Contrasting Homeostasis in Plants and Animals

Autotrophs rely on either light or chemical energy to turn CO2 into organic carbon molecules. Photoautotrophs are photosynthesizing organisms such as algae and higher plants that use light for this process. Heterotrophs, in contrast, obtain their chemical energy from preexisting organic molecules. Examples ofheterotrophs include bacteria, which absorb organic substances from their surroundings, and many different animals, which consume and digest other organisms. These two major contrasting nutritional strategies of autotrophy and het-erotrophy also contrast in their stoichiometric flexibility. Autotrophs obtain carbon, energy, and nutrients from different, somewhat independent sources, whereas many heterotrophs obtain all of these at once from the same food parcels. This contrasting flexibility in turn has a great bearing on the specifics of how stoichiometry enters into ecology.

Photosynthesis relies on light energy to fix CO2 into organic molecules such as sugars. From these building blocks many other biochemicals can be made. Carbon:nutrient stoichiometry (C:N or C:P ratios) in individual autotroph species can be quite variable. Biochemicals such as carbohydrates and many lipids, which contain only C, H, and O, are made without incorporation of nutrients such as N or P. An autotroph in the light and with adequate access to CO2 can make a plentiful supply of these compounds (starches, oils, organic acids, etc.) without investment of other critical resources.

It is often observed that autotrophs growing in high-light, low-nutrient environments will possess a great abundance of these molecules, so much so in fact that the C-content of the autotroph will be elevated under those types of conditions. Carbon:nutrient ratios within such plants can be exceedingly high (>1500 C:P, for example). When a slow-growing, nutrient-limited autotroph suddenly is exposed to high nutrient availability, it will take up those nutrients much faster than its growth rate. That is, nutrients are taken up in excess compared to growth requirements and in some extreme cases stored in specialized structures such as vacuoles or in specialized molecules such as polyphosphate. High carbon:nutrient ratios are also characteristic of large autotrophs such as trees, which require substantial investment in wood and ancillary tissues having high C:nutrient ratio. Ecological implications of these stoichiometric responses to light:-nutrient ratios are discussed below.

Autotroph nutrient content is related to growth rate (p, g g-1 d-1). A quota (Q) is the mass or molar quantity of nutrients per cell (this discussion assumes a constant cell size). In unicellular autotrophs, the 'cell quota' concept relates these two variables. The quota of the element that regulates growth rate will be very tightly related to growth rate by a relationship referred to as the Droop formula:

where p' is a theoretical maximum growth, never attained, associated with infinite quota, and k is the minimum quota occurring at zero growth.

Under strongly nutrient limiting conditions where growth rate is low, quota of the limiting nutrient will be low, meaning a low nutrient:C or high C:nutrient ratio (see cellular C:P, Figure 1, top panel). The minimum cell quota (k) is set by the level of nutrient-containing bio-chemicals necessary for basic metabolism, and nutrient requirements for growth are added to this basal level. A true upper level for nutrient content (less than p') will be set by some combination of the composition of protoplasm at high growth rate or the ability of an autotroph to store excess quantities of any nutrient not currently needed for growth. In autotrophs, growth involves at least two specific major stoichiometric components, and probably more. The first is N for proteins involved in photosynthesis, especially the enzyme RUBISCO, which can be a major portion of cellular biomass. Metabolism in vascular plants relates more strongly and consistently to N than biomass or C. The second is P for ribosomes, which are needed to manufacture additional proteins.

In addition to these patterns relating content of the limiting nutrient to growth rate, the ratio of nutrient elements in an autotroph varies positively with the ratio of those nutrients in the environment. Soils or water of

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Figure 1 Autotroph nutrient content as a function of both growth rate and nutrients in the external environment. (a) Experiments with the unicellular alga Dunaliella tertiolecta. Symbols refer to different N:P in the growth medium (5-50). (b) Experiments with two species of grasses, one (Dactylis glomerata) fast-growing and the other (Brachypodium pinnatum) slow-growing. In the upper part of (a), note that cellular C:P declines with increasing growth rate, and is highest at low growth rate and where the environmental N:P is greatest. Similarly, both panels of (a) show that environmental N:P has a positive effect on algal N:P at all growth rates. In panel (b), note again that environmental N:P has a positive influence on tissue N:P. Panel (b) also shows that for any given environmental N:P, the fast-growing species has lower N:P than the slow-growing species.

