The Importance of Initial Conditions in Batch Assays for Estimating the in Situ Biodegradation Kinetics of Chlorinated Ethene-Respiring Cultures

The reported chlorinated ethene utilization kinetics for dehalorespiring cultures vary considerably. Some of the variability in the parameter estimates may reflect true differences in the inherent kinetic characteristics of different dehalorespiring organisms. However, three interrelated factors, culture history, parameter identifiability, and the nature of the assay procedure, can also impact kinetic parameter estimates. In non-steady-state batch assays, the influence of these factors on the parameter estimates is related to the initial conditions, which are captured by the initial substrate (S₀) to biomass (X₀) ratio, S₀/X₀. The S₀/X₀ ratio also affects the physiological state of the cells. To obtain kinetic parameter estimates that reflect the activity of a culture in situ, the physiological state of the cells in a batch assay must not be allowed to change, which requires a very small S₀/X­₀.

Monod kinetic parameters were estimated for tetrachloroethene (PCE)-dehalorespiring strains, including Desulfuromonas michiganensis, in batch assays with low S₀/X₀ to predict PCE biotransformation in the source environment. However, evaluation of the correlation coefficients for the parameter estimates revealed that they were highly correlated. The usefulness of correlated Monod parameters is limited because different combinations of estimated parameter values may fit the experimental equally well, and the estimated values are particularly sensitive to measurement errors. A series of batch experiments were undertaken to determine the relationship between S₀/X₀ and parameter correlation (or identifiability). When the S₀/X₀ ratio became sufficiently large, the estimated parameter values became independent of the source environment.