Acquisition of Nutrients

While all biota contain the same elements, the mode of acquisition differs in a fundamental way between plants and animals. The dominating uptake mechanism in plants is uptake of inorganic ions, whereas metazoan animals typically consume particles that can be considered as parceled nutrients (Figure 1). Bacteria, archaea, fungi, and many other eukaryotic microorganisms are capable of uptake of inorganic compounds, but they can also process small organic molecules that can be used either as an energy source or as a source of nutrients for biosynthesis. Even though only small organic molecules are transported across the cell membranes of these organisms, microorganisms have a potential to utilize large molecules or even particles because these can be digested by extracellular enzymes. Some unicellular eukaryotes have the capacity to ingest particles such as bacteria or small algae by means of phagocytosis, a mode of acquisition that resembles animal feeding in the sense that whole packages of nutrients are ingested. This facultative phagotrophy can be an important mechanism for incorporating iron(Fe) and/or P in environments with low dissolved nutrient concentrations, as well as an important contribution of organic matter.

Nutrient Acquisition in Plants

The uptake of inorganic nutrients from the environment across the plant cell membrane is either by a transport through selective ion channels or by active transport by carrier proteins. While the ion channels allow rapid uptake of, for example, Na+, K+, Ca2+, and CP, and require no energy because the transport is downhill across electrochemical gradients, the carrier proteins may transport compounds against concentration gradients, using energy from electrochemical gradients, ATP, or light to drive the reactions. The activity of ion channels and carrier proteins can change in response to energy availability, ion gradients, and specific activators and inhibitors. Uptake capacity also changes as an effect of changes in gene expression, and as a consequence, changes both the type and number of ion channels and carrier proteins. For example, plant nitrate uptake is performed by several different transport systems with different affinities for nitrate. Some of these systems are facultative whereas others are induced as a result of N stress. This large flexibility in physiological capacity, as well as the large variations in environmental concentrations of the ions, results in large differences in uptake rates both within and among species. Because the uptake of different ion species is not directly coupled, uncoupled nutrient uptake rates for different elements enhance the variation in elemental stoichiometry of plants.

Under nutrient-limited conditions, plants face problems in acquiring enough of the limiting nutrient to sustain growth, and both intra- and interspecific competition may be intense. Consequently, high-affinity nutrient uptake systems have evolved as an adaptive strategy to cope with low nutrient availability. Species with efficient high-affinity uptake systems may eventually suppress the concentration of the limiting nutrient to the extent that its competitors cannot subsist. The uptake of nutrients generally follows Michaelis-Menten kinetics and is dependent on both the nutrient concentration and the physiological capacity of the organism. Under steady-state conditions in chemostat cultures, the growth rate of microalgae is proportional to the uptake rate of the limiting nutrient. In that case, the specific growth rate (biomass increase rate per unit biomass) can be described as a function of the limiting nutrient concentration in the medium according to the Monod model:

p pmax T

S + ks where p is the achieved specific growth rate at the concentration S of the limiting nutrient, pmax is the maximum specific growth rate of the organism, and ks is the halfsaturation constant, that is, the nutrient concentration that sustains a growth rate equivalent to pmax/2. The growth rate approaches the maximum physiological growth capacity of the organism asymptotically at high nutrient concentrations, whereas the growth rate declines at lower nutrient concentrations, finally reaching zero growth rate when the resource is absent (Figure 3). Apart from a qualitative similarity among species in the shape of these growth curves, there are species-specific differences both in the maximum specific growth rate as well as in the affinity for nutrients. For example, the green alga Chlorella has a higher pmax than the diatom Synedra, and thus grows more rapidly at high P concentrations (Figure 3). At low P concentrations, the low ks (i.e., a high affinity and thus an efficient nutrient uptake) of Synedra makes it grow at rates close to its pmax already at low P concentrations, making it a

Figure 3 Specific growth rates of the green alga Chlorella and the diatom Synedra across a supply gradient of phosphorus. The lines show Monod curves fitted to experimental data obtained from chemostat experiments. Chlorella has a ^max of 2.15 per day and a ks of 0.22 mM P, and Synedra has a ^max of 0.65 per day, and a ks of 0.003 mM P. Data from Tilman D, Kilham SS, and Kilham P (1982) Phytoplankton community ecology: The role of limiting nutrients. Annual Review of Ecology and Systematics 13: 349-372.

