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Sinking Trichodesmium colonies fix nitrogen (N2) in the dark ocean, contributing to mesopelagic ecosystems. These cyanobacteria utilize stored carbon to maintain N2 fixation at depth, supporting other microbes.

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Area of Science:

  • Marine microbiology
  • Ocean biogeochemistry
  • Cyanobacterial physiology

Background:

  • The cyanobacterium Trichodesmium is a key nitrogen (N2) fixer in the surface ocean.
  • Trichodesmium has been detected in deeper ocean layers, but its N2 fixation ability in the dark remains unexplored.

Purpose of the Study:

  • To investigate the capacity of sinking Trichodesmium colonies to fix N2 in the dark ocean's mesopelagic zone.
  • To determine the contribution of Trichodesmium to N2 fixation at depth and identify other active diazotrophs.

Main Methods:

  • 15N2 incubations in sediment traps at multiple depths (170-1000 m) in the South Pacific.
  • cDNA nifH amplicon sequencing to identify active diazotrophs.
  • Laboratory experiments simulating deep-sea conditions (pressure, temperature) on Trichodesmium.
  • Cell metabolism modeling to predict carbon utilization for N2 fixation.

Main Results:

  • Sinking Trichodesmium colonies exhibited significant N2 fixation rates in the mesopelagic zone, comparable to surface rates.
  • Trichodesmium contributed substantially to measured N2 fixation, alongside other diazotrophs like Chlorobium and Deltaproteobacteria.
  • N2 fixation in Trichodesmium was reduced but not inhibited by increased hydrostatic pressure and decreased temperature.
  • Model predictions indicate Trichodesmium uses stored carbon for N2 fixation during descent.

Conclusions:

  • Sinking Trichodesmium colonies are an active source of new nitrogen in the dark ocean.
  • Trichodesmium supports mesopelagic microbial communities by providing ammonium, dissolved organic matter, and biomass.
  • The study highlights the ecological significance of vertical carbon and nitrogen cycling mediated by Trichodesmium.