Biological scaling in green algae: the role of cell size and geometry

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.

Solving the grand challenge of phenotypic integration: allometry across scales

“…This is explained by simple geometric considerations: the surface area, which allows exchanges of resources and energy between the internal medium and the environment, increases according to a square function; while the volume, a priori linked to the number of cells and thus to energy needs, increases according to a cubic function. As a result, the surface-to-volume ratio decreases with volume and mass, whatever the shape of the object (but see Bestova et al 2021 ). In living things, these geometric constraints have profound consequences for resource use strategies.…”

Shared and contrasting associations in the dynamic nano‐ and picoplankton communities of two close but contrasting sites from the Bay of Biscay

“…While this has been recognized for some time, recent studies are transforming our understanding of these organisms in marine ecosystems, revealing the underestimated complexity in their dynamics, interactions and trophic strategies (Lima‐Mendez et al, 2015 ; Needham & Fuhrman, 2016 ; Worden et al, 2004 ), including the widespread occurrence of mixotrophy and photoheterotrophy in this smaller size fraction (Arandia‐Gorostidi et al, 2020 ; Flynn et al, 2019 ). Smaller plankton are favoured when nutrients are limited due to their higher surface area‐to‐volume ratio, and smaller thickness of the diffusion boundary, both enhancing the efficiency of nutrient uptake (Potter et al, 1997 ; Levin & Angert, 2015 ; Sym, 2015 , but see also Bestová et al, 2021 ). In contrast, they are typically poorer competitors in nutrient‐rich environments due to lower maximum rates of nutrient uptake (Marañón et al, 2012 ).…”