P concentration (pM)

Figure 3 Specific growth rates of the green alga Chlorella and the diatom Synedra across a supply gradient of phosphorus. The lines show Monod curves fitted to experimental data obtained from chemostat experiments. Chlorella has a ^max of 2.15 per day and a ks of 0.22 mM P, and Synedra has a ^max of 0.65 per day, and a ks of 0.003 mM P. Data from Tilman D, Kilham SS, and Kilham P (1982) Phytoplankton community ecology: The role of limiting nutrients. Annual Review of Ecology and Systematics 13: 349-372.

superior competitor for P compared with Chlorella. Such physiologically based differences among phytoplankton species in maximum growth rates and nutrient uptake capability are an important part of the explanation of species succession patterns in phytoplankton communities that typically occur in temperate lakes.

In addition to the uptake of inorganic ions, some prokaryotes have evolved the physiological capacity to assimilate N2 which is ubiquitous in the atmosphere. Because N fixation is energetically costly compared to ammonium and nitrate uptake, it is not an efficient strategy in environments with high availability of nitrate or ammonium and/or low energy supply. On the other hand, N-fixing cyanobacteria are superior competitors in aquatic environments with low concentrations of inorganic N and high concentrations of other nutrients. N fixation is also important in terrestrial habitats, where N-fixing Rhizobium bacteria live in symbiosis with some species of vascular plants (e.g., legumes). Rhizobium provides the plant with N, and gets energy in the form of organic carbon from the plant.

Nutrient Acquisition in Metazoan Animals

Animals feed on particles which are parcels containing a mix of nutrients. This means that they can fulfill their nutritional requirements by ingesting nutritionally balanced particles. However, consumers that do not feed on a nutritionally balanced diet will encounter difficulties in obtaining nutrients that are deficient in the food. In this context, it is also important to note that in addition to elements, animals also need to ingest some essential biochemical compounds that they cannot synthesize. These biochemicals include some amino acids, polyunsaturated fatty acids, vitamins, and sterols.

Carnivores often feed on prey with a similar nutrient and biochemical composition as their own, suggesting that the diet often is close to balanced. In contrast, herbivores and detritivores frequently encounter a nutritionally imbal-anced diet both in terms of elemental and biochemical composition. They, therefore, need to maximize the uptake of the limiting nutrient. This can be achieved either by selective feeding or by selective digestion and absorption of the limiting substance. Many animals are capable of selecting food particles of high quality. A prerequisite for this ability is that they must have a sensory system that enables them to assess the quality of the food particles as well as an ability to capture suitable particles. Such a system may be energetically costly to produce, maintain, and use. This appears to be a drawback, but this energetic cost can be counteracted by a higher growth efficiency when feeding on a high-quality diet compared to feeding on a low-quality diet. Food particles may also have imbalanced but complementary nutritional values to consumers, and in such a scenario the consumer has to be able to mix the diet in order to balance its nutrient intakes and to obtain the optimum intake ratio, given the available choices.

Once ingested, the food particles are digested, that is, the organic matter is broken down to monomers. This is achieved by enzymatic hydrolysis, which can be both intracellular (in food vacuoles) or extracellular (in a gas-trovascular cavity or a digestive tract). Ions and organic monomers can then be absorbed by the gastrodermic cells by similar uptake mechanisms as involved in the uptake of nutrients in autotrophs. Both in the digestive and absorption processes, dietary imbalances may be compensated for by regulation of the expression and/or the activity of the hydrolytic and uptake enzymes. Although such regulatory mechanisms have been documented, it is, at present, unclear to what extent this mechanism actually is used to balance the intake of nutrients.

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