Metabolic Pathways of Amino Acids, Monosaccharides and Organic Acids in Soils assessed by Position-Specific Labeling

Transformation of low molecular weight organic substances (LMWOS) is one of the most important steps in biogeochemical cycles since all high molecular substances pass this stage during their decomposition. Microbial utilization is the most relevant sink for LMWOS in soils and thus knowledge about microbial transformations of LMWOS is crucial for understanding the soil organic carbon (SOC) cycle and predicting its reaction to changes in controlling environmental parameters. Previous studies focused on determining fluxes through the LMWOS pool, but they rarely identified transformation steps. This thesis aims to establish position-specific isotope labeling as a tool in soil science to trace the pathways of LMWOS transformations. In a mediumterm field experiment six position-specific 13C-labeled LMWOS from the three main LMWOS classes were applied: two amino acids (alanine and glutamate), two monosaccharides (glucose and ribose) and two organic acids (acetate and palmitate). C-13 remaining in soil and that incorporated into microbial biomass and specific microbial cellular compounds (phospholipid fatty acids (PLFA) and amino sugars) was determined by bulk and compound-specific C-13 analyses. Therefore, a new instrument coupling, an ion chromatograph with an isotope ratio mass spectrometer (IC-O-IRMS), and the respective methods for amino sugar analysis were established. The effect of altered environmental conditions and the relevance of further LMWOS sinks (sorption or plant uptake) were evaluated in several additional laboratory experiments based on position-specific C-14-labeling. The divergence index (DI) was established to compare the position-specific fate of individual substances in various studies independent of the isotopic approach or experimental design used or the pool investigated. Microbial utilization was the fastest process in the removal of LMWOS from soil solution and neither plant uptake nor sorption could out-compete microorganisms. The incorporation of individual molecule positions in soils, microbial biomass and distinct compound classes was clearly defined by the microbial metabolism: Glycolysis, oxidation by pyruvate dehydrogenase and the citric acid cycle were identified as the main metabolic processes. However, in addition to these oxidizing catabolic pathways, the anabolic pathways, i.e. building-up new cellular compounds, occurred in soils simultaneously. This involved an intensive C recycling and turnover within the microorganisms that was observed not only for cytosolic compounds but also for cell wall polymers. Intensive modifications and transformation within metabolic side branches, like the fatty acid formation and transformation pathways, were identified. These results for fatty acid transformations are crucial for their application as plant biomarkers in studies on palaeoenvironmental reconstruction. The combination of position-specific C-13-labeling with compound-specific isotope analysis of microbial biomarkers allowed the further identification of specific pathways of individual functional microbial groups in soils. Fungal metabolism was shown to be slower than bacterial intracellular C recycling and turnover, which provides the metabolic reason for the slow-cycling fungal-based and fast-cycling bacteria-based branch of the soil food web. Shifts in C allocation through various metabolic pathways were dependent on environmental factors: a gradient of C metabolism from starvation pathways via maintenance metabolism to metabolic pathways characteristic for microbial growth was observed with increasing substrate concentration. Sorption, also limiting the bioavailability of a substrate, caused similar shifts in metabolic pathways: the lower the bioavailability (e.g. due to sorption), the more C was allocated towards anabolic biosynthesis, i.e. into microbial products. Thus, these studies revealed that position-specific labeling is not only a valuable tool in biochemistry for metabolic flux analysis, but also enables the reconstruction of metabolic pathways of LMWOS within diverse microbial communities in complex media such as soil. Processes occurring simultaneously in soil i.e. 1) within individual, reversible metabolic pathways, 2) in various microbial groups or 3) in specific microhabitats (like on mineral surfaces, at the soil-plant interface or at hot-spots versus bulk soil) could be traced by position-specific labeling in soils in situ. The main metabolic pathways of microbial LMWOS transformation by cata- and anabolism were traced by position-specific labeling. These pathways and their regulating factors are crucial for assessing C flows towards mineralization versus the formation of microbial bio-mass, the prerequisite for the formation of microbially-derived SOC. This molecular knowledge of transformation steps and their regulating factors is crucial to predict (i.e. by new process-based modelling approaches) and manipulate C allocation and stabilization in soils.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2013

Author dc:creator

• Dippold, Michaela

Contributors dc:contributor

• Kuzyakov, Yakov

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/49/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:49