Inorganic N Assimilation and Ammonium Pools in a Deep-Sea Mussel Containing Methanotrophic Endosymbionts

Area of Science:

• Marine biology
• Biogeochemistry
• Microbial ecology

Background:

• Undescribed mussels (seep mytilid 1a) thrive near Gulf of Mexico hydrocarbon seeps.
• These mussels host methanotrophic endosymbionts and utilize high environmental ammonium and nitrate.

Purpose of the Study:

• Investigate nitrogen assimilation pathways in seep mussels.
• Determine if ammonium and nitrate uptake involves active mechanisms.

Main Methods:

• Enzyme activity measurements (glutamine synthetase, nitrate reductase).
• 15N tracer experiments using 15NH3 and 15NO3-.
• Quantification of internal nitrogen pools and assimilation rates.

Main Results:

• Glutamine synthetase activity was consistently detected.
• 15NH3 exposure led to high internal 15NH3 pools, indicating active uptake.
• 15NO3- exposure resulted in rapid reduction to 15NH3, which entered internal ammonium pools.
• Internal ammonium concentrations were high, inconsistent with depletion-diffusion models.

Conclusions:

• Seep mussels exhibit active ammonium uptake mechanisms.
• Nitrate assimilation involves rapid reduction and potential ammonium production via dissimilatory nitrate reduction.
• These pathways may be common in other chemosynthetic symbioses.