101
of Fe needed for nitrogenase has been a major theme in marine N 2 fixation research,
from global to cellular scales (Berman-Frank et al. 2001a; Kustka et al. 2002).
N 2 -fixers draw on an essentially unlimited supply of N 2 from the atmosphere, so
the ability to compete for non-N nutrients is important for determining the success
of N 2 -fixers and their activities in the environment (Sohm et al. 2011). However,
since N 2 fixation is energetically expensive in terms of ATP and reductant, it has
generally been assumed that the presence of fixed inorganic N compounds inhibits
N 2 fixation and that N 2 -fixing microorganisms are not ecologically competitive
under those conditions (Karl et al. 2002). However, fixing N 2 may not be much more
costly than using NO 3
−
(Gutschick 1978; Silsbury 1977) in terms of energetic
investment, and in recent years, evidence has been accumulating from culture work
and the environment that N 2 -fixing microorganisms can be present and active when
modest concentrations of fixed N in the form of NO 3
−
are present (Knapp 2012).
Some marine N 2 -fixing microorganisms such as Trichodesmium and Crocosphaera
can supplement their N needs by taking up dissolved inorganic N (Mulholland and
Capone 2000). It can be favorable for Trichodesmium growth to use fixed N
(Boatman et al. 2018). Recent modeling studies have also shown that it can be beneficial for heterotrophs to fix N 2 and use fixed inorganic N under some conditions
(Inomura et al. 2018). The UCYN-A symbiosis is found in many environments with
elevated NO 3
−
, such as coastal waters (Moisander et al. 2010; Moreira-Coello et al.
2017; Shiozaki et al. 2018).The UCYN-A symbiosis does not take up nitrate and is
found in waters containing higher concentrations of this nutrient than usually found
in regions where diazotrophs are abundant (Mills et al. 2020).
Sediments are another habitat where N 2 fixation occurs, but which often contain
high concentrations of dissolved ammonium (and sometimes nitrate near the oxic
layer) (Capone 1988). Diverse nifH genes have been detected in both bare and vegetated sediments and N 2 fixation appears to be due to anaerobic heterotrophic bacteria such as sulfate reducers whose nifH gene transcripts have been detected in
various sediment environments (see Chap. 4).
Phosphorus in the oceans is present as phosphate and organic P compounds,
including nucleic acids (Benitez-Nelson 2000; Karl 2014). Concentrations are very
low and difficult to measure in the oligotrophic oceans yet vary dramatically
between the Atlantic and Pacific Ocean basins. P form and concentration can be a
major factor determining the distribution of diazotrophs (Deutsch et al. 2007;
Sañudo-Wilhelmy et al. 2001) as well as non-diazotrophs (Martiny et al. 2006)
(Chaps. 7 and 8). The availability of P relative to N (N:P ratio) is important as well
as the absolute concentration of dissolved inorganic N or P compounds. It has been
assumed that diazotrophs have elevated P requirements compared to non- diazotrophs
that would preclude their co-existence under certain N:P ratios (Landolfi et al. 2015;
Tilman 1980; White et al. 2006). However, some marine diazotrophs have high
affinity phosphate transport systems, and enzymes for using organic phosphorus
that enable them to be competitive in oligotrophic oceans areas such as the
N. Atlantic with high N:P ratios and low P availability (Landolfi et al. 2005).
In addition to the availability of fixed N, dissolved Fe availability is key in constraining primary productivity in large regions of the ocean (Behrenfeld 1999;
6.4 Nutrients
of Fe needed for nitrogenase has been a major theme in marine N 2 fixation research,
from global to cellular scales (Berman-Frank et al. 2001a; Kustka et al. 2002).
N 2 -fixers draw on an essentially unlimited supply of N 2 from the atmosphere, so
the ability to compete for non-N nutrients is important for determining the success
of N 2 -fixers and their activities in the environment (Sohm et al. 2011). However,
since N 2 fixation is energetically expensive in terms of ATP and reductant, it has
generally been assumed that the presence of fixed inorganic N compounds inhibits
N 2 fixation and that N 2 -fixing microorganisms are not ecologically competitive
under those conditions (Karl et al. 2002). However, fixing N 2 may not be much more
costly than using NO 3
−
(Gutschick 1978; Silsbury 1977) in terms of energetic
investment, and in recent years, evidence has been accumulating from culture work
and the environment that N 2 -fixing microorganisms can be present and active when
modest concentrations of fixed N in the form of NO 3
−
are present (Knapp 2012).
Some marine N 2 -fixing microorganisms such as Trichodesmium and Crocosphaera
can supplement their N needs by taking up dissolved inorganic N (Mulholland and
Capone 2000). It can be favorable for Trichodesmium growth to use fixed N
(Boatman et al. 2018). Recent modeling studies have also shown that it can be beneficial for heterotrophs to fix N 2 and use fixed inorganic N under some conditions
(Inomura et al. 2018). The UCYN-A symbiosis is found in many environments with
elevated NO 3
−
, such as coastal waters (Moisander et al. 2010; Moreira-Coello et al.
2017; Shiozaki et al. 2018).The UCYN-A symbiosis does not take up nitrate and is
found in waters containing higher concentrations of this nutrient than usually found
in regions where diazotrophs are abundant (Mills et al. 2020).
Sediments are another habitat where N 2 fixation occurs, but which often contain
high concentrations of dissolved ammonium (and sometimes nitrate near the oxic
layer) (Capone 1988). Diverse nifH genes have been detected in both bare and vegetated sediments and N 2 fixation appears to be due to anaerobic heterotrophic bacteria such as sulfate reducers whose nifH gene transcripts have been detected in
various sediment environments (see Chap. 4).
Phosphorus in the oceans is present as phosphate and organic P compounds,
including nucleic acids (Benitez-Nelson 2000; Karl 2014). Concentrations are very
low and difficult to measure in the oligotrophic oceans yet vary dramatically
between the Atlantic and Pacific Ocean basins. P form and concentration can be a
major factor determining the distribution of diazotrophs (Deutsch et al. 2007;
Sañudo-Wilhelmy et al. 2001) as well as non-diazotrophs (Martiny et al. 2006)
(Chaps. 7 and 8). The availability of P relative to N (N:P ratio) is important as well
as the absolute concentration of dissolved inorganic N or P compounds. It has been
assumed that diazotrophs have elevated P requirements compared to non- diazotrophs
that would preclude their co-existence under certain N:P ratios (Landolfi et al. 2015;
Tilman 1980; White et al. 2006). However, some marine diazotrophs have high
affinity phosphate transport systems, and enzymes for using organic phosphorus
that enable them to be competitive in oligotrophic oceans areas such as the
N. Atlantic with high N:P ratios and low P availability (Landolfi et al. 2005).
In addition to the availability of fixed N, dissolved Fe availability is key in constraining primary productivity in large regions of the ocean (Behrenfeld 1999;
6.4 Nutrients