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Light and depth dependency of nitrogen fixation by the non-photosynthetic, symbiotic cyanobacterium UCYN-A

Lookup NU author(s): Professor Sam WilsonORCiD

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

Abstract

© 2021 The Authors. Environmental Microbiology published by Society for Applied Microbiology and John Wiley & Sons Ltd.The symbiotic cyanobacterium UCYN-A is one of the most globally abundant marine dinitrogen (N2)-fixers, but cultures have not been available and its biology and ecology are poorly understood. We used cultivation-independent approaches to investigate how UCYN-A single-cell N2 fixation rates (NFRs) and nifH gene expression vary as a function of depth and photoperiod. Twelve-hour day/night incubations showed that UCYN-A only fixed N2 during the day. Experiments conducted using in situ arrays showed a light-dependence of NFRs by the UCYN-A symbiosis, with the highest rates in surface waters (5–45 m) and lower rates at depth (≥ 75 m). Analysis of NFRs versus in situ light intensity yielded a light saturation parameter (Ik) for UCYN-A of 44 μmol quanta m−2 s−1. This is low compared with other marine diazotrophs, suggesting an ecological advantage for the UCYN-A symbiosis under low-light conditions. In contrast to cell-specific NFRs, nifH gene-specific expression levels did not vary with depth, indicating that light regulates N2 fixation by UCYN-A through processes other than transcription, likely including host–symbiont interactions. These results offer new insights into the physiology of the UCYN-A symbiosis in the subtropical North Pacific Ocean and provide clues to the environmental drivers of its global distributions.

Publication metadata

Author(s): Gradoville MR, Cabello AM, Wilson ST, Turk-Kubo KA, Karl DM, Zehr JP

Publication type: Article

Publication status: Published

Journal: Environmental Microbiology

Year: 2021

Volume: 23

Issue: 8

Pages: 4518-4531

Print publication date: 01/08/2021

Online publication date: 06/07/2021

Acceptance date: 16/06/2021

Date deposited: 06/12/2021

ISSN (print): 1462-2912

ISSN (electronic): 1462-2920

Publisher: John Wiley and Sons Inc

URL: https://doi.org/10.1111/1462-2920.15645

DOI: 10.1111/1462-2920.15645

PubMed id: 34227720

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