Linking the active fraction of the soil microbial community with substrate-induced respiration

Soil microorganisms carry out many biological functions that are fundamental to soil ecosystems; however, less than 2% are estimated to be translationally active at any point in time. Soil tests for proxies of microbial activity such as substrate-induced respiration (SIR) can only give information at the bulk community level. Here, we enumerate active bacterial cells from an agricultural soil using the novel Bioorthogonal Non-canonical Amino Acid Tagging (BONCAT) method, in which cells incorporate a labeled amino acid into newly synthesized proteins, which then chemically bind a fluorescent dye and are counted by flow cytometry. We aimed to link bulk respiration rates with numbers of active cells. To accomplish this, we incubated a soil with 3 substrate treatments and periodically monitored respiration and active cell counts for 24 h.
We found an overall strong positive correlation between respiration rate and numbers of active cells, yet this relationship differed across substrates. This study advances understanding of how bulk soil measurements relevant for soil health monitoring are related to microbial activity at the cellular level.