冻融作用对冻土区微生物生理和生态的影响

Physiological and ecological effects of freezing and thawing processes on microorganisms in seasonally-froze ground and in permafrost

不同时空尺度的冻融过程导致冻土温度变化及水的相变和迁移,改变着微生物生境的物理和化学性质。冻融过程可改变细胞内外渗透压平衡,且冰晶生长过程能损伤细胞膜和细胞器。限于营养、氧气等其它极端不利条件,不少细胞逐渐转入休眠状态以度过难关;微生物DNA、蛋白质的合成和能量代谢仅用于维持细胞生存。冻融作用通过改变细胞代谢模式而影响微生物参与的寒区碳氮元素的生物地球化学循环。多年冻土层保存了不同地质时期微生物种群的多样性,作为物理和生物地球化学屏障,可有效削弱地表过程和地壳本底辐射对微生物的影响。在长期的适应过程中,微生物发展了相应的耐受机制,从结构和功能方面,细胞和分子水平上应对冻土环境和冻融过程。这可为寻找地外寒冷星球上可能的生命形式提供一些线索。

The freezing and thawing processes occurring at various spatio-temporal scales in soils can lead to cyclical changes in their temperature and moisture fields. The changes result in alterations of physical and chemical properties in microbial habitats, as well as direct or indirect injuries to the cell membranes and organelles. These injuries are caused by ice crystalization disrupting the cellular walls and changes in the osmotic balances. Due to restriction of the environments in seasonally frozen soils and in permafrost, cell becomes dormant, and DNA, protein synthesis and energy supply are maintained only at extremely low levels needed for cell survival. The biogeochemical cycles of carbon and nitrogen, mediated and modulated by microbial activity, are influenced by the freeze-thaw process in the seasonally-frozen ground and the active layer of permafrost. The permafrost environments tend to conserve biological objects and diversity at temperatures below 0℃ and serve as physical and chemical barriers which sharply restrict the influences of external factors, including background radiation, on the microorganisms. During the long process of adaptation, microbes correspondingly developed tolerant mechanisms and structures and functions at both cellular and molecular levels consistent with the permafrost environments. This provides important implications in the search for past or extant life in the presumed permafrost on Mars,which apparently had a water environment earlier in its history, or perhaps in the near-surface of some moons with a methane environment.