Rhizosphere bacterial carbon turnover is higher in nucleic acids than membrane lipids: implications for understanding soil carbon cycling.

Ashish A Malik,Helena Dannert,Bruce C Thomson,Robert I Griffiths,Gerd Gleixner

doi:10.3389/fmicb.2015.00268

Ashish A Malik, Helena Dannert + Show 3 more

Open Access

https://doi.org/10.3389/fmicb.2015.00268

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Abstract

Using a pulse chase 13CO2 plant labeling experiment we compared the flow of plant carbon into macromolecular fractions of rhizosphere soil microorganisms. Time dependent 13C dilution patterns in microbial cellular fractions were used to calculate their turnover time. The turnover times of microbial biomolecules were found to vary: microbial RNA (19 h) and DNA (30 h) turned over fastest followed by chloroform fumigation extraction-derived soluble cell lysis products (14 days), while phospholipid fatty acids (PLFAs) had the slowest turnover (42 days). PLFA/NLFA 13C analyses suggest that both mutualistic arbuscular mycorrhizal and saprophytic fungi are dominant in initial plant carbon uptake. In contrast, high initial 13C enrichment in RNA hints at bacterial importance in initial C uptake due to the dominance of bacterial derived RNA in total extracts of soil RNA. To explain this discrepancy, we observed low renewal rate of bacterial lipids, which may therefore bias lipid fatty acid based interpretations of the role of bacteria in soil microbial food webs. Based on our findings, we question current assumptions regarding plant-microbe carbon flux and suggest that the rhizosphere bacterial contribution to plant assimilate uptake could be higher. This highlights the need for more detailed quantitative investigations with nucleic acid biomarkers to further validate these findings.

Highlights

Soil microorganisms are a crucial link between the above and belowground components of ecosystems
13C in Microbial Deoxyribonucleic acids (DNA) and Ribonucleic acids (RNA) The incorporation of 13C into DNA and RNA was measured to assess the turnover of these compounds in soil microorganisms
We did not analyze root exudates, based on the respiratory pattern we expected that the pulse 13C would be rapidly translocated into the rhizosphere as labile substances in the form of root exudates, which would serve as rhizodeposited C sources for rhizosphere microorganisms

Summary

Introduction

Soil microorganisms are a crucial link between the above and belowground components of ecosystems. Despite the relatively large size and temporal sensitivity of the soil organic carbon pool and its importance in terrestrial ecosystem functioning (including maintenance of agricultural productivity and mitigation of atmospheric CO2 levels), our knowledge of the soil processes involved in the formation and decomposition of SOM remains very limited (Allison and Martiny, 2008; Lal, 2010). It has been estimated that up to 20% of photoassimilates can be utilized by AM fungi, making mycorrhizal hyphal turnover a substantial process for carbon input into SOM (Bago et al, 2000; Godbold et al, 2006) These results point to a lack of consensual knowledge concerning microbial trophic interactions and the flow of carbon through the plant-rhizosphere-soil continuum. A reassessment of the root associated microbial food web is warranted

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