Quantification of Aquatic Unicellular Diazotrophs by Immunolabeled Flow Cytometry

Quantifying the number of aquatic diazotrophs is highly challenging and relies mainly on microscopical approaches and/or molecular tools that are based on nif genes. However, it is still challenging to count diazotrophs, especially the unicellular fraction, despite their significant contribution to the aquatic nitrogen cycle. In this study a new method was developed to quantify unicellular diazotrophs by immunolabeling the nitrogenase enzyme followed by identification and quantification via flow cytometry. The new quantification method was initially developed using a diazotrophic monoculture ( Vibrio natriegens ) and verified by various auxiliary approaches. It was found that only 15-20% of the total number of V. natriegens cells have synthesized the nitrogenase enzyme, even though the media was anaerobic, and N limited. This approach was further tested in samples from marine and freshwater environments. It was found that the ratio of diazotrophs to total bacteria was 0.1% in the Mediterranean Sea, while 4.7% along the Jordan River. In contrast, the specific N2 fixation per unicellular diazotrophs was highest in the Mediterranean Sea (88 attomole N cell-1 d-1) while the total N2 fixation rates were lowest in the lake and the river (0.2 nmole N L-1 d-1). Overall, we expect that this direct quantification approach will provide new insights on the number and contribution of unicellular diazotrophs to total N2 fixation in marine and freshwater environments under various conditions. ### Competing Interest Statement The authors have declared no competing interest.