3.13 Catabolic Diversity Chemolithotrophy Uses inorganic chemicals as electron donors Examples include hydrogen sulfide (H2S), hydrogen gas (H2), ferrous iron (Fe2+), ammonia (NH3) Begins with oxidation of inorganic electron donor Uses an electron transport chain and transmembrane ion gradient

Dissimilative Iron Oxidizers are chemolithotrophs (13.9 and 14.15) Oxidize Fe2+ to Fe3+ Very widely distributed in many environments where Fe2+ is available Autotrophic or heterotrophic Aerobic or anaerobic Archaea or Bacteria

Acidithiobacillus ferrooxidans is a representative iron oxidizer Acidophile at pH 2-3 Acid environments with Fe2+ Fe2+ -> Fe3+ -> FeOH3

Figure 13.24 (pH 2) (pH 6) + ATP ATP e– e– Out Outer membrane Electron transport generates proton motive force. Rusticyanin e– Periplasm Reverse e– flow e– NAD+ Q cyt bc1 cyt c cyt aa3 Figure 13.24 Electron flow during Fe2+ oxidation by the acidophile Acidithiobacillus ferrooxidans. In (pH 6) + ATP NADH Cell material ADP ATP Figure 13.24

Figure 13.23 Iron-oxidizing bacteria.

3.17 Nitrogen Fixation (Sec 3.17 also pp 438-439) Living systems require nitrogen in the form of NH3 or R-NH2 “Fixed” or “reduced” nitrogen, not N2 Numerous types of prokaryotes can fix atmospheric nitrogen: Diazotrophs Assimilative not dissimilative

3.17 Nitrogen Fixation Energetically expensive (8 ATP per N atom) Requires electron donor, often pyruvate Reaction is catalyzed by nitrogenase Sensitive to the presence of oxygen Fe plus various metal cofactors Can catalyze a variety of reactions

3.17 Nitrogen Fixation Cyanobacteria most important nitrogen-fixers at ecological level Numerous types and habitats: terrestrial, aquatic, marine Can be unicellular or colonial Most species free-living But some cyanobacteria are symbionts

3.17 Nitrogen Fixation Some nitrogen fixers are symbiotic Rhizobia provide best example Bradyrhizobium japonicum colonizes soybean roots Induces plant to form specialized structures called root nodules Leghemoglobin

Rhizobium radiobacter aka Agrobacterium radiobacter aka Agrobacterium tumefaciens is another important member but not a diazotroph Soil bacterium that causes plant tumors called crown galls Agro uses chemotaxis to accumulate around plant roots Transfers some of its DNA (called T-DNA) into plant genome In plant cell T-DNA causes synthesis of plant hormones and other compounds not usually found in plants