Root exuded carbon is taken up by bacteria and bacterial DNA turns over faster than fatty acids in soil

In a recent article in Frontiers in Microbiology, Malik and colleagues highlight that rhizosphere microbial cellular components have variable C turnover. The study shows that bacterial C turnover is higher in nucleic acids (19 h for RNA and 30 h for DNA) than in membrane phospholipid fatty acids (42 d). The authors used a pulse-chase 13CO2 plant labeling experiment and traced the 13C from plants through roots into the rhizosphere soil microorganisms.

Roots exude labeled carbon molecules, which are taken up by bacteria and incorporated into DNA and fatty acids. Upon the death of the bacteria, DNA and fatty acids are released into the soil and eventually incorporated into soil organic matter (SOM). Figure by RBN.
The results highlight two ecological aspects:

1) Since soil microbial biomass is thought to be important in maintenance of soil organic matter (SOM), the findings that microbial fractions have variable turnover rate suggest that their macromolecular structure and composition is important in determining the degree of microbial contribution to SOM formation. To put it in simple words, what constitutes the microbial cells could influence how much of it remains in soil which acts as a carbon sink.

2) Higher turnover of bacterial nucleic acids than fatty acids suggests that bacterial role in the initial assimilation of plant C could be higher than current estimates. This is because lipid fatty acids which have been widely used to investigate plant-rhizosphere C flow are biased against bacteria particularly the Gram positive ones which have a very complex cell envelope. The authors suggest that future studies relating to C flow consider the inherent turnover rates of microbial biomolecules while choosing the target biomarker. They also highlight the need for more detailed quantitative investigations with nucleic acid biomarkers to reappraise the rhizosphere microbial food web.

--Written text contributed by Ashish Malik as a solicited RhizoView