171
© 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_10
Chapter 10
Summary and Conclusions
10.1 Introduction
N 2 fixation plays a key role in the biogeochemical cycle of N in the marine environment just as it does in the terrestrial environment. N 2 fixation provides a direct
nutritional source of N for some microorganisms, termed diazotrophs, that have the
capability of reducing atmospheric N 2 to ammonia using the enzyme nitrogenase.
Given that the oceans cover so much of the Earth’s surface, marine BNF is an
important source of readily available N in Earth’s biome at large.
N 2 is abundant in the atmosphere and is very a stable molecule. Although the
reduction of N 2 to ammonia with H 2 is exergonic, it has a high activation energy.
Nitrogenase is a complex, highly regulated enzyme that involves a family of nif
genes that not only encode the subunits of the 2 proteins which catalyze N 2 fixation
but for multiple other processes including some that facilitate the assembly of the
enzyme, its regulation, transport and other genes involved with symbionts.
BNF is a metabolically expensive process that involves multiple structural and
other proteins that have been highly conserved through evolution. The nitrogenase
(nif) genes are widely distributed throughout the Bacteria and Archaea, but no
Eukaryotes can fix N 2 except in symbiosis with Bacteria or Archaea. Although
widely distributed, nif genes are scattered throughout the Bacterial and Archaeal
phylogenetic trees such that phylogenetic affiliation cannot generally predict diazotrophy. Closely related taxa may differ in whether or not they are diazotrophs. As
in soils, marine diazotrophs are physiologically diverse using many different types
of metabolisms, including phototrophy, chemolithotrophy and chemoheterotrophy.
In contrast to the terrestrial environment, symbiosis in the marine environment is
dominated by unicellular or filamentous microbial symbioses with unicellular plants
(planktonic algae), except in benthic environments, where symbiosis occurs with
diverse invertebrates, including corals and shipworms. The vast majority of marine
microbes, including the diazotrophs, are uncharacterized and very few representative forms have been brought into culture.
© 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_10
Chapter 10
Summary and Conclusions
10.1 Introduction
N 2 fixation plays a key role in the biogeochemical cycle of N in the marine environment just as it does in the terrestrial environment. N 2 fixation provides a direct
nutritional source of N for some microorganisms, termed diazotrophs, that have the
capability of reducing atmospheric N 2 to ammonia using the enzyme nitrogenase.
Given that the oceans cover so much of the Earth’s surface, marine BNF is an
important source of readily available N in Earth’s biome at large.
N 2 is abundant in the atmosphere and is very a stable molecule. Although the
reduction of N 2 to ammonia with H 2 is exergonic, it has a high activation energy.
Nitrogenase is a complex, highly regulated enzyme that involves a family of nif
genes that not only encode the subunits of the 2 proteins which catalyze N 2 fixation
but for multiple other processes including some that facilitate the assembly of the
enzyme, its regulation, transport and other genes involved with symbionts.
BNF is a metabolically expensive process that involves multiple structural and
other proteins that have been highly conserved through evolution. The nitrogenase
(nif) genes are widely distributed throughout the Bacteria and Archaea, but no
Eukaryotes can fix N 2 except in symbiosis with Bacteria or Archaea. Although
widely distributed, nif genes are scattered throughout the Bacterial and Archaeal
phylogenetic trees such that phylogenetic affiliation cannot generally predict diazotrophy. Closely related taxa may differ in whether or not they are diazotrophs. As
in soils, marine diazotrophs are physiologically diverse using many different types
of metabolisms, including phototrophy, chemolithotrophy and chemoheterotrophy.
In contrast to the terrestrial environment, symbiosis in the marine environment is
dominated by unicellular or filamentous microbial symbioses with unicellular plants
(planktonic algae), except in benthic environments, where symbiosis occurs with
diverse invertebrates, including corals and shipworms. The vast majority of marine
microbes, including the diazotrophs, are uncharacterized and very few representative forms have been brought into culture.