Diazotrophs Under PrEssure

DUPE investigates the potential contribution of sinking eukaryotic diazotrophs on dark ocean N2 fixation

N2 fixation was long attributed solely to photosynthetic diazotrophic cyanobacteria that inhabit the warm and oligotrophic surface waters of the (sub)tropical oceans. However, several studies have measured active N2 fixation in the mesopelagic layer (200-1000 m depth), expanding considerably the habitat extent where nitrogen inputs need to be considered and added to current global budgets. Indeed, it has been estimated that aphotic N2 fixation could contribute up to 67% of global nitrogen inputs.

Light in the ocean extincts about 150 m of depth. Below this depth, photosynthetic organisms such as diazotrophic cyanobacteria are theoretically dead. Hence, dark ocean N2 fixation has been attributed to non-cyanobacterial diazotrophs such as bacteria and archaea.

However, mesopelagic samples often contain diazotrophic cyanobacteria DNA and even RNA!

Does that mean they are active down there?

The presence of active diazotrophic cyanobacteria in the mesopelagic layer could be linked to the formation of aggregates at the surface, which would increase their sinking speed.

To test this Emilie Poletti is performing experiments using a particle sinking simulator (PASS) in collaboration with Marc Garel and Christian Tamburini. Trichodesmium and Crocosphaera cultures are submitted to increasing depths to simulate their descent along the water column to the dark ocean. We can't wait to see her results!

PASS setup including water baths, cryostat, piloted pressure generator and device for high pressure bottles (HPBs) half a revolution. On the left photo three water bath are dedicated to PASS experiment. On the right photo HPBs are in the device creating automatic half a revolution. Temperature and oxygen are measured continuously within the HPBs.