A cross-section of Blood Falls showing how microbial communities survive without light or oxygen. Image courtesy of Zina Deretsky / NSF.
In Antarctica, a stream of red brine leaks through the Taylor Glacier into Lake Bonney. Known as Blood Falls, this mixture of ice, salt crystals, and rust flowing from a subglacial ecosystem 400 meters below the glacier has been the center of much scientific fascination.
This unique subglacial ecosystem, estimated to be 1-1.5 million years old, provides a modern living analogue to the ecosystems of the Earth over 530 million years ago when the Earth’s ocean had similar chemistry.
Using Blood Falls as a model, Dartmouth earth sciences professor Jill Mikucki has developed a unique theory describing how the metabolic systems of organisms in primitive Earth make life possible. The finding was published last month in Science.
The red color of Blood Falls comes from iron that oxidizes upon contact with the oxygen in the air. The source of the brine comes from an oxygen-lacking, highly saline, and completely dark ecosystem. Though life seems hard to sustain in those harsh conditions, it is an ecosystem that supports various microbial organisms.
Usually, anaerobic organisms use sulfate or iron as the final electron acceptor in lieu of oxygen. Sulfate reduces to hydrogen sulfide while Fe (III) reduces to Fe (II). Both sulfate and iron are known to be present in this ecosystem. The chemical composition of marine water makes it likely to be a source for sulfur compounds but not for iron. Mikucki predicts that the bedrock upon which the subglacial ecosystem rests serves as the source for iron.
The main mystery comes from the fact that hydrogen sulfide and Fe (III) typically react to form iron sulfides, such as pyrite; one consequence would be the depletion of Fe (III) so that none could accumulate and no Fe (II) would have the opportunity to form at the surface. Yet plenty of iron flows out of Blood Falls, evidenced both visually and chemically.
To elucidate this phenomenon, Mikucki used isotopes of sulfur and oxygen to track the reactants and products of sulfur compounds in the microbial metabolism. The findings lead her to believe that instead of reducing sulfate to hydrogen sulfide like every other known organism, these microbes cut the chain reaction short.
Mikucki hypothesizes that the reduction of sulfate to sulfite creates a catalytic cycle that facilitate the reduction of Fe (III) to Fe (II) in some modified method of metabolism.
A study of the microbes’ phylogenetic tree shows many genetic similarities with other anaerobic microorganisms that have sulfate and iron reducing enzymes, suggesting a shared evolutionary descent.
Mikucki states, “It’s absolutely incredible how life finds creative pathways to grow even in the most extreme conditions.”