Unusual marine cyanobacteria/haptophyte symbiosis relies on N2 fixation even in N-rich environments

The microbial fixation of N₂ is the largest source of biologically available nitrogen (N) to the oceans. However, it is the most energetically expensive N acquisition process, and can be inhibited when other forms of N, such as dissolved inorganic N (DIN), are available. Curiously, the cosmopolitan N₂-fixing UCYN-A/haptophyte symbiosis grows in DIN-replete waters, but the sensitivity of their N₂ fixation to DIN is unknown. We used stable isotope incubations, catalysed reporter deposition fluorescence in-situ hybridization (CARD-FISH), and nanoscale secondary ion mass spectrometry, to investigate the N source used by the haptophyte host and sensitivity of UCYN-A N₂ fixation in DIN-replete waters. We demonstrate that unlike other eukaryotic phytoplankton, the haptophyte hosts of two UCYN-A sublineages do not assimilate nitrate and meet little of their N demands via ammonium uptake. Instead the UCYN-A/haptophyte symbiosis relies on UCYN-A N₂ fixation to supply large portions of the haptophyte’s N requirements, even under DIN-replete conditions. Furthermore, UCYN-A N₂ fixation rates, and haptophyte host carbon fixation rates, were at times stimulated by nitrate additions in N-limited waters suggesting a link between the activities of the bulk phytoplankton assemblage and the UCYN-A/haptophyte symbiosis. N₂ fixation is thus an evolutionarily viable strategy for diazotroph-eukaryote symbioses, even in N-rich coastal or high latitude waters.