Optimization of the bicinchoninic acid assay for quantifying carbohydrates of soil extracellular polymeric substances

Background and aims The bicinchoninic acid (BCA) method was not yet applied on soil extracts of extracellular polymeric substances (EPS) to quantify polysaccharides, although this might be possible by introducing a cleavage step to produce monosaccharides. A pre-extraction with CaCl₂ to remove interfering substances is usually performed before extracting EPS. The main objective of this study was to optimize the BCA assay for total carbohydrates quantification by applying a hydrolysis step to the EPS extracts, while also testing carbohydrate contents of CaCl₂ pre-extracts.

Methods Total carbohydrates were quantified with BCA in EPS extracts of three soils, after hydrolysis with H₂SO₄, using two acid concentrations (0.75 and 1.0 M), three hydrolysis temperatures (100, 120 and 130 °C), and five hydrolysis times (10, 30, 50, 70, and 90 min). EPS were extracted with the cation exchange resin (CER) method adapted to soils. Two versions of pre-extraction with CaCl₂ were tested twice consecutively.

Results More carbohydrates were measured after hydrolysis with 0.75 M H₂SO₄ at below 100 °C and after 10 min for all soils. Decreasing values were seen after longer reaction times and higher temperatures. CaCl2 extracted no or negligible amounts of carbohydrates from the soil.

Conclusion The pre-extraction step can be done without in most cases. The BCA assay is free of toxicity and easily performed, while also tolerant to interferences from most compounds in EPS extracts. These characteristics highlight the potential of this method for a rapid quantification of carbohydrates in studies of extractable polymers in several areas of soil biogeochemistry.