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

Lookup NU author(s): Professor Sam Wilson

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This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).

Abstract

© 2020, The Author(s).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 is believed inhibited when less energetically expensive forms, like 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, 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 N2 fixation in DIN-replete waters. We demonstrate that under our experimental conditions, the haptophyte hosts of two UCYN-A sublineages do not assimilate nitrate (NO3−) and meet little of their N demands via ammonium (NH4+) 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 NO3− 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 N2 fixation may be an evolutionarily viable strategy for diazotroph–eukaryote symbioses, even in N-rich coastal or high latitude waters.

Publication metadata

Author(s): Mills MM, Turk-Kubo KA, van Dijken GL, Henke BA, Harding K, Wilson ST, Arrigo KR, Zehr JP

Publication type: Article

Publication status: Published

Journal: ISME Journal

Year: 2020

Volume: 14

Issue: 10

Pages: 2395-2406

Print publication date: 01/10/2020

Online publication date: 10/06/2020

Acceptance date: 27/05/2020

Date deposited: 15/12/2021

ISSN (print): 1751-7362

ISSN (electronic): 1751-7370

Publisher: Springer Nature

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

DOI: 10.1038/s41396-020-0691-6

PubMed id: 32523086

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