From first principles to the functions of plankton ecosystems

Ken H. Andersen, Professor of theoretical marine ecology at Technical University of Denmark, DTU Aqua, Denmark 

Plankton ecosystems are extremely complex and diverse ecosystems. Unicellular plankton span all three domains of life: primaryproducers, herbi-/carnivores, and decomposers (bacteria). However, individual cells often fill more than one of these functional roles, e.g., by combining phototrophy with osmo-heterotrophy or phagotrophy. 

Together, unicellular plankton perform half of the photosynthesis on the planet and are important in the regulation of climate through the biological carbon pump. How can that diversity be described in simple models? Here I show that the major functional diversity can be captured by focusing on cell size, which ranges 8 orders of magnitude in mass. The primary assumption is that evolution optimizes the dominant organisms until they are limited by constraints from first principles: diffusion, cell geometry, fluid mechanics, reaction kinetics, and metabolic capacity. The cell level model scales directly up to the function of the entire unicellular plankton ecosystem: primary production, respiration and losses, and carbon available to production of higher trophic levels. Size-based simulation models rely on a small set of generic parameters that are universal across regions and changing climate.