103
The availability of other elements can also be important for N 2 -fixing microorganisms (Morel and Price 2003). Although Mo concentrations in seawater are not
extremely low, it had been hypothesized that Mo availability actually limits N 2 -
fixers in coastal waters because of a competitive interaction between sulfate and Mo
by transporters of Mo and perhaps additional enzymes (Howarth and Cole 1985).
However, this has been controversial and there is evidence that challenges that
hypothesis (Paerl et al. 1987; Ter Steeg et al. 1986). Marino and Howarth (2016)
summarized their multi-decade studies of lakes and estuaries regarding limiting factors for N 2 fixation. The aggregate conclusion from their studies was that they could
not confirm this hypothesis, but that several enzyme systems were involved which
were expressed as a function of sulfate concentration (Marino and Howarth 2016).
Alternative nitrogenases use V or Fe instead of Mo, but V concentrations are also
low. Some studies have recently shown that there are environments where V nitrogenases may be significant in the marine environment (McRose et al. 2017). Nickel
(Ni) is used in hydrogenase, an enzyme that is important for N 2 -fixing microorganisms to recycle the H 2 evolved by nitrogenase. In one study, growth and N 2 fixation
of Trichodesmium relied on adequate Ni (Rodriguez and Ho 2014).
6.5 Dissolved Organic Matter
Dissolved organic matter (DOM) is a critical source of energy in the marine environment from the surface to deep ocean and in sediments, but still remains poorly
understood (Church et al. 2002; Hansell and Carlson 2015). DOM concentrations
are higher than particulate matter concentrations, but comprised of diverse, poorly
characterized compounds. Concentrations and fluxes vary between the surface and
deep ocean and in sediments. Much of DOM is recalcitrant and not readily degraded
by microbial metabolism. However, labile or semi-labile organic matter provides
the energy for heterotrophic metabolism in sediments and surface waters
(Kujawinski 2011).
DOM has long been known to be linked with N 2 fixation in marine waters (Paerl
et al. 1987), and although difficult to study, new methods are enabling detailed characterization of chemical compounds and their relationship to microbial diversity
(Kujawinski 2011). Some organic matter is excreted by diazotrophs, releasing fixed
N to the environment (Glibert and Bronk 1994; Capone et al. 1994). Organic matter
is an important source of energy and reductant needed to fuel the nitrogenase reaction, particularly for heterotrophs. However, DOM can also be utilized by the cyanobacterium Trichodesmium, either directly or indirectly from epibiotic bacteria
(Benavides et al. 2017) and stimulates gene expression (Benavides et al. 2020).
DOM additions have also stimulated UCYN-A growth rates (Moisander et al.
2011). DOM additions in surface and mesopelagic waters sometimes stimulated N 2
fixation (Benavides et al. 2015; Dekaezemacker et al. 2013), but this is not always
observed (Benavides et al. 2018). Similarly, a wide range of organic compounds
have been shown to stimulate N 2 fixation in marine sediments (Capone 1988; O’Neil
6.5 Dissolved Organic Matter
The availability of other elements can also be important for N 2 -fixing microorganisms (Morel and Price 2003). Although Mo concentrations in seawater are not
extremely low, it had been hypothesized that Mo availability actually limits N 2 -
fixers in coastal waters because of a competitive interaction between sulfate and Mo
by transporters of Mo and perhaps additional enzymes (Howarth and Cole 1985).
However, this has been controversial and there is evidence that challenges that
hypothesis (Paerl et al. 1987; Ter Steeg et al. 1986). Marino and Howarth (2016)
summarized their multi-decade studies of lakes and estuaries regarding limiting factors for N 2 fixation. The aggregate conclusion from their studies was that they could
not confirm this hypothesis, but that several enzyme systems were involved which
were expressed as a function of sulfate concentration (Marino and Howarth 2016).
Alternative nitrogenases use V or Fe instead of Mo, but V concentrations are also
low. Some studies have recently shown that there are environments where V nitrogenases may be significant in the marine environment (McRose et al. 2017). Nickel
(Ni) is used in hydrogenase, an enzyme that is important for N 2 -fixing microorganisms to recycle the H 2 evolved by nitrogenase. In one study, growth and N 2 fixation
of Trichodesmium relied on adequate Ni (Rodriguez and Ho 2014).
6.5 Dissolved Organic Matter
Dissolved organic matter (DOM) is a critical source of energy in the marine environment from the surface to deep ocean and in sediments, but still remains poorly
understood (Church et al. 2002; Hansell and Carlson 2015). DOM concentrations
are higher than particulate matter concentrations, but comprised of diverse, poorly
characterized compounds. Concentrations and fluxes vary between the surface and
deep ocean and in sediments. Much of DOM is recalcitrant and not readily degraded
by microbial metabolism. However, labile or semi-labile organic matter provides
the energy for heterotrophic metabolism in sediments and surface waters
(Kujawinski 2011).
DOM has long been known to be linked with N 2 fixation in marine waters (Paerl
et al. 1987), and although difficult to study, new methods are enabling detailed characterization of chemical compounds and their relationship to microbial diversity
(Kujawinski 2011). Some organic matter is excreted by diazotrophs, releasing fixed
N to the environment (Glibert and Bronk 1994; Capone et al. 1994). Organic matter
is an important source of energy and reductant needed to fuel the nitrogenase reaction, particularly for heterotrophs. However, DOM can also be utilized by the cyanobacterium Trichodesmium, either directly or indirectly from epibiotic bacteria
(Benavides et al. 2017) and stimulates gene expression (Benavides et al. 2020).
DOM additions have also stimulated UCYN-A growth rates (Moisander et al.
2011). DOM additions in surface and mesopelagic waters sometimes stimulated N 2
fixation (Benavides et al. 2015; Dekaezemacker et al. 2013), but this is not always
observed (Benavides et al. 2018). Similarly, a wide range of organic compounds
have been shown to stimulate N 2 fixation in marine sediments (Capone 1988; O’Neil
6.5 Dissolved Organic Matter