Effects of PVA and boric acid on denitrification The effects of boric acid and PVA on denitrification with resting cells are shown in Figs. 3 and 4. In Fig. 3a, nitrate reduction was suppressed with the increase in the boric acid concentration. The resting cells were soaked
for ca. 20 h in a boric acid-containing denitrifying medium and then they were incubated for 15 d in a boric acid-free denitrifying medium. A small amount of nitrite accumulation was observed in the control experiment only (Fig. 3a). When nitrite was used as an electron acceptor in place of nitrate (Fig. 3b), the level of nitrite consumption was almost the same as that in the control experiment. These results suggest that boric acid seems to act as an inhibitor for nitrate reductase during the 20-h soaking period to harden the PVA gel beads.