Moderate heating renders 7.8-million-year-old sedimentary organic matter bioavailable

Area of Science:

• Geochemistry
• Microbiology
• Organic Geochemistry

Background:

• Marine sediments store vast amounts of recalcitrant organic matter.
• Subsurface microbial life exists at extreme depths and temperatures.
• Mechanisms for supplying bioavailable substrates in these environments are poorly understood.

Purpose of the Study:

• To investigate the role of temperature on the transformation of dissolved organic matter (DOM) in marine sediments.
• To elucidate the pathways of bioavailable substrate generation under simulated burial conditions.

Main Methods:

• Incubation of 7.8-million-year-old marine sediment at temperatures ranging from 20°C to 85°C.
• Analysis of dissolved organic matter using 3D fluorescence spectroscopy and ultrahigh-resolution mass spectrometry.
• Monitoring of metal ion release and DOM bioavailability.

Main Results:

• At 35°C, humic-like DOM with low bioavailability was released.
• At 55°C, abiotic decomposition yielded smaller, more bioavailable DOM, enhancing fermentation.
• At 85°C, abiotic processes generated labile H2 and acetate from nitrogen-containing humic compounds, bypassing fermentation.

Conclusions:

• Abiotic thermal processes are crucial for activating refractory organic matter in marine sediments.
• Temperature-driven DOM transformation influences microbial activity and substrate availability.
• These findings advance understanding of long-term carbon sequestration and global carbon cycling.