Cold-loving fungi fight frostbite, but can’t take the heat
To the unaided eye, Antarctic soil and alpine glaciers may appear to be barren wastelands devoid of life. But some microbes call hostile habitats like these home. Research on one such organism, published in the latest issue of G3, reveals some of the mechanisms behind cold adaptation—and explains why these otherwise hardy creatures can’t survive at temperatures that would be comfortable for us.
The research’s star subject is Mrakia psychrophila, a type of yeast found on the Tibetan Plateau that grows best at 12-15°C (54-59°F) and cannot grow over 20°C (68°F). Cold stress is distinct from cold adaptation—cold stress occurs when an organism is transferred from a higher temperature to a lower temperature over a short time, while cold adaptation (the process at play in M. psychrophila) is a steady-state phenomenon that occurs when an organism is kept at a cold temperature over an extended period.
By sequencing the cold-adapted yeast’s genome and comparing it to those of organisms that grow at moderate temperatures (mesophiles), high temperatures (thermophiles) and others that grow at cold temperatures (psychrophiles), the researchers observed some interesting trends. Like many cold-adapted and cold-tolerant microbes, the fungus exhibits high expression of genes involved in producing unsaturated fatty acids, which are critical for keeping membranes fluid at low temperatures. One major difference between M. psychrophila and other organisms is a bias in codon usage. This may influence mRNA structure, which also depends on temperature.
Comparing the transcriptome and proteome of the fungus when grown at different temperatures unveiled other secrets behind its adaptation to cold. Alternative splicing is highly influenced by growth temperature in this yeast. Genes related to energy metabolism are also upregulated in response to cold in M. psychrophila, in contrast to the mesophilic bacterial species Pseudomonas putida, which previous research showed conserves energy under cold stress. An increase in functions related to energy metabolism may reflect a greater need for ATP to fuel processes such as biosynthesis of unsaturated fatty acids.
Most intriguingly, the researchers found that at 20°C (68°F), M. psychrophila showed evidence of endoplasmic reticulum (ER) stress, a state characterized by the accumulation of unfolded proteins in the ER. This could explain why the fungi can’t tolerate moderate temperatures: ER stress, if not relieved, can induce cell death.
In addition to apparent quirks of M. psychrophila, these findings shed light on what may be some common mechanisms of cold adaptation. But that’s not all: research on psychrophilic organisms also informs the field of astrobiology, which aims to understand what life on other planets, if it exists, might be like. So while these researchers focused on an exotic yeast, the implications of their findings could be out of this world.
Su, Y.; Jiang, X.; Wu, W.; Wang, M.; Hamid, M. I.; Xiang, M.; Liu, X. Genomic, Transcriptomic, and Proteomic Analysis Provide Insights into the Cold Adaptation Mechanism of the Obligate Psychrophilic Fungus Mrakia psychrophila.
G3, 6(11), 3603–3613.