Optical density measurement at 540 nm was used to monitor cell growth with a UV/Visible spectrophotometer (Pharmacia Biotech Ultrospec 2000). Biomass concentration was calculated by a regression equation: y = 0.4155x (R2 = 0.9933, P < 0.05), where y (g L−1) is the dry cell weight, x is the absorbance of the suspension at 540 nm.