Resistance and Removal of Chromium (Cr) and Cadmium (Cd) by Bacillus cereus and Aspergillus niger Isolated from Polytrichum commune Rhizosphere

Abstract

Bryophytes indicate the presence of metals in the environment because of their biosorption capacity. It is therefore, hypothesized that microbial communities in bryophyte rhizosphere can be highly metal-resistant; and this was tested using Bacillus cereus and Aspergillus niger isolated from the rhizosphere of a  bryophyte, Polytrichum commune. Nutrient agar and Malt Extract agar were used for isolation of  bacteria and microfungi, respectively. Identification was by cultural, morphological and biochemical tests.  A total of 10 B. cereus and A. niger were isolated from P. commune rhizospheres and soils at 10m away from bryophyte locations for comparison. The Minimum Inhibitory Concentrations (MIC) in mg/L were determined with Cr and Cd and the rhizosphere strains of B. cereus and A. niger that had the highest MIC were tested for their ability to reduce Cr and Cd concentrations singly, and in combination using Nutrient and Malt Extract broths containing 1000-2,000 mg/L. Compared to soil isolates, the MIC of the rhizosphere strains which stood at: B. cereus, Cr=650-750, Cd=720-840; and A. niger, Cr=410-450-750, Cd=480-540 mg/L were significantly higher (P=0.000). While the MICs of both metals markedly  varied with soil sites of isolation of B. cereus and A. niger, it did not with P. commune locations. The removal of Cr and Cd from aqueous medium containing 1,000-1500 mg/L for each metal, stood at 65.5-74.5, 75.6-79.4 and 92.5-99.8% for B. cereus, A. niger and combined B. cereus/A. niger, respectively. It sharply dropped to <36% when metal concentration increased to 2000 mg/L. It is concluded that the level of MICs is a sufficient ground to sustain the hypothesis that bryophyte rhizosphere harbor microorganisms that are metal-resistant. Thus, bryophytes can be sources of organisms that can be useful in bioremediation application.