A fluorescence-based microplate assay to quantify DOM-induced catabolic activity

This note describes a novel method to quickly quantify the dissolved organic matter (DOM)-induced catabolic activity from low-volume samples. The concept is based on the catabolic response profiles (CRP) assay and is described as an inverse CRP, where the reactivity of a complex and diverse mixture of organic compounds towards single strains of bacteria is quantified. A strain of Pseudomonas fluorescens was grown and then transferred to an organic carbon-free mineral salt medium. 90 microL of a fluorogenic redox indicator was added to 90 microL of the bacterial suspension in a well on a 96-well microplate. The DOM sample (90 microL) was added to the well and the fluorescence emitted by the reduced indicator was read over the period of incubation. Only 0.8 mL of the DOM sample, including controls and replicates, was required to quantify the activity of each sample. Results are presented for a surface soil DOM sample and they were compared to glucose samples of various concentrations. The detection limit was reached for samples containing as little as 55 microM of glucose (0.3 mg C L(-1)). The assay showed that only 9% of the total carbon of the soil surface DOM sample was readily biodegradable.