Superoxide-Producing Fungus Sponges Up Mine Metals

Fungus, not bacteria, may be the star microbial agent that cleans up metals in some coal mine drainage systems, according to work from geochemists (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1203885109).

Colleen M. Hansel, a microbial geochemist at Harvard University and Woods Hole Oceanographic Institution, and colleagues report that a common ascomycete fungus found in mine drainage treatment systems promotes production of manganese oxides. These compounds are well-known environmental sponges that scavenge and sequester toxic metals such as lead, copper, and zinc.

Combining light microscopy with synchrotron-based X-ray absorption spectroscopy and fluorescence microscopy, the researchers showed that the fungus oxidizes soluble Mn(II) by producing superoxide extracellularly during asexual reproduction. This superoxide reacts with Mn(II), causing precipitation of brown Mn(III) and Mn(IV) oxides on the base of the fungus’ reproductive structures.

The work could help in bioremediating mine drainage, says Bradley M. Tebo, a microbiologist at Oregon Health & Science University.

The research began when Hansel teamed up with William Burgos, a Pennsylvania State University environmental engineer who was helping figure out why some mine treatment sites in central Pennsylvania worked and some didn’t, Hansel says. Some sites added corn cobs and straw to limestone treatment beds, creating an environment that may have selected for fungi, she says.

In the treatment beds where the researchers found the superoxide-producing fungi, Hansel says, “the water coming out the other side is as clean as you can get.”

Cara M. Santelli, a coauthor of the study who is now at the Smithsonian Institution, is researching what carbon and nutrient sources could encourage these fungi to oxidize Mn(II) more efficiently, Hansel says.

Hansel and colleagues have also found other types of fungi and a marine bacterium that oxidize Mn(II) by producing superoxide.