代谢异速生长理论及其在微生物生态学领域的应用

新陈代谢是生物的基本生理过程,影响生物在不同环境中参与物质循环和能量转化的过程.代谢速率作为生物体重要的生命过程指标,几乎影响所有的生物活性速率,且在很多研究中均表现出异速生长现象.所谓代谢异速是指生物体代谢速率与其个体大小(或质量)之间存在的幂函数关系.代谢异速生长理论的提出,从机制模型角度解释了代谢异速关系这一普遍存在的生命现象.该理论利用分形几何学及流体动力学等原理,从生物能量学角度阐释了异速生长规律的机理,证实了3/4权度指数的存在；但同时有研究表明,权度指数因环境因素等影响处于2/3-1范围之间而非定值.随着研究工作的深入,代谢异速生长理论研究从起初的宏观动植物领域拓展到了微生物领域,在研究微生物的代谢异速生长理论时,可将微生物的可操作分类单元(Operational taxonomic unit,OTU)或具有特定功能的功能群视为一个微生物个体,基于其遗传多样性和功能多样性特征进行表征,以便于将微生物群落多样性与其生态功能性联系起来,使该理论在微生物生态学领域得到有效的补充和完善.尽管细菌具有独特的生物学特性,但与宏观生物系统中观测到的现象表现出明显的一致性.有研究表明,3个农田土壤细菌基于遗传多样性的OTU数的平均周转率分别为0.71、0.80和0.84,介于2/3与1之间,可能与生物代谢异速指数有一定关联,为微生物代谢异速指数的研究提出了一个参考解决方案.鉴于微生物个体特征和生物学特性,在分析代谢速率与个体大小关系中,从微生物单位个体的定义、个体大小表征到计量单位的统一,仍需更多的理论支持.分析了代谢异速生长理论在微生物与生态系统功能关系研究中的可能应用,延伸了该理论的应用范围,并对尚待加强的研究问题进行了评述和展望.

Metabolic scaling theory and its application in microbial ecology

Metabolisms are fundamental processes of organisms. These processes affect material recycling and energy transfer of the organisms in different environments. Metabolism determines the demands that organisms place on their environment for all resources, and sets powerful constraints on allocation of resources to all components of fitness. As the important index of life process, metabolic rate, setting the pace of life, was found to follow the allometric scaling relationship in many studies. For a heterotroph, the metabolic rate is equal to the rate of respiration because heterotrophs obtain energy by oxidizing carbon compounds. For an autotroph, the metabolic rate is equal to the rate of photosynthesis because the same reaction is run in reverse using energy to fix carbon. Metabolic rate determines the rates of almost all biological activities. The metabolic scaling relationship is the power function existed between the metabolic rate and the body size (or mass) of an organism. Many data including most of the range of biological diversity on the planet, from bacteria to elephant and from algae to sapling trees, showed that metabolism displays a striking degree of homeostasis across all life, despite the enormous biochemical, physiological, and ecological differences between the surveyed species that vary over 10²⁰-fold in body mass, metabolic rates of major taxonomic groups displayed at physiological rest converge on a narrow range from 0.3 to 8.7 W/kg. The metabolic scaling theory was proposed as a mechanism model to explain allometric scaling relationship, which was a fundamental phenomenon of life. This theory based on the fractal-like distribution network models and fluid dynamics theory, and explained the allometric scaling relationship using bioenergetics. The fact that metabolic rate scales as three-quarter power was proved in many organisms, but there were also many results indicated that the scale exponent was not a fixed value of 3/4, but between 2/3 and 1 due to the influence of environmental factors such as temperature and stoichiometry. Based on the successful applications on macroorganisms, the metabolic scaling theory was extended to analyze the microorganisms, and revealed the possible universality of the theory to all the organisms in the nature. Much of the variation among ecosystems, including their biological structures, chemical compositions, energy and material fluxes, population processes, and species diversities, depends on the metabolic characteristics of the organisms that are present. Because the body size and other biological characters of the microorganisms were not well defined as the macroorganisms, it was important to define the taxon, the unit of body size and measurement standard of the microorganisms for the studies of their allometric scaling relationships. In the field of microbial ecology, microbial taxon could be defined as a phylogenetic or functional unit, and the application of metabolic scaling theory in the microbial ecology should focus on the consortium or functional community levels instead of individual strain level. Our preliminary work indicated that the microbial diversity was linked with the ecosystem functionality and well expressed using a power law equation with the scale exponents between 0.71 and 0.84 for different soils. These exponents (the microbial turnover rate) were well fell in the range of values for the metabolic scaling law, suggesting the possible universality of metabolic scaling theory for the microbial world. This paper introduced metabolic scaling theory and explored its preliminary application to the microbial ecology field, extended and deepened our understanding of the theory. We also foresaw the perspectives of the metabolic scaling theory in the field of microbial ecology.