Metabolic level and size scaling of rates of respiration and growth in unicellular organisms

1.  Metabolic rate is conventionally assumed to scale with body mass to the 3/4-power, independently of the metabolic level of the organisms being considered. However, recent analyses in a variety of animals and plants indicate that the power (log–log slope) of this relationship varies significantly with metabolic level, ranging from c . 2/3 to 1.
2.  Here I show that the scaling slopes of rates of respiration and growth are related to the metabolic level of a variety of unicellular organisms, as similarly occurs for respiration rates in multicellular organisms.
3.  The recently proposed 'metabolic-level boundaries hypothesis' provides insight into these effects of metabolic level. As predicted, the scaling slopes for resting (endogenous) respiration rate in prokaryotes, algae and protozoans are negatively related to metabolic level; and in protozoans, the scaling slope increases with starvation. Also as predicted, the scaling slopes of growth rate in algae and protozoans are negatively related to growth level. Unexpectedly, opposite effects of starvation on the metabolic scaling slopes of unicellular prokaryotes (compared to that of eukaryotes) may be a spurious result of respiration measurements that did not adequately consider the effects of rapid cell multiplication in prokaryotes with extremely short generation times.
4.  Analyses of both unicellular and multicellular organisms show that there is no universal metabolic scaling relationship, and that variation in metabolic scaling relationships is systematically and possibly universally related to metabolic level.