Fractionation of stable carbon isotopes during anaerobic production and degradation of propionate in defined microbial cultures

In many anoxic environments propionate is, after acetate, the second most important fermentation product, being degraded further to finally result in CH₄ production. In principle, isotope discrimination can be used to assess the path of organic matter degradation to acetate, CO₂ and CH₄. However, nothing is known about the isotope fractionation in primary and secondary fermentation steps involving propionate, although it is an important precursor of acetate. We therefore studied the degradation of propionate with a syntrophic coculture of Syntrophobacter fumaroxidans and Methanobacterium formicicum. The isotope enrichment factor for propionate degradation to acetate, CO₂ and CH₄ was almost negligible (ε_prop 0.9‰). The fermentative production of propionate was studied in cultures with Opitutus terrae growing on pectin, xylan and starch. These polysaccharides were fermented to acetate, succinate, propionate, H₂ and CO₂. While the δ¹³C value of the initially produced propionate was similar to that of the organic substrates (ca. −28 to −25‰), the δ¹³C value of the other fermentation products was higher. The δ¹³C values of all products generally decreased during the course of fermentation. Finally, a small depletion in ¹³C (ca. 6‰) with respect to the organic substrate was observed for propionate, while the other fermentation products where slightly enriched. Overall, stable carbon isotope discrimination was small during both fermentative production and consumption of propionate in the anaerobic microbial cultures, so that propionate turnover probably only marginally affects isotope fractionation during anaerobic degradation of organic matter.