Subzero activity of cold-adapted yeasts

Abstract

Cold adaptation is usually interpreted as ability of microorganisms to grow at temperature around zero or recover from deep freezing. This chapter focuses on a recently discovered physiological phenomenon of microbial growth and metabolism in a frozen environment cooled down to-20 °C and deeper. Still, there are numerous controversies about subzero activity (SZA); therefore, the chapter starts from extensive introduction into the SZA phenomenon beyond the yeasts' taxonomic boundary. Critical review of available techniques for SZA detection resulted in a precautionary note that some reports based on account of frozen soil respiration or methane generation could be considerably overestimated. More reliable measurements of SZA are obtained by using methods based on direct microscopy and uptake of labelled gaseous substrates, e.g. 14CO2. Two types of below-zero habitats are recognized: homogeneous icy environment (polar sea ice, glaciers, snow) where microbial cells can survive in supercooled liquid veins and permanently frozen soils and sediments containing buried organic matter and allowing slow gas exchange through aeration micropores. Fungi including yeasts are the most successful colonizers of the second type of habitats. Solid-state frozen enrichment culture, phylogenetic survey of soils, round-year seasonal community dynamics and specific inhibitors of protein synthesis-all indicate that fungi have a competitive advantage over prokaryotic organisms in frozen soils. None of the known molecular mechanisms of cold adaptation, e.g. membrane structure, heat-and cold-stress proteins, cold-adapted enzymes, could be uniquely attributed to fungi. Instead, the author discusses wide opportunities given to all fungi by bigger cell size and colonization of frozen heterogeneous environments via mycelial or pseudomycelial growth.