The medium pH was maintain at 5 by addition of potassiumhydroxide (2 M) or orthophosphoric acid 21% (v/v), through Peripexperistaltic pumps (Bioengineering, Switzerland). Dissolved oxy-gen concentration was measured with a polarographic-membraneprobe (InPro 6000, Mettler Toledo, USA) using the BioScadaLab soft-ware