Jill Mikucki’s work on the geobiology of Blood Falls was published in Science last week, and since then, her article has received attention from the lay press as well. And rightly so, I think, because her work is fascinating stuff. A bacterial community thrives in a salty, iron-rich, anaerobic, ice-cold subglacial lake that is apparently completely isolated from Earth’s atmosphere! Waters in the lake do occasionally reach the surface through poorly understood subglacial fluid flows, and when they do, the iron-rich waters rapidly oxidize on exposure to air, forming blood-red mineral deposits. This outflow thus provides at least two things to this ecosystem: a cool name — Blood Falls — as well as a means for Mikucki to sample the otherwise inaccessible, isolated subglacial waters.
I am no chemical oceanographer, but if I understand her paper correctly, she and her co-workers are saying that the sulfate in the subglacial lake catalytically mediates the microbial reduction of triply-charged iron ions to doubly-charged iron ions. Mikucki and her co-workers think that the sulfate is reduced to sulfite or other intermediate-oxidation-state sulfur compounds, after which it gets re-oxidized back to sulfate, by transferring its newly acquired electrons to triply charged iron. This recycle keeps sulfate levels in the ecosystem constant, and its a very new idea; usually in anaerobic ecosystems, sulfate reduction goes all the way to hydrogen sulfide; sulfate is not regenerated.
The take-away is not just that life exists down there underneath the glaciers. The discovery of an extant, non-sulfidic, iron-rich microbial ecosystem based on sulfate cycling lends support to the idea that such conditions may have prevailed during Earth’s past, especially during proposed (and still controversial) Snowball Earth scenarios.
The big remaining question is, what’s the electron donor to the ecosystem? Mikucki’s earlier articles on the Blood Falls site contain some possible clues. First, the most prevalent microbes at the site are very closely related to Thiomicrospira arctica, a known CO2-eating “autotroph”. So it is likely that not all of the reducing power feeding this ecosystem is in the form of organic carbon. Dissolved organic carbon would be eventually be depleted anyway, if, as is believed, no new sources of DOC have ever fed in fresh carbon to the subglacial lake. Reduced sulfur compounds might be electron donors, but sulfides seemed impossible to detect in the Blood Falls waters. Karsten Pedersen and others have proposed that hydrogen gas drives a microbial ecosystem in the pore waters of hot subterranean granitic rocks. Perhaps hydrogen may turn out to be important in cold subglacial lakes too.