Figure 1 Autotroph nutrient content as a function of both growth rate and nutrients in the external environment. (a) Experiments with the unicellular alga Dunaliella tertiolecta. Symbols refer to different N:P in the growth medium (5-50). (b) Experiments with two species of grasses, one (Dactylis glomerata) fast-growing and the other (Brachypodium pinnatum) slow-growing. In the upper part of (a), note that cellular C:P declines with increasing growth rate, and is highest at low growth rate and where the environmental N:P is greatest. Similarly, both panels of (a) show that environmental N:P has a positive effect on algal N:P at all growth rates. In panel (b), note again that environmental N:P has a positive influence on tissue N:P. Panel (b) also shows that for any given environmental N:P, the fast-growing species has lower N:P than the slow-growing species.

high N:P ratio will generally support plants or algae with high N:P ratio. This positive relationship derives in part from shifts in species across gradients such as these, with competition favoring species that have similar nutrient ratios as the supply ratio in the environment. It also derives from intraspecific, physiological shifts associated with differing storage and utilization of the two nutrients similar to those described for quota above. Figure 1 summarizes these different influences on autotroph nutrient content.

Samplings of whole assemblages of autotroph biomass have been examined in terrestrial, freshwater, and marine ecosystems, and have included microscopic as well as macroscopic species. Terrestrial ecosystems, with their larger, cellulose-rich, and woody plant species have higher and more variable C:P and C:N ratios than aquatic ecosystems. In the aquatic realm, offshore marine environments characteristically have low and less variable C:P and C:N ratios in their suspended matter, which contains a strong signal of autotroph biomass. We saw this relative constancy in the offshore marine realm when we discussed the Redfield ratio above. Redfield described the marine plankton to have a C:N:P ratio of 106:16:1. Today there is continuing interest in the Redfield ratio in the ocean, and it is known that it is not a true constant but rather varies with several factors, including climate. Freshwater ecosystems can be thought of as being intermediate in their stoichiometric patterns of C:N:P between terrestrial ecosystems and offshore marine ecosystems.

Animals and other heterotroph species also vary in their chemical content. Large shifts in C:N or C:P ratios in heterotrophs can follow from storage of large amounts of chemical energy in the form of lipids. Some invertebrates in seasonal environments, for instance, may assimilate and store lipids to the point where they are approximately half of organism mass. When those lipids are subsequently catabolized, dramatic shifts in C:N or C:P result. However, in contrast to the great stoichio-metric flexibility often observed in autotrophs, unicellular and multicellular heterotrophs come closer to approaching an idealized, strictly homeostatic, abstract 'molecule' of defined chemical composition. Reasons for this contrast between plants and animals are not well understood but might involve lack of specialized storage vacuoles in animal cells and the fact that animals obtain carbon, energy, and nutrients from living or recently living material, which is less chemically variable than the abiotic sources of carbon, energy, and nutrients used by plants.

Metazoan animal species exhibit a wide range of N:P ratios. Small, poorly skeletonized organisms such as tadpole stages of amphibians have N:P of ^20 whereas some fish species that are heavily endowed with calcium phosphate apatite mineral both in their internal skeleton and in their scales have N:P of ~5. Fish in fact are a highly stoichiometrically variable group. From that minimum N:P of about 5, different species of lower structural P content range upward to N:P of 15. Within fish, the Ca:P ratios are highly constrained, indicating that most of the stoichiometric differences in this group result from evolutionary pressures on structure and hardness of the integument.

These inter- and intraspecific patterns of elemental content combine in food webs of many species. Stoichiometric imbalance, where resource and consumer differ radically in their nutrient content, generates interesting ecological dynamics that we will consider next.

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Responses

  • Cherubino
    What molecules in plants contain a low c:n ratio?
    7 years ago

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