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1. UCYN-A/haptophyte symbioses dominate N2 fixation in the Southern California Current System

   https://doi.org/10.1038/s43705-021-00039-7  

   Turk-Kubo, Kendra A. ; Mills, Matthew M. ; Arrigo, Kevin R. ; van Dijken, Gert ; Henke, Britt A. ; Stewart, Brittany ; Wilson, Samuel T. ; Zehr, Jonathan P. ( August 2021 , ISME Communications)

   The availability of fixed nitrogen (N) is an important factor limiting biological productivity in the oceans. In coastal waters, high dissolved inorganic N concentrations were historically thought to inhibit dinitrogen (N₂) fixation, however, recent N₂fixation measurements and the presence of the N₂-fixing UCYN-A/haptophyte symbiosis in nearshore waters challenge this paradigm. We characterized the contribution of UCYN-A symbioses to nearshore N₂fixation in the Southern California Current System (SCCS) by measuring bulk community and single-cell N₂fixation rates, as well as diazotroph community composition and abundance. UCYN-A1 and UCYN-A2 symbioses dominated diazotroph communities throughout the region during upwelling and oceanic seasons. Bulk N₂fixation was detected in most surface samples, with rates up to 23.0 ± 3.8 nmol N l⁻¹ d⁻¹, and was often detected at the deep chlorophyll maximum in the presence of nitrate (>1 µM). UCYN-A2 symbiosis N₂fixation rates were higher (151.1 ± 112.7 fmol N cell⁻¹ d⁻¹) than the UCYN-A1 symbiosis (6.6 ± 8.8 fmol N cell⁻¹ d⁻¹). N₂fixation by the UCYN-A1 symbiosis accounted for a majority of the measured bulk rates at two offshore stations, while the UCYN-A2 symbiosis was an important contributor in three nearshore stations. This report of active UCYN-A symbioses and broad mesoscale distribution patterns establishes UCYN-A symbioses as the dominant diazotrophs in the SCCS, where heterocyst-forming and unicellular cyanobacteria are less prevalent, and provides evidence that the two dominant UCYN-A sublineages are separate ecotypes.

    

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

   https://doi.org/10.1038/s41396-020-0691-6  

   Mills, Matthew M. ; Turk-Kubo, Kendra A. ; van Dijken, Gert L. ; Henke, Britt A. ; Harding, Katie ; Wilson, Samuel T. ; Arrigo, Kevin R. ; Zehr, Jonathan P. ( October 2020 , The ISME Journal)

   null (Ed.)

   Abstract 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. 

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3. A coastal N2 fixation hotspot at the Cape Hatteras front: Elucidating spatial heterogeneity in diazotroph activity via supervised machine learning

   https://doi.org/10.1002/lno.11727  

   Selden, Corday R. ; Chappell, P. Dreux ; Clayton, Sophie ; Macías‐Tapia, Alfonso ; Bernhardt, Peter W. ; Mulholland, Margaret R. ( April 2021 , Limnology and Oceanography)

   In the North Atlantic Ocean, dinitrogen (N₂) fixation on the western continental shelf represents a significant fraction of basin‐wide nitrogen (N) inputs. However, the factors regulating coastal N₂fixation remain poorly understood, in part due to sharp physico‐chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N₂fixation on the western North Atlantic shelf, at the confluence of Mid‐ and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the¹⁵N₂bubble release method and used to build empirical models of regional N₂fixation via a random forest machine learning approach. N₂fixation rates were then predicted from high‐resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N₂fixation rates (> 100 nmol N L⁻¹d⁻¹), which were likely driven by the haptophyte‐symbiont UCYN‐A. Above and below this hotspot, N₂fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N₂uptake rates, a biomass‐independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N₂fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients.

    

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4. Symbiotic unicellular cyanobacteria fix nitrogen in the Arctic Ocean

   https://doi.org/10.1073/pnas.1813658115  

   Harding, Katie ; Turk-Kubo, Kendra A. ; Sipler, Rachel E. ; Mills, Matthew M. ; Bronk, Deborah A. ; Zehr, Jonathan P. ( December 2018 , Proceedings of the National Academy of Sciences)

   Biological dinitrogen (N₂) fixation is an important source of nitrogen (N) in low-latitude open oceans. The unusual N₂-fixing unicellular cyanobacteria (UCYN-A)/haptophyte symbiosis has been found in an increasing number of unexpected environments, including northern waters of the Danish Straight and Bering and Chukchi Seas. We used nanoscale secondary ion mass spectrometry (nanoSIMS) to measure¹⁵N₂uptake into UCYN-A/haptophyte symbiosis and found that UCYN-A strains identical to low-latitude strains are fixing N₂in the Bering and Chukchi Seas, at rates comparable to subtropical waters. These results show definitively that cyanobacterial N₂fixation is not constrained to subtropical waters, challenging paradigms and models of global N₂fixation. The Arctic is particularly sensitive to climate change, and N₂fixation may increase in Arctic waters under future climate scenarios.

    

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5. Unexpected presence of the nitrogen‐fixing symbiotic cyanobacterium UCYN‐A in Monterey Bay, California

   https://doi.org/10.1111/jpy.13045  

   Cabello, Ana M. ; Turk‐Kubo, Kendra A. ; Hayashi, Kendra ; Jacobs, Lucien ; Kudela, Raphael M. ; Zehr, Jonathan P. ; Mock, ed., T. ( August 2020 , Journal of Phycology)

   In the last decade, the known biogeography of nitrogen fixation in the ocean has been expanded to colder and nitrogen‐rich coastal environments. The symbiotic nitrogen‐fixing cyanobacteria group A (UCYN‐A) has been revealed as one of the most abundant and widespread nitrogen‐fixers, and includes several sublineages that live associated with genetically distinct but closely related prymnesiophyte hosts. The UCYN‐A1 sublineage is associated with an open ocean picoplanktonic prymnesiophyte, whereas UCYN‐A2 is associated with the coastal nanoplanktonic coccolithophoreBraarudosphaera bigelowii, suggesting that different sublineages may be adapted to different environments. Here, we study the diversity ofnifHgenes present at the Santa Cruz Municipal Wharf in the Monterey Bay (MB), California, and report for the first time the presence of multiple UCYN‐A sublineages, unexpectedly dominated by the UCYN‐A2 sublineage. Sequence and quantitative PCR data over an 8‐year time‐series (2011–2018) showed a shift toward increasing UCYN‐A2 abundances after 2013, and a marked seasonality for this sublineage which was present during summer‐fall months, coinciding with the upwelling‐relaxation period in the MB. Increased abundances corresponded to positive temperature anomalies in MB, and we discuss the possibility of a benthic life stage of the associated coccolithophore host to explain the seasonal pattern. The dominance of UCYN‐A2 in coastal waters of the MB underscores the need to further explore the habitat preference of the different sublineages in order to provide additional support for the hypothesis that UCYN‐A1 and UCYN‐A2 sublineages are different ecotypes.

    

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