Biological stoichiometry from genes to ecosystems

Ecological stoichiometry is the study of the balance of multiple chemical e lements in ecological interactions. This paper reviews recent findings in t his area and seeks to broaden the stoichiometric concept for use in evoluti onary studies, in integrating ecological dynamics with cellular and genetic mechanisms, and in developing a unified means for studying diverse organis ms in diverse habitats. This broader approach would then be considered "bio logical stoichiometry". Evidence supporting a hypothesised connection betwe en the C:N:P stoichiometry of an organism and its growth rate (the "growth rate hypothesis") is reviewed. Various data indicate that rapidly growing o rganisms commonly have low biomass C:P and N:P ratios. Evidence is then dis cussed suggesting that low C:P and N:P ratios in rapidly growing organisms reflect increased allocation to P-rich ribosomal RNA (rRNA), as rapid prote in synthesis by ribosomes is required to support fast growth. Indeed, diver se organisms (bacteria, copepods, fishes, others) exhibit increased RNA lev els when growing actively. This implies that evolutionary processes that ge nerate, directly or indirectly, variation in a major life history trait (sp ecific growth rate) have consequences for ecological dynamics due to their effects on organismal elemental composition. Genetic mechanisms by which or ganisms generate high RNA, high growth rate phenotypes are discussed next, focusing on the structure acid 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 parti cular, under conditions favouring increased growth or yield, the number of repeat units ("enhancers") increases land the IGS increases in length), and transcription rates of rRNA increase. In addition, there is evidence in th e literature that increased numbers of copies of rDNA genes are associated with increased growth and production. Thus, a combination of genetic mechan isms 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 varia tion 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 c onditions of energy input and nutrient supply.