Differential gene expression in the model Actinomycete Streptomyces coelicolor A3(2) during soil organic carbon priming

Soil organic carbon (SOC) priming by microorganisms influences ecosystem SOC processing. Priming is considered as either an increase (positive) or decrease (negative) in basal SOC catabolism in response to the input of organic matter, although what occurs at the cellular level remains elusive. One hypothesis suggests that priming is intricately linked to nitrogen (N) availability, whereby priming is a result of microbial mining of soil organic matter (SOM) for N due to N-limitation. We paired 13C isotope tracing and differential gene expression analysis to monitor priming in the model soil Actinomycete Streptomyces coelicolor A3(2) in response to substrates of varying N concentration: cellobiose (0% N), wheat (1.2% N; C:N 35.7) and mungbean (2.2% N; C:N 17.3). Positive priming occurred in cellobiose and wheat treatments, whereas negative priming occurred under mungbean. During positive priming, S. coelicolor A3(2) significantly increased transcription of extracellular enzymes involved in C and N acquisition (cellulases, chitinase, N-acetyl glucosaminidase), increased metabolism of mineral N (nitrite reductase), yet showed a significant decrease in cellular N assimilation (glutamate dehydrogenase) relative to negative priming. These results support the nutrient mining hypothesis, in that S. coelicolor A3(2) catabolised greater quantities of SOM as the N concentration of input organic matter decreased.