Radhey Shyam Sharma

Bioresources and Environmental Biotechnology Laboratory

Department of Environmental Biology

University of Delhi, Delhi - 110 007, INDIA

Alteration in the natural plant communities is the first visible symptom of degraded habitats. Such habitats are characterized by reduction in biodiversity (above and below ground) and a multitude of abiotic stresses (nutrients, moisture, etc.). The key ecological functions responsible for sustenance of the ecosystem are also lost (Boyer and Wratten, 2010). The loss of these functions is primarily due to changes in soil properties (physico-chemical and biological) which determine the productivity of the ecosystem. Ecologically significant functional groups of soil bacteria affect the soil structure, nutrient availability and organic matter content which in turn determine the plant recruitment, establishment and hence the productivity of stressed  and degraded habitats (Sharma et al., 2010; Rau et al., 2009). Therefore, stress-induced changes in above-ground biodiversity are primarily due to loss of below-ground biodiversity (Bardgett et al., 2005). Consequent loss of above and below ground biodiversity creates an extremely harsh environment which is non-conducive for natural regeneration of the native ecosystem. In fact, such places become a favourite place for weedy exotic species which further adds to the stress on the habitat. Due to such alterations in the native ecosystems, an array of ecosystem goods and services available to mankind also get diminished, which affect the quality of life (Wainger and Price, 2004). Therefore, such stressed-habitats are an ecological and economic burden on society.

Ecological restoration of degraded habitats has been recognized as one of the components of sustainable development which ensures long-term supply of the ecosystem services for mankind, minimizes the rate of climate change and increases the socio-economic status of native communities (Schmidhuber and Tubiello, 2007). Wild grasses have been considered the ‘nurse species’, which can colonize extreme habitats. They are bottom-up control species and regulate ecosystem functioning. These species can thrive in nutritionally poor habitats, contribute to soil formation by enhancing the decomposable biomass and are able to coexist and give way to other native species making rapid ecosystem development (Sharma et al., 2010; Rau et al., 2009). Wild grasses are among the few naturally colonizing native species at such sites but their spread is generally slow, therefore the sites remain mostly barren even after several decades. Establishment of wild grasses having economic and ecological significance would be a viable strategy to convert the degraded habitats into biologically and sociologically significant habitats. Presence of ecologically diverse functional group of rhizobacteria might contribute to the ecological success of the wild grasses. Therefore, the spread of wild grasses could be facilitated by using ecologically diverse functional group of rhizobacteria, which contribute to ecological success of the wild grasses. They help the host plant and process the habitat directly and indirectly by: (i) nutrient enrichment (release of phosphate, iron etc. from insoluble mineral complexes, fixation of atmospheric nitrogen, etc.), (ii) biological storage of phosphate and iron (polyphosphate and hemophores etc.), (iii) chelation and mobilization of nutrients to the host plant (siderophore production), (iv) enhancing nutrient acquisition and colonization potential of host plant (production of phytohormones, reduction of stressed-ethylene by production of 1-aminocyclopropane-1-carboxylate deaminase), and(v) protecting the host plant from biotic stresses such as pests and pathogens (production of HCN, siderophore and antibiotics etc.) (Rau et al., 2009). However, limited studies are available on rhizosphere microbes of wild grasses of arid and semi-arid regions.

Keeping this in view, field surveys of stressed/degraded habitats in Delhi region were carried out and Saccharum munja and S. ravennae were identified as among the naturally colonizing native wild grasses (Fig.1). They are perennial wild grasses which are excellent soil binders due to their extensive root network system and forms tall thick clumps with high biomass tufts. Moreover, it is also an integral component of the socio-economic fabric (used for making rope, hand fans, baskets, broom, mats, shield for crop protection etc.) of the native people (Sharma et al., 2005). To fasten the colonization of these grasses at stressed sites, there is an urge to identify and characterize its rhizobacteria. Therefore, the study reported: (i) the diversity in rhizobacteria of these grasses colonizing selected stressed sites; (ii) the variations in plant growth promoting traits and tolerance to different metals among rhizobacteria; and (iii) the significance of rhizobacteria in the establishment of these grasses in stressed environment.

Fig. 1. Saccharum munja (A) and S. ravennae (B) native wild grasses naturally colonizing abandoned Bhatti mine and fly ash dump (Delhi), respectively

Characterization of the rhizobacteria of native grasses naturally colonizing abandoned mine sites may help in identification of microbial inoculants for ecological-restoration programmes. Eighty one strains of Saccharum munja rhizobacteria isolated from an abandoned mine located on Aravalli mountain and 50 from bulk-region were identified using 16S rRNA sequence analyses. Based on chemical and biological-assays they were categorized into ecologically diverse functional groups (siderophore-, IAA-, ACC-deaminase-, HCN-, polyphosphate producers; phosphate-solubilizer; antagonistic). Eight genera, 25 species from rhizosphere and 2 genera, and 5 species from bulk-region were dominated by Bacillus spp. (B. barbaricus, B. cereus, B. firmus, B. flexus, B. foraminis, B. licheniformis, B. megaterium, B. pumilus, B. subtilis, B. thuringiensis) and Paenibacillus spp. (P. alvei, P. apiarius, P. lautus, P. lentimorbus, P. polymyxa, P. popillae). Siderophore-producers were common in, rhizosphere and bulk soil, whereas IAA producers, N2-fixers and FePO4-solubilizers dominated rhizosphere samples. During the reproductive phase (winter) of S. munja, siderophore-, ACC-deaminase and polyP-producers were predominant; however dominance of HCN-producers in summer might be associated with termite-infestation. In vivo ability of selected rhizobacteria (B. megaterium BOSm201, B. subtilis BGSm253, B. pumilus BGSm157, P. alvei BGSm255, P. putida BOSm217, P. aeruginosa BGSm306) to enhance seed-germination and seedling-growth of S. munja in mine-spoil suggest their significance in natural colonization and potential for ecological restoration of Bhatti mine (Sharma et al., 2010).

