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Figure 4 Organismal RNA content and specific growth rate in a phylogenetically diverse set of taxa. Adapted from Vrede T, Dobberfuhl DR, Kooijman SALM, and Elser JJ (2004) Fundamental connections among organism C:N:P stoichiometry, macromolecular composition, and growth. Ecology 85:1217-1229.

There is also a positive relationship between organism RNA and P content (Figure 5). It should be noted that the slope of the relationship between organism total P content and RNA-P content is close to unity both within and among taxa. This means that differences in P content among organisms can be interpreted as an effect of differences in allocation to RNA. The distance (parallel to the j-axis) from a data point to the 1:1 line represents the organism content of non-RNA-P (e.g., DNA, ATP, and phospholi-pids), and it is evident from the graph that RNA constitutes a major fraction of P in many organisms, in particular, in those with high growth rate. Along with this increase in cellular P content with increasing growth rate, the N content also increases because RNA is also rich in N. However, this increase is smaller relative to overall N content because a significant fraction of the cellular N is allocated to protein. An effect of this less-pronounced increase in N is that the N:P ratio decreases with increasing growth rate.

Despite the wide diversity in phylogenetic origin of the organisms studied, there are apparently fundamental similarities in basic biosynthetic machinery and processes, which are associated with different stoichiometric patterns as well as organism life history.

Stoichiometry of Autotroph Growth

The patterns described above are valid for organisms that regulate their stoichiometry homeostatically, but is the same also true for nonhomeostatic organisms, that is, plants? Although less is known about RNA content in

♦ Drosophila melanogaster A Daphnia pulicaria o Lake bacteria O Sibinia setosa

Figure 5 Relationship between organismal RNA-P content and total P content. The broken line indicates a 1:1 relationship between RNA-P and total organismal P, that is, that all cellular P is allocated to RNA. The dashed lines are linear regression curves fitted to each species, and the solid line is a linear regression curve fitted to all data. Because the P content of RNA is —10% by weight, the RNA content of the organism can be calculated as 10 x RNA-P. Adapted from Elser JJ, Acharya K, Kyle M, et al. (2003) Growth rate-stoichiometry couplings in diverse biota. Ecology Letters 6: 936-943.

Figure 5 Relationship between organismal RNA-P content and total P content. The broken line indicates a 1:1 relationship between RNA-P and total organismal P, that is, that all cellular P is allocated to RNA. The dashed lines are linear regression curves fitted to each species, and the solid line is a linear regression curve fitted to all data. Because the P content of RNA is —10% by weight, the RNA content of the organism can be calculated as 10 x RNA-P. Adapted from Elser JJ, Acharya K, Kyle M, et al. (2003) Growth rate-stoichiometry couplings in diverse biota. Ecology Letters 6: 936-943.

Similar relationships between plant growth and elemental stoichiometry have also been modeled for microalgae. In the Droop model, the relationship between the cellular content of the limiting nutrient and the growth rate is expressed as f I 1 <min \ p - ^ - —j where the specific growth rate, p, is a function of the cell quota, ^(the intracellular amount of the limiting nutrient, expressed as nutrient per cell or per unit biomass), the minimum cell quota, Qmin (below which growth and cell division is not possible), and the theoretical asymptotic maximum growth rate, p' m, that is reached at infinite Q From a physiologial point of view, the minimum cell quota can be interpreted as the minimum amount of P that is needed for the genome (DNA), phospholipids in cell membranes, and other P compounds (including rRNA and ATP) required for retaining a basal metabolism that permits survival. Because Qcannot be infinite (the organism cannot consist of 100% of any element, and even then Qis not greater than 1 if it is expressed as a fraction of total biomass), p'm cannot be reached. However, for elements that can be stored in large quantities in relation to the cellular demands (e.g., micronutrients), pmax approaches p 'm. Another way of expressing the Droop model is to

A Daphnia galeata • Escherichia coli □ Zooplankton species

o ra

—^ P (% of dry mass)

----N (% of dry mass)

-N/P (atomic)

0.1 1 Specific growth rate (d-1)

Figure 6 Correlations between plant N and P content and N:P ratio with specific growth rate. Based on data from Nielsen SL, Enriquez S, Duarte CM, and Sand-Jensen K (1996) Scaling maximum growth rates across photosynthetic organisms. Functional Ecology 10: 167-175.

relation to growth of vascular plants, observed N:P ratios, and growth rates of autotrophs covering the whole range from microalgae to trees are consistent with the GRH: the growth rate is positively correlated with plant P and N content, and the N:P ratio decreases with increasing growth rate (Figure 6).

calculate the relative growth rate (RGR) as a function of the inverse of Q^RGR is the ratio between p and p'm, 1/Q is equal to M:X, the biomass:nutrient ratio of nutrient X in the cell (or the C:nutrient ratio), and the inverse of is equal to M:Xmax:

