Describing the Fate of Diazotroph-derived New Nitrogen

Abstract

Marine diazotrophs play an important role in marine biogeochemical cycles by fixing N2 into bioavailable forms, thus sustaining oceanic productivity over broad timescales through maintenance of bioavailable nitrogen stores. However, as assessments of diazotrophic organisms are traditionally constrained to the upper ocean, the fate of diazotroph-derived new nitrogen is not clear. Many previous assessments of the fate of diazotrophs has assumed that the majority of new nitrogen produced in these organisms is recycled in the upper ocean through the microbial loop and that diazotroph contribution to export is minimal except following blooms of diazotrophic diatom associations (DDAs). In this study, a combination of light microscopy, transmission electron microscopy, and qPCR of sinking particulate material from the subtropical and tropical Atlantic Ocean and Gulf of Mexico has revealed that filamentous, heterocystous and unicellular cyanobacterial diazotrophs are present below 100 m, and provides some of the first evidence that this appears to be a widespread occurrence. Herein we identify the mechanisms by which diazotrophs are exiting the mixed layer via passive sedimentation, aggregation, and incorporation in faecal material. Diazotrophs also appear to be contributing to the export of particulate organic nitrogen with Trichodesmium composing up to 3% of PON standing stock and 1 – 17.5% of PON flux at 10 m below the mixed layer in the (sub-)tropical Atlantic Ocean. The likelihood that the subsequent remineralisation of diazotroph-derived material at depth is contributing to the N* anomaly observed in the thermocline in the North Atlantic sub-tropical gyre is also discussed. This work provides some of the first descriptions of mechanisms by which diazotrophs contribute to these anomalous nutrient distributions, such as through remineralisation of diazotroph biomass following cellular lysis. These results aid in the elucidation of the extent to which Trichodesmium and other
diazotrophs are contributing to the biogeochemistry of deeper waters and provides novel insight into the cycling of fixed nitrogen in the oligotrophic ocean.

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