In autotrophs, excess nutrients are normally stored, but there are also disposal mechanisms present that enable them to dispose of excess organic carbon. First, organic matter can be released by diffusion or active transport across the cell membrane. This release of dissolved organic matter may be significant; phytoplankton release on average 13% of the assimilated C as dissolved organic carbon, but release rates as high as 80% of total primary production have been reported. In lacustrine plankton, the percentage of total C assimilation that is released increases with decreasing nutrient levels, suggesting that the release may serve as a way of disposing of excess C. Plants with symbiotic N-fixing Rhizobium bacteria release carbohydrates to the bacteria and receive N in exchange. The release of excess C thus serves as a way to promote the uptake of a limiting nutrient. Second, there are metabolic pathways, futile cycles, that provide alternatives to normal catabolic pathways. In these cycles, excess C is respired without producing new biomass or performing biochemical work. For example, in the alternative oxidase (AOX) pathway, the enzyme AOX is an electron acceptor that is not coupled to the generation of a proton motive force, which is generated by the normal oxidative phos-phorylation pathway. The seemingly wasteful AOX pathway allows the mitochondrion to modulate its ATP production rate and to reduce the rate of production of reactive oxygen species. The AOX activity is induced by N and P deficiency, thus increasing the respiration rate and decreasing the C-use efficiency.
Animals homeostatically regulate their nutrient balance not only by selective uptake, digestion or absorption, but also by selectively releasing nutrients in excess. The main excretory products containing P and N are phosphate (P), ammonium (N), and urea (N). In this way, the C:N:P ratios of the animal are regulated at relatively fixed levels. When the C:nutrient ratio of the assimilated food is higher than the demands of the consumer, the excess C has to be expelled either by increased respiration or by selective excretion of dissolved organic matter. Increased respiation using the oxidative phosphorylation pathway produces an energy surplus that has to be used in some way. Although data are rather anecdotal, increased physiological activity such as intensified filtration and swimming may account for some extra energy consumption. However, similar to the AOX pathway in plants, there are also futile cycles in animal metabolism that provide a decoupling of energy production and respiration. The other way of disposing excess C is to excrete it. Both these mechanisms lead to a depression of the energetic growth efficiency, but there is a tradeoff because the stoichiometry can be maintained at a level balancing the nutritional demands. Although data are scarce both on respiration rates and excretion rates as a function of food quality, there is considerable evidence that growth efficiency varies inversely with food C:nutrient ratio, and that the disposal of C at high C:nutrient ratios is due to both increased respiration and excretion. Similar results have also been obtained from a modeling study of C, N, and P turnover in animals, based on analysis of major physiological processes including assimilation, maintenance metabolism, growth, respiration, and excretion. Thus, physiological processes and associated allocation patterns are able to explain observed patterns in elemental stoichiome-try of nutrient release and C metabolism in animals.
See also: Ecological Stoichiometry: Overview; Evolutionary and Biochemical Aspects; Excretion; Growth Constraints: Michaelis-Menten Equation and Liebig's Law; Microbial Models; Plant Growth Models; Population and Community Interactions; Respiration; r-Strategist/K-Strategists.
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