Effects of intensified freezing-thawing cycles on arctic soil microbiota investigated via soil chips

Arctic ecosystems are experiencing a strong and fast warming in the realms of climate change, and understanding the involved processes are important to predict impacts and feedbacks on their C cycling. Winter warming leads to frequent and reoccurring snow melts and as a consequence exposed bare ground. This leads to accelerated freeze-thaw cycles, since the snow cover that was insulating the soil below to temperature variations around a few degrees minus now can be exposed to much harsher freezes. We experimentally exposed soil crusts from Greenland to freezing-thawing cycles of different intensities and frequencies and measured the abundance of the three soil microbial groups bacteria, fungi and protists with help of microfluidic soil chips. The soil chips are brought into tight contact with the soil sample, and the microbial community colonizes their transparent pore spaces which enable us to image-based analysis of microbial abundance and interactions. We found that increased freezing frequency (daily versus bi-weekly) strikingly reduced bacterial populations, stronger than increased freezing intensity (-5°C vs -18°C). We also exposed the soil chips to live-freezing under the microscope to analyze direct effects of the approaching ice front on the microbial community. At intermediate freezing temperatures, dead-end pockets in the pore space remained liquid-filled and could act as refugia for the organisms. Fast approaching ice fronts caught fleeing organisms and in some cases led to detrimental outcomes, especially for protists. Disturbances in the trophic network differently affecting predators and pray may thus also contribute to changes in the bacterial carbon cycling.