Nutrient-Colimited Trichodesmium as a Nitrogen Source or Sink in a Future Ocean

Area of Science:

• Marine microbiology
• Biogeochemical cycles
• Oceanography
• Climate change impacts

Background:

• Nitrogen-fixing (N₂) cyanobacteria are crucial for supplying nitrogen to nutrient-limited ocean regions.
• These organisms are sensitive to environmental changes, particularly iron (Fe) and phosphorus (P) availability.
• Global change drivers like elevated CO₂ can influence nitrogen acquisition strategies.

Purpose of the Study:

• To investigate the adaptive responses of the N₂-fixing cyanobacterium *Trichodesmium* to long-term high-CO₂ conditions under Fe and/or P limitation.
• To understand how these adaptations alter nitrogen metabolism and its implications for ocean biogeochemical cycles.

Main Methods:

• Culturing *Trichodesmium* under controlled high-CO₂ conditions for extended periods (7 years).
• Subsequent exposure to iron and/or phosphorus (co)limitation.
• Analysis of global transcriptomic and proteomic shifts to assess metabolic changes.

Main Results:

• High-CO₂ adaptation under nutrient (co)limitation induced a fundamental shift in *Trichodesmium*'s nitrogen metabolism.
• Significant downregulation of the nitrogenase enzyme (key for N₂ fixation) was observed.
• Concurrently, a major upregulation of enzymes involved in scavenging organic nitrogen, specifically trimethylamine (TMA), occurred.

Conclusions:

• Under projected future ocean conditions (high CO₂, nutrient limitation), *Trichodesmium* prioritizes organic nitrogen uptake over N₂ fixation.
• This metabolic shift reduces the input of new nitrogen into marine food webs.
• The findings suggest profound implications for future ocean nitrogen and carbon cycling, potentially altering ecosystem structure and function.