49
The specific organisms driving potential heterotrophic N 2 fixation in the oceans
are largely unknown. nifH gene sequences from PCR amplicons and from metagenomic surveys show a great diversity of non-cyanobacterial diazotrophs in marine
environments, including taxa from the proteobacteria, archaea, and planctomycetes
(Delmont et al. 2018; Farnelid et al. 2011; Zehr et al. 1998a). However, very few of
these organisms have been isolated in culture. In most cases, even the genomic content of putative heterotrophic N 2 fixers is unknown, although metagenome amplified
genomes show promise as way to obtain the genomes of heterotrophic diazotrophs
(Delmont et al. 2018).
Despite this lack of knowledge, certain heterotrophic nifH sequence types do
appear widely distributed in the oceans. For example, the nifH genes and transcripts
belonging to “Gamma-A” a gamma-proteobacterial diazotroph, have been detected
in surface waters throughout the tropics and subtropics (Cornejo-Castillo and Zehr
2020; Langlois et al. 2015). The apparent restriction of Gamma-A to the euphotic
zone and to the large (>3 μm) size fraction suggests that Gamma-A may attach to
phytoplankton or other particles, be filamentous, and/or be capable of photoheterotrophy. Meanwhile, Gamma-A has not been cultivated or definitively shown to fix N 2
in the marine environment. Another heterotrophic marine diazotroph, the alphaproteobacterium S. castanea, has been detected in numerous studies sampling at or
near OMZs (Fernandez et al. 2015; Gradoville et al. 2017; Shiozaki et al. 2014;
Zhang et al. 2011) and was recently cultivated (Martínez-Pérez et al. 2018); however, single-cell analyses detected no N 2 fixation by this organism in its natural
environment (Martínez-Pérez et al. 2018). A new group of heterotrophic hydrocarbon degrading diazotrophs have been recently discovered within the gammaproteobacteria (Karthikeyan et al. 2019).
4.2 Oxygen Deficient Zones (ODZs)
Modeling studies have suggested that low O 2 regions may be important in conditioning upwelled surface waters to promote N 2 fixation (Deutsch et al. 2007).
Researchers have also looked directly within ODZs for the presence of N 2 fixation
as the reduced concentrations of O 2 and combined N would also make a suitable
habitat for some diazotrophs. Significant but low rates of N 2 fixation have been
reported in several hypoxic and anoxic marine environments typically well below
the euphotic zone, along with evidence of genes of heterotrophic diazotrophs
(Bonnet et al. 2013; Farnelid et al. 2013; Hamersley et al. 2011; Löscher et al.
2014). In contrast, two very recent studies reported no N 2 fixation in the Eastern
Tropical North Pacific (ETSP) ODZ (Selden et al. 2019) despite the presence of
nifH sequences of putative heterotrophs (Jayakumar et al. 2017). The presence of
measured N 2 fixation in low O 2 waters extends the habitats known to be important
for N 2 fixation in the marine environment and potentially a role for heterotrophic
bacteria.
4.2 Oxygen Deficient Zones (ODZs)
The specific organisms driving potential heterotrophic N 2 fixation in the oceans
are largely unknown. nifH gene sequences from PCR amplicons and from metagenomic surveys show a great diversity of non-cyanobacterial diazotrophs in marine
environments, including taxa from the proteobacteria, archaea, and planctomycetes
(Delmont et al. 2018; Farnelid et al. 2011; Zehr et al. 1998a). However, very few of
these organisms have been isolated in culture. In most cases, even the genomic content of putative heterotrophic N 2 fixers is unknown, although metagenome amplified
genomes show promise as way to obtain the genomes of heterotrophic diazotrophs
(Delmont et al. 2018).
Despite this lack of knowledge, certain heterotrophic nifH sequence types do
appear widely distributed in the oceans. For example, the nifH genes and transcripts
belonging to “Gamma-A” a gamma-proteobacterial diazotroph, have been detected
in surface waters throughout the tropics and subtropics (Cornejo-Castillo and Zehr
2020; Langlois et al. 2015). The apparent restriction of Gamma-A to the euphotic
zone and to the large (>3 μm) size fraction suggests that Gamma-A may attach to
phytoplankton or other particles, be filamentous, and/or be capable of photoheterotrophy. Meanwhile, Gamma-A has not been cultivated or definitively shown to fix N 2
in the marine environment. Another heterotrophic marine diazotroph, the alphaproteobacterium S. castanea, has been detected in numerous studies sampling at or
near OMZs (Fernandez et al. 2015; Gradoville et al. 2017; Shiozaki et al. 2014;
Zhang et al. 2011) and was recently cultivated (Martínez-Pérez et al. 2018); however, single-cell analyses detected no N 2 fixation by this organism in its natural
environment (Martínez-Pérez et al. 2018). A new group of heterotrophic hydrocarbon degrading diazotrophs have been recently discovered within the gammaproteobacteria (Karthikeyan et al. 2019).
4.2 Oxygen Deficient Zones (ODZs)
Modeling studies have suggested that low O 2 regions may be important in conditioning upwelled surface waters to promote N 2 fixation (Deutsch et al. 2007).
Researchers have also looked directly within ODZs for the presence of N 2 fixation
as the reduced concentrations of O 2 and combined N would also make a suitable
habitat for some diazotrophs. Significant but low rates of N 2 fixation have been
reported in several hypoxic and anoxic marine environments typically well below
the euphotic zone, along with evidence of genes of heterotrophic diazotrophs
(Bonnet et al. 2013; Farnelid et al. 2013; Hamersley et al. 2011; Löscher et al.
2014). In contrast, two very recent studies reported no N 2 fixation in the Eastern
Tropical North Pacific (ETSP) ODZ (Selden et al. 2019) despite the presence of
nifH sequences of putative heterotrophs (Jayakumar et al. 2017). The presence of
measured N 2 fixation in low O 2 waters extends the habitats known to be important
for N 2 fixation in the marine environment and potentially a role for heterotrophic
bacteria.
4.2 Oxygen Deficient Zones (ODZs)