The effect of free nitrous acid on the anabolic and catabolic processes of glycogen accumulating organisms

Nitrite/Free Nitrous Acid (FNA) has previously been shown to inhibit aerobic and anoxic phosphate uptake by polyphosphate accumulating organisms (PAOs). The inhibitory effect of FNA on the aerobic metabolism of Glycogen Accumulating Organisms (GAOs) is investigated. A culture highly enriched (92 ± 3%) in Candidatus Competibacter phosphatis (hereafter called Competibacter) was used. The experimental data strongly suggest that FNA likely directly inhibits the growth of Competibacter, with 50% inhibition occurring at 1.5 × 10⁻³ mgN-HNO₂/L (equivalent to approximately 6.3 mgN-NO₂⁻/L at pH 7.0). The inhibition is well described by an exponential function. The organisms ceased to grow at an FNA concentration of 7.1 × 10⁻³ mgN-HNO₂/L. At this FNA level, glycogen production, another anabolic process performed by GAOs in parallel to growth, decreased by 40%, while the consumption of polyhydroxyalkanoates (PHAs), the intracellular carbon and energy sources for GAOs, decreased by approximately 50%. FNA likely inhibited either or both of the PHA oxidation and glycogen production processes, but to a much less extent in comparison to the inhibition on growth. The comparison of these results with those previously reported on PAOs suggest that FNA has much stronger inhibitory effects on the aerobic metabolism of PAOs than on GAOs, and may thus provide a competitive advantage to GAOs over PAOs in enhanced biological phosphorus removal (EBPR) systems.