Title:

Detecting Low Abundance Plastic Degrading Soil Microbes through Response Ratio Analysis and DNA Stable Isotope Probing

The decomposition of plastics by microbes is slow and energetically inefficient. As a result, these microbes may be slow-growing, making them exceptionally difficult to study. In 2023, we completed an 8-week incubation of agricultural soil with microplastics, adding a heavy isotope in the final week as an active microbe tracer. DNA stable isotope probing (SIP) was utilized to isolate the active portion of the microbial community under microplastic pollution, but we had insufficient isotope incorporation into microbial DNA. As such, we were unable to detect the active microbial community, but certain low-abundance taxa may have still increased in abundance with plastic addition. This year, with response ratio analysis, we determined that 15 taxa increased in abundance under plastic exposure, 11 of which were known plastic degraders or previously shown to increase in abundance under microplastic treatment conditions. However, we were unable to detect these active taxa using DNA SIP, meaning they were not active enough to incorporate the isotope tracer and are thus slow-growing. Longer isotope incubations are needed when using DNA SIP to allow for sufficient isotope incorporation in active microbes, such as slow-growing plastic degrading taxa. Given this limitation of DNA SIP, response ratios should be utilized in pollution research to detect proliferating taxa, as some slow-growing taxa could have pollutant degradation potential.

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