In this form, the model predicts a linear decrease in RGR with increasing M:X, and the slope is equal to — 1/ M:Xmax. The Droop model has been developed and tested in chemostat systems with steady state, and there are many empirical examples where it produces a good fit to data. It has also been shown that the Droop model successfully predicts growth rates of natural phytoplankton that presumably are not at steady state. However, there are examples when algal growth deviates from the predictions of the Droop model, and the RGR is nonlinearly related to M:X. Such deviations point to the fact that autotroph stoichiometry is related not only to growth, but that other processes such as nutrient storage also may have a large influence on their stoichiometry. Although not mechanistically connecting growth with cellular nutrient content, the predictions of the Droop model when growth is P-limited are qualitatively similar to the relationship between cellular P content and growth rate of the GRH.

Growth rate is also affected by temperature because enzymatic process rates generally are temperature-dependent. Growth rate therefore co-varies with temperature even if the nutrient supply rate and the macromolecular allocation pattern are constant. The availability of other resources such as light and inorganic carbon also affects the elemental stoichiometry of autotrophs. At a given supply rate of the limiting nutrient, the C:nutrient ratio increases with increasing light intensity (but only at light intensities below those where photoinhibition occurs). There are physiological adaptations to high light intensities, such as downregulation of chlorophyll synthesis and consequently lower chlorophyll:biomass ratios. However, this decrease in photosynthetically active systems only partially compensates for the increased C fixation, and therefore the C yield per nutrient atom increases with increasing light intensity. At low light intensities, the C:nutrient ratio decreases. This can be understood as an effect of lower photosynthetic rates because of lower C assimilation rates, but it can also be an effect of increased allocation to photosynthetic machinery (i.e., chloroplasts, which are rich in N-rich chlorophyll and proteins like rubisco and cytochromes). Because chloroplasts contain a high fraction of N but little P, this allocation pattern will result in low C:N and high N:P ratios, and is consistent with observed C:N and N:P ratios of phytoplankton cells. The high N:P ratios of N-fixing cyanobacteria, which have been explained as an effect of their ability to take up N in an otherwise N-deficient environment, may also be explained as an effect of allocation patterns. Because N fixation is very energy demanding, N-fixing cyanobacteria have to allocate considerable resources to photosynthetic machinery, and therefore their N:P ratio can be expected to be high.

The C:nutrient ratio of terrestrial plants increases also with increasing partial pressure of carbon dioxide (pCO2). Similar patterns may prevail in phytoplankton too, especially under nutrient-limited conditions, but consistent data are at present lacking. From a physiological point of view this increase in plant C content at elevated pCO2 can be understood as an effect of increased availability of substrate for rubisco, and hence increased C assimilation rates.

Nutrient Storage

Animals generally have a limited ability to store chemical elements for later use. The main exception to this is the storage of lipids (with high C:nutrient ratios) in lipid droplets or adipose tissue for subsequent use as energy reserves. In contrast to the limited nutrient storage capacity of animals, this ability is very well developed in autotrophs, thus increasing the range of variation in elemental stoichiometry of plants. Nutrient storage does not normally take place under rapid growth, when biosyn-thetic requirements balance nutrient uptake, but when growth is resource-limited there is a luxury uptake of nutrients that are available in excess. Even though nutrient uptake systems of nonlimiting nutrients are generally downregulated, nutrient uptake continues in facultative uptake systems. These excess nutrients, which cannot be used for immediate growth and biosynthesis, can be stored in many different forms in plant cells: as ions or organic compounds in the cytoplasm, as large intracellu-lar deposits ofmacromolecules, or in the vacuole. C that is produced in excess can be stored as nonpolar lipids, starch, glycogen, or other low-N and low-P organic molecules. A large store of nutrient-poor organic matter can be of great importance for the plants because this makes C:nutrient ratios so high that herbivore growth cannot be sustained on such low-quality food. P can be stored as increased concentrations of vacuolar phosphate and small P-rich organic molecules or as polyphosphate granules. N is stored as, for example, nitrate or amino acids. The presence of vacuole in autotrophs is of special importance in the context of nutrient storage. This cell compartment can store large quantities of nutrients that can be transported back to the cytoplasm when the nutrients are needed. This provides autotrophs with an alternate strategy to the high-affinity uptake system (see the Monod model discussed above) to compete for nutrients. When nutrient availability is high, luxury uptake provides an internal nutrient store that can be used for growth when the resource is limiting. This strategy can be observed in some phytoplankton that are capable of vertical migration between nutrient-rich but dark deep water layers and well-illuminated but nutrient-depleted surface waters. In higher plants, the metabolism and storage of nutrients and organic compounds is even more complex and multifaceted, since it both involves reallocation of storage products among different tissues, and differences in nutritional requirements among tissues. Thus, nutrient storage products and strategies specific for plants have evolved. For example, storage proteins and phytate deposited in seeds provide germinating seeds with N and P.

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