The Effect of Flavonoids on the Microbial Mineralisation of Polycyclic Aromatic Hydrocarbons in Soil

Abstract

The effect of flavonoids (flavone, morin hydrate and 3-hydroxyflavone)on the microbial mineralisation of polycyclic aromatic hydrocarbons (PAHs) in soil slurry by the indigenous microbial communities has been investigated. The rates and extents of 14C–PAHs (14C–naphthalene, 14C–phenanthrene and 14C–pyrene)mineralisation in artificially spiked soils were monitored in the absence and presence of flavonoids applied at three different concentrations (50, 100 and 200 µg kg-1) either as single compounds or as a mixture of flavonoids (flavone, morin hydrate and 3-hydroxyflavone at a 1:1:1 ratio). Respirometric and microbial assays were monitored in fresh (0 d) and pre–incubated (28 d) artificially spiked soils following amendment with flavonoids. The highest extents of14C–naphthalene, 14C–phenanthrene and 14C–pyrenemineralisation (P< 0.001) were obtained in fresh artificially spiked soils amended with 100 µg kg-13-hydroxyflavonecompared to those obtained in unamended soils. However, amendment of fresh artificially spiked soils with higher concentrations of morin hydrate (≥ 100 µg kg-1) inhibited mineralisation of14C–pyrene by indigenous microbial communities.Apart from naphthalene and pyrene spiked soils amended with morin hydrate, pre–incubated artificially spiked soils amended with 100 µg kg-1 flavonoids had the highest extents of microbial mineralisation of PAHs compared to those obtained in unamended soils. The results obtained showed enhanced microbial mineralisation of 14C–PAHs in fresh artificially spiked soils amended with 50 – 100 µg kg-1 flavonoidsand in addition, low molecular weight PAHs (containing < 4 aromatic rings) exhibited higher extents of microbial mineralisation compared to high molecular weight PAH.Depending on its (bio) available concentration and solubility in soil, flavonoids can either stimulate or inhibit mineralisation of PAHs by indigenous soil microbial communities. This study suggests that phytochemical–microbe interactions are essential drivers of ecosystem functions relevant for the biodegradation of petroleum–derived or organic contaminants in soils.