Table 1 Free-living and symbiotic nitrogen fixing bacteria

Methanosarcinales

Methanosarcina sp.

n.d.

n.d.

---

[18]

Methanobacteriales

Methanothermobacter sp.

n.d.

n.d.

---

[19,20]

Methanobacterium sp.

Cyanobacteria

Nostoc sp.

Nostoc sp.

Bryophytes (e.g. hornworts)

extracellular (within cavities of the gametophyte)

[79]

Anabaena sp.

Anabaena sp.

Pteridophytes (Azolla)

extracellular (within cavities of the dorsal leaves)

[80,85,88]

Trichodesmium sp.

Cyanothece sp.

Gymnosperms (cycads)

extracellular (within coralloid roots)

[81]

Synechococcus sp.

Angiosperms (Gunnera)

intracellular (within cells of the stem gland)

[87]

Cyanothece sp.

Fungi (cyanolichens)

extracellular (in cephalodia or in the thallus)

[50,51]

Diatoms (R. gibba)

intracellular

[97,102]

Sponges (Dysidea spp.)

extracellular

[30,31]

Actinobacteria

Frankia sp.

Frankia sp.

Actinorhizal plants

root-nodules

[57]

Proteobacteria

α: Sinorhizobium sp., Mesorhizobium sp.

Bradyrhizobium sp.

Legumes

intracellular (in root-nodules)

[63,64,68]

β: Azoarcus sp., Burkholderia sp.

Rhizobium sp.

Legumes

intracellular (in root-nodules)

γ: Azotobacter sp., Pseudomonas sp.

Sinorhizobium sp.

Legumes

intracellular (in root-nodules)

Klebsiella pneumoniae, Erwinia sp.

Azorhizobium sp.

Legumes

intracellular (in root-nodules)

δ: Gloeobacter sp., Desulfovibrio sp.

Burkholderia sp.

AM fungi

intracellular

[54]

Azospirillum sp.

Grasses/nonleguminous crops

extracellular (w/o nodulation)

[76]

Azoarcus sp.

Grasses/nonleguminous crops

inter- and intracellular (w/o nodulation)

[77]

Klebsiella pneumoniae

nonleguminous crops

extracellular

[107]

Firmicutes (Clostridia)

Clostridium sp.

n.d.

n.d.

---

[108]

Bacteroidetes/Chlorobiales

Chlorobium sp.

n.d.

n.d.

---

[109]

Spirochaetales

Treponema sp.

Treponema ZAS-9

Termites

extracellular (in the hindgut)

[43]

Chloroflexi

Dehalococcoides sp.

n.d.

n.d.

---

[110]