Organismal and Molecular Stoichiometry

Stoichiometric Homeostasis in Organisms

All organisms are made of organic matter containing the same basic set of elements, but there is a fair amount of variation in elemental stoichiometry. Some of this variation is due to interspecific differences among organisms with a homeostatically regulated stoichiometry. However, there are also organisms that have a chemical composition that is not homeostatically regulated, and in which the elemental content closely reflects the availability of elements. An example of this lack of homeostatic control is the N:P ratio of the green alga Scenedesmus, which shows a very tight correlation between its cellular N:P ratio and the N:P ratio of the medium (Figure 2a). Although there certainly are limits to this lack of homeostatic regulation, the investigated range in N:P ratios is relevant from an

Figure 2 Relationships between nutrient supply ratios and cellular nutrient ratios (molar ratios) showing either lack of homeostatic control or strong homeostatic control. Broken lines indicate a 1:1 ratio between resource and cell nutrient ratios. Solid lines show linear regression lines of log-transformed data. If the slope of the solid line is equal to 1, there is a complete lack of homeostatic regulation, and if the slope is 0, the nutrient ratio is constant. (a) The N:P ratio of the green alga Scenedesmus closely resembles that of its nutrient medium, indicating a lack of homeostatic regulation. (b) The C:N ratio of the bacterium Pseudomonas fluorescens does not depend on the substrate C:N ratio, indicating a homeostatic regulation of its C:N ratio. (a) Data from Rhee G-Y (1978) Effects of N:P atomic ratios and nitrate limitation on algal growth, cell composition and nitrate uptake. Limnology and Oceanography 23: 10-25. (b) Data from Chrzanowski TH and Kyle M (1996) Ratios of carbon, nitrogen and phosphorus in Pseudomonas fluorescens as a model for bacterial element ratios and nutrient regeneration. Aquatic Microbial Ecology 10: 115-122.

Figure 2 Relationships between nutrient supply ratios and cellular nutrient ratios (molar ratios) showing either lack of homeostatic control or strong homeostatic control. Broken lines indicate a 1:1 ratio between resource and cell nutrient ratios. Solid lines show linear regression lines of log-transformed data. If the slope of the solid line is equal to 1, there is a complete lack of homeostatic regulation, and if the slope is 0, the nutrient ratio is constant. (a) The N:P ratio of the green alga Scenedesmus closely resembles that of its nutrient medium, indicating a lack of homeostatic regulation. (b) The C:N ratio of the bacterium Pseudomonas fluorescens does not depend on the substrate C:N ratio, indicating a homeostatic regulation of its C:N ratio. (a) Data from Rhee G-Y (1978) Effects of N:P atomic ratios and nitrate limitation on algal growth, cell composition and nitrate uptake. Limnology and Oceanography 23: 10-25. (b) Data from Chrzanowski TH and Kyle M (1996) Ratios of carbon, nitrogen and phosphorus in Pseudomonas fluorescens as a model for bacterial element ratios and nutrient regeneration. Aquatic Microbial Ecology 10: 115-122.

ecological point of view, and thus indicates that the N:P stoichiometry of this autotroph varies substantially within this range of nutrient supply ratios. Other photoautotrophic organisms, for example, vascular plants, microalgae, and cyanobacteria, show a similar lack of homeostatic regulation of their N:P, and C:nutrient ratios, with variation within species that can be of a similar magnitude as the total variation across autotroph taxa.

In contrast to the large intraspecific variation in auto-troph C:N:P stoichiometry, the elemental composition of metazoan animals varies much less. Even when the food is stoichiometrically imbalanced compared to the nutritional demands, the stoichiometry of the animals varies only within a relatively narrow range. Thus, metazoan elemental composition appears to be homeostatically controlled (albeit not strictly) and there are large interspecific differences. Likewise, the stoichiometry of bacterial cells varies much less than that of their substrates, suggesting a homeostatic control of bacterial stoichiometry. This is exemplified by the bacterium Pseudomonas fluorescens which does not change its biomass C:N ratio even when its substrate C:N stoichiometry varies more than one order of magnitude (Figure 2b).

Although the P and N content of resources may be high under conditions of high nutrient availability, the C:N and C:P ratios of food resources are frequently higher than those of herbivores, detritivores, and bacteria, both in terrestrial and aquatic systems. This elemental imbalance, as well as the limited intraspecific variation in animal and bacterial elemental composition, suggests that there is a potential for resource quality limitation of consumer growth. Consequently, there is a need for physiological adaptations helping consumers to cope with their elementally imbalanced diet. To achieve an understanding of the mechanisms behind the stoichiometric patterns among organisms, we need to consider the physiological processes that organisms use for acquiring, incorporating, and releasing nutrients, and how these are connected to biochemical allocation patterns.

Allocation Patterns Affect Organism Stoichiometry

Organisms consist mainly ofmacromolecules, but there are also monomeric precursors to the macromolecules, a multitude of other small organic metabolites, and inorganic compounds. The organic matter contains the elements hydrogen (H), carbon (C), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S) in various proportions depending on the type of molecule. The macromolecules have functions as structural components of the cells (e.g., cellulose, phospholipids, and some proteins), metabolically active substances (e.g., enzymes and ribosomal RNA), carriers of genetic information (e.g., DNA and messenger RNA), or storage products (e.g., starch and some lipids). In addition to organic compounds, organisms also contain inorganic compounds such as free ions involved in osmoregulation, signal transduction, and other electrochemical reactions, or larger inorganic molecules used for nutrient storage, as structural components of cell walls, or vertebrate and invertebrate supportive tissues. Each of these molecules has a specific elemental composition (reviewed in more detail in Evolutionary and Biochemical Aspects). The most conspicuous differences are that proteins are rich in N but contain little P, nucleic acids are rich in both N and P, and carbohydrates and lipids contain only minor fractions ofN and P.

A consequence of the distinct patterns in elemental stoichiometry of these organic and inorganic molecules is that the allocation of these will have a strong influence on the elemental stoichiometry of organisms. For example, storage of lipids or starches as energy reserves, and allocation to structural tissues containing cellulose, lignin, and chitin, will result in high C:N, C:P, and C:S ratios of the organism. Likewise, a large allocation to protein results in high N content and intermediate C content, and thus low C:N ratio, and high C:P and N:P ratios. Allocation to large quantities of nucleic acids (which contain the major part of the organism P) will result in high P and N contents, and low C:P, C:N, and N:P ratios. Therefore, any stressors that change the allocation to different macromolecules and inorganic compounds will also affect the elemental stoi-chiometry of organisms.

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