Effect of substrate concentration and nitrate inhibition on product release and heavy metal removal by a Citrobacter...

A Citrobacter sp. accumulates heavy metals as cell-bound metal phosphates, utilizing phosphate released by the enzymatic cleavage of a phosphomonoester substrate. The effect of increased substrate (glycerol 2-phosphate, G2P) concentration on phosphate release and heavy metal accumulation was evaluated using a stirred tank reactor (STR) and a plug flow reactor (PFR). A significant improvement in metal removal was achieved with increased substrate concentration using immobilized Citrobacter cells in the PFR, which was not observed using free cells in the STR. Nitrate is an inhibitor of the Citrobacter phosphatase. This inhibition 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 conditions (different nitrate and G2P concentrations) can be described by a equation developed from Michaelis-Menten kinetics. The concentration of substrate required for restoration of maximum velocity, V(max), in a batch and continuous-flow system can be predicted by substitution and calculation; this was confirmed by an experiment in model systems using cell suspensions and polyacrylamide gel immobilized cells in a flow-though 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 with a Michaelis-Menten-based model to describe removal of lanthanum from nitrate-supplemented flow in a PFR.