In past decades, environmental nitrogen fixation has been attributed almost exclusively to the action of enzymes in the well-studied molybdenum-dependent nitrogen fixation system. However, recent evidence has shown that nitrogen fixation by alternative pathways may be more frequent than previously suspected. In this study, the nitrogen fixation systems employed by lichen-symbiotic cyanobacteria were examined to determine whether their diazotrophy can be attributed, in part, to an alternative pathway. The mining of metagenomic data (generated through pyrosequencing) and PCR assays were used to determine which nitrogen-fixation systems are present in cyanobacteria from the genus Nostoc associated with four samples from different geographical regions, representing different lichen-forming fungal species in the genus Peltigera. A metatranscriptomic sequence library from an additional specimen was examined to determine which genes associated with N2 fixation are transcriptionally expressed. Results indicated that both the standard molybdenum-dependent system and an alternative vanadium-dependent system are present and actively transcribed in the lichen symbiosis. This study shows for the first time that an alternative system is utilized by cyanobacteria associated with fungi. The ability of lichen-associated cyanobacteria to switch between pathways could allow them to colonize a wider array of environments, including habitats characterized by low temperature and trace metal (e.g. molybdenum) availability. We discuss the implications of these findings for environmental studies that incorporate acetylene-reduction assay data.
|Number of pages||9|
|Journal||European Journal of Phycology|
|Publication status||Published - Jan 2014|
Bibliographical noteFunding Information:
Paul Bishop and Telisa Loveless are thanked for sparking B.P.H.’s interest in alternative nitrogen fixation pathways. We would like to thank Lisa Bukovnik, Bernie Ball, Jolanta Miadlikowska, Kathryn Picard, Tami McDonald, Daniele Armaleo, Terri Porter, Greg Bonito, Jason Jackson, Neil Gottel, Chris Schadt, Sarah Hodkinson and Molly McMullen, all of whom have provided important research assistance. This work was supported in part by Duke Biology Grants-in-Aid to B.P. H. from the Keever Endowment, a Mycological Society of America Graduate Fellowship to B.P.H., a grant from the Icelandic Research Fund, and a subcontract (112442) to Daniele Armaleo, Fred Dietrich and F.L. as part of the Pacific Northwest National Laboratory (PNNL) foundational scientific focus area (FSFA) under DOE-BER's genomic sciences programme in collaboration with Scott Baker and Jon Magnuson. Analyses were conducted using the Duke Shared Cluster Resource (DSCR), with outstanding services provided by John Pormann and Tom Milledge, and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by a grant from the National Science Foundation (OCI-1053575) and was made available to B.P.H. through XSEDE Allocation Awards (DEB110024 & DEB130002). Contributions by L.F. and B.G. were made possible through NSF award DEB-0919284.
- alternative nitrogenase
- dinitrogen fixation