Competition between vacuolated and mixotrophic unicellular plankton

Abstract

Trait-based ecology allows much of the complexity of ecosystems to be projected onto a low-dimensional trait space. We conjecture that three key traits capture the main aspects of diversity of unicellular planktonic organisms: cell size, trophic strategies (relative investment in photosynthesis and phagotrophy) and vacuolation. The three selected traits are representative of two groups: mixotrophic protists, which in addition to phototrophy, use phagotrophic grazing as a food source, and diatoms that use fluid-filled vacuoles to increase their physical size relative to carbon biomass. We construct a trait-based model related to the three traits and determine the optimal trait values of cells of a given carbon size in a given environment. We also perform fully dynamic simulations to study the self-assembled trait distribution at steady state and throughout a seasonal cycle. Diatoms’ strategy is shown to be advantageous among small cells and under early spring conditions, in which highly vacuolated diatoms are simulated, while mixotrophy is dominant among larger cells when nutrients are limiting (summer conditions). The novelty of this approach lies in the mechanistic understanding of plankton physiology that is successfully used to capture how community trait structure emerges from trade-offs that constrain the different strategy among unicellular plankton.