Effects of intense freeze-thaw cycles on Arctic biological soil crusts as studied by Raman microspectroscopy

Soil microbial metabolism is extremely important to large scale processes such as nutrient cycling and climate change. At the same time, the changing climate influences soil structure and function, especially in the Arctic region, which has been experiencing faster and more intense warming compared to anywhere else in the world. To better understand the microscale processes and how they are affected by changing temperatures and extreme weather events, we use soil microchips, mimicking the soil structure and providing visual access to the soil systems, and incubate them with microorganisms from Arctic biological soil crusts. These chips then are subjected to different freezing and thawing cycles (FTCs), and we follow the microbial activity and metabolism by the means of optical microscopy and Raman microspectroscopy.

The samples for this experiment were collected in summer from a dry heath tundra ecosystem in Blæsedalen on Disko Island, West Greenland. The plots where the soil was sampled had been warmed during the previous winter in a winter warming experiment which showed some increased activity of microbes from the warmed plots. During the laboratory experiments, the chips containing soil microbes were placed at +5 °C (control), as well as -5 °C and -18 °C (freezing) temperatures. The frozen chips were thawed at two different frequencies – one daily and one biweekly. During the six weeks of the freezing and thawing cycles, the chips were observed in an optical microscope in order to follow the microbial growth and community changes. After the treatment was finished, the chips were analysed by Raman microspectroscopy.

Raman microspectroscopy can be employed to study the chemical composition and metabolic processes of individual live microorganisms in near real time. The microbial metabolic activity was monitored using SIP (stable isotope probing) Raman microspectroscopy. We injected SIP labelled substrates into the soil microchips and followed the intensity of SIP related spectral bands as microorganisms incorporated the labelled substances. The results show significant differences between control and FTC treatment chips, with microbes from control chips metabolizing injected substances much faster, especially in the case of bacteria. The differences among the treated chips are less pronounced. However, the microbes in the chips that had been thawed daily exhibit stronger fluorescence signal, suggesting their different protective responses to the stronger environmental stressor.

All in all, soil chips allow the visual observation of microbial community changes in response to FTCs, while SIP Raman makes it possible to estimate metabolic activity rates of individual organism groups. Although currently limited in scale, in the future this information could be used to better describe the role of microbial communities in larger scale climate models.