Assimilation of toluene carbon along a bacteria–protist food chain determined by 13C-enrichment of biomarker fatty acids

A food chain consisting of toluene, toluene-degrading Pseudomonas sp. PS+ and a bacterivorous flagellated amoebae Vahlkampfia sp. was established in a batch culture. This culture was amended with [U- 13C]toluene and served as a model system to elucidate the flux of carbon in the food chain by quantifying bacterial biovolumes and 13C enrichment of phospholipid fatty acid (PLFA) biomarkers of the bacteria and the heterotrophic protists. Major PLFA detected in the batch co-culture included those derived from Pseudomonas sp. PS+ (16:1ω7c and 18:1ω7c) and Vahlkampfia sp. (20:4ω6c and 20:3ω6c). A numerical model including consumption of toluene by the bacteria and predation of the bacteria by the heterotrophic protists was adjsuted to the measured toluene carbon, bacterial carbon and δ 13C values of bacterial and protist biomass. Using this model, we estimated that 28±7% of the consumed toluene carbon was transformed into bacterial biomass, and 12±4% of the predated bacterial carbon was incorporated into heterotrophic protist biomass. Our study showed that the 13C enrichment of PLFA biomarkers coupled to biomass determination via biovolume calculations is a suitable method to trace carbon fluxes in protist-inclusive microbial food chains because it does not require the separation of protist cells from bacterial cells and soil particles.