Unusual marine cyanobacteria/haptophyte symbiosis relies on N(2) fixation even in N-rich environments

The microbial fixation of N(2) is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N-acquisition process and is believed inhibited when less energetically expensive forms, like dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N(2)-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N(2) fixation to DIN is unknown. We used stable isotope incubations, catalyzed reporter deposition fluorescence in-situ hybridization (CARD-FISH), and nanoscale secondary ion mass spectrometry (nanoSIMS), to investigate the N source used by the haptophyte host and sensitivity of UCYN-A N(2) fixation in DIN-replete waters. We demonstrate that under our experimental conditions, the haptophyte hosts of two UCYN-A sublineages do not assimilate nitrate (NO(3)(−)) and meet little of their N demands via ammonium (NH(4)(+)) uptake. Instead the UCYN-A/haptophyte symbiosis relies on UCYN-A N(2) fixation to supply large portions of the haptophyte’s N requirements, even under DIN-replete conditions. Furthermore, UCYN-A N(2) fixation rates, and haptophyte host carbon fixation rates, were at times stimulated by NO(3)(−) additions in N-limited waters suggesting a link between the activities of the bulk phytoplankton assemblage and the UCYN-A/haptophyte symbiosis. The results suggest N(2) fixation may be an evolutionarily viable strategy for diazotroph–eukaryote symbioses, even in N-rich coastal or high latitude waters.