INTRODUCTION:

Global agricultural soil pollution by heavy metals poses a significant challenge to achieving sustainable development in developing countries. Heavy metals are characterized by their high densities and atomic weights (greater than 50). It can originate from both natural processes and human activities, ultimately contaminating soil water, and air^1-3. Soil pollution from heavy metals threatens the environment and food security, driven by rapid industrial and agricultural expansion, as well as the disruption of natural ecosystems due to increased anthropogenic activity and growing population⁴. The presence of heavy metals poses numerous risks to ecosystems and human health, affecting the safety and quality of the food chain, and impeding the use of land for agricultural purposes. This, in turn, impacts food security and exacerbates issues related to land tenure ⁵. Chromium (VI) is highly toxic and water-soluble, allowing it to rapidly penetrate cell membranes and interact with proteins and nucleic acids⁶. This accumulation of toxic heavy metals in humans can lead to serious health issues, including carcinogenesis, cognitive impairments, renal dysfunction, and other abnormalities⁷. Mycoremediation offers an economical solution that does not produce hazardous waste. Its effectiveness relies on selecting the appropriate fungal species for targeting specific heavy metals or pollutants. This approach presents a viable and cost-effective method for addressing the escalating problem of soil and water contamination. Using chromium-tolerant fungi for mycoremediation represents a promising strategy to implement green technologies for treating contaminated soils and waters⁸.

Numerous studies have identified various bacterial species capable of bioremediating chromium and have explored the mechanisms behind this process⁹ . Evidence suggests that chromium-resistant fungi employ a range of strategies to manage chromium-induced stress, including enzymatic reduction, biosorption, bioaccumulation, biotransformation, and efflux. Due to their small size, microorganisms offer a high surface area relative to their volume, which enhances their ability to assimilate metals from their environment ¹⁰. Many microorganisms found in industrial effluent discharge areas have developed mechanisms to protect themselves from heavy metal toxicity^11-14 . These organisms—such as bacteria, fungi, algae, and protozoa—utilize various strategies to counteract heavy metal toxicity, including metal uptake, adsorption, oxidation, methylation, and reduction to less toxic forms ^11,15. Notably, the reduction of toxic Cr(VI) to the less harmful Cr(III) is a common mechanism employed by many organisms to survive in Cr(VI)-contaminated environments ¹⁴.

This study aims to isolate chromium-tolerant fungi from chromium-contaminated areas and evaluate their potential for chromium adsorption. The goal is to further develop these fungi for use in the bioremediation of contaminated soil and water. During bioremediation, fungi utilize mechanisms such as biosorption, bioaccumulation, enzymatic reduction, and precipitation to address chromium contamination. They produce organic acids and extracellular enzymes that help immobilize chromium in less bioavailable forms while converting Cr(VI) to Cr(III). These processes not only mitigate the toxicity of chromium but also prevent its leaching into groundwater, thereby safeguarding ecosystem health and maintaining water quality.

MATERIAL AND METHOD:

Isolation of fungi from industrial site

Samples collection

Soil samples from different places of industrial area of Raipur city's that is Urla were collected after 15-20 cm deep from the surface. The samples were collected in sterile zipper polythene bags and stored in refrigerator at 4 °C.

Isolation and identification of fungi

Potato dextrose agar (PDA) media was taken for the isolation of fungi and serial dilution plating technique is used as isolation techniques. Soil dilution method: diluted with 1g soil is in 10ml of sterile distilled water.1ml of suspension was added to sterile petriplates in triplicates containing sterile Potato Dextrose Agar the plates were incubated at 28^oC for 5-7 days. Fungi growing on the agar plates were purified by subculturing and transferred to PDA slants and then maintained as a stock culture. The isolated fungi were identified to the genus level on the basis of their morphological characters and microscopic analysis.

