Biochemistry of hexose and pentose transformations in soil analyzed by position-specific labeling and 13C-PLFA

Microbial transformations are key processes of soil organic matter (SOM) formation, stabilization and decomposition. Combination of position-specific 13C labeling with compound-specific 13C-PLFA analysis is a novel tool to trace metabolic pathways. This combination was used to analyze short-term transformations (3 and 10 days after tracer application) of two key monosaccharides: glucose and ribose in soil under field conditions. Transformations of sugars were quantified by the incorporation of 13C from individual molecule positions in bulk soil, microbial biomass (by CFE) and in cell membranes of microbial groups classified by 13C-PLFA. The 13C incorporation in the Gram negative bacteria was higher by one order of magnitude compared to all other microbial groups. All of the 13C recovered in soil on day 3 was allocated in microbial biomass. On day 10 however, a part of the 13C was recovered in non-extractable microbial cell components or microbial excretions. As sugars are not absorbed by mineral particles due to a lack of charged functional groups, their quick mineralization from soil solution is generally expected. However, microorganisms transformed sugars to metabolites with a slower turnover. The 13C incorporation from the individual glucose positions into soil and microbial biomass showed that the two main glucose utilizing pathways in organisms – glycolysis and the pentose phosphate pathway – exist in soils in parallel. However, the pattern of 13C incorporation from individual glucose positions into PLFAs showed intensive recycling of the added 13C via gluconeogenesis and a mixing of both glucose utilizing pathways. The pattern of position-specific incorporation of ribose C also shows initial utilization in the pentose phosphate pathway but is overprinted on day 10, again due to intensive recycling and mixing. This shows that glucose and ribose – as ubiquitous substrates – are used in various metabolic pathways and their C is intensively recycled in microbial biomass. Analyzing the fate of individual C atoms by position-specific labeling deeply improves our understanding of the pathways of microbial utilization of sugars (and other compounds) by microbial groups and so, of soil C fluxes.