Nitrogen fixation and nifH diversity in human gut microbiota

Katsura Igai, Manabu Itakura, Suguru Nishijima, Hirohito Tsurumaru, Wataru Suda, Takumi Tsutaya, Eriko Tomitsuka, Kiyoshi Tadokoro, Jun Baba, Shingo Odani, Kazumi Natsuhara, Ayako Morita, Minoru Yoneda, Andrew R. Greenhill, Paul F. Horwood, Jun Ichi Inoue, Moriya Ohkuma, Yuichi Hongoh, Taro Yamamoto, Peter M. SibaMasahira Hattori, Kiwamu Minamisawa, Masahiro Umezaki

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Abstract

It has been hypothesized that nitrogen fixation occurs in the human gut. However, whether the gut microbiota truly has this potential remains unclear. We investigated the nitrogen-fixing activity and diversity of the nitrogenase reductase (NifH) genes in the faecal microbiota of humans, focusing on Papua New Guinean and Japanese individuals with low to high habitual nitrogen intake. A 15 N 2 incorporation assay showed significant enrichment of 15 N in all faecal samples, irrespective of the host nitrogen intake, which was also supported by an acetylene reduction assay. The fixed nitrogen corresponded to 0.01% of the standard nitrogen requirement for humans, although our data implied that the contribution in the gut in vivo might be higher than this value. The nifH genes recovered in cloning and metagenomic analyses were classified in two clusters: one comprising sequences almost identical to Klebsiella sequences and the other related to sequences of Clostridiales members. These results are consistent with an analysis of databases of faecal metagenomes from other human populations. Collectively, the human gut microbiota has a potential for nitrogen fixation, which may be attributable to Klebsiella and Clostridiales strains, although no evidence was found that the nitrogen-fixing activity substantially contributes to the host nitrogen balance.

Original languageEnglish
Article number31942
JournalScientific reports
Volume6
DOIs
Publication statusPublished - 2016 Aug 24

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    Igai, K., Itakura, M., Nishijima, S., Tsurumaru, H., Suda, W., Tsutaya, T., Tomitsuka, E., Tadokoro, K., Baba, J., Odani, S., Natsuhara, K., Morita, A., Yoneda, M., Greenhill, A. R., Horwood, P. F., Inoue, J. I., Ohkuma, M., Hongoh, Y., Yamamoto, T., ... Umezaki, M. (2016). Nitrogen fixation and nifH diversity in human gut microbiota. Scientific reports, 6, [31942]. https://doi.org/10.1038/srep31942