The University of Southampton
University of Southampton Institutional Repository

Describing the Fate of Diazotroph-derived New Nitrogen

Describing the Fate of Diazotroph-derived New Nitrogen
Describing the Fate of Diazotroph-derived New Nitrogen
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.
Sargent, Elizabeth Colby
03e33822-812a-4ba5-a62a-9f13a919d2f7
Sargent, Elizabeth Colby
03e33822-812a-4ba5-a62a-9f13a919d2f7
Poulton, Alex
14bf64a7-d617-4913-b882-e8495543e717

Sargent, Elizabeth Colby (2014) Describing the Fate of Diazotroph-derived New Nitrogen. University of Southampton, Ocean and Earth Science, Doctoral Thesis, 166pp.

Record type: Thesis (Doctoral)

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.

PDF
Sargent, E_2014_PhD.pdf - Other
Download (6MB)

More information

Published date: 6 May 2014
Organisations: University of Southampton, Ocean and Earth Science

Identifiers

Local EPrints ID: 365470
URI: http://eprints.soton.ac.uk/id/eprint/365470
PURE UUID: 40f2b210-aa34-4cdd-bcdc-e1e44236ef0c

Catalogue record

Date deposited: 10 Jun 2014 09:34
Last modified: 18 Jul 2017 02:21

Export record

Contributors

Author: Elizabeth Colby Sargent
Thesis advisor: Alex Poulton

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×