Biological stoichiometry provides a mechanistic theory linking cellular and biochemical features of co‐evolving biota with constraints imposed by ecosystem energy and nutrient inputs. Thus, understanding variation in biomass carbon : nitrogen : phosphorus (C : N : P) stoichiometry is a major priority for integrative biology. Among various factors affecting organism stoichiometry, differences in C : P and N : P stoichiometry have been hypothesized to reflect organismal P‐content because of altered allocation to P‐rich ribosomal RNA at different growth rates (the growth rate hypothesis, GRH). We tested the GRH using data for microbes, insects, and crustaceans and we show here that growth, RNA content, and biomass P content are tightly coupled across species, during ontogeny, and under physiological P limitation. We also show, however, that this coupling is relaxed when P is not limiting for growth. The close relationship between P and RNA contents indicates that ribosomes themselves represent a biogeochemically significant repository of P in ecosystems and that allocation of P to ribosome generation is a central process in biological production in ecological systems.

Ecological stoichiometry is the study of the balance of multiple chemical elements in ecological interactions. This paper reviews recent findings in this area and seeks to broaden the stoichiometric concept for use in evolutionary studies, in integrating ecological dynamics with cellular and genetic mechanisms, and in developing a unified means for studying diverse organisms in diverse habitats. This broader approach would then be considered``biological stoichiometry''. Evidence supporting a hypothesised connection between the C:N:P stoichiometry of an organism and its growth rate (the``growth rate hypothesis'') is reviewed. Various data indicate that rapidly growing organisms commonly have low biomass C:P and N:P ratios. Evidence is then discussed suggesting that low C:P and N:P ratios in rapidly growing organisms reflect increased allocation to P-rich ribosomal RNA (rRNA), as rapid protein synthesis by ribosomes is required to support fast growth. Indeed, diverse organisms (bacteria, copepods, fishes, others) exhibit increased RNA levels when growing actively. This implies that evolutionary processes that generate, directly or indirectly, variation in a major life history trait (specific growth rate) have consequences for ecological dynamics due to their effects on organismal elemental composition. Genetic mechanisms by which organisms generate high RNA, high growth rate phenotypes are discussed next, focusing on the structure and organisation of the ribosomal RNA genes (the``rDNA''). In particular, published studies of a variety of taxa suggest an association between growth rate and variation in the length and content of the intergenic spacer (IGS) region of the rDNA tandem repeat unit. In particular, under conditions favouring increased growth or yield, the number of repeat units (``enhancers'') increases (and the IGS increases in length), and transcription rates of rRNA increase. In addition, there is evidence in the literature that increased numbers of copies of rDNA genes are associated with increased growth and production. Thus, a combination of genetic mechanisms may be responsible for establishing the growth potential, and thus the RNA allocation and C:N:P composition, of an organism. Furthermore, various processes, during both sexual and asexual reproduction, can generate variation in the rDNA to provide the raw material for selection and to generate ecologically significant variation in C:N:P stoichiometry. This leads us to hypothesize that the continuous generation of such variation may also play a role in how species interactions develop in ecosystems under different conditions of energy input and nutrient supply.

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Summary 1.We examined the relative importance of resource composition (carbon : phosphorus molar ratios which varied between 9 and 933) and growth rate (0·5-1·5 h − 1 ) to biomass carbon : nitrogen : phosphorus stoichiometry and nucleic acid content in Escherichia coli grown in chemostats, and in other heterotrophic prokaryotes using published literature. 2. Escherichia coli RNA content and the contribution of RNA-P to total cellular P increased with increasing growth rate at all supply C : P ratios. Growth rate had a much stronger effect on biomass C : P than did supply C : P, and increased RNA content resulted in low biomass C : P and N : P ratios. 3. However, we observed only twofold variations in biomass C : P and N : P ratios in the experiments, despite a difference of two orders of magnitude in C : P and N : P supply. The response of biomass C : P and N : P ratios to alteration of the supply C : P and N : P ratios revealed that E. coli was strongly homeostatic in its elemental composition. 4. This result, and a literature survey, suggest that each heterotrophic bacterial strain regulates its elemental composition homeostatically within a relatively narrow range of characteristic biomass C : P and N : P ratios. 5. Thus shifts in the dominance of different bacterial strains in the environment are probably responsible for the large variation in bacterial biomass C : P, as has been suggested for crustacean zooplankton. These findings indicate that bacteria are more like animals than plants in terms of biomass C : P and N : P homeostasis.

Summary 1.Temperature strongly affects virtually all biological rate processes, including many central to organismal fitness such as growth rate. A second factor related to growth rate is organismal chemical composition, especially C : N : P stoichiometry. This association arises because high rates of growth require disproportionate investment in N-and P-rich biosynthetic cellular structures. Here the extent to which these factors interact is examined -does acclimation temperature systematically affect organismal chemical composition? 2. A literature survey indicates that cold-acclimated poikilotherms contain on average 30-50% more nitrogen [N], phosphorus [P], protein and RNA than warm-exposed conspecifics. The primary exception was bacteria, which showed increases in RNA content but no change in protein content at cold temperatures. 3. Two processes -changes in nutrient content (or concentration) and in organism size -contribute to the overall result. Although qualitatively distinct, both kinds of change lead to increased total catalytic capacity in cold-exposed organisms. 4. Temperature-driven shifts in nutrient content of organisms are likely to resonate in diverse ecological patterns and processes, including latitudinal and altitudinal patterns of nutrient content, foraging decisions by organisms living in strong temperature gradients, and patterns of biodiversity.