9
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
J. P. Zehr, D. G. Capone, Marine Nitrogen Fixation,
https://doi.org/10.1007/978-3-030-67746-6_2
Chapter 2
Fundamentals of N 2 Fixation
2.1 Microbiology
Although there are no known eukaryotes capable of fixing N 2 , there are diverse freeliving and symbiotic Archaea and Bacteria that have nitrogenase genes and are
capable of fixing N 2 (Table 2.1) (Dixon and Kahn 2004; Leigh 2002). These microorganisms, also termed diazotrophs, span broad physiological groups and include
aerobes and anaerobes (De Bruijn 2015) (Table 2.1). N 2 -fixing microorganisms are
present and important in most ecosystems (Encarnación 2005). Much of what we
know about the microbiology, biochemistry and genetics was learned from decades
of research on diverse free-living and symbiotic terrestrial N 2 -fixing microorganisms because of its importance to agriculture. Most if not all phylogenetic groups
that contain N 2 -fixing species also have representative taxa in the marine environment (Zehr and Paerl 2008).
Diazotrophs have diverse physiologies ranging from chemoheterotrophy and
chemolithotrophy to photoheterotrophy and photoautotrophy (Dixon and Kahn
2004). Sources of energy and reductant and O 2 protection are particularly important
for N 2 -fixing microorganisms. Nitrogenase was first isolated from the anaerobe
Clostridium pasteurianum and subsequent early studies focused on the genetics of
Klebsiella pneumoniae and Azotobacter vinelandii (Eady et al. 1975; Fisher and
Newton 2002). N 2 fixation was then discovered to be associated with heterocysts,
the specialized N 2 -fixing cells in some cyanobacteria, and subsequently demonstrated in many unicellular and filamentous nonheterocyst-forming cyanobacteria
(Fay 1992) (see also Chap. 3).
N 2 fixation is ecologically advantageous and while no known eukaryotes can fix
N 2 , there are diverse symbiotic relationships in unicellular eukaryotes (protists) and
multicellular plants and animals, in both terrestrial and aquatic environments. N 2
fixation and its effects on crop fertility were first discovered in association with
plant roots and nodules (Elmerich 2007). Symbioses between bacteria and legumes
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
J. P. Zehr, D. G. Capone, Marine Nitrogen Fixation,
https://doi.org/10.1007/978-3-030-67746-6_2
Chapter 2
Fundamentals of N 2 Fixation
2.1 Microbiology
Although there are no known eukaryotes capable of fixing N 2 , there are diverse freeliving and symbiotic Archaea and Bacteria that have nitrogenase genes and are
capable of fixing N 2 (Table 2.1) (Dixon and Kahn 2004; Leigh 2002). These microorganisms, also termed diazotrophs, span broad physiological groups and include
aerobes and anaerobes (De Bruijn 2015) (Table 2.1). N 2 -fixing microorganisms are
present and important in most ecosystems (Encarnación 2005). Much of what we
know about the microbiology, biochemistry and genetics was learned from decades
of research on diverse free-living and symbiotic terrestrial N 2 -fixing microorganisms because of its importance to agriculture. Most if not all phylogenetic groups
that contain N 2 -fixing species also have representative taxa in the marine environment (Zehr and Paerl 2008).
Diazotrophs have diverse physiologies ranging from chemoheterotrophy and
chemolithotrophy to photoheterotrophy and photoautotrophy (Dixon and Kahn
2004). Sources of energy and reductant and O 2 protection are particularly important
for N 2 -fixing microorganisms. Nitrogenase was first isolated from the anaerobe
Clostridium pasteurianum and subsequent early studies focused on the genetics of
Klebsiella pneumoniae and Azotobacter vinelandii (Eady et al. 1975; Fisher and
Newton 2002). N 2 fixation was then discovered to be associated with heterocysts,
the specialized N 2 -fixing cells in some cyanobacteria, and subsequently demonstrated in many unicellular and filamentous nonheterocyst-forming cyanobacteria
(Fay 1992) (see also Chap. 3).
N 2 fixation is ecologically advantageous and while no known eukaryotes can fix
N 2 , there are diverse symbiotic relationships in unicellular eukaryotes (protists) and
multicellular plants and animals, in both terrestrial and aquatic environments. N 2
fixation and its effects on crop fertility were first discovered in association with
plant roots and nodules (Elmerich 2007). Symbioses between bacteria and legumes