Marine sediments represent the largest reservoir of organic carbon on Earth. For a long time, it was believed that the organic matter in these deep sediments was “refractory” and stable, meaning it was resistant to biological decomposition and contributed to long-term sequestration. carbon. However discovery of the deep biosphere – an ecosystem of microorganisms that survive at depths of up to 2.5 kilometers below the seabed and in temperatures of up to 120°C – has challenged this traditional view. New research sheds light on the mechanisms by which this once-unavailable carbon can become biologically active, with major implications for the global carbon cycle and climate change.

Study examined 7.8 million-year-old sediment from IODP site C0012 in the Nankai Trough, Japan, incubated at four different temperatures (20°C, 35°C, 55°C, and 85°C) to simulate burial temperatures. The findings revealed that moderate warming significantly affects the bioavailability of sedimentary organic matter:

• At 35°C Humic acid was released together with metal ions, but with low bioavailability. At this stage, protein DOM degradation dominated over its production.
• At 55°C abiotic decomposition of humic compounds led to the formation of smaller and more bioavailable dissolved organic matter (DOM), which supported fermentation. It was shown that thermal decomposition of humic DOM at 55 °C can provide suitable substrates for fermenters, compensating for the decrease in biotic hydrolysis at this temperature.
• At 85°C The decomposition of large nitrogen-containing humic compounds occurred, producing labile hydrogen (H2) and acetate. These processes were predominantly abiotic and bypassed fermentation. The abiotic production of H2 and acetate was so rapid that it led to their accumulation, surpassing microbial consumption.

These findings suggest that abiotic thermal processes activate a reservoir of refractory organic matterThe change in the reactivity of sedimentary organic matter, driven by temperature, rather than its age, is key to understanding its availability.

The most significant consequence of this process is reversing key carbon pumps:

• Mineral carbon pump, which normally stabilizes organic matter in sediments, can reverse at temperatures as low as 35 °CThis leads to the release of manganese, humic and protein DOM, and ammonia, increasing their availability to microbes.
• Microbial carbon pump, which usually converts labile DOM to refractory, is also reverses at temperatures above 55 °C. Thermal decomposition of refractory organic matter abiotically transforms it back into more biologically available DOM, facilitating its utilization by the deep biosphere. At 85 °C, these processes are significantly accelerated.

The consequences for climate change are significantIf long-term sequestered carbon in marine sediments becomes bioavailable due to rising temperatures (e.g., through gradual burial of sediments or regional warming), this could lead to releasing this carbon back into the Earth's active carbon cycle. Although some of the carbon may also be transformed into more resistant aromatic compounds, the primary mechanism is to increase bioavailability. This dynamic behavior of deep-seated carbon changes our understanding of global carbon fluxes and highlights the complex nature of the carbon cycle in deep subsurface environments, which is distinct from surface processes. The potential release of this carbon could have long-term impacts on the greenhouse gas balance and the planet's overall climate. JRi

The findings, published in the journal Science Advances