10
have been well-studied, but there are many symbioses between bacteria and other
plants, that range from endosymbiotic to epibiotic or loosely associated with plant
roots (Fisher and Newton 2002). Bacteria have been isolated from a variety of plant
roots and soils for over a hundred years (Elmerich 2007). These include species in
the genera Clostridium, Azotobacter, Klebsiella, Aerobacter, Bacillus,
Rhodospirillum and the cyanobacterial genus Nostoc.
Some bacterial genera are “associative” with plant roots since they do not lead to
the development of specialized structures or invade the plant tissues (Elmerich
2007). However, other bacteria were shown to be fixing N 2 within the plant root tissues (endophytes) (Elmerich 2007). Endophytes have been found in legumes, but
Table 2.1 Examples of typical terrestrial and aquatic N 2 -fixing genera, showing diversity of
metabolism and phylogeny
Metabolism
Genus
Phylogeny
Strict anaerobes
Clostridium
Firmicutes
Desulfovibrio
Delta proteobacteria
Desulfotomaculum
Firmicutes
Phototrophic anaerobes
Chromatium
Gamma proteobacteria
Chlorobium
Bacteriodetes/Chlorobi
Thiopedia
Gamma proteobacteria
Ectothiorhodospira
Gamma proteobacteria
Facultative heterotrophs
Klebsiella
Gamma proteobacteria
Bacillus
Firmicutes
Citrobacter
Gamma proteobacteria
Enterobacter
Gamma proteobacteria
Propionibacterium
Actinobacteria
Microaerophiles
Mycobacterium
Actinobacteria
Thiobacillus
Beta proteobacteria
Spirillum
Beta proteobacteria
Aquaspirillum
Beta proteobacteria
Methanosinus
Rhizobium
Alpha proteobacteria
Microaerophilic phototrophs
Lyngbya
Cyanobacteria
Oscillatoria
Cyanobacteria
Plectonema
Cyanobacteria
Spirulina
Cyanobacteria
Aerobic heterotrophs
Azotobacter
Gamma proteobacteria
Azotococcus
Gamma proteobacteria
Azomonas
Gamma proteobacteria
Beijerinckia
Alpha proteobacteria
Derxia
Beta proteobacteria
Aerobic phototrophs
Dolichospermum (Anabaena)
Cyanobacteria
Calothrix
Cyanobacteria
Gloeocapsa
Cyanobacteria
Nostoc
Cyanobacteria
2 Fundamentals of N 2 Fixation