Metabolic tracing unravels pathways of fungal and bacterial amino sugar formation in soil

Abstract

Summary The present study aimed to reveal the steps in the formation of fungal and bacterial amino sugars (AS) from glucose. Glucose labelled uniformly and position‐specifically at C‐1, C‐2, C‐4 and C‐6 was applied to an agricultural soil. Fungal and bacterial pathways of AS formation were reconstructed by a new approach: 13C recovery from individual C positions of glucose in AS by IC‐O‐IRMS. Only 0.75% of 13C from initially applied glucose was incorporated into AS within 10 days and followed the order: glucosamine > galactosamine > muramic acid. Relative incorporation of 13C (% of AS‐C) had the largest values for bacterial muramic acid and the smallest for fungal galactosamine. This reflects faster turnover of bacterial than fungal cell walls. A maximum of 55% ± 19% of the 13C incorporated in glucosamine was derived from intact, untransformed glucose. The smallest recovery was observed for C‐1 and C‐4. Recovery of C‐1 even decreased from day 3 to day 10, reflecting the metabolization by the pentose phosphate pathway. To be incorporated into AS, metabolites resulting from either the pentose phosphate pathway or glycolysis needed to be reconstructed to glucose by the reversible pathways of gluconeogenesis. The direct formation of AS (e.g. intact recovery of the glucose precursor), as well as amino sugar formation from recycled metabolites allocated to the hexose pool by back flux, occurred in parallel. Bacterial muramic acid showed the most variation in the recovery of individual C positions during 10 days, which reflects its intensive transformation by glycolysis, pentose phosphate pathway and gluconeogenesis. It is explained by the lower metabolic activity of fungi than bacteria at steady‐state conditions. The present approach enabled us to analyse the biochemical pathways in bacteria and fungi, which will considerably improve our understanding of the formation and stabilization of microbially derived SOM compounds. Highlights Fungal and bacterial pathways of the formation of amino sugars from glucose are reconstructed Intact use of glucose is distinguished from recycling by a metabolic tracing approach Half of the precursor (glucose) is metabolized prior to incorporation into amino sugars Intracellular recycling plays a crucial role in microbial SOM formation

Phosphorus rather than nitrogen enhances CO2 emissions in tropical forest soils: Evidence from a laboratory incubation study

“…The lack of change in the ratio of fungal to bacterial gene copy numbers in both soils implies a stable microbial composition in response to C addition (Table 5). Although these data imply that more microorganisms were involved with degrading the added C substrate, shifts in composition within the bacterial and fungal lineages cannot be ruled out with this approach and would require further studies using geochemical biomarkers such as phospholipid fatty acids (PLFAs), ergosterol and amino sugars, and sequence-based methods (Dippold et al, 2019;Hicks, Rahman, Carnol, Verheyen, & Rousk, 2018;Schlatter, Bakker, Bradeen, & Kinkel, 2015).…”

On maintenance and metabolisms in soil microbial communities

“…Under anoxic conditions, assuming fermentation only, the higher ATP yield of glycolysis over ED pathway may be critical, while the production of NADPH to fight oxidative stress is less important. It should be noted that measurement of CO2 production per C atom as described here can be supplemented with analysis of position-specific isotope incorporation into biosynthesis products, for example phospholipid fatty acids, thus revealing additional details of soil C metabolism (Apostel et al 2015;Dippold et al 2019;Wu et al 2020).…”

Calculation of fungal and bacterial inorganic nitrogen immobilization rates in soil

“…However, mainly as a result of the variation in the composition of tissues of massive microbial species, but also within each species under different growth conditions, the actual contents of GlcN and MurN in the respective biomasses of fungi and bacteria in soil are almost unobtainable (Appuhn and Joergensen 2006, Engelking et al 2007, Glaser et al 2004, Joergensen 2018. It is also still unclear how fast do the cell N-containing components turn over intracellularly and extracellularly in soil (Dippold et al 2019, Engelking et al 2007, Gunina et al 2017. As a consequence, converting the synthesis rates of 15 N-labeled amino sugars specific for fungi and bacteria to the actual inorganic N immobilization rates in soil is challenging.…”