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 C-13 labeling with compound-specific C-13-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 C-13 from individual molecule positions in bulk soil, microbial biomass (by CFE) and in cell membranes of microbial groups classified by C-13-PLFA. The C-13 incorporation in the Gram negative bacteria was higher by one order of magnitude compared to all other microbial groups. All of the C-13 recovered in soil on day 3 was allocated in microbial biomass. On day 10 however, a part of the C-13 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 C-13 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 C-13 incorporation from individual glucose positions into PLFAs showed intensive recycling of the added C-13 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. (C) 2014 Elsevier Ltd. All rights reserved.