Flash heating boosts the potential for mechanochemical energy sources for subglacial ecosystems

Subglacial environments harbour a diversity of microbial ecosystems capable of influencing biogeochemical cycles. However, the darkness and isolation of subglacial environments limit the energy sources available for microbial metabolism. A recently recognised energy source for these microbes in wet-based regions is the rock-water reactions that occur after the mechanical fracturing of glacial bedrock. These mechanochemical reactions produce H 2 and H 2 O 2 at 0°C from reactions with mineral surface defects (Si• and SiO•) and release Fe from within the mineral structures, providing electron donors and acceptors for microbial metabolism. However, the production of H 2 O 2 and H 2 may be underestimated as temperatures at rock abrasion sites can increase substantially above 0°C as glaciers “slip and grind” rocks, potentially accelerating the rates of mechanochemical reactions. Despite this, the effect of rapid heating on subsequent low-temperature mechanochemical reactions has yet to be examined. Here, we investigate H 2 , H 2 O 2 , and Fe production during low-temperature (0 °C) incubations of water with a range of ground rocks and minerals following “flash heating” to 30, 60, or 121 °C. We show that transient increases (as little as 5–10 min of heating) to moderate temperatures (30 or 60 °C) can significantly increase the rate of H 2 production, while short-term heating to 121 °C generates larger bursts of H 2 . In addition, pyrite is easily crushed, potentially releasing large quantities of Fe 2+ into subglacial systems and promoting mechanochemical reactions due to the resulting large surface area (10× larger than other materials). We provide the first evidence for H 2 production from water reactions with crushed pyrite and suggest that crushed pyrite has a greater influence on subglacial H 2 O 2 production than silicates. We conclude that electron donors in the form of Fe 2+ and H 2 bursts can be produced in subglacial ecosystems, which may be coupled to substantial concentrations of H 2 O 2 produced from crushed pyrite. This suggests that rock–water mechanochemical reactions may be a greater source of energy for subglacial environments than previously recognised.