From cell size and first principles to structure and function of unicellular plankton communities

Here we review, synthesize, and analyse the size-based approach to model unicellular plankton cells and communities. We connect cell-level processes to cell size and to first principles of diffusion, geometry, and fluid mechanics. We scale cell-level processes up to the plankton community size distribution to show that first principles determine overall plankton community structure and function. We first review how cell size influences processes of the individual the cell: uptake of dissolved nutrients, phototrophy, phagotrophy, and metabolism. We parameterise processes primarily from first principles using a synthesis of existing data only when needed, and show how these processes determine minimum and maximum cell size and limiting resource concentrations. The cell level processes scale directly up to the structure and function of the entire unicellular plankton ecosystem. The structure is described by the Sheldon size spectrum and by the emergent trophic strategies. We develop an analytical approximate solution of the biomass size spectrum and show how the trophic strategies depend on the resource environment. The functions of the plankton community are the production, respiration and losses, and carbon available to production of higher trophic levels. We finally discuss strengths and limitations of size-based representations and models of plankton communities. Highlights Review of how cell size affects resource uptake, losses, and metabolism. Derivation of most cell-level parameters from first principles. Closed-form solutions of limiting resources and size spectrum Scaling from cell-level processes to size structure and ecosystem functions.