The Paradox of the Prochlorococcus: A Trait-Based Approach to Modeling Ecotype Niche Differentiation Via Light and Nutrient Resource Competition

The habitat of Prochlorococcus, the most abundant photosynthetic organism on Earth, extends throughout the heterogeneous conditions of the euphotic zone of the tropical and subtropical oligotrophic ocean. Observations reveal that Prochlorococcus ecotypes are organized by vertical niche-partitioning in a seasonally dynamic, but annually stable, pattern. To mechanistically understand how ecotypes are structured, we employ a modified resource-ratio theory (Huisman & Weissing, 1994) to explore inter-ecotype competition for the essential resources of nutrients and light. We predict resource supply regimes related to the exclusion and co-existence of multiple Prochlorococcus ecotype-analogs defined using traits determined in the laboratory and ask if these predictions are consistent with the observed patterns in the North Atlantic and North Pacific subtropical gyres. The model helps identify the traits of ecotypes which thrive during seasonal mixing events.