Multiple Pathways Drive CH₄ and N₂O Emissions in Coastal Macroalgal Ecosystems

Area of Science:

• Marine Ecology
• Biogeochemistry
• Climate Science

Background:

• Coastal macroalgal ecosystems are known for carbon sequestration.
• Their role as sources of potent greenhouse gases, methane (CH4) and nitrous oxide (N2O), is poorly understood.
• Green tides, dominated by Ulva prolifera, present a unique case study in the Yellow Sea.

Purpose of the Study:

• To investigate the emission of methane (CH4) and nitrous oxide (N2O) from macroalgal ecosystems, specifically during Ulva prolifera blooms.
• To elucidate the mechanisms and pathways driving CH4 and N2O production under varying dissolved oxygen (DO) conditions.
• To assess the contribution of macroalgal blooms to non-CO2 greenhouse gas fluxes.

Main Methods:

• Field measurements of CH4 and N2O concentrations in surface waters dominated by Ulva prolifera.
• Laboratory cultivation experiments to study CH4 release dynamics during algal growth and decay stages.
• Isotopic tracing experiments to identify pathways of N2O production (ammonia oxidation and denitrification).

Main Results:

• Waters with Ulva prolifera showed significantly higher CH4 (346%) and N2O (147%) concentrations compared to non-macroalgal waters, even with high DO levels (>6.0 mg/L).
• CH4 release occurred in two stages: algal photosynthesis/ROS during growth (DO > 5.0 mg/L) and increased microbial degradation of precursors during decay (DO > 4.0 mg/L).
• Methanogenic archaea were found in anoxic micro-niches within the macroalgal matrix.
• N2O production was linked to ammonia oxidation and denitrification during growth, decreasing with nitrogen depletion during decay.

Conclusions:

• Coastal macroalgal ecosystems, particularly during blooms, are significant sources of methane (CH4) and nitrous oxide (N2O).
• Multiple aerobic pathways contribute to CH4 and N2O emissions, driven by both algal and microbial processes.
• These previously unrecognized greenhouse gas fluxes must be considered alongside carbon sequestration when evaluating the climate impact of macroalgae.