53
benthic Sargassum (Raut et al. 2018) as well as the giant kelp, Macrocystis pyrifera
(Hamersley et al. 2015).
While one earlier study reported diazotrophic isolates from a saltmarsh that
apparently utilized alternative nitrogenases based on growth in the absence of Mo,
and the conversion of acetylene to ethane (a diagnostic test for some alternative
nitrogenases, see Chap. 5) (Tibbles and Rawlings 1994), a more recent study has
more definitively identified genes for alternative nitrogenases in Everglades sediments and Sippewisett marsh sediments (McRose et al. 2017).
Deep Sea Benthos
The earliest study considering deep sea sediments was that of Hartwig and Stanley
(1978) who retrieved deep sea sediments and performed C 2 H 2 reduction assays on
deck and finding very low to unmeasureable rates. The potential for N 2 fixation in
the deep benthos was renewed by the detection of nif genes at hydrothermal vents
(Mehta et al. 2003) followed by cultivation of a thermophilic isolate (Mehta and
Baross 2006). Diazotrophy was found to occur in the unique methanogen-sulfate
respiring associations discovered at a range of deep hydrocarbon seeps (Dang et al.
2009; Miyazaki et al. 2009). N 2 fixation was demonstrated in these habitats using
direct isotope uptake methods in tandem with direct visualization of uptake with
nanoscale secondary ion mass spectrometry (nanoSIMS) (