On relating physical limits to the carbon: nitrogen ratio of unicellular algae and benthic plants

Unicellular algae such as phytoplankton and benthic microalgae have an elemental ratio of carbon to nitrogen to phosphorus (C/N/P) of approximately 106:16:1, known as the Redfield ratio. Benthic plants, including benthic macroalgae and seagrass, have a significantly different and more variable C/N/P ratio, with a median of 550:30:1, herein called the Atkinson ratio. In this paper, the implications of physically limited light absorption and nutrient uptake, combined with the biological constraint of a relatively fixed elemental stoichiometry, are investigated. Calculations reveal that: (1) for randomly oriented convex-shaped unicellular algae, if nutrient uptake is a linear function of surface area and light absorption is a linear function of projected area, then the ratio of light absorption to nutrient uptake is constant; (2) for benthic plants, the ratio of the rates of light absorption to nutrient uptake varies depending on plant morphology and hydrodynamic conditions; and (3) underlying other environmental influences, there is a constant factor of four difference in the ratio of the rates of light absorption to nutrient uptake of benthic plants, relative to unicellular algae, that can be attributed to the two- and three-dimensional environments they respectively experience. The contrasting geometric properties of unicellular algae and benthic plants, combined with the biological constraint of a relatively fixed stoichiometry, appear to exert a significant evolutionary pressure on the magnitude and variability of their C/N ratios.