Ocean's Hidden Fixers: Unveiling the Mystery of Nitrogen Fixation

 Nitrogen is a necessary component of all living beings and inhibits life in the water. Although atmospheric N2 gas is the biggest reservoir of publicly accessible nitrogen, it is only biologically available to bacteria that contain the nitrogenase metalloenzyme, which can fix N2 into ammonia. Despite the fact that many marine bacteria and archaea encode nitrogenase, cyanobacteria are responsible for the majority of nitrogen fixation in the ocean. These phototrophs can live both freely and symbiotically, and they can directly or indirectly contribute to carbon fixation and export production in areas where they are plentiful, such as oligotrophic coastal waters and subtropical gyre margins. Cyanobacterial N2 fixers are too uncommon to account for the observed rates of N2 fixing. Instead, non-cyanobacterial N2 fixers have been implied, based on the abundance of nitrogenase-encoding gene sequences (nifH), the majority of which are found in uncultured proteobacteria. The role of non-cyanobacterial N2 fixation in the tropical North Atlantic was explored during a January-February 2020 excursion. This region accounts for around 20% of oceanic N2 fixation, and cyanobacteria can only explain roughly half of the rates observed in the region. Surface waters had high N2 fixation rates of up to 40 nmol. Metagenomic sequencing showed the presence of both cyanobacterial and heterotrophic N2 fixers, including gamma-A. Gamma-A nifH sequences were only found in particles larger than 3 µm, indicating attachment or interaction with a host organism. 

A nearly-full metagenome-assembled genome was recovered, including the gamma-A nifH gene and a complete cluster of rRNA genes. Although the recovered nifH sequence was originally claimed to be in the Gammaproteobacteria, both 16S-rRNA-gene-based and whole-genome taxonomy placed this MAG firmly into the alphaproteobacterial family Hyphomicrobiaceae. This family is part of the Rhizobiales order, which includes the most notable rhizobial symbionts of terrestrial legumes that form nodules. In addition to nifH, the majority of the genes in the nif regulon are of gammaproteobacterial source, including nifD and nifK, which code the catalytic part of the nitrogenase; nifE, nifN, and nifB, which encode the iron-molybdenum cofactor assembly proteins; and nifS, and that is involved in metallocluster biosynthesis. Almost all of the genes in the gamma-A MAG are of alphaproteobacterial origin. Based on these findings, it is concluded that the gamma-A N2 fixer is an alphaproteobacterium that gained nitrogenase genes by horizontal gene transfer from a gammaproteobacterial donor. Aside from gamma-A, several other bacteria, including members of the Rhizobiales order, acquired nitrogenase genes via horizontal gene transfer from a gammaproteobacterial donor. Other N2 fixers have previously been reported to have acquired nitrogenase genes through horizontal gene transfer across classes.

The discovery of a non-cyanobacterial N2-fixing symbiont, 'Candidatus Tectiglobus diatomicola', which feeds fixed nitrogen to its diatom host in exchange for photosynthetic carbon, is described. The N2-fixing symbiont is from the order Rhizobiales, and its relationship with a unicellular diatom broadens the known hosts for this order beyond the well-known N2-fixing rhizobia-legume symbioses on land. The findings indicate that rhizobia-diatom symbioses can contribute as much fixed nitrogen as cyanobacterial N2 fixers in the tropical North Atlantic, and that they may be responsible for N2 fixation in vast areas of the ocean where cyanobacteria are too rare to account for the measured rates. 

REFERENCE:

Tschitschko B, Esti M, Philippi M, Kidane AT, Littmann S, Kitzinger K, Speth DR, Li S, Kraberg A, Tienken D, Marchant HK. Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean. Nature. 2024 May 9:1-3.

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cover image:  https://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Diatoms_through_the_microscope.jpg/390px-Diatoms_through_the_microscope.jpg