No abstract

Optimum concentrations of heavy metals like copper, cadmium, lead, chromium, and zinc in soil are essential in carrying out various cellular activities in minimum concentrations and hence help in sustaining all life forms, although higher concentration of these metals is lethal to most of the life forms. Galerina vittiformis, a macrofungus, was found to accumulate these heavy metals into its fleshy fruiting body in the order Pb(II) > Cd(II) > Cu(II) > Zn(II) > Cr(VI) from 50 mg/kg soil. It possesses various ranges of potential cellular mechanisms that may be involved in detoxification of heavy metals and thus increases its tolerance to heavy metal stress, mainly by producing organic acids and phytochelatins (PCs). These components help in repairing stress damaged proteins and compartmentalisation of metals to vacuoles. The stress tolerance mechanism can be deduced by various analytical tools like SEM-EDX, FTIR, and LC-MS. Production of two kinds of phytochelatins was observed in the organism in response to metal stress.

Hexavalent chromium contamination is a serious problem due to its high toxicity and carcinogenic effects on the biological systems. The enzymatic reduction of toxic Cr(VI) to the less toxic Cr(III) is an efficient technology for detoxification of Cr(VI)-contaminated industrial effluents. In this regard, a chromate reductase enzyme from a novel Ochrobactrum sp. strain Cr-B4, having the ability to detoxify Cr(VI) contaminated sites, has been partially purified and characterized. The molecular mass of this chromate reductase was found to be 31.53 kD, with a specific activity 14.26 U/mg without any addition of electron donors. The temperature and pH optima for chromate reductase activity were 40°C and 8.0, respectively. The activation energy (Ea) for the chromate reductase was found to be 34.7 kJ/mol up to 40°C and the activation energy for its deactivation (Ed) was found to be 79.6 kJ/mol over a temperature range of 50-80°C. The frequency factor for activation of chromate reductase was found to be 566.79 s(-1), and for deactivation of chromate reductase it was found to be 265.66 × 10(3) s(-1). The reductase activity of this enzyme was affected by the presence of various heavy metals and complexing agents, some of which (ethylenediamine tetraacetic acid [EDTA], mercaptoethanol, NaN3, Pb(2+), Ni(2+), Zn(2+), and Cd(2+)) inhibited the enzyme activity, while metals like Cu(2+) and Fe(3+) significantly enhanced the reductase activity. The enzyme followed Michaelis-Menten kinetics with Km of 104.29 µM and a Vmax of 4.64 µM/min/mg.

A B S T R A C TNocardia hydrocarbonoxydans NCIM 2386 (Nhy) can grow using phenol as a sole carbon source and has a strong ability to degrade phenol. The paper presents the main metabolism pathways and mechanism of phenol degradation by Nhy. Phenol was found to be degraded via meta cleavage of catechol by the action of enzyme catechol 2,3-dioxygenase. The enzyme was found to be both extracellular and cell bound. The cell bound and extracellular enzymes actively degraded phenol even in the absence of the organism. The rate of phenol degradation by extracellular enzymes as sole enzymatic process (in the absence of cells) was found to be almost similar to that with the whole cells, indicating the prominence of extracellular enzymes. Michaelis-Menten model was found to fit the degradation rate kinetics of total phenol for total phenol concentrations of less than 100 mg L −1 and also the degradation rate kinetics of catechol at catechol concentrations of less than 80 mg L −1 during the exponential growth phase of the organism. Michaelis-Menten model was found to fit the kinetics of catechol formation rate which is also equal to the actual rate of phenol degradation to catechol. Both phenol and catechol were found to be substrate inhibitory.

Soil heavy metal contamination, a major threat due to industrialization, can be tackled by an efficient and economical process called bioremediation. Mushrooms are employed to accumulate heavy metals from soil due to their high metal accumulation potential and better adaptability. The bioaccumulation potential of Galerina vittiformis was already reported for individual metals. At natural conditions, since soil consists of more than one polluting metal, more focus has to be given to multimetal systems. In this study, multimetal accumulation potential was analyzed using central composite design, and the responses obtained were analyzed using response surface methodology. Heavy metals such as Cu(II), Cd(II), Cr(VI), Pb(II), and Zn(II) were subjected to biosorption at 10-250 mg/kg concentrations along with pH 5-8. The results showed that the preference of the organism for the five metals under study was in the order Pb(II) > Zn(II) > Cd(II) > Cu(II) > Cr(VI) at pH 6.5 under multimetal condition. The study also indicates that the metal interaction pattern in multimetal interaction is a property of their ionic radii. The response surface methodology clearly explains the effect of interaction of heavy metals on the accumulation potential of the organism using three-dimensional response plots. The present work suggests that the fungus Galerina vittiformis could be employed as a low-cost metal removal agent from heavy metal-polluted soil.