Novel molecular insights into the fate of N2 fixed by diazotrophic plankton

Next Monday, 13th of August, Liz Sargent will do a practice run of her talk for the Challenger Conference: Novel molecular insights into the fate of N2 fixed by diazotrophic plankton
 
Marine diazotrophs play an important role in oligotrophic surface oceans by fixing N2 into bioavailable forms.  To date, studies of the three main groups of N2-fixing organisms in the ocean (i.e. filamentous, heterocystous, and unicellular diazotrophs) have generally been limited to assessment of their presence and function in the euphotic zone, while the role these organisms have in the export of material to the ocean's interior has seldom been addressed.  Optical assessments of sinking particulate material from the eastern subtropical and tropical Atlantic demonstrated that Trichodesmium and Richelia intracellularis were commonly present below 100 m and as deep as 500 m.  Real-time quantitative PCR analysis with TaqMan probes was carried out on extracts of sinking particulate samples to 500 m to assess the presence of 5 nifH phylotypes: a single filamentous cyanobacterial probe specific to Trichodesmium, two heterocystous cyanobacterial probes specific to Richelia-Rhizosolenia and ¬Richelia¬-Hemiaulus (diazotrophic diatom associations), and two unicellular cyanobacterial probes specific to the uncultured Group A and Group C cyanobacteria.  These analyses revealed the presence of all of these diazotrophs below the mixed layer, which indicates previous assessments of the vertical distributions of these organisms may have overlooked the presence of diazotrophs at depth.  Contrary to previous expectations, results suggest that all three groups of marine diazotrophs are constituents of sinking material and are exported out of the euphotic zone in the subtropical and tropical Atlantic Ocean, providing novel insight into the cycling of fixed nitrogen in the oligotrophic ocean.