Screening and identification of rhizobacteria isolated from heavy metal contaminated soils
INTRODUCTION As a result of human activities such as mining, agriculture and industrial activity, heavy metal pollution has become one of the most serious environmental problems today. Elevated levels of heavy metals not only decrease soil microbial activity and crop production, but also threaten human health through the food chain. Soil microorganisms can degrade organic contaminants, while metals need immobilization or physical removal. Although many metals are essential, all metals are toxic at higher concentrations, because they cause oxidative stress by formation of free radicals. Another reason why metals may be toxic is that they can replace essential metals in pigments or enzymes disrupting their function. Thus, metals render the land unsuitable for plant growth and destroy the biodiversity. Though several regulatory steps have been implemented to reduce or restrict the release of pollutants in the soil, they are not sufficient for checking the contamination. Metal contaminated soil can be remediated by chemical, physical and biological techniques. These can be grouped into two categories, ex-situ and in-situ method. One of the insitu method that has been used a lot to remediate heavy metal contamination is the phytoremediation method. Phytoremediation is the technology of remediation involving the use of green plants and their associated biota for treatment of contaminated soil. However, according to recent studies, the efficiency of metals uptake by plants used for phytoremediation can be enhanced by aid of plant promoting growth rhizobacteria. Therefore, main objective of this study is to identify tolerant heavy metals bacteria to be used in phytoremediation via supporting plants uptake of heavy metals in contaminated soils.
PROCEDURE A. Screening of rhizobacteria with heavy metal tolerant potential
Objective: To screen rhizobacteria tolerant to heavy metals arsenic, mercury and lead.
Method: Rhizobacteria samples chosen for this experiment were isolated from different plant roots at sludge farm and land farm of Petronas Penapisan Melaka Sdn. Bhd. (PPMSB). After screening for their ability to degrade hydrocarbon, 35 isolates from five different plant roots were selected and were used for screening of rhizobacteria which can tolerate in heavy metals condition. The isolates were grown on TSA for 18 – 24 hours at 37˚C, and then subcultured onto MSM agar plates containing arsenic, mercury or lead heavy metals with concentration of 10 ppm. Agar plates were incubated for 18 – 24 hours at 37˚C. Then, the isolates were determined to have tolerance towards heavy metals via qualitative observation of bacterial growth on the agar plates. Steps were repeated for grown isolates with increasing concentration of selected heavy metals in range 20 ppm to 250 ppm.
Results: Isolates SF-S1-R3 3 4 5 6 7 8 10 17 Heavy metals (ppm) Mercury (30 ppm) No growth No growth Growth No growth No growth Growth No growth Growth
Arsenic (250 ppm) No growth
Lead (250 ppm) No growth Growth
No growth No growth No growth No growth Growth Growth
18 19 60 61 62
Growth Growth Growth Growth No growth
Growth Growth Growth Growth
Growth Growth Growth Growth Growth
No growth Growth Growth 63 Table 1: Summarize of qualitative observation of isolates growth on medium containing
heavy metals. Discussion: Out of 35 isolates from five different plants, isolates SF-S1-R3 sample from Paspalum vaginatum Sw. plant root, showed the most tolerance towards heavy metals tested. Therefore, 14 isolates of SF-S1-R3 sample were analyzed for this study. The highest concentration of arsenic and lead heavy metals tested for this screening step is 250 ppm for both, while concentration tested for mercury was up to 30 ppm only. Isolates were determined its tolerance towards...
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