The Paradox of the Plankton: Coexistence of Structured Microbial Communities

In the framework of resource-competition models, it has been argued that the number of species stably coexisting in an ecosystem cannot exceed the number of shared resources. However, plankton seems to be an exception of this so-called “competitive-exclusion principle”. In planktic ecosystems, a large number of different species stably coexist in an environment with limited resources. This contradiction between theoretical expectations and empirical observations is often referred to as “The Paradox of the Plankton”. This project aims to investigate biophysical models that can account for the large biodiversity observed in real ecosystems in order to resolve this paradox. A model is proposed that combines classical resource competition models, metabolic trade-offs and stochastic ecosystem assembly. Simulations of the model match empirical observations, while relaxing some unrealistic assumptions from previous models. Paradox: from Greek para : “distinct from”, and doxa : opinion. Sainsbury (1995) defines a paradox as “an apparently unacceptable conclusion derived by apparently acceptable reasoning from apparently acceptable premises”. Paradoxes are useful research tools as they suggest logical inconsistencies. In order to spot the flaw, the validity of all the premises has to be carefully assessed. Plankton: refers to the collection of organisms that spend part or all of their lives in suspension in water (Reynolds 2006). Plankton, or plankters , are “organisms that have velocities significantly smaller than oceanic currents and thus are considered to travel with the water parcel they occupy” (Lombard et al. 2019). Phytoplankters refer to the members of the plankton that perform photosynthesis.