Carbon Metabolism of Soil microorganisms at Low Temperatures: Position-Specific 13C Labeled Glucose Reveals the Story

Abstract:

Metabolic pathway activities at low temperature are not well understood, despite the fact that the processes are relevant for many soils globally and seasonally.

To analyze soil metabolism at low temperature, isotopomeres of position-specifically ¹³C labeled glucose were applied at three temperature levels; +5, -5 -20 ^oC. In additon, one sterilization treatment with sodium azide at +5 ^oC was also performed. Soils were incubated for 1, 3 and 10 days while soil samples at -20 ^oC were additionally sampled after 30 days. The ¹³C from individual molecule position in respired CO₂ was quantifed. Incorporation of ¹³C in bulk soil, extractable microbial biomass by chloroform fumigation extraction (CFE) and cell membranes of different microbial communities classified by ¹³C phospholipid fatty acid analysis (PLFA) was carried out.

Our ¹³CO₂ data showed a dominance of C-1 respiration at +5 °C for treatments with and without sodium azide, but total respiration for sodium azide inhibited treatments increased by 14%. In contrast, at -5 and -20 ^oC metabolic behavior showed intermingling of preferential respiration of the glucose C-4 and C-1 positions. Therefore, at +5 °C, pentose phosphate pathway activity is a dominant metabolic pathway used by microorganisms to metabolize glucose. The respiration increase due to NaN₃ inhibition was attributed to endoenzymes released from dead organisms that are stabilized at the soil matrix and have access to suitable substrate and co-factors to permit their funtions. Our PLFA analysis showed that incorporation of glucose ¹³C was higher in Gram negative bacteria than other microbial groups as they are most competitive for LMWOS. Only a limited amount of microbial groups maintained their glucose utilizing activity at -5 and -20 °C and they strongly shifted towards a metabolization of glucose via both glycolysis and pentose phosphate pathways indicating both growth and cellular maintenance. This study revealed a remarkable microbial acitivity at low temperatures which differs significantly from our observations at ambient temperature, which could be unraveled based on position-specific labeling.