EXOMET: An extracellular oxidative metabolism that substantially contributes to soil CO2 emissions

Background/Question/Methods

The respiratory release of CO₂ from soils is a major determinant of the global carbon cycle. It is traditionally considered that this respiration is an intracellular metabolism consisting of complex biochemical reactions carried out by numerous enzymes and co-factors. However, some studies have shown that substantial emission of CO₂ can persist for several weeks in soils where microbial life has been strongly reduced by exposition to toxics (ClCH₃, orange acridine) or irradiation. The objective of this study is to understand the cause of CO₂ emission in soils where microbial life has been suppressed with an approach combining soil incubations with yeast-extracted respiratory enzymes, electron microscopy, fluorescence in-situhybridization (TSA FISH) and enzyme assays with soil particles.

Results/Conclusions

Our results show that the respiratory enzymes released from dead organisms are stabilized by soil particles (minerals, humus) and have access to suitable substrates and co-factors to permit function. These enzymes reconstitute an extracellular oxidative metabolism (Exomet) that may substantially contribute to soil respiration (16 to 48% of CO₂ released from soils in the present study). Thus, CO₂ emissions from soils are apparently driven by two major oxidative metabolisms: 1) the well-known respiration of soil biota, 2) an Exomet carried out by enzymes released from dead organisms and stabilized by soil particles. Exomet and respiration from living organisms should be considered separately when studying effects of environmental factors on the C cycle because Exomet shows specific properties such as resistance to high temperature and toxics.