Ali, Saiqa Mubeen; (1996) Thermophilic biodegradation of phenolic compounds. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Several thermophilic microorganisms capable of degrading phenol as the sole carbon source were isolated from sewage effluent collected from Crossness sewage treatment site. The isolates were aerobic, rod-shaped bacteria characterised as Bacillus species with a growth temperature optimum of 50–60°C. The enzyme catalysing the second step in the phenol degradation meta-cleavage pathway, catechol 2,3-dioxygenase, was detected in all isolates grown in the presence of phenol. Three isolates were selected and their ability to grow in the presence of a range of phenolic compounds was investigated. In addition, their induction and substrate specificities were determined. Bacillus strain Cro3.2 was capable of degrading phenol, o-, m-, and p-cresol via the meta-pathway. This strain was selected for further studies on the basis of its relative substrate versatility. Bacillus strain Cro3.2 exhibited a high level of phenol tolerance, as shown by the ability of this strain to degrade phenol at concentrations up to 0.1% (w/v) without any detectable growth inhibition. Phenol degradation by Bacillus strain Cro3.2 did not appear to be coupled to growth, as demonstrated by the induction of at least the first two steps of the meta-pathway in mid-stationary phase. Resting cell biodegradation experiments were carried out with phenol as the sole carbon source. The effect of oxygen concentration, temperature, pH and buffer molarity on the rate of phenol degradation was investigated. Optimum rates of phenol degradation were achieved at pure oxygen concentrations of 0.6–1.0 v/v/min, at temperatures of 45–60°C and at a pH of 6.2-7.2. However, the enzymes catalysing the second and third steps in the phenol degradative pathway appeared to be unstable at high oxygen concentrations and temperatures. This instability was marked by large losses in catechol 2,3-dioxygenase activity and an extracelluar accumulation of pathway intermediates; catechol and 2-hydroxymuconic semialdehyde. Whole cells of Bacillus strain Cro3.2 were immobilised in (i) calcium-alginate, (ii) polyacrylamide gel and (iii) agarose gel in an attempt to stabilise the phenol-degrading pathway. The initial rate of phenol degradation determined from experiments with whole cells immobilised in calcium-alginate, polyacrylamide gel and agarose gel were 6.01, 0.10 and 1.37 μmoles phenol / min / gram of immobilised cells, respectively.

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