1.5. Phosphatase activity
The phosphatase activity of cultured biofilms was estimated mainly using a modified procedure by Turner et al. (2001) and Ellwood et al. (2012). The determination used para-nitrophenyl phosphate (pNPP) as an analogue substrate. Biofilm samples from each treatment were scraped from the slides, placed into 50-mL tubes containing 25.8 mL of buffered medium, thoroughly mixed, then equally divided into 6 aliquots in 10-mL glass tubes. The six small (10-mL) tubes were then placed in a shaking incubator at 25°C for 20 min. The assay was then initiated by adding 0.18 mL pNPP (final concentration 250 μmol/L) into four tubes, and the same amounts of ultrapure water for the other two tubes as control. Next, all samples were incubated at 25°C for 3 hr, then 0.25 mL of 0.5 mol/L NaOH was added to terminate the assay. The absorbance of p-nitrophenol was measured spectrophotometrically at 405 nm. The values of the blank (substrate and buffer only) and control tubes were subtracted from the final measured value.
For dry weight (DW) analysis, the scraped biofilm samples were first centrifuged at 10,000 r/min for 10 min, and then the pellets were rinsed with distilled water, dried at 105°C for 24 hr, and finally weighed.