On the other hand, when PVA was added without soaking in boric acid, the nitrate comsumption in the PVA added experiments was comparable to that in the control experiment, however, nitrite was accumulated in the PVA-added (5 and 10%) experiments (Fig. 4a). In addition, when nitrite was used as an electron acceptor in place of nitrate (Fig. 4b), the nitrite consumption in the PVA-added experiment was reduced as compared with the control experiment. These results indicate that PVA mainly inhibits nitrite reductase.