Degradation of microcystin in sediments at oxic and anoxic, denitrifying conditions

The potent toxin microcystin is frequently released during cyanobacterial blooms in eutrophic waters and may impose a risk to human health, when surface water is used for drinking water. For removal of microcystin in surface waters, infiltration through sediment is commonly used. In the present study, mineralization of 14C-labelled microcystin (accumulation of 14CO2) and concentration changes (protein phosphatase inhibition assay) demonstrated that indigenous microorganisms in the sediment of a water recharge facility were capable of degrading microcystin. At oxic or microaerophilic (<2% O2) conditions, microcystin added to sediment slurries at 70 (mu)g l^-1 was reduced to <20 (mu)g l^-1 in 1–2 weeks, and less than 3 (mu)g l^-1 after 7 weeks. At anoxic conditions (<0.3% O2) and with addition of nitrate, the degradation was significantly stimulated, reducing microcystin from 100 to <20 (mu)g l^-1 within 1 day. The simultaneous production of N2O in the samples suggests that the microcystin degradation was coupled to dissimilative nitrate reduction (denitrification). Since aquifers and sediments beneath drinking water reservoirs often are anoxic, nitrate respiration may be an important process in removal and detoxification of microcystin.