User Guide to Thresholds and Classification - Environmental ...

368User Guide for Thresholds and Classificationsaffinity for the substrate but lower growth rates and higher generation times than eutrophic organisms(OECD, 1995). Moreover, oligotrophs are unable to degrade chemicals in concentrations higher than 1 mg/Land may even be inhibited at high concentrations. Opposite to that, eutrophs require higher substrateconcentrations before mineralisation begins and they thrive at higher concentrations than oligotrophs. Thus,the lower threshold limit for degradation in the aquatic environment will depend on whether the microbialpopulation is an oligotroph or an eutroph population. It is, however, not clear whether oligotrophs andeutrophs are different species or whether there is only an oligotrophic and an eutrophic way of life (OECD,1995). Most pollutants reach the aquatic environment directly through discharge of wastewater andconsequently, these recipients are mostly eutrophic.From the above discussion it may thus be concluded that the chance of presence of competent degraders isgreatest in highly exposed environments, that is, in environments continuously receiving substances (whichmore frequently occurs for high production volume chemicals than for low production volume chemicals).These environments are often eutrophic and therefore, the degradation may require relatively highconcentrations of substances before onset. On the other hand, in pristine waters competent species may belacking, especially species capable of degradation of chemicals only occasionally released as low productionvolume chemicals.Substrate-related factorsConcentration of test substanceIn most laboratory tests, the test substance is applied in very high concentrations (2–100 mg/L) compared tothe concentrations in the lower μg/L range that may be expected in the aquatic environment. In general,growth of micro-organisms is not supported when a substrate is present in concentrations below a thresholdlevel of around 10 μg/L and at lower concentrations, even the energy requirement for maintenance is not met(OECD, 1995). The reason for this lower threshold level is possibly a lack of sufficient stimulus to initiate anenzymatic response (Scow, 1982). This means in general that the concentrations of many substances in theaquatic environment are at a level where they are too low to be the primary substrate for degrading microorganisms.Moreover, the degradation kinetics depends on substance concentration (S0) compared with the saturationconstant (Ks) as described in the Monod equation. The saturation constant is the concentration of thesubstrate resulting in a specific growth rate of 50% of the maximum specific growth rate. At substrateconcentrations much lower than the saturation constant, which is the normal situation in most of the aquaticenvironment, the degradation can be described by first order or logistic kinetics (OECD, 1995). When a lowdensity of micro-organisms (lower than 103–105 cells/mL) prevails (for example, in oligotrophic waters), thepopulation grows at ever decreasing rates which is typical of logistic kinetics. At a higher density ofmicroorganisms (for example, in eutrophic waters), the substrate concentration is not high enough to supportgrowth of the cells and first order kinetics apply, that is, the degradation rate is proportional with thesubstance concentration. In practice, it may be impossible to distinguish between the two types ofdegradation kinetics due to uncertainty of the data (OECD, 1995).January 2012 EPA0109