Incorporation and Turnover of Elements

After the nutrients have entered into the cells, they are processed or used in different anabolic and catabolic pathways depending on the physiological status and biosynthesis and energy requirements of the cell. These processes include biological reactions such as photosynthesis, respiration, biosynthesis, and accumulation of storage compounds. With the exception of photosynthesis, which only takes place in photoautotrophs, there is a backbone of major biochemical pathways common to all biota. The regulation and coordination of these biochemical processes is under genetic control by numerous feedback mechanisms that includes inhibition or activation of enzymes as well as changes in gene expression. This basic similarity in biochemistry among organisms puts constraints on variation in elemental stoichiometry, but changes in the rates of these processes ultimately affect allocation patterns of major biochemical compounds and thus, overall elemental composition. The focus of the following sections will be on growth of heterotrophs and autotrophs and on nutrient storage in autotrophs. The reasons for this are that these processes have a profound impact on organismal stoichiometry, and these topics have also been the subject for some detailed study. However, conceptually similar connections between elemental stoichiometry and other physiological processes involved in nutrient turnover and biosynthesis can be hypothesized to exist.

Stoichiometry of Heterotroph Growth

Growth (i.e., the biosynthesis of new organic matter that is allocated either to biomass increase or to reproductive output) is a fundamental process in all biota, and it is important for the retention and turnover of nutrients in ecosystems as well as for population growth. Protein synthesis makes up a significant proportion of this biosynthesis simply because proteins are so abundant, both as structural components and enzymes, constituting as much as 30-75% of the dry mass of organisms. Consequently, the synthesis of proteins is a core biosynthetic pathway in all organisms. Protein synthesis takes place in ribosomes, which consists of ribosomal RNA (rRNA) and proteins, approximately 50% by weight of each. The protein synthesis rate of an organism is proportional to the number of ribosomes. Growth rate should, therefore, be closely related to protein synthesis rate as well as to its number of ribosomes. In each cell, there are thousands of ribosomes, which makes rRNA the most abundant nucleic acid. Because RNA is very rich in P (—10% by weight), a large number of ribosomes should also be reflected in a high P content. These mechanistic links between organism P content, RNA content, and growth rate have been proposed as the growth rate hypothesis (GRH).

Empirical data from metazoan animals, bacteria, and eukaryotic microorganisms support the GRH, although there are some exceptions. A positive relationship between growth rate and RNA content is observed among species representing as phylogenetically diverse organisms as bacteria, eukaryotic microorganisms, crustaceans, insects, and mollusks (Figure 4). Similar positive relationships between RNA content and growth rate have also been observed within species, for example, in the bacterium Escherichia coli, the insect Blatella, and the crustacean Daphnia. However, the relationship between RNA and growth rate may break down under some circumstances. This can be the case, for example, if the N supply is limited and therefore the supply of amino acids constrains protein synthesis rate.

TO o

O Bacteria

□ Unicellular eukaryotes X Insects A Mollusks ♦ Crustaceans

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