N2 Fixation in Trichodesmium Does Not Require Spatial Segregation from Photosynthesis
Weicheng Luo1, Keisuke Inomura2, Han Zhang3
1State Key Laboratory of Marine Environmental Science and College of Ocean and Earth Sciences, Xiamen Universitygrid.12955.3a, Xiamen, China.
Msystems
|July 21, 2022
Summary
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.
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.
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