Oxidative stress and culture atmosphere effects on bioactive compounds and laccase activity in the white rot fungus Phlebia radiata on birch wood substrate

Wood-decaying white rot fungi live in changing environmental conditions and may switch from aerobic to fermentative metabolism under oxygen depletion. Decomposition of wood and lignocellulose by fungi is dependent on enzymatic and oxidative biochemistry including generation of reactive oxygen species. In this study, we subjected semi-solid wood-substrate cultures of the white rot fungus Phlebia radiata to oxidative stress by addition of hydrogen peroxide under aerobic and anaerobic cultivation conditions. Wood decomposition and fungal metabolism were followed by analysis of extracellular organic compounds, mycelial growth, and laccase activity. Under both atmospheric conditions, accumulation of bioactive aromatic compounds from birch wood occurred into the culture supernatants after hydrogen peroxide treatment. The supernatants inhibited both fungal growth and laccase activity. However, the fungus recovered from the oxidative stress quickly in a few days, especially when cultivated under regular aerobic conditions. With repeated hydrogen peroxide treatments, laccase suppressive-recovery effect was observed. Culture supernatants demonstrated antioxidant and antimicrobial effects, in concert with emergence of chlorinated birch-derived organic compounds. Bioactivities in the cultures disappeared in the same pace as the chlorinated compounds were transformed and de-chlorinated by the fungus. Our results indicate tolerance of white rot fungi against excessive oxidative stress and wood-derived, growth-inhibiting and harmful agents.