Coexistence of dominant marine phytoplankton species sustained by nutrient specialization

Abstract Prochlorococcus and Synechococcus are the two dominant picocyanobacteria in the low-nutrient surface waters of the subtropical ocean, but the basis for their coexistence in these biomes is still unclear. Here we combine in situ microcosm experiments and an ecological model to show that this coexistence can arise from specialization in the uptake of distinct nitrogen (N) substrates. In field incubations, the response of both Prochlorococcus and Synechococcus to nanomolar N amendments demonstrates N limitation of growth in both populations, but Prochlorococcus showed a higher affinity to ammonium whereas Synechococcus was more adapted to nitrate uptake. A simple ecological model demonstrates that the differential nutrient affinity of these species can explain their coexistence. Phylogenetic analysis of the presence of nitrate reductase and nitrite reductase further support the higher nitrate affinity of Synechococcus compared to Prochlorococcus. Our study suggests that the evolution of differential nutrient affinities is an important mechanism for sustaining coexistence of species under resource competition.