Price P Buford, Sowers Todd
Physics Department, University of California, Berkeley, CA 94720, USA.
Proc Natl Acad Sci U S A. 2004 Mar 30;101(13):4631-6. doi: 10.1073/pnas.0400522101.
Our work was motivated by discoveries of prokaryotic communities that survive with little nutrient in ice and permafrost, with implications for past or present microbial life in Martian permafrost and Europan ice. We compared the temperature dependence of metabolic rates of microbial communities in permafrost, ice, snow, clouds, oceans, lakes, marine and freshwater sediments, and subsurface aquifer sediments. Metabolic rates per cell fall into three groupings: (i) a rate, microg(T), for growth, measured in the laboratory at in situ temperatures with minimal disturbance of the medium; (ii) a rate, microm(T), sufficient for maintenance of functions but for a nutrient level too low for growth; and (iii) a rate, micros(T), for survival of communities imprisoned in deep glacial ice, subsurface sediment, or ocean sediment, in which they can repair macromolecular damage but are probably largely dormant. The three groups have metabolic rates consistent with a single activation energy of approximately 110 kJ and that scale as microg(T):microm(T):micros(T) approximately 10(6):10(3):1. There is no evidence of a minimum temperature for metabolism. The rate at -40 degrees C in ice corresponds to approximately 10 turnovers of cellular carbon per billion years. Microbes in ice and permafrost have metabolic rates similar to those in water, soil, and sediment at the same temperature. This finding supports the view that, far below the freezing point, liquid water inside ice and permafrost is available for metabolism. The rate micros(T) for repairing molecular damage by means of DNA-repair enzymes and protein-repair enzymes such as methyltransferase is found to be comparable to the rate of spontaneous molecular damage.
我们的研究工作是受原核生物群落发现的启发，这些群落能在冰和永久冻土中以极少的养分生存，这对火星永久冻土和木卫二冰层中过去或现在的微生物生命具有启示意义。我们比较了永久冻土、冰、雪、云、海洋、湖泊、海洋和淡水沉积物以及地下含水层沉积物中微生物群落代谢率的温度依赖性。每个细胞的代谢率分为三类：（i）一种生长速率，即微生长速率（microg(T)），在实验室原位温度下、对培养基干扰最小的情况下测量得到；（ii）一种维持功能的速率，即微维持速率（microm(T)），在养分水平过低而无法生长的情况下；（iii）一种群落生存速率，即微生存速率（micros(T)），适用于被困在深层冰川冰、地下沉积物或海洋沉积物中的群落，在这些环境中它们可以修复大分子损伤，但可能大多处于休眠状态。这三类代谢率与约110 kJ的单一活化能一致，且比例为微生长速率（microg(T)）：微维持速率（microm(T)）：微生存速率（micros(T)）约为10^6：10^3：1。没有证据表明代谢存在最低温度。在-40℃的冰中的速率相当于每十亿年细胞碳约10次周转。冰和永久冻土中的微生物代谢率与相同温度下的水、土壤和沉积物中的相似。这一发现支持了这样一种观点，即在远低于冰点的情况下，冰和永久冻土中的液态水可用于代谢。通过DNA修复酶和蛋白质修复酶（如甲基转移酶）修复分子损伤的微生存速率（micros(T)）被发现与自发分子损伤速率相当。
