The performance of phenol biodegradation by Candida Tropicalis RETL-cr1 using batch and fed-batch fermentation techniques

Abstract

Phenol is a toxic compound found in many industrial-waste effluents. A locally isolated yeast strain RETL-Cr1 from the effluent of the Exxon Mobil Oil Refinery wastewater treatment plant was investigated for phenol degradation using batch and fedbatch fermentation under aerobic condition. Based on a BLASTN search of GenBank, the complete sequences of ITS1-5.8S rDNA-ITS2 regions and portions of I8S and 28S for the purified DNA products of RETL-Cr1 shared 98% similarity with C. tropicalis. This yeast strain RETL-Cr1 was redesignated C. tropicalis RETL-Cr1 and was deposited at the GenBank under the accession number AY725426. The optimum condition for phenol degradation was at 30oC, pH 6.5 in RM in the absence of glucose. The highest phenol biodegradation efficiency in shake-flask cultures with IPC of 3mM was 100% achieving a degradation rate of 0.0257 g L-1 h-1 at µ 0.3718 h -1 after 14 h cultivation. Degradation of phenol was faster by 1.5-fold in bioreactor than in shake-flask whereby degradation rate was improved to 0.0395 g L-1 h-1 at µ 0.5391 h-1 after 10 hours of incubation. When tested at various IPC (0.0028 – 0.94 g L-1), inhibition was evident at IPC levels above 5 mM (0.470 g L-1). The fed-batch system in a bioreactor offered an 85 times fold degradation rate (2.3 g L-1 h-1) over shake-flask culture (0.0257 g L-1 h-1) and 61-fold over 2L bioreactor (0.0395 g L-1 h-1) batch system. It was observed that kinetically phenol degradation by RETL-Cr1 was significantly high in fed-batch culture as indicated by high degradation rate (2.3 g L-1 h-1) and substrate yield (Yx/s = 0.71-4.48 g g-1). However, a lower product yield (Ypc/s = 1.6x10-4 – 2.1x10-3 g g-1; Ypc/x = 3.5x10-5 – 1.4 x10-3 g g-1; YccMA/s = 1.0x10-4 – 2.0x10-4 g g -1; YccMA/x = 4.4x10-5 – 1.8x10-4 g g-1) and productivity (catechol = 1.2x10-5 – 5.3x10-5 g L-1 h-1; ccMA = 1.4x10-5 – 2.6 x10-5 g L-1 h-1) were achieved. When catechol and ccMA were analysed to determine whether an ortho or meta pathway was taken, it was found that these two metabolites were present in low amounts. This probably indicates further degradation of the metabolites. Hence, RETLCr1 strain metabolizes phenol via ortho-cleavage pathway. The optimum condition for both phenol hydroxylase and catechol 1,2-dioxygenase were at 30oC, pH 6.5. The most distinctive feature of this yeast strain is that it has a very high tolerance limit towards phenol reaching up to 60 mM. Based on the observations, RETL-Cr1 has a good potential to be used for treatment of phenol in industrial effluent