EFFECT OF SUBSTRATE CONCENTRATION AND NITRATE INHIBITION ON PRODUCT RELEASE AND HEAVY-METAL REMOVAL BY A CITROBACTER SP

A Citrobacter sp. accumulates heavy metals as cell-bound metal phospha tes, utilizing phosphate released by the enzymatic cleavage of a phosp homonoester substrate. The effect of increased substrate (glycerol 2-p hosphate, G2P) concentration on phosphate release and heavy metal accu mulation was evaluated using a stirred tank reactor (STR) and a plug f low reactor (PFR). A significant improvement in metal removal was achi eved with increased substrate concentration using immobilized Citrobac ter cells in the PFR, which was not observed using free cells in the S TR. Nitrate is an inhibitor of the Citrobacter phosphatase. This inhib ition was concentration dependent and reversible. The rate of product release was restored by increasing the concentration of substrate (G2P ). The ratio of rates of phosphate release under two different conditi ons (different nitrate and G2P concentrations) can be described by a e quation developed from Michaelis-Menten kinetics. The concentration of substrate required for restoration of maximum velocity, V-max, in a b atch and continuous-flow system can be predicted by substitution and c alculation; this was confirmed by an experiment in model systems using cell suspensions and polyacrylamide gel immobilized cells in a flow-t hough column. For use in industrial situations it may be uneconomical or infeasible to supply additional substrate. Bioreactor activity was also restored by increasing the flow residence time, in accordance wit h a Michaelis-Menten-based model to describe removal of lanthanum from nitrate-supplemented flow in a PFR. (C) 1997 John Wiley & Sons, Inc.