Measuring soil protist respiration and ingestion rates using stable isotopes

Protists have a direct effect on soil nutrient cycling due to their abundance, diversity and assimilation rates of bacterial biomass. Here for the first time stable isotopes have been utilised to quantify respiration and ingestion rates of soil protists. We show that microcosms can generate values for these variables within the instrument detection range. Through consumption of enriched bacteria, indigenous agricultural grassland soil protists obtained an enrichment of 6.8 atom% (±1.67) for 13C and 16.4 atom% (±4.34) for 15N. Bacteria were consumed at a rate of 41 (±0.04) bacteria h−1 protist−1 during the initial 24 h period of incubation. Protist respiration monitored over time equated to an increase of 0.15 atom% excess (±0.036) in 13CO2 respired per minute and an accumulation of 0.7 fg (±0.36) CO2–C protist−1 min−1. These results provide numbers quantifying the assimilation efficiencies of soil protists, their effect on labile biomass turnover and the flow of C and N through the soil food web. ► Protists where enriched by 6.8 atom% for 13C and 16.4 atom% for 15N. ► Assimilation rates were calculated to yield a net production efficiency of 36.8%. ► Protist respiration equated to an increase of 0.15 atom% excess 13CO2 respired min−1. ► Protist respiration amounted to an accumulation of 0.7 fg CO2–C protist−1 min−1. ► First time amount of CO2 respired was measured for soil protists to this accuracy.