Critical evaluation of the microbial turnover to biomass approach for the estimation of biogenic non-extractable residues (NER)

Background: Persistence is a key criterion for the risk assessment of chemicals. In degradation tests, microbial biodegradation of labeled test chemicals leads to the incorporation of the label in microbial biomass, resulting in biogenic non-extractable residues (bioNER), which are not considered as harmful in persistence assessment. The amount of bioNER can be estimated using the microbial turnover to biomass (MTB) model. MTB estimates the biomass growth during productive degradation of a compound from theoretical growth yield and CO2-formation and gives an upper and a lower value for bioNER formation. The aim of this study is use available experimental data for bioNER to assess the validity, accuracy and precision of the MTB method as new tool in persistence assessment. Results: We collected experimental data in order to test accuracy and precision of this estimation method. In total, 16 experimental studies were found in literature where bioNER was experimentally quantified. Hereof, 13 studies used the amount of label recovered from total amino acid (tAA) content as proxy for bioNER. Unfortunately, the comparison with experimental data was difficult due to the variety of employed methods. A conversion factor is required to extrapolate from tAA on bioNER, and this factor may vary during the experiment and between experiments. The bioNER formation for all compounds tested was calculated with the MTB method, and the outcome was compared to measured tAA as proxy for bioNER. The relation between predicted and measured bioNER was significant, but no better correlation was obtained than with CO2 to tAA. The mean absolute error of the prediction (low MTB versus tAA) was 5% applied label (range 0.3 to 16%). Some deviation between measured results and calculated bioNER could be contributed to uncertainties in the experimental determination, as shown by variance in replicates (bromoxynil) or high background of label in sterile samples (sulfadiazine). Conclusions: MTB thus provides a robust model for determining of the potential amounts of biomass and bioNER formed from the degradation of organic chemicals.