Abstract
The Metabolic Scaling Theory (MST), hypothesizes limitations of resource-transport networks in organisms and predicts their optimization into fractal-like structures. As a result, the relationship between population growth rate and body size should follow a cross-species universal quarter-power scaling. However, the universality of metabolic scaling has been challenged, particularly across transitions from bacteria to protists to multicellulars. The population growth rate of unicellulars should be constrained by external diffusion, ruling nutrient uptake, and internal diffusion, operating nutrient distribution. Both constraints intensify with increasing size possibly leading to shifting in the scaling exponent. We focused on unicellular algae Micrasterias. Large size and fractal-like morphology make this species a transitional group between unicellular and multicellular organisms in the evolution of allometry. We tested MST predictions using measurements of growth rate, size, and morphology-related traits. We showed that growth scaling of Micrasterias follows MST predictions, reflecting constraints by internal diffusion transport. Cell fractality and density decrease led to a proportional increase in surface area with body mass relaxing external constraints. Complex allometric optimization enables to maintain quarter-power scaling of population growth rate even with a large unicellular plan. Overall, our findings support fractality as a key factor in the evolution of biological scaling.
Highlights
The Metabolic Scaling Theory (MST), hypothesizes limitations of resource-transport networks in organisms and predicts their optimization into fractal-like structures
As metabolism sustains biomass production for growth and reproduction, organismal allometry relationships can be extended to population growth r ate[1,5], The MST hypothesis underlying the scaling from individual energetics to population growth rate are that: (i) population growth is fueled by the acquisition and allocation of energy, and (ii) the acquisition and allocation of energy are constrained by body size
We found that the scaling of population growth rate with body mass followed Kleiber’s law
Summary
The Metabolic Scaling Theory (MST), hypothesizes limitations of resource-transport networks in organisms and predicts their optimization into fractal-like structures. The population growth rate of unicellulars should be constrained by external diffusion, ruling nutrient uptake, and internal diffusion, operating nutrient distribution Both constraints intensify with increasing size possibly leading to shifting in the scaling exponent. DeLong et al.[5] highlighted changes in the allometric scaling of metabolic rate and μmax along main evolutionary transitions, linked with “innovations in metabolic design”, such as cell compartmentalization and multicellularity Their meta-analysis validated Kleiber’s law for metazoans only, whereas bacteria and protists deviated from the − 1/4 exponent. At the upper limit of protist size, external and internal constraints should lead to reduced metabolic efficiency (a shift in metabolic scaling) and competitive superiority of multicellular o rganisms[5] Both constraints could be minimized by body plan optimizations, especially by changing body morphology and diluting cell content[14,17]. Those changes in body plan certainly increase surface to active cell volume ratio[9,17], but it is little known to what extent such allometric adjustments influence the scaling of organismal processes
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