Background/Question/Methods   Biological stoichiometry attempts to understand the underlying rules that establish the coupling of multiple chemical elements in living systems and to consider their consequences for ecological dynamics.  Living things exhibit a highly constrained coupling of carbon (C), nitrogen (N), and phosphorus (P) in biomass because they share a major set of biochemical mechanisms inherited from their common ancestors.  In contrast, ongoing human actions often lead to a fundamental decoupling of element cycles in the biosphere, likely because industrial processes rely on novel mechanisms divorced from the core biology of life. For example, fossil fuel combustion has mobilized considerable amounts of fixed N in the atmosphere, with major impacts even on ecosystems far from direct human impact. 
Results/Conclusions .   In recently completed studies of lakes in Colorado and Norway, the impact of industrial decoupling is exemplified by qualitative shifts in the nature of phytoplankton nutrient limitation in which N limitation seems to have been replaced by P limitation in response to elevated N deposition.  Thus, ecosystems removed from immediate agricultural and waste impacts may be experiencing enhanced P limitation due to effects of global increases in atmospheric CO₂ and N deposition. On much longer time scales, effects of human alteration of large-scale nutrient cycling may also be perceived in the core molecules that execute the "central dogma of molecular biology" in plants.  In a series of whole-genome analyses, it is seen that plant proteomes exhibit a significant usage bias towards low-N amino acids in comparison to animal proteomes.  This low-N bias seems to have been relaxed in crop species, which exhibit N-use patterns in proteins, RNA, and DNA that are more similar to animals than non-domesticated plants.  Thus, chronic alteration of nutrient supplies due to human activity can even re-model the core structures of biological systems. We are just beginning to understand the rules that underpin the coupling of multiple chemical elements in molecules, cells, and ecosystems. However, the urgency of the task is highlighted by ongoing disruptions in the C, N, and P cycles that are occurring disproportionately and at diverse spatial and temporal scales that impinge on the ecological and evolutionary future of life on Earth.