Metal-rich fly ash dumps may serve as repository of ecologically useful multi-functional rhizobacteria having potential use in the development of vegetation at the dumps. Therefore, bacteria from the rhizosphere of a wild perennial grass (S. ravennae) colonizing Indraprastha and Badarpur fly ash dumps of Delhi region were purified, identified and functionally characterized. The fly ash had low levels of nutrients, moisture and organic matter coupled with toxic levels of heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn). Both the dumps were mostly barren except for a few patches of Saccharum ravennae and some weedy species. Sixty five dominant, morphologically distinct rhizobacteria were purified, which belonged to 18 genera and 38 species. Gram-positive bacteria were dominating in the fly ash environment. Bacillus spp. and Paenibacillus spp. were common at both the dumps. Multi-metal tolerance was shown by diverse bacterial taxa. The minimum inhibitory concentration (MIC) was highest for As (12.5 – 20.0 mM) and Pb (7.5 – 10.0 mM), although many rhizobacteria also possessed significant tolerance to Cr, Zn, Ni, Cu, Co and Cd. The tolerance profiles of rhizobacteria to different metals may be ranked in the decreasing order as As > Pb > Cr > Zn > Ni > Cu > Co > Cd > Hg. Majority of rhizobacteria showed good siderophore activity. Multiple-metal tolerance was also coupled with high siderophore production in some of the isolates (Microbacterium barkeri IPSr74, Serratia marcescens IPSr90 and IPSr82, Enterococcus casseliflavus BPSr32, Bacillus sp. IPSr80, Pseudomonas aeruginosa BPSr43 and Brochothrix campestris BPSr3). Most of the bacteria could grow on nitrogen-deficient medium. However, the dominant nitrogen fixers reported from the rhizosphere of other Saccharum species were not detected. S. marcescens IPSr90 was the only rhizobacterium, which showed ACC-deaminase (ACCD) activity. Proportion of phosphate solubilizing bacteria was high. Considerable improvement in the seedling establishment, plant weight and shoot length in rhizobacterial inoculated plants of S. ravennae in fly ash environment indicated the significance of rhizobacteria in its colonization and spread to the dumps. Representative rhizobacteria, with high MIC (for most of the metals) and good plant growth promoting (PGP) traits comparable to commercially useful bacterial inoculants were identified as S. marcescens IPSr82 and IPSr90, P. aeruginosa BPSr43, Paenibacillus larvae BPSr106, Arthrobacter ureafaciens BPSr55, Paenibacillus azotofixans BPSr107 and E. casseliflavus BPSr32. S. ravennae and some of these rhizobacteria may be potentially useful for the development of inoculation technologies for conversion of barren fly ash dumps into ecologically and economically productive habitats (Rau et al., 2009).

In conclusion, the stressed and degraded habitats harbour ecologically unique wild grasses which support taxonomically and functionally diverse group of rhizobacteria. These rhizobacteria possess multiple plant growth promoting traits and show tolerance towards a range of toxic heavy metals. They are involved in the natural colonization and spread of these grasses on stressed habitats. Wild grasses and their associated plant growth promoting rhizobacteria may be potentially useful for the development of ecological restoration technologies to convert barren, stressed - and degraded-habitats into ecologically and economically productive habitats.

References

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Boyer, S. and Wratten, S.D. (2010) The potential of earthworms to restore ecosystem services after opencast mining–A review. Basic and Applied Ecology. 11, 196 - 203.

Rau, N., Mishra, V., Sharma, M., Das, M.K., Ahaluwalia, K. and Sharma, R.S. (2009) Evaluation of functional diversity in rhizobacterial taxa of a wild grass (Saccharum ravennae) colonizing abandoned fly ash dumps in Delhi urban Ecosystem. Soil Biology and Biochemistry. 41, 813 - 821.

Schmidhuber, J. and Tubiello, F.N. (2007) Global food security under climate change. Proceedings of Natural Academy of Sciences USA. 104, 19703 - 19708.

Sharma, M., Mishra V., Rau, N. and Sharma, R.S (2010) Functionally diverse rhizobacteria of Saccharum munja (a native wild grass) colonizing abandoned morrum mine in Aravalli hills (Delhi). Plant and Soil. DOI 10.1007/s11104 - 010 - 0657 - y.

Sharma, M., Rau, N., Mishra, V. and Sharma, R.S. (2005) Unexplored ecological significance of Saccharum munja. Species. 43, 22.

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