Nitrogen fixation and nitrogenase (nifH) expression in tropical waters of the eastern North Atlantic

• Published in: The ISME Journal
• Main Authors: A Turk, Kendra, Rees, Andrew P, Zehr, Jonathan P, Pereira, Nicole, Swift, Paul, Shelley, Rachel, Lohan, Maeve, Woodward, E Malcolm S, Gilbert, Jack
• Format: Text
• Language: English
• Published: Nature Publishing Group 2011
• Subjects: Original Article; North Atlantic; North East Atlantic
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3146282; http://www.ncbi.nlm.nih.gov/pubmed/21228888; https://doi.org/10.1038/ismej.2010.205

Expression of nifH in 28 surface water samples collected during fall 2007 from six stations in the vicinity of the Cape Verde Islands (north-east Atlantic) was examined using reverse transcription-polymerase chain reaction (RT-PCR)-based clone libraries and quantitative RT-PCR (RT-qPCR) analysis of seven diazotrophic phylotypes. Biological nitrogen fixation (BNF) rates and nutrient concentrations were determined for these stations, which were selected based on a range in surface chlorophyll concentrations to target a gradient of primary productivity. BNF rates greater than 6 nmolN l−1 h−1 were measured at two of the near-shore stations where high concentrations of Fe and PO43− were also measured. Six hundred and five nifH transcripts were amplified by RT-PCR, of which 76% are described by six operational taxonomic units, including Trichodesmium and the uncultivated UCYN-A, and four non-cyanobacterial diazotrophs that clustered with uncultivated Proteobacteria. Although all five cyanobacterial phylotypes quantified in RT-qPCR assays were detected at different stations in this study, UCYN-A contributed most significantly to the pool of nifH transcripts in both coastal and oligotrophic waters. A comparison of results from RT-PCR clone libraries and RT-qPCR indicated that a γ-proteobacterial phylotype was preferentially amplified in clone libraries, which underscores the need to use caution interpreting clone-library-based nifH studies, especially when considering the importance of uncultivated proteobacterial diazotrophs.