Abstract

The effect of heavy metal concentration and contact time (exposure period) on heavy metal up take by pure cultures of three bacteria (Pseudomonas, Bacillus and Aeromonas ) isolated from a crude oil impacted brackish aquatic system in the Niger Delta were investigated. Heavy metals employed included metals found in this Bonny light crude oil (Fe, Zn, Cd, Cu, Ni and Pb). Accumulation of these metals was gradual and the amount increased in direct proportion to initial metal concentration up to an extent that ranged from 1 - 100 (mg/l) after which uptake remained either constant or declined. Maximum uptake of Fe, Pb, Cd, Cu, Zn and Ni were obtained at initial concentrations of 10, 10, 10, 100, 100 and 10 (mg/l), respectively and the values were 8.75, 0.01, 1.3, 0.06, 4.2 and 0.001 milligram per gram dry weight (mg/g dry wt) of Bacillus cells. For Pseudomonas sp. initial metal concentration that resulted in maximum uptake were 10 mg/l (Fe), 1 mg/l (Zn,) 10 mg/l (Pb), 0.1 mg/l (Cd), 10 mg/l (Cu) and 10 mg/l (Ni). Values accumulated at these concentrations were 14, 0.7, 1.0, 0.08, 0.3 and 0.11 (mg/g dry wt), respectively. Whereas, maximum amounts accumulated by Aeromonas sp. were 1.65, 0.1, 0.001, 0.9, 0.2 and 0.013 (mg/g dry wt) respectively. The respective initial concentration that yielded these uptake values was 100, 10, 1.0, 10, 10 and 10 (mg/l). Contact duration increased the amount of metal bioconcentrated by each test organism. At all tested concentrations maximum uptake of Fe, Zn, Cu, Cd, Pb and Ni by Bacillus were at the 8^th, 8^th, 8^th, 4^th, 4^thand 4^th hours of exposure respectively. Slight decreases in uptake were noticed on further incubation beyond these durations. Maximum accumulation of Fe, Zn, Cu, Cd, Pb and Ni by Pseudomonas sp. were obtained at incubation durations of 8^th, 12^th, 12^th, 8^th, 2^nd and 24^th hours. 12^th, 12^th, 12^th, 4^th, 8^th and 4^th h were the incubation periods that resulted in maximum bioconcentration of Fe, Zn, Cu, Cd, Pb and Ni by Aeromonas sp. The three test organisms presented distinct uptake capacities which decreased thus: Pseudomonas sp. > Bacillus sp. ≥ Aeromonas sp. Affinities of Bacillus sp., Pseudomonas sp. andAeromonas sp. for the various heavy metals followed the pattern Fe > Zn > Cd > Cu > Ni > Pb, Fe > Pb ≥ Zn > Cu > Ni ≥ Cd and Fe > Cd > Cu ≥ Zn > Ni > Pb respectively. Results showed that heavy metal concentrations between 10 – 100 mg/l and exposure periods of between 4 - 12 h depending on the metal and the test organism rapidly promoted accumulation in heavy metal polluted sites.

Key words: Accumulation bioconcentration, heavy metal, toxicity, contact time, bacteria.