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Response of Subantarctic microbes to new versus regenerated Fe in a cold-core eddy
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  • Marion Fourquez,
  • Robert Strzepek,
  • Michael J Ellwood,
  • Christel Hassler,
  • Damien Cabanes,
  • Sam Eggins,
  • Imojen Pearce,
  • Stacy Deppeler,
  • Thomas W. Trull,
  • Philip W Boyd,
  • Matthieu Bressac
Marion Fourquez
Aix Marseille Univ., Universite de Toulon

Corresponding Author:[email protected]

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Robert Strzepek
University of Tasmania
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Michael J Ellwood
Australian National University
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Christel Hassler
University of Geneva
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Damien Cabanes
Univ. de Geneva
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Sam Eggins
Research School of Earth Sciences
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Imojen Pearce
Australian Antarctic Division, AAD, Kingston, TAS, Australia
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Stacy Deppeler
National Institute of Water and Atmospheric Research
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Thomas W. Trull
Commonwealth Scientific and Industrial Research Organisation (CSIRO)
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Philip W Boyd
Institute for Marine and Antarctic Studies
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Matthieu Bressac
Sorbonne Université
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In the Subantarctic sector of the Southern Ocean, vertical entrainment of dissolved iron (DFe) triggers the seasonal productivity cycle. However, diminishing physical supply of new Fe during the spring to summer transition rapidly drives epipelagic microbial communities to rely upon recycled DFe for growth. Hence, subpolar waters evolve seasonally from a high fe ratio system (i.e., [uptake of new Fe]/[uptake of new+recycled Fe]) to a low fe ratio system. Here, we tested how resident microbes within a cyclonic eddy respond to different Fe/ligand inputs which mimic entrained new DFe (Fe-NEW), diffusively-supplied regenerated DFe (Fe-REG), and a control with no addition of DFe (Fe-NO). After 6 days, 3.5 (Fe-NO, Fe-NEW) to 5-fold (Fe-REG) increases in Chl a were observed despite ~2.5-fold range between treatments of initial DFe. Marked differences were also evident in the proportion of in vitro DFe derived from recycling to sustain phytoplankton growth (Fe-REG, 30% recycled c.f. 70% Fe-NEW, 50% Fe-NO). This trend supports the concept that DFe/ligands released from subsurface particles are more bioavailable than new DFe collected at the same depth. This additional recycling may be mediated by bacteria. Indeed, by day 6 bacterial production (BP) was comparable between Fe-NO and Fe-NEW but~2 fold higher in Fe-REG. Interestingly, a preferential response of phytoplankton (haptophyte-dominated) relative to bacteria was also found in Fe-REG. In contrast, in Fe-NEW and Fe-NO the proportion of diatoms increased. Hence, different modes of Fe/ligand supply modify BP and Fe bioavailability to phytoplankton that may drive distinctive floristic shifts and biogeochemical signatures.