Evaluation of bioflocculant-producing potential of bacillus pumilus strain isolated from Tyume River in the Eastern Cape Province of South Africa

Abstract

A bioflocculant is a kind of metabolite produced by microorganisms during their growth. Most bioflocculants are mainly composed of high polymers such as extracellular polysaccharides, glycoprotein, protein and nucleic acids. Bioflocculants promote flocculation by forming bridges between suspended particles in solutions resulting in precipitation of the suspended particles. Generally, when suspended particles are flocculated into large flocs, they settle down thus resulting in a clarified solution and can easily be removed. In this study, a bacterial strain named F45 was investigated for its ability to produce a bioflocculant. Source samples for isolation of bioflocculant-producing microorganisms were collected from the Tyume River, Eastern Cape Province of South Africa. The collected samples included water samples, rock scrapings and sediment samples. In total, 144 isolates were obtained from all the samples in which 13 were found to be bioflocculant producers as evidenced by the ability of the produced metabolite to flocculate kaolin suspension. Notable among these isolates was a bacterial strain F45 which was obtained from rock scrapings and whose bioflocculant exhibited a flocculating activity above 60%. Identification of the F45 strain revealed it to have 95% similarity to Bacillus pumilus strain ZAP 028. The optimum culture conditions for bioflocculant production by this strain were inoculum size of 4% (v/v), maltose as carbon source, multiple nitrogen source composed of yeast extract, urea and ammonium sulphate. The highest flocculating activity was recorded at an initial pH of 7. A bioflocculant yield of about 0.289 g/l was recovered from the fermented broth. The purified bioflocculant was a white powder which showed high flocculating activity (96.5%) against kaolin suspension at a dosage of 0.1 mg/ml. All the cations tested stimulated the flocculating activity of the purified bioflocculant except for Fe3+ which only showed a low flocculating activity of 21%. Thermal stability test of the purified bioflocculant proved it to be stable as it could retain more than 90% of its activity after being heated at 100ºC for 1 hour. Fourier-transform infrared (FTIR) spectroscopy analysis revealed that purified bioflocculant contained hydroxyl groups, carboxyl groups and uronic acid. The bioflocculant was composed of sugar (75.4%), protein (5.3%) and uronic acid (15.4%). Scanning electron microscopy (SEM) revealed a dendritic bioflocculant structure and elemental analysis of the purified bioflocculant showed that the weight fractions of elements C, N, O, S and P were 22.71%, 11.56%, 41.60%, 0.51% and 7.98% respectively. The results obtained suggest that this bioflocculant could be utilized as an alternative for harmful synthetic flocculants in various industrial applications.