Cr(VI) reduction by an Aspergillus tubingensis strain: role of carboxylic acids and implications for natural attenuation and biotreatment of Cr(VI) contamination.

Experiments conducted in shake flask cultures, in minimal medium of pH 5.3 containing 50 microg mL(-1)Cr(VI) with glucose as a carbon source, indicated that the biomass of Aspergillus sp. strain Ed8, a chromate-tolerant fungal strain previously isolated from a chromium-polluted soil, responds to the presence of citrate in the medium by increasing the rate of Cr(VI) reduction; this effect required the use of live biomass and was not observed in medium with lactate. Other natural carboxylic acids or non-natural metal chelating agents showed a stimulatory effect of Cr(VI) reduction by Ed8 biomass; salicylate, tartrate and citrate were the stronger stimulators of the specific rate of Cr(VI) reduction, with about 12, 8 and 7-fold stimulatory effects, respectively, as compared to control cultures without additions. A procedure for Cr(VI) removal from a diluted electroplating effluent was devised, based on the use of growth medium amended with citrate or a mixture of salycilate-tartrate and cycles of recharge of growth medium-diluted effluent. In addition, conditions were adjusted in a 2-L bioreactor to reach a 20-fold increase in the volume of the reduction system with no loss of efficiency. Strain Ed8 was identified as an Aspergillus tubingensis isolate (included in Aspergillus niger species complex) on the basis of the ITS1-5.8s rDNA-ITS2 sequence similarity.