Elemental Composition at Higher Levels of Organization

Stoichiometric variation also occurs at higher levels of internal organization because organisms have different allocations to cellular structures and tissues having distinct biomolecular mixtures. A description of this variation is valuable because it helps to link elemental composition more directly to phenotypes.

A focal point in ecological stoichiometry has been the compositional difference between ribosomes and other cellular components. Ribosomes are the centers of protein synthesis; they catalyze peptide bond formation between amino acids in sequences determined by genetic information received from messenger RNA. The rate of protein synthesis often depends more on the number of ribosomes in cells than on the efficiency of individual ribosome molecules. As a result, ribosome concentration may largely determine protein synthesis rate, a trait with important evolutionary consequences (see below).

Ribosomes have the highest P concentration of any organelle. Ribosomes consist almost entirely of RNA

and protein, with RNA making up a sizable fraction of ribosome mass (the RNA:protein ratio in eukaryotic ribo-somes is about 1.2; in prokaryotes, it is 1.8). Because RNA is P rich (—10%), the biomolecular makeup of ribosomes results in a particularly P-rich structure (eukaryotes: 41.8% C, 16.3% N, 5% P; prokaryotes: 40% C, 16.1% N, 5.6% P).

Other cellular components likely also influence whole-organism stoichiometry. For example, the mammalian nucleus contains about 12.8% N and 2.3% P, which reflects an abundance of high-N nuclear proteins and the high P content of DNA. Mitochondria and chloroplast are also high-N organelles, as each contain about 11 % N. They also contain very little P (0.31% and 0.32% P, respectively). Animal cell membranes have a fairly high amount of phospholipids relative to protein, and as a result have moderately high P content (59.5% C, 9.5% N, 1.5% P). Finally, plant cell walls consist mostly of N- and P-free cellulose and lignin, although they do contain small amounts of proteins and lipids. The estimated composition of plant cell walls is 35-38% C, <0.5% N, and -0% P. These calculations suggest that adaptive allocation to particular cellular components will create a link between elemental composition and biological function.

Clear compositional differences also exist among different tissues. For instance, the N content in the leaves of apple trees (—1.2% dry mass) is considerably higher than N levels in stems and roots, which contain higher concentrations of cellulose and lignin (Table 3 a). N content is particularly low in a tree's older woody tissue. In the crayfish, Astacus astacus, there are substantial differences in C:P and N:P ratios among the hepatopancreas (a digestive organ) and other major tissues (Figure 2). Finally, N and P levels differ among human tissues, and similar differences are likely in most other mammals (Table 3b). P content is particularly high in bone and other bony tissues such as teeth. Bone makes up roughly 10% of the biomass in mammals, and the skeleton may contain as much as 85% of a mammal's P content.

Table 3a N concentrations in different components of apple treesa

Table 3b N and P concentrations (% dry mass) in human tissues

Table 3a N concentrations in different components of apple treesa

Tissue

Age (year)

% N

Leaves

1.23

Spurs

1.04

Wood

1

0.93

11-18

0.16

Roots

1-6

1.24

14-18

0.32

aData from Murneek AE (1942) Quantitative distribution of nitrogen and carbohydrates in apple trees. Missouri Agricultural Experimental Station Research Bulletin 348, as reported in Sterner RW and Elser JJ (2002) Ecological Stoichiometry. Princeton, NJ: Princeton University Press.

Tissue

%N

%P

Kidney

7.2a

0.70a

Liver

7.2a

0.94a

Muscle

7.2a

0.3-0.85a

Bone

4.3a

~12.00a

Skin

16.0a

~0.10a

Hair

15.7b

0.01b

Nails

14.0c

0.01c

aBowen HJM (1979) Environmental Chemistry of the Elements. London: Academic Press.

Johnston FA, Debrock L, and Diao EK (1958) The loss of calcium, phosphorus, iron, and nitrogen in hair from the scalp of women. American Journal of Clinical Nutrition 6: 136-141. cIyengar GV (1978) The Elemental Composition of Human Tissues and Body Fluids. New York: Verlag Chemie.

aBowen HJM (1979) Environmental Chemistry of the Elements. London: Academic Press.

Johnston FA, Debrock L, and Diao EK (1958) The loss of calcium, phosphorus, iron, and nitrogen in hair from the scalp of women. American Journal of Clinical Nutrition 6: 136-141. cIyengar GV (1978) The Elemental Composition of Human Tissues and Body Fluids. New York: Verlag Chemie.

500

450

400

350

300

CL

250

o

200

150

100

50

0

90

80

70

60

CL

50

Z

40

30

20

10

B"

Hepato- Cara- Gill Abdom Claw Claw pancreas pace muscle muscle shell

Major organs

Figure 2 C:P and N:P ratios for major tissue of adult Astacus astacus crayfish. Extensions of boxes represent the 25th and 75th quartiles. Different characters on top of each box indicate statistically significant difference. From Faerovig PJ and Hessen DO (2003) Allocation strategies in crustacean stoichiometry: The potential role of phosphorus in the limitation of reproduction. Freshwater Biology 48: 1782-1792.

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