How much hydrogen is consumed by ocean-floor microbes?

Running beneath the Atlantic Ocean almost from pole to pole is a mountainous ridge about 2 kilometers high. The Mid-Atlantic Ridge and Earth’s other mid-ocean ridges form when abutting tectonic plates separate. Magma wells up through the gap, piles up, spreads out, and cools to form fresh rock, the ridge.

In the 1970s copious amounts of hydrogen gas were discovered in and around mid-ocean ridges. In the 2000s several lines of evidence pointed to the existence of hydrogen-eating microbes that live in rock beneath the seafloor. How large could the global population of those microbes be? To find out, Stacey Worman of Duke University and her colleagues recently conducted an inventory of all significant sources and sinks of H₂ at the bottom of the ocean.

Given that H₂ does not appear to accumulate in underground deposits, any excess production is plausibly being consumed by microbes. Worman and her colleagues identified 10 distinct sources and 9 distinct sinks and fed their estimates into a box model. Among the sources is the anaerobic oxidation of ferrous iron–rich ions to make greenish minerals known as serpentines (shown in the photo). The process contributes on the order of 10¹² moles of H₂ a year. Among the sinks is the escape of H₂ from the seafloor through hydrothermal vents, where deep-sea microbes rapidly consume it. That flux is also on the order of 10¹² mol H₂/y.

Much of the deep ocean remains unexplored. Given the paucity of field data, all estimates were crude, but they nevertheless shed light on the relative importance of the processes at play. Worman and her colleagues were also conservative in their approach. They derived a net difference between sources and sinks of about 2 × 10¹² mol H₂/y, which suggests that microbes are consuming at least 30% of total H₂ production. Such a vast amount suggests the microbes of the mid-ocean ridges are not only voracious en masse; they are also significant actors in the geochemistry of Earth’s ocean–atmosphere system. (S. L. Worman et al., Proc. Natl. Acad. Sci. USA, in press.)