Award: OCE-1559152

Primary production by marine phytoplankton is limited by nitrogen availability throughout much of the global open oceans. As a result, phytoplankton have evolved metabolisms for utilizing different chemical forms of nitrogen (e.g. nitrate, ammonium, or urea). One important nitrogen source for nutrient depleted waters is nitrogen fixation, the conversion of nitrogen gas (N2) into a biologically available form (ammonia) and is performed by only some prokaryotic microbes called diazotrophs. Marine N2 fixation was once thought to be dominated by the tropical/subtropical cyanobacterium Trichodesmium sp. and cyanobacterial symbionts of some diatoms. This paradigm changed with the discovery that nitrogen fixation is also carried out by the unicellular nitrogen fixing cyanobacterial ?group A? (UCYN-A), which lives in symbiosis with single-celled eukaryotic phytoplankton hosts from the Haptophyte group. UCYN-A is unusual in that it has undergone genomic streamlining, including the loss genes for carbon fixation, the entire tricarboxylic acid cycle, and other metabolic pathways. The phytoplankton host provides photosynthetically fixed carbon to UCYN-A in exchange for nitrogen supplied by UCYN-A from N2 fixation. UCYN-A symbionts include two genetically-distinct sublineages, UCYN-A1 and UCYN-A2, with similarly streamlined genomes, that are associated with morphologically and physiologically distinct haptophyte hosts. The fixation of nitrogen is energetically expensive compared to other nitrogen sources. Previous studies show that nitrogen fixation can be inhibited at elevated concentrations of nitrate and ammonium. The distribution of the UCYN-A/haptophyte symbiosis extends into nitrogen-rich environments not typically considered important for nitrogen fixation, including cold high latitude waters, coastal shelves, and upwelling regions. It is unclear why the UCYN-A/haptophyte symbiosis is competitive in nitrogen enriched waters and whether its growth is supplemented by a nitrogen source other than UCYN-A N2 fixation. The specific aims of this project were to quantify UCYN-A nitrogen fixation rates and to understand how different chemical (nutrient) and physical (light) factors regulate nitrogen fixation by UCYN-A. To determine the nitrogen source(s) used for growth by the UCYN-A/haptophyte symbioses we conducted a series of experiments in southern coastal waters of the California Current System. We used UCYN-A specific catalysed reporter deposition fluorescence in-situ hybridization (CARD-FISH) assays combined with nanoscale secondary ion mass spectrometry (nanoSIMS) to measure nitrogen and carbon fixation rates of individual cells within UCYN-A/haptophyte symbioses under nitrogen deplete and replete conditions and. Surprisingly, the haptophyte host assimilated no nitrate and little ammonium, but did exhibit higher carbon fixation rates in the nitrogen replete conditions indicating that the haptophyte cells were indirectly stimulated by the nitrogen addition. Additionally, UCYN-A abundances increased, as did the average per cell rate of nitrogen fixation in the nitrogen replete experiments. We estimate that nitrogen fixation by UCYN-A provides up to ~ 60% of the haptophyte host?s daily nitrogen requirement. This is the first direct evidence that the widely-distributed UCYN-A/haptophyte symbiosis uses little ammonium and no nitrate but continues to fix nitrogen in nitrogen-rich waters. Our findings challenge the commonly-held assumption that the energetic-tradeoffs of fixing nitrogen vs. assimilating other nitrogen forms leads to inhibition of nitrogen fixation in. Current ecosystem and biogeochemical models predict little nitrogen fixation in nitrogen-rich environment (e.g. high latitude and temperate coastal regions) despite recent evidence to the contrary. With this new insight, we can improve parameterizations of the UCYN-A/haptophyte symbiosis in models and improve our ability to predict the distribution of nitrogen fixation in the ocean and the response of the nitrogen cycle to environmental perturbations. Lastly, in this project we worked with collaborators from the University of California at Santa Cruz and the Virginia Institute of Marine Sciences to make the first direct measurements of nitrogen fixation by UCYN-A in Arctic waters. These measurements are further north than any previously reported nitrogen fixing marine cyanobacteria. The results show that nitrogen fixing cyanobacteria are not constrained to subtropical waters and challenges commonly held ideas about the distributions of global marine nitrogen fixation. Last Modified: 05/06/2019 Submitted by: Kevin R Arrigo