Intriguing size distribution of the uncultured and globally widespread marine non-cyanobacterial diazotroph Gamma-A

Abstract

Non-cyanobacterial diazotrophs (NCDs) have recently emerged as potentially important contributors to marine nitrogen fixation. One of the most widely distributed NCDs is Gamma-A, yet information about its autecology is still scarce and solely relies on the PCR-based detection of its nitrogenase (nifH) gene in seawater, since previous metagenomic surveys targeting free-living planktonic size fractions (<3 μm) have not detected it. Here, we explore the diversity, biogeography, size-distribution, and nitrogenase gene expression of Gamma-A across four larger planktonic size-fractions (0.8–5, 5–20, 20–180, and 180–2000 μm) using metagenomes and metatranscriptomes from the Tara Oceans. We detected a single variant of a complete Gamma-A nifH gene along with other nitrogenase-related genes (nifKDT) within a metatranscriptomic-based contig of the Marine Atlas of Tara Ocean Unigenes. Gamma-A was detected in tropical and subtropical oceanic regions across all the size-fractions. However, the highest gene and transcript abundances were found in the 0.8–5 and 5–20 μm size-fractions at the surface, whereas abundances at the deep chlorophyll maximum were lower and similar across all size-fractions. The ubiquitous presence of active Gamma-A in large planktonic size-fractions suggests a filamentous or particle-attached lifestyle and places its potential to fix nitrogen in larger planktonic compartments.

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Fig. 1: Distribution of Gamma-A-MATOU in the surface ocean.
Fig. 2: Abundance and expression of the Gamma-A-MATOU nitrogenase gene cluster across size fractions and depths.

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Acknowledgements

We specially thank Ana M. Cabello, Rosie Gradoville, Katie Harding, Marine Landa, Kendra Turk-Kubo and Clara Ruiz-Gonzalez for their helpful comments and suggestions on the manuscript. JPZ was supported by Simons Collaboration on Ocean Processes and Ecology (SCOPE, grant 329108). FMC-C was supported by a Marie Curie Individual Global Fellowship—Horizon 2020 European Framework Programme (UCYN2PLAST, grant 749380).

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Correspondence to Francisco M. Cornejo-Castillo or Jonathan P. Zehr.

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Cornejo-Castillo, F.M., Zehr, J.P. Intriguing size distribution of the uncultured and globally widespread marine non-cyanobacterial diazotroph Gamma-A. ISME J (2020). https://doi.org/10.1038/s41396-020-00765-1

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