plant growth promotion and release of enzymes under salt stress. Halophiles have thus potential to remediate salt affected soils, enhancing plant growth and yield under high salt containing soils where plant growth is restricted. This eco-friendly approach for bio-remediation of salt affected soils to optimize crop yields under stress through halophiles has gained importance among researchers in recent past. Key words: Halophiles, bio-remediation, saline soils, salt tolerant bacteria. INTRODUCTION Life exists over the whole range of salt concentrations 1996). Salt accumulation in soil is characterized by saline encountered in natural habitats: from freshwater soil, contains high amount of soluble salts Ca2+, Mg2+, K+ + - - 2- 3- environments to hypersaline lakes or seas, and other and Na salt of Cl , NO3 , SO4 and CO3 etc; Sodic soil, places saturated with respect to sodium chloride dominated by Na+ salt and saline-sodic soil that have especially saline and saline-alkali soils. In many cases, high salt of Ca2+, Mg2+ and K+ as well as Na+. Salt- the soil properties are the most definitory of the limi- affected soils occupy an estimated 952.2 million ha of tations for the ecosystem functioning and, quite always, land in the world which constitutes nearly 7% of the total the soil is the component of the ecosystem more resilient land area and nearly 33% of the area of potential arable to changes. The influence of the high salt concentrations land. In India, the salt affected soils account for 6.727 masks other soil forming processes or soil properties and million ha (2.1%) of geographical area of the country. A environmental conditions, often altering them. build-up of soluble salts in the soil may influence its The soil that contains excess salts which impairs its behaviour for crop production through changes in the productivity is called salt-affected (Figure 1) (Chhabra, proportions of exchangeable cations, soil reaction, physical Corresponding author. E-mail: [email protected]. Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Arora et al. 3071 Figure 1. Salt affected soils. properties and the effects of osmotic and specific ion Halophilic microbes toxicity. Physical and chemical methods of their reclamation are The existence of high osmotic pressure, ion toxicity, not cost-effective and also the availability of gypsum or unfavourable soil physical conditions and/or soil flooding, other chemical amendments is a problem. The applications are serious constraints to many organisms and therefore of halophilic bacteria include recovery of salt affected salt-affected ecosystems are specialised ecotones. The soils by directly supporting the growth of vegetation thus organisms found there have developed mechanisms to indirectly increasing crop yields in salt affected soils. All survive in such adverse media, and many endemisms. halophilic microorganisms contain potent transport The halophilic microorganisms or "salt-loving" micro- + + mechanisms, generally based on Na /H antiporters, to organisms live in environments with high salt concen- expel sodium ions from the interior of the cell (Oren, 2002). tration that would kill most other microbes. Halotolerant Also, some halophiles express ACC deaminase activity and halophilic microorganisms can grow in hypersaline that removes stress, ethylene from the rhizosphere and environments, but only halophiles specifically require at some produce auxins that promote root growth. Halophilic least 0.2 molar (M) of salt for their growth. Halotolerant microbes are also found to remove salt from saline soils microorganisms can only tolerate media containing <0.2 (Bhuva et al., 2013). M of salt. Distinctions between different kinds of There are reports that potential salt tolerant bacteria halophilic microorganisms are made on the basis of their isolated from soil or plant tissues and having plant growth level of salt requirement and salt tolerance. According to promotion trait, helps to alleviate salt stress by promoting Kushner (1993) classification, microbes responsive to salt seedling growth and increased biomass of crop plants are defined under five groups: grown under salinity stress (Ravikumar et al., 2007; Chakraborty et al., 2011). 1. Non-halophilic, <0.2 M (~1%) salt, Endophytic salt tolerant bacteria residing within plant 2. Slight halophiles, 0.2-0.5 M (~1-3%) salt, tissues have been reported to be promoting the plant 3. Moderate halophiles, 0.5-2.5 M (~3-15%) salt, growth directly or indirectly through production of phyto- 4. Borderline extreme halophiles, 1.5-4.0 M (~9-23%) hormones, bio-control of host plant diseases and salt, and improvement of plant nutritional status (Pandey et al., 5. Extreme halophiles, 2.5-5.2 M (~15-32%) salt. 2008; Rosenblueth and Romero, 2006; Arora et al., 2014). They also possess the capacity to solubilize phosphates The halotolerant grow best in media containing <0.2 M (Hung and Annapurna, 2004; Son et al., 2006). (~1%) salt and also can tolerate high salt concentrations. Plant-microbe interaction is beneficial association This definition is widely referred to in many reports between plants and microorganisms and also a more (Arahal and Ventosa, 2002; Ventosa et al., 1998; Yoon et efficient method used for the reclamation of salt affected al., 2003). soils. Bacteria are the most commonly used microbes in this technique. However, archaea, fungi, actinomycetes, Soil microbial diversity etc that may be found in environments with high salt concentrations can also be effective. Thus, there is high Microbial community in the soil are not distributed at potential for bio-remediation applications for salt affected random. Factors such as soil composition, organic soils using halophilic microbes. matter, pH, water and oxygen availability, along with the 3072 Afr. J. Microbiol. Res. host plant, play major role in the selection of the natural solutes are low-molecular weight organic compounds flora (Ross et al., 2000). The soil gains importance, such as polyols, amino acids, sugars and betaines that especially in saline agricultural soils, where high salinity the halophilic and halotolerant bacteria accumulate results from irrigation practices and application of intracellularly to achieve osmotic balance. On the other chemical fertilizer. This effect is always more pronounced hand, halophilic bacteria tolerant to heavy metals could in the rhizosphere as a result of increased water uptake be used as bioassay indicator organisms in saline- by the plants due to transpiration. Hence, the polluted environments. Several halotolerant and halo- rhizobacteria form a group of the best adapted philic bacteria isolated from hypersaline soils tolerate microorganisms (Tripathi et al., 1998). high concentrations of different metals such as Co, Ni, Saline or hypersaline soils have yielded many Gram- Cd, or Cr (Nieto et al., 1989; Rios et al., 1998). positive species, and these have been characterized In the biological treatment, the micro-organisms taxonomically. The microbiota of hypersaline soils is conventionally used show only poor degradative efficacy more similar to those of nonsaline soils than to the due to the highly saline conditions. The potential of microbiota from hypersaline waters. This suggests that halophilic organisms in effluent treatment offers the general features of the environments are more important promise of innovative research. Other than that, in determining the microbiota in a particular habitat than halophilic is also used to recover saline soil by directly are individual factors such as high salinity (Quesada et supporting the growth of vegetation, thus indirectly al., 1983). increasing crop yields in saline soil. The structure and function of plant root system contri- butes to the establishment of the rhizosphere microbial population (Russell, 1977) and rhizosphere microbial DIVERSITY OF HALOPHILIC SOIL MICROORGANISMS communities are mainly determined by plant species (Marshner et al., 2001; Miethling et al., 2000) and soil Halophilic soil bacteria characteristics (Degens et al., 2000). The microbial community found around the roots of plants or rhizo- The soil is an important habitat for bacteria. Soil bacteria sphere is generally much greater than in the surrounding can be found as single cells or as microcolonies, soil environment (Nihorimbere et al., 2011). It reflects that embedded in a matrix of polysaccharides. Bacteria the rhizosphere supports higher microbial growth rates inhabiting soil play a role in conservation and restoration and activities as compared