Gas biosensors have been developed that “see” into soil and follow the behavior of the microbial communities in the soil. Genetically engineered bacteria was used that release methyl halide gases to monitor microbial gene expression in soil samples in the lab.
The bacteria are programmed using synthetic biology to release gas to report when they exchange DNA through horizontal gene transfer, the process by which organisms share genetic traits without a parent-to-child relationship. The biosensors allow researchers to monitor such processes in real time without having to see into or disturb a lab soil sample. The technique will serve the same purpose for environmental scientists that fluorescent reporter proteins serve for biochemists who track protein expression and other processes in biological systems.
The process of horizontal gene transfer controls a number of things that are important to humans either because they're good (how rhizobia trade the genes they need to fix nitrogen and support plant growth), or they're bad (how bacteria trade antibiotic resistance in soils).
The researchers expect scientists will use gas biosensors in the lab to study nitrogen fixing in agriculture, antibiotic exchange in wastewater treatment, gene transfer in conditions where nutrients are scarce, and the relationship between gene expression in soil and the release of greenhouse gases.
Releasing and sensing methyl halide gas represents an easy proof-of-concept. The next step is to obtain higher-resolution information about other types of biological events by creating more sophisticated genetic programs using synthetic biology. The ability to test agricultural soil samples to help fine-tune crop growth through more efficient watering and fertilizer use is the next goal.
These are tools for soil studies within lab environments; the synthetic microbes are destroyed once the results are obtained. Soil samples were tested after adding Escherichia coli bacteria programmed to release gas upon transfer of their DNA to another microbe. Signals from the gas were up to 10,000 times the lab's detection limit. The gas sensors were effective in anoxic — or oxygen-depleted — conditions, unlike green fluorescent protein, which requires oxygen to work. It is anticipated the reporter proteins can be used in many kinds of soil microbes, and some are currently being tested.
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