Using anvi’o, a state-of-the-art, open-source bioinformatics platform to analyze metagenomes (the pool of DNA sequences that represent all the microbial organisms found in an environment), the team revealed insights into previously unknown marine microbes with nitrogen fixation capabilities affiliated with Proteobacteria as well as Planctomycetes, a prevalent bacterial phylum that has never been linked to nitrogen fixation before.

These newly described microbial populations occur widely and are particularly abundant in the Pacific Ocean, where they average an estimated 700,000 cells per liter of seawater and up to 3 million cells per liter—orders of magnitude more than previous estimates for non-cyanobacterial nitrogen fixers in the open ocean.

Using data generated from the Tara Oceans expedition from 2009 to 2013, Delmont and colleagues reconstructed about 1,000 microbial genomes from more than 30 billion short metagenomic sequences. Of those 1,000 genomes, nine contained the six genes that are required for nitrogen fixation, and yet lacked the genes needed for photosynthesis. This is the first genomic database of non-photosynthetic microorganisms inhabiting the open ocean and capable of fixing nitrogen.

Because the team reconstructed and used near-complete genomes for their investigation (rather than using a single marker gene for nitrogen fixation), they could resolve the taxonomic affiliations of these nitrogen-fixing populations. They could also investigate their abundance and distribution patterns in the oceans and seas from which the samples came (the Atlantic, Pacific, Indian and Southern oceans and the Mediterranean and Red seas).

“We can now use these population genomes to guide the laboratory cultivation of nitrogen-fixing Planctomycetes and Proteobacteria from the open ocean,” Delmont said. “This will help us understand the conditions in which they fix nitrogen, the complexity of their functional lifestyles, and other aspects of their ecology that we cannot comprehend by just looking at their genomes, genes and inferred functions.”

Meren and Delmont began this research at the Marine Biological Laboratory in 2015 with the support of a University of Chicago Lillie Innovation Award. Meren and his group continue to develop anvi’o, the open-source software platform used in this and other studies investigating the ecology and evolution of microbes through complex environmental sequencing data.

In addition to Delmont, Meren and Quince, other collaborators in this study include Alon Shaiber, Özcan C. Esen, and Sonny T. M. Lee of the University of Chicago, Michael S. Rappé of the University of Hawaii at Mānoa, Sandra L. McLellan of the University of Wisconsin-Milwaukee, and Sebastian Lücker of Radboud University, The Netherlands.

Citation: Delmont, T.O. et al (2018) “Nitrogen-fixing populations of Planctomycetes and Proteobacteria are abundant in surface ocean metagenomes.” Nature Microbiology, doi: 10.1038/s41564-018-0176-9.