Download Nitrogen Fixation

Chemolithotrophy
Chemolithotrophs are organisms that oxidize an inorganic electron donor
They transport the electrons down a chain of carriers to form gradient that powers ATP synthesis
Dissimilative iron oxidizers are excellent examples of chemolithotrophs
Widely distributed, they oxidize Fe2+ to Fe3+ and harvest released energy
Acidithiobacillus ferredoxans is a good example of a dissimilative iron oxidizer
An acidophile, the Fe3+ it produces spontaneously converts to Fe(OH)3 an insoluble orange material
which is visible in many Pennsylvania streams
Nitrogen Fixation
Organisms that reduce N2 to R-NH2 are called diazotrophs. The reduction process is called nitrogen
fixation.
R-NH2 compounds are needed by cells (not waste products) so this is not a dissimilative process.
Cyanobacteria are free-living diazotrophs (oceans, streams, lakes, ponds).
Bradyrhizobia are symbiotic diazotrophs. They live inside plant roots and fix nitrogen for the plants. The
plants provide the bacteria with reduced carbon in return. Leguminous plants like soybeans house
bradyrhizobia.
Nitrogenase is the name of the enzyme that actually fixes nitrogen. It is another one of those
temperamental “diva” enzymes and does not work well with oxygen around.
To maintain low oxygen and keep nitrogenase happy….
Cyanobacteria fix nitrogen only in special cells called heterocysts, which have extra thick walls to keep
oxygen out.
Plants with bradyrhizobia grow special organs called root nodules where the bacteria live. The plants
maintain low oxygen in the nodules using a special protein named leghemoglogin (related to human
hemoglobin) that removes oxygen from the nodule and allows nitrogen fixation to occur.