Assessment of Chromium tolerance

Chromium tolerance of isolated strains is determined by agar well diffusion method. Spore suspension of isolated strains were prepared in sterilized distilled water and 0.1 ml of the suspension was spread over the surface of the sterilized and solidified PDA media containing plates, five small wells were prepared in this inoculated plated with cork borer. Each well was filled with 100 µl of different concentrations of potassium dichromate solution. Well filled with 100µl distilled water was taken as control. Plates were incubated at 28 ± 1^οCfor 5 – 7 days.

Adsorption of Chromium

Adsorption of Chromium from the liquid medium containing 2mg /l K₂ Cr₂O₇was performed in screw capped tubes by inoculating the PD broth with fungal culture. Optical density of medium was measured after centrifugation in 0,4,8,12 and 15^th day with the help of spectrophotometer.

RESULT AND DISCUSSION:

Table 1: - Zone of Inhibition of Rhizopus sp. at different Concentrations of K₂ Cr₂O₇

Zone of Inhibition
Concentration of K₂Cr₂O₇	*Rhizopus*	*Aspergillus niger*
100mg/l	Nil	nil
200mg/l	Nil	5mm
400mg/l	Nil	10mm
600mg/l	3 mm	15 mm
700mg/l	15 mm	18 mm
800 mg/l	12 mm	23 mm

Figure 1. % reduction of K₂Cr₂O₇from medium after different incubation period

Fig 2: % reduction of K₂Cr₂O₇from medium after different incubation period

Two fungal isolates Aspergillus niger and Rhizopus spp, were identified on the basis of colony morphology and microscopic examination and found to shown the tolerance against Chromium. As indicated in the table 1 Rhizopus sp. can tolerate high conc. of K₂Cr₂O₇i e. 400mg/l where as Aspergillus sp. can tolerate up to 200mg/l conc. of K₂Cr₂O₇

It is revealed from fig. 1. that that 21% reduction of K₂Cr₂O₇is seen after 4^thday of incubation after that 29%, 36%, and 59% reduction is observed after 8^th,12^th, and 15^thday of incubation respectively. While in case of Aspergillus sp. Fig. 2. indicates around 3.61% reduction of K₂Cr₂O₇salt is seen after 4^thday of incubation after that 7.95%, 16.95%, and 17.27% reduction is observed after 8^th,12^th, and 15^thday of incubation respectively. This suggest that Aspergillus spp. the retention time for removal of Cr from medium should be 12^th day after that it shows a very minor reduction of Cr concentration. It is also clear from the present study is Rhizopus sp. is able to reduce a considerable amount of chromium from medium (59%) in 15 days of incubation. In a similar study it is reported that Trichoderma koningiopsis LBM 253 isolated from chromium contaminated soil demonstrated the highest tolerance of chromium in solid medium and showed high removal efficiency of chromium in liquid media¹⁶. Absolutely, microorganisms play a crucial role in bioremediation of heavy metal by breaking down or transforming pollutants into less harmful substances. Different microorganisms employ various mechanisms to achieve this, each with its unique set of requirements, advantages, and potential drawbacks¹⁷.

CONCLUSION:

Rhizopus sp. and Aspergillus sp. demonstrate exceptional tolerance to chromium, making them effective candidates for bioremediation of chromium-contaminated soils. These fungi can not only survive but thrive in polluted environments by employing mechanisms such as biosorption, bioaccumulation, and biotransformation to remove and detoxify harmful substances. Their resilience and ability to remediate soil contaminants underscore their potential for contributing to sustainable environmental cleanup and the restoration of polluted ecosystems.However, optimizing fungal bioremediation for large-scale applications remains challenging. Factors such as the presence of other pollutants, varying environmental conditions, and the selection of suitable fungal species must be carefully considered. Further research is essential to refine these bioremediation techniques and ensure their effectiveness under diverse environmental conditions.

REFERENCES:

1. Masindi V, Muedi KL. Environmental contamination by heavy metals. Heavy Metals. 2018 Jun 27; 10(4): 115-33.

2. Aziz KH, Mustafa FS, Omer KM, Hama S, Hamarawf RF, Rahman KO. Heavy metal pollution in the aquatic environment: efficient and low-cost removal approaches to eliminate their toxicity: a review. RSC Advances. 2023; 13(26): 17595-610.

3. Assubaie FN. Assessment of the levels of some heavy metals in water in Alahsa Oasis farms, Saudi Arabia, with analysis by atomic absorption spectrophotometry. Arabian Journal of Chemistry. 2015 Mar 1; 8(2): 240-5.

4. Sarwar N, Imran M, Shaheen MR, Ishaque W, Kamran MA, Matloob A, Rehim A, Hussain S. Phytoremediation strategies for soils contaminated with heavy metals: modifications and future perspectives. Chemosphere. 2017 Mar 1;171:710-21

5. Wuana RA, Okieimen FE. Heavy metals in contaminated soils: a review of sources, chemistry, risks and best available strategies for remediation. International Scholarly Research Notices. 2011; 2011(1): 402647..

6. Markiewicz B, Komorowicz I, Sajnóg A, Belter M, Barałkiewicz D. Chromium and its speciation in water samples by HPLC/ICP-MS–technique establishing metrological traceability: a review since 2000. Talanta. 2015 Jan 15; 132: 814-28.

7. Rosen BP. Transport and detoxification systems for transition metals, heavy metals and metalloids in eukaryotic and prokaryotic microbes. Comparative Biochemistry and Physiology Part A: Molecular and Integrative Physiology. 2002 Nov 1; 133(3): 689-93.

8. Akhtar N, Mannan MA. Mycoremediation: expunging environmental pollutants. Biotechnology Reports. 2020 Jun 1;26:e00452.

9. Baldiris R, Acosta-Tapia N, Montes A, Hernández J, Vivas-Reyes R. Reduction of hexavalent chromium and detection of chromate reductase (ChrR) in Stenotrophomonas maltophilia. Molecules. 2018 Feb 13; 23(2):406.

10. Zouboulis AI, Loukidou MX, Matis KA. Biosorption of toxic metals from aqueous solutions by bacteria strains isolated from metal-polluted soils. Process Biochemistry. 2004 Apr 30; 39(8):909-16.

11. Zahoor A, Rehman A. Isolation of Cr (VI) reducing bacteria from industrial effluents and their potential use in bioremediation of chromium containing wastewater. Journal of Environmental Sciences. 2009 Jan 1; 21(6): 814-20.

12. Sun W, Xiao E, Krumins V, Häggblom MM, Dong Y, Pu Z, Li B, Wang Q, Xiao T, Li F. Rhizosphere microbial response to multiple metal (loid) s in different contaminated arable soils indicates crop-specific metal-microbe interactions. Applied and Environmental Microbiology. 2018 Dec 15; 84(24): e00701-18.

13. Watts MP, Khijniak TV, Boothman C, Lloyd JR. Treatment of alkaline Cr (VI)-contaminated leachate with an alkaliphilic metal-reducing bacterium. Applied and Environmental Microbiology. 2015 Aug 15; 81(16): 5511-8.

14. Fernández PM, Viñarta SC, Bernal AR, Cruz EL, Figueroa LI. Bioremediation strategies for chromium removal: current research, scale-up approach and future perspectives. Chemosphere. 2018 Oct 1; 208: 139-48.

15. Sturm G, Brunner S, Suvorova E, Dempwolff F, Reiner J, Graumann P, Bernier-Latmani R, Majzlan J, Gescher J. Chromate resistance mechanisms in Leucobacter chromiiresistens. Applied and Environmental Microbiology. 2018 Dec 1; 84(23): e02208-18.

16. Tatarin AS, Aranguiz C, Sadañoski MA, Polti MA, Fonseca MI. Fungal species originating from chromium contaminated soil for ecofriendly and biotechnological processes. Applied Soil Ecology. 2024 Mar 1;195:105231.

17. Kapahi M, Sachdeva S. Bioremediation options for heavy metal pollution. Journal of Health and Pollution. 2019 Dec 1; 9(24): 191203

Received on 11.06.2024 Modified on 24.08.2024

Research J. Science and Tech. 2024; 16(3):270-273.

DOI: 10.52711/2349-2988.2024.00038