Teacher resources and professional development across the curriculum

Teacher professional development and classroom resources across the curriculum

Monthly Update sign up
Mailing List signup
Search
Follow The Annenberg Learner on LinkedIn Follow The Annenberg Learner on Facebook Follow Annenberg Learner on Twitter
Rediscovering Biology Logo
Home
Online TextbookCase StudiesExpertsArchiveGlossarySearch
Online Textbook
Back to Unit Page
Unit Chapters
Genomics
Proteins & Proteomics
Evolution & Phylogenetics
Microbial Diversity
Introduction
Microbes as the First Organisms
The Diversity of Microbial Metabolism
Archaea and Bacteria
The Universal Tree of Life
Studying Unculturable Microbes with PCR
Microbes and the Carbon Cycle
Microbes and the Cycling of Nitrogen
Biofilms
Biofilms Formation and Bacterial Communication
Impact of Biofilms on Humans
Communication Between Bacteria and Eukaryotes
Microbes in Mines
Microbial Leaching of Ores
Coda
Emerging Infectious Diseases
HIV & AIDS
Genetics of Development
Cell Biology & Cancer
Human Evolution
Neurobiology
Biology of Sex & Gender
Biodiversity
Genetically Modified Organisms
Microbes in Mines

Pyrite (FeS2), otherwise known as "fool's gold," may not look like lunch to you but it does to the chemolithotrophic bacteria Acidithiobacillus ferrooxidans (formerly Thiobacillus ferrooxidans). These bacteria extract energy from the oxidation of ferrous ions (Fe2+) to ferric ions (Fe3+). Pyrite is one of the most common forms of iron in nature, and is very common in bituminous coals and in many ore bodies. When pyrite is exposed, as in a mining operation, it reacts with oxygen to generate ferrous ions, sulfate, and hydrogen ions.

2FeS2 + 7O2 + 2H2O ---> 2Fe2+ + 4SO42- + 4H+

Ferrous ions - lunch! And, the hydrogen ions generated in this reaction do not faze A. ferrooxidans. This acidophile prefers a pH below 3.5. It is able maintain a relatively neutral internal pH by actively pumping protons between the cytoplasm and external environment against a steep pH gradient.

Acid mine drainage, which causes serious ecological damage to rivers and lakes is in part a result of the presence of A. ferrooxidans. The ferric ions generated by the bacteria are soluble in the acid environment and easily react with additional pyrite.

FeS2 +14Fe3+ + 8H2O ---> 15Fe2+ + 2SO42- + 16H+

The additional acid formed from this reaction is just one of the resultant pollutants. The ferric ions (Fe2+) that are generated precipitate in a complex mineral called jarosite [HFe3(SO)4,(OH)6]. The unsightly stains in mine drainages, called "yellow boy" by U.S. miners, are jarosite.

Acid mine drainage and its associated pollution do not form unless pyrite is exposed to oxygen. Only upon mining does the initial reaction generating ferrous ions provide an environment in which A. ferrooxidans will thrive.

To reduce toxic metal content in acid mine drainage, scientists are turning to sulfate-reducing bacteria, which occur naturally in anoxic soils. These bacteria use sulfate as an electron acceptor instead of oxygen, in a form of metabolism known as anaerobic respiration. Hydrogen sulfide is generated in the process. At a bioremediation site in southeast Idaho Dan Kortansky and his colleagues set up a series of ponds separated by berms (embankments) of crushed limestone, straw, and manure. The goal was to convert the sulfate in the drainage to sulfide. This reacts with the dissolved metals to form metal sulfides, such as ferrous sulfide. The limestone in Kortansky's bioremediation system lowers the pH as metal-laden water passes through the berms and sulfate-reducing bacteria thrive. Results have been encouraging. Iron concentrations in the drainage at this site were reduced 65% and copper residues were reduced by nearly 100 percent.

Back Next


© Annenberg Foundation 2014. All rights reserved. Legal Policy