REAL AND PSEUDO OXYGEN GRADIENTS IN CA-ALGINATE BEADS MONITORED DURING POLAROGRAPHIC PO-2-MEASUREMENTS USING PT-NEEDLE MICROELECTRODES

Polarographic microcoaxial needle electrodes were used to measure inte rnal profiles of dissolved oxygen tension (Po-2) within single Ca-algi nate beads of different diameter containing entrapped cells of Sacchar omyces cerevisiae. For the investigations, single beads coming from va riable growing conditions and distinct cultivation stages were fixed i n a special holding device. In dependence on microbial growth steep ox ygen gradients were observed. The oxygen penetration depth at steady s tate lay between 50 and 100 mu m. After 8 h of cultivation time, the a naerobic space within the beads (phi 2 mm; cultivation in a packed bed reactor) is beginning at similar to 130 mu m, whereas the anaerobic s pace within the beads (phi 2 mm) coming from the shaker flask culture is located similar to 440 mu m below the bead surface. Surprisingly, s teep gradients were also observed, when recording profiles from cell-f ree Ca-alginate beads of different diameter and alginate concentration s. The steep oxygen gradients apparently had to be interpreted as pseu do-Po-2-gradients. These results were borne by several effects, such a s formation of artifacts and diffusion barriers in front of the electr ode tip or oxygen ''availability'' at the tip and consumption of oxyge n by the electrode itself. These phenomena could be documented by micr oscopic observation and photography. Thus, to obtain real Po-2-profile s it is important to be exactly informed about the physical, chemical, and biological properties of the material to be investigated. Further more, it is necessary to apply a special stepwise puncture technique w ith distinct step-in/step-out movements of the electrode; e.g., unidir ectional or contradirectional puncture techniques. (C) 1994 John Wiley and Sons, Inc.