Nano-remediation of toxic heavy metal contamination: Hexavalent chromium [Cr(VI)].

The inexorable industrialization and modern agricultural practices to meet the needs of the increasing population have polluted the environment with toxic heavy metals such as Cr(VI), Cu2+, Cd2+, Pb2+, and Zn2+. Among the hazardous heavy metal(loid)s contamination in agricultural soil, water, and air, hexavalent chromium [Cr(VI)] is the most virulent carcinogen. The metallurgic industries, tanneries, paint manufacturing, petroleum refineries are among various such human activities that discharge Cr(VI) into the environment. Various methods have been employed to reduce the concentration of Cr(VI) contamination with nano and bioremediation being the recent advancement to achieve recovery at low cost and higher efficiency. Bioremediation is the process of using biological sources such as plant extracts, microorganisms, and algae to reduce the heavy metals while the nano-remediation uses nanoparticles to adsorb heavy metals. In this review, we discuss the various activities that liberate Cr(VI). We then discuss the various conventional, nano-remediation, and bioremediation methods to keep Cr(VI) concentration in check and further discuss their efficiencies. We also discuss the mechanism of nano-remediation techniques for better insight into the process.

Optimization of production of caffeine demethylase by Pseudomonas sp. in a bioreactor.

The effect of pH, aeration rate, and agitation rate on specific productivity of caffeine demethylase from Pseudomonas sp. was studied in a bioreactor. Maximum specific productivity of caffeine demethylase of 2,214 U g cell dry weight(-1) h(-1) was obtained at 0.27 vvm, 700 rpm, and pH 7.0. Under these conditions, volumetric oxygen transfer coefficient was 74.2 h(-1), indicating that caffeine demethylase production by Pseudomonas sp. was highly oxygen-dependent. Different metabolite formation at different agitation and aeration rates can be used as a strategy for recovery of pharmaceutically important metabolites from caffeine by manipulation of conditions in a bacterial culture. This is the first report on production of high levels of caffeine demethylase in bioreactors.

Inducible nature of the enzymes involved in catabolism of caffeine and related methylxanthines.

Previously isolated strain of Pseudomonas sp. has the capability of utilizing caffeine as the sole source of carbon and nitrogen and degrading caffeine at higher concentrations (>10 g l(-1)). In this study, an assay has been developed to study the enzymatic conversion of caffeine to subsequent methylxanthines by cell free extracts of Pseudomonas sp., the activity of which has been stabilized by use of stabilizers in the lysis buffer. Growth of the strain in various methylxanthines and later enzyme assay demonstrated that the enzyme(s) involved in degradation of caffeine and other methylxanthines were inducible in nature. The results also indicated that more than one enzyme are involved in degradation of caffeine to xanthine, which constitute the primary steps in bacterial caffeine catabolism.

Chemotaxis of Pseudomonas sp. to caffeine and related methylxanthines.

Pseudomonas sp. isolated from soil of coffee plantation area has been shown to degrade higher concentrations of caffeine ( approximately 15 g l(-1)) by N-demethylation at a rate higher than what has been reported for any strain so far. This strain exhibits positive chemotaxis towards caffeine (1,3,7-trimethylxanthine) in swarm plate assay and modified capillary assay in a dose dependant manner. Related methylxanthines and xanthine also act as chemoattractants for the strain with the highest relative chemotactic response (RCR) seen for xanthine. Chemotaxis in Pseudomonas sp. is possibly plasmid mediated as indicated by positive chemotaxis of plasmid transformed E. coli DH5alpha. The chemotactic abilities of Pseudomonas sp. combined with higher rates of degradation of caffeine can be used in the development of strategies for biodecaffeination of caffeine containing wastes.

Degradation kinetics of caffeine and related methylxanthines by induced cells of Pseudomonas sp.

In this study, the kinetics of degradation of caffeine and related methylxanthines by induced cells of Pseudomonas sp. was performed. The kinetics data showed that degradation of caffeine, theobromine, and 7-methylxanthine followed Michealis-Menten kinetics. The values of K (m) are low for caffeine and 7-methylxanthine and high for theobromine. Degradation of caffeine and theobromine was enhanced in the presence of NADH and NADPH, whereas the degradation of 7-methylxanthine was unaffected. Among the various metal ions tested, Fe(2+) was found to enhance the rate of degradation for all three substrates, whereas Zn(2+) and Cu(2+) inhibited the degradation of caffeine and theobromine but not 7-methylxanthine. The differences in kinetic parameters and cofactor requirement suggest the possibility of the involvement of more than one N-demethylases in the caffeine catabolic pathway in Pseudomonas sp. The induced cells can serve as effective biocatalysts for the development of biodecaffeination techniques.

Catabolic pathways and biotechnological applications of microbial caffeine degradation.

Catabolism of caffeine (1,3,7-trimethylxanthine) in microorganisms commences via two possible mechanisms: demethylation and oxidation. Through the demethylation route, the major metabolite formed in fungi is theophylline (1,3-dimethylxanthine), whereas theobromine (3,7-dimethylxanthine) is the major metabolite in bacteria. In certain bacterial species, caffeine has also been oxidized directly to trimethyl uric acid in a single step. The conversion of caffeine to its metabolites is primarily brought about by N-demethylases (such as caffeine demethylase, theobromine demethylase and heteroxanthinedemethylase), caffeine oxidase and xanthine oxidase that are produced by several caffeine-degrading bacterial species such as Pseudomonas putida and species within the genera Alcaligenes, Rhodococcus and Klebsiella. Development of biodecaffeination techniques using these enzymes or using whole cells offers an attractive alternative to the present existing chemical and physical methods removal of caffeine, which are costly, toxic and non-specific to caffeine. This review mainly focuses on the biochemistry of microbial caffeine degradation, presenting recent advances and the potential biotechnological application of caffeine-degrading enzymes.