In situ observation of the distribution and activity of microorganisms in frozen soil

Frozen soils were observed in situ using fluorescence microscopy, and the suitability of applying 5-sulfofluorescein diacetate dye (sFDA) to study frozen soil was verified. We examined the temperature dependency of microbial activity in soil around 0 °C using the fluorescent area as an index. Microbial activity decreased proportionally with lowered soil temperature and the rate of decrease changed at 0 °C, becoming steeper at subzero temperatures. We observed more microbial activity in samples with more water content at above-freezing temperatures, but found no significant differences in activity by water content at subzero temperatures. When soil was directionally frozen, not only did microbial activity decrease with lowering temperature but also the distribution varied by freezing rate and the shape of ice formation; the microorganism was unevenly distributed in warmer regions near the freezing front. These results indicate that soil microorganisms might migrate in frozen ground via soil water flow due to soil freezing and ice exclusion near the freezing front.