Molybdenum, an emerging pollutant, has being demonstrated recently to be toxic to spermatogenesis in several animal model systems. Metal mines especially gold mine often use cyanide and hence isolation of metal-reducing and cyanide-degrading bacteria can be useful for the bioremediation of these pollutants. Preliminary screening shows that three cyanide-degrading bacteria were able to reduce molybdenum to molybdenum blue (Mo-blue) when grown on a molybdate low phosphate minimal salts media. Phylogenetic analyses of the 16S rRNA gene of the best reducer indicates that it belongs to the Serratia genus. A variety of mathematical models such as logistic, Gompertz, Richards, Schnute, Baranyi-Roberts, von Bertalanffy, Buchanan three-phase and Huang were used to model molybdenum reduction, and the best model based on statistical analysis was modified Gompertz with lowest values for RMSE and AICc, highest adjusted R2 values, with Bias Factor and Accuracy Factor nearest to unity (1.0). The reduction constants obtained from the model will be used to carry out secondary modelling to study the effect of various parameters such as substrate, pH and temperature to molybdenum reduction.

The presence of both heavy metals and organic xenobiotic pollutants in a contaminated site justifies the application of either a multitude of microbial degraders or microorganisms having the capacity to detoxify a number of pollutants at the same time. Molybdenum is an essential heavy metal that is toxic to ruminants at a high level. Ruminants such as cow and goats experience severe hypocuprosis leading to scouring and death at a concentration as low as several parts per million. In this study, a molybdenum-reducing bacterium with amide-degrading capacity has been isolated from contaminated soils. The bacterium, using glucose as the best electron donor reduces molybdenum in the form of sodium molybdate to molybdenum blue. The maximal pH reduction occurs between 6.0 and 6.3, and the bacterium showed an excellent reduction in temperatures between 25 and 40 oC. The reduction was maximal at molybdate concentrations of between 15 and 25 mM. Molybdenum reduction incidentally was inhibited by several toxic heavy metals. Other carbon sources including toxic xenobiotics such as amides were screened for their ability to support molybdate reduction. Of all the amides, only acrylamide can support molybdenum reduction. The other amides; such as acetamide and propionamide can support growth. Analysis using phylogenetic analysis resulted in a tentative identification of the bacterium as Pseudomonas sp. strain 135. This bacterium is essential in remediating sites contaminated with molybdenum, especially in agricultural soil co-contaminated with acrylamide, a known soil stabilizer.

The issue of heavy metal contamination and toxic xenobiotics has become a rapid global concern. This has ensured that the bioremediation of these toxicants, which are being carried out using novel microbes. A bacterium with the ability to reduce molybdenum has been isolated from contaminated soils and identified as Serratia marcescens strain DR.Y10. The bacterium reduced molybdenum (sodium molybdate) to molybdenum blue (Mo-blue) optimally at pHs of between 6.0 and 6.5 and temperatures between 30°C and 37°C. Glucose was the best electron donor for supporting molybdate reduction followed by sucrose, adonitol, mannose, maltose, mannitol glycerol, salicin, myo-inositol, sorbitol and trehalose in descending order. Other requirements include a phosphate concentration of 5 mM and a molybdate concentration of between 10 and 30 mM. The absorption spectrum of the Mo-blue produced was similar to the previously isolated Mo-reducing bacterium and closely resembles a reduced phosphomolybdate. Molybdenum reduction was inhibited by Hg (ii), Ag (i), Cu (ii), and Cr (vi) at 78.9, 69.2, 59.5 and 40.1%, respectively. We also screen for the ability of the bacterium to use various organic xenobiotics such as phenol, acrylamide, nicotinamide, acetamide, iodoacetamide, propionamide, acetamide, sodium dodecyl sulfate (SDS) and diesel as electron donor sources for aiding reduction. The bacterium was also able to grow using amides such as acrylamide, propionamide and acetamide without molybdenum reduction. The unique ability of the bacterium to detoxify many toxicants is much in demand, making this bacterium a vital means of bioremediation.

Bacterial based remediation of environmental toxicants is a promising innovative technology for molybdenum pollution. To date, the enzyme responsible for molybdate reduction to Mo-blue from bacteria show that the Michaelis-Menten constants varies by one order of magnitude. It is important that the constants from newer enzyme sources be characterized so that a comparison can be made. The aim of this study is to characterize kinetically the enzyme from a previously isolated Mo-reducing bacterium; Bacillus pumilus strain Lbna. The maximum activity of this enzyme occurred at pH 5.5 and in between 25 and 35 oC. The Km and Vmax of NADH were 6.646 mM and 0.057 unit/mg enzyme, while the Km and Vmax of LPPM were 3.399 mM and 0.106 unit/mg enzyme. The results showed that the enzyme activity for Bacillus pumilus strain Lbna were inhibited by all heavy metals used. Zinc, copper, silver, chromium, cadmium and mercury all caused more than 50% inhibition to the Mo-reducing enzyme activity with copper being the most potent with an almost complete inhibition of enzyme activity observed.

Kinetic modelling of bacterial reduction process reveals vital parameters like specific reduction rate, hypothetical maximum reduction and deduce whether high substrate (molybdenum) concentration affects the lag phase of the reduction. The commonly used natural logarithmic transformation to linearize the reduction process seems inaccurate as it gives only an approximate value for the specific growth rate. This work for the first time utilized eight different primary models such as Gompertz, Baranyi-Roberts, Logistic, Von Bertalanffy, Richards, Schnute, Buchanan three-phase Huang to obtain values of the kinetic constants that could further be used for secondary modelling. Baranyi-Roberts model was the best model fitting Mo-blue production curve in Bacillus sp. strain Neni-10 based on statistical values for RMSE (root-mean-square error), R2 (adjusted coefficient of determination), AICc (corrected Akaike Information Criterion) BF (bias factor) and AF (accuracy factor). The fitting parameters obtained were lag time (l), maximal Mo-blue production (Ymax) and maximum Mo-blue production rate (mm). The use of microbial growth models to get accurate Mo-blue production rate is entirely new to molybdenum reduction (detoxification) process and the kinetic constants obtained could be very useful secondary modelling. This work has revealed the usefulness of these models in modelling bacterial Mo-blue production