Kleiber's Law, that metabolic rate of organisms scales with the ¾-power of body mass, is said to apply to organisms from the smallest bacteria to the largest mammals. Thus, organisms increase in metabolic rate more slowly than they increase in size. Unicellular organisms have historically been thought to scale with the same slope, b, as multicellular organisms, but with a lower y-intercept. More recent studies suggest the scaling of unicells is closer to isometric. However, the metabolic scaling of unicells needs to be reassessed, as early studies did not include many bacteria, combined data from a wide range of physiological states, and used multiple points per species. In addition, no study has evaluated the scaling with statistical techniques that acknowledge the large error in cell mass estimates. We compiled a data set combining both prokaryote and eukaryote heterotrophic unicells, and reevaluated the scaling for unicells as a group and for prokaryotes and eukaryotes separately. We restricted our analysis to active metabolic rates (not measured under starvation conditions), used a single average value for each species, and used reduced major axis regression to account for the error in body mass estimates.  

Results/Conclusions

The body mass and metabolic rates of unicells span ten orders of magnitude, approximately half the size range in microbes-to-monsters plots. As a whole, unicells showed isometric scaling (b = 0.99, R² = 0.94). However, there was a clear break in the scaling for prokaryotes and eukaryotes. Prokaryotes showed super-linear scaling (b = 1.79) and eukaryotes showed slightly sublinear scaling (b = 0.92). Metabolic scaling theories do not predict these patterns. It is likely that the transition in scaling reflects a shift in energetic constraints that accompany the evolution of eukaryoty and the attendant increase in cell size. We suggest that the superlinear scaling in prokaryotes is caused by the increasing number of genes, and corresponding metabolic processes, as size increases in prokaryotes. Although the total length of the genome may increase with size in eukaryotes, it is not clear that the total number of genes increases as it does in prokaryotes, and thus there would not be a gene number effect in eukaryotes. In contrast, the augmentation of surface area achieved by the increasing complexity of membrane structures in eukaryotes may allow these species to have greater resource uptake than simple surface area constraints would imply.