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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. |
