Nitrogen fixation and nitrogenise expression in the eastern North Atlantic

Nirtogen is a key element in the oceans. Atmospheric nitrogen must be converted into ammonium before it can be used. This is called biological nitrogen fixation (BNF). BNF is carried out by a small group of microorganisms called diazotrophs. Little is known about this group and they were identified using polymerase chain reaction amplification of the nifH gene which encodes the enzyme nitrogenase. Nitrogenase enzymes have been shown to require iron by previous studies and it is believed that the availability of iron and phosphorous may be important limiting or co-limiting factors in BNF.

This study by Turk et al (2011) focused on BNF and nifH expression near the Cape Verde islands in the eastern North Atlantic; it is a unique study because it is the first to look at nifH expression in these waters. This region generally has depleted nutrient levels but is heavily influenced by Saharan dusts which may supply iron and phosphorus to the area.

Samples were collected from 6 stations chosen to give a gradient from near shore to open waters. Nitrogen fixation rates, background nitrogen levels, total dissolved iron and nitrogen levels were measured at all 6 stations to be used later.  The study aimed to do three main things firstly describe the diversity of nifH-containing organisms actively transcribing nitrogenise by Reverse-transcription PCR(RT-PCR), secondly quantifying  the nifH transcripts from 7 major cyanobacteria by quantitative RT-PCR and thirdly measuring BNF.

The results of this study showed a positive correlation between BNF rates and dissolved iron levels but it was not show to correlate with phosphorus. This provides evidence for the link between BNF and Saharan dust. A total of 605 nifH transcripts were isolated from the samples of which 76% were from 6 operational taxonomic units including Trichodesmonium, uncultivated UCYN-A and 4 non-cyanobacterial diazotrophs. UCYN-A  (a type of cyanobacteria) was found to contribute most significantly to the pool of nifH in coastal and oligitrophic waters which indicates that they may be important contributors to BNF in this region. This contrasts with other studies carried out in the eastern equatorial Atlantic where nifH expression is dominated by Trichodesmium.

The authors of this paper recognise that the their methods have some limitations for example the PCR method means that only certain thing are targeted by the qPCR assays therefore other non targeted phylotypes may contribute to the pool of nifH transcripts. The methods used to measure nitrogen fixation rates have also recently been brought into question as this method may underestimate fixation rates.

Overall I found this paper quite difficult to interpret because of the way it was written which made it hard to follow. I chose to review this paper because it linked with our lectures on the nitrogen cycle.

Kendra A Turk, Andrew P Rees, Jonathan P Zehr, Nicole Pereira, Paul Swift,  Rachel Shelley, Maeve Lohan, E Malcolm S Woodward and Jack Gilbert (2011) The ISME Journal (2011) 5, 1201–1212; doi:10.1038/ismej.2010.205

Here is a link to the original paper:

http://www.nature.com/ismej/journal/v5/n7/full/ismej2010205a.html