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N2 Fixation in Trichodesmium Does Not Require Spatial Segregation from Photosynthesis.

Weicheng Luo1, Keisuke Inomura2, Han Zhang3

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Marine cyanobacteria Trichodesmium fixes nitrogen during the day, despite oxygen sensitivity. A new model shows Trichodesmium uses stored carbon and low oxygen to enable nitrogen fixation without spatial segregation of processes.

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Area of Science:

  • Marine microbial ecology
  • Biogeochemical cycles
  • Photosynthesis and nitrogen fixation

Background:

  • The marine cyanobacterium *Trichodesmium* is a key oceanic nitrogen (N2) fixer, performing both photosynthesis and N2 fixation.
  • A paradox exists as nitrogenase, the enzyme for N2 fixation, is oxygen-sensitive, yet *Trichodesmium* photosynthesizes and fixes N2 concurrently during daylight.
  • Previous research debated whether specialized cells (diazocytes) spatially segregate these processes, with conflicting evidence.

Purpose of the Study:

  • To investigate the metabolic mechanisms enabling *Trichodesmium* to perform oxygen-sensitive N2 fixation alongside oxygenic photosynthesis.
  • To evaluate the necessity of spatial segregation of N2 fixation and photosynthesis within *Trichodesmium* filaments.
  • To model *Trichodesmium*'s metabolism to understand its physiological adaptations.

Main Methods:

  • Construction and simulation of a systematic metabolic model for *Trichodesmium*.
  • Analysis of carbon accumulation, respiratory protection, and oxygen regulation.
  • Incorporation of cell membrane properties and alternative electron transfer pathways in the model.

Main Results:

  • The model indicates that spatial segregation is likely unnecessary for *Trichodesmium* growth and N2 fixation.
  • *Trichodesmium* accumulates fixed carbon in the morning, utilizing it for respiratory protection to lower intracellular oxygen mid-day.
  • Cellular mechanisms like oxygen barriers and alternative electron transfer contribute to maintaining low internal oxygen levels.

Conclusions:

  • A physiological mechanism exists for *Trichodesmium* to perform N2 fixation concurrently with photosynthesis without spatial segregation.
  • The proposed mechanism involves temporal regulation of photosynthesis and carbon reserves for respiratory oxygen scavenging.
  • Hypothetical spatial segregation may be ineffective if intercellular substance transfer is inefficient.