The growth rate hypothesis (GRH) links elemental content to biochemistry and in turn links both of those to life history. It concerns the specific growth rate of body mass (M, g g^1 d^1). The GRH comes about because of the almost uniquely low N:P ratio of nucleic acids compared to all other major biochemicals. According to the central dogma of molecular biology, information flows from DNA to proteins by way of several forms of RNA. Protein is a large fraction of cell mass and to achieve high specific growth rates of cell mass, a great deal of ribosomal RNA is needed to manufacture large amounts of protein at high rates. In rapidly growing Escherichia coli, RNA makes up approximately 35 % of total cell mass, and as much as 90% cellular P is in RNA. Such a large proportion of cell mass in a single biochemical of unusually low N:P generates a distinct stoichiometric signature for the whole organism. Due to this stoichio-metric imprint of RNA, the GRH posits positive relationships among three variables: specific biomass growth rate, P-content, and RNA content. Support for the GRH comes from multiple groups of organisms ranging from unicells to small invertebrates such as Drosophila and Daphnia (Figure 2).
Applicability of the GRH to larger organisms is still unclear. When organisms are examined over a range of many orders of magnitude in body size, RNA content is observed to decline greatly with body size, as does growth rate. Whole-organism P content, however, does not decline as strongly with body size in such a range. Hence, in considering a range of organisms from microbes to large multicellular animals, there must be a shift in P-containing pools along body-size gradients from RNA to other substances. As we will see in more detail below, in vertebrates bone replaces RNA as a major P-reservoir.
There is evidence too that the GRH applies to auto-trophs as well as heterotrophs. In a study with the unicellular alga Selenastrum, high growth rate was associated with a shift in the boundaries between N- and P-limitation; the N:P of the boundary was lower at high growth rate. This is consistent with a need for larger quantities of low N:P RNA at high growth rate. Similarly, a model of unicellular algal growth was analyzed where cellular investment in two broad classes of biochemical machinery was examined: (1) assembly
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Figure 2 Positive relationships among specific biomass growth rate (p, d 1) and both P-content and RNA content. Each dotted line represents a different species. Though species differ from one another, these variables are often linked in a way consistent with the GRH.
machinery, for example, ribosomes with low N:P ratios, and (2) resource-acquisition machinery, which is made up mainly of protein containing N but little or no P. Shifts in optimal N:P ratios in the model were associated with growth rate. Low N:P was favored at high growth, whereas high N:P was favored under low growth and strong resource scarcity. These studies of microscopic autotrophs are consistent with the GRH. In one study looking at foliar N and P in more than 100 vascular plants, N:P was lower in species that have higher growth rate. Figure 1b showed one such contrast.
The GRH thus proposes a material basis to a key life-history parameter: growth rate. All else being equal, specific biomass growth rate will be closely coupled to such life-history parameters as age and size at first reproduction, and therefore is directly tied to fitness itself. Because it is especially P among all the elements that is disproportionately required for high growth, we can further hypothesize that high-growth phenotypes might be particularly sensitive to the presence or absence of phosphorus from the environment. A fascinating biomedical example of the GRH in action is in cancerous tumors, which in many cases are high in RNA. P-content of tumors has not often been studied, but evidence for high P-content in tumors has been seen.
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