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Aquatic Microbial Ecology

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AME 77:125-138 (2016)  -  DOI:

New insights into the ecology of the globally significant uncultured nitrogen-fixing symbiont UCYN-A

Hanna Farnelid1,2,*, Kendra Turk-Kubo1, María del Carmen Muñoz-Marín1,3, Jonathan P. Zehr1

1Ocean Sciences Department, University of California, Santa Cruz, CA 95064, USA
2Centre for Ecology and Evolution in Microbial Model Systems (EEMiS), Linnaeus University, 392 34 Kalmar, Sweden
3Departamento de Bioquímica y Biología Molecular, Edificio Severo Ochoa, planta 1, Universidad de Córdoba, 14071-Córdoba, Spain
*Corresponding author:

ABSTRACT: Cyanobacterial nitrogen-fixers (diazotrophs) play a key role in biogeochemical cycling of carbon and nitrogen in the ocean. In recent years, the unusual symbiotic diazotrophic cyanobacterium Atelocyanobacterium thalassa (UCYN-A) has been recognized as one of the major diazotrophs in the tropical and subtropical oceans. In this review, we summarize what is currently known about the geographic distribution of UCYN-A, as well as the environmental factors that govern its distribution. In addition, by compiling UCYN-A nifH sequences from the GenBank no. database as well as those from nifH gene amplicon next generation sequencing studies, we present an in-depth analysis of the distribution of defined UCYN-A sublineages (UCYN-A1, UCYN-A2 and UCYN-A3) and identify a novel sublineage, UCYN-A4, which may be significant in some environments. Each UCYN-A sublineage exhibited a remarkable global distribution pattern and several UCYN-A sublineages frequently co-occurred within the same sample, suggesting that if they represent different ecotypes they have overlapping niches. Recently, single cell visualization techniques using specific probes targeting UCYN-A1 and UCYN-A2 and their respective associated eukaryotic partner cells showed that the size of the consortia and the number of UCYN-A cells differed between these 2 sublineages. Combined, the results highlight that UCYN-A sublineages likely have different physiological requirements, which need to be accounted for in future studies. Furthermore, based on our increasing knowledge of the diversity of the UCYN-A lineage, we discuss some of the limitations of currently used cultivation-independent molecular techniques for the identification and quantification of UCYN-A.

KEY WORDS: UCYN-A · Symbiosis · nifH · qPCR · Next generation amplicon sequencing · CARD-FISH · Braarudosphaera bigelowii

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Cite this article as: Farnelid H, Turk-Kubo K, Muñoz-Marín MC, Zehr JP (2016) New insights into the ecology of the globally significant uncultured nitrogen-fixing symbiont UCYN-A. Aquat Microb Ecol 77:125-138.

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