Colourful coexistence of red and green picocyanobacteria in lakes and seas

Hutchinson’s paradox of the plankton inspired many studies on the mechanisms of
species coexistence. Recent laboratory experiments showed that partitioning of white
light allows stable coexistence of red and green picocyanobacteria. Here, we investigate
to what extent these laboratory findings can be extrapolated to natural waters. We
predict from a parameterized competition model that the underwater light colour of
lakes and seas provides ample opportunities for coexistence of red and green
phytoplankton species. To test this prediction, we sampled picocyanobacteria of 70
aquatic ecosystems, ranging from clear blue oceans to turbid brown peat lakes. As
predicted, red picocyanobacteria dominated in clear waters, whereas green picocyanobacteria
dominated in turbid waters. We found widespread coexistence of red and green
picocyanobacteria in waters of intermediate turbidity. These field data support the
hypothesis that niche differentiation along the light spectrum promotes phytoplankton
biodiversity, thus providing a colourful solution to the paradox of the plankton.