Nitrate concentration was determined according to the Ultraviolet
Spectrophotometric Screening Method [27]. A sample (1 mL)
collected from photobioreactor was centrifuged at 10 1000 g
for 5 min. The supernatant was collected and optical density was
measured at 275 nm and 220 nm by using spectrophotometer (Shimadzu
UV mini-1240). Then, the absorbance reading at 275 nm
was subtracted two times from the reading at 220 nm to obtain
the actual absorbance caused by the presence of nitrate. Dry potassium
nitrate (KNO3) at different concentrations was used for
calibration purposes.