Stable isotope probing: technical considerations when resolving (15)N-labeled RNA in gradients

Area of Science:

• Microbiology
• Molecular Biology
• Environmental Science

Background:

• Stable isotope probing (SIP) identifies active microbial populations assimilating labeled compounds.
• (15)N-RNA-SIP is a novel method with potential but untested applications.
• Understanding nitrogen cycling microbes is crucial for environmental processes.

Purpose of the Study:

• To evaluate the efficacy and limitations of (15)N-RNA-SIP for analyzing microbial communities.
• To define challenges associated with using (15)N-labeled substrates in SIP.
• To propose modifications for improving (15)N-RNA-SIP.

Main Methods:

• Incubation of mixed microbial communities with (15)N-ammonium chloride or (15)N(2).
• Isopycnic centrifugation of RNA in caesium trifluoroacetate (CsTFA) gradients.
• Analysis using terminal-restriction fragment length polymorphism (T-RFLP) and fluorescent in situ hybridisation (FISH).

Main Results:

• Increased isotopic labeling enhanced buoyant density (BD) separation between (15)N- and (14)N-RNA.
• Definitive resolution of labeled from unlabeled RNA was not achieved through gradient fractionation.
• T-RFLP and FISH provided insights into nitrogen-fixing organisms.

Conclusions:

• While (15)N-RNA-SIP shows promise for studying nitrogen assimilation, limitations in RNA resolution persist.
• Further methodological development is needed to optimize its application in diverse nitrogen-fixing environments.
• The study highlights the need for robust techniques to track microbial activity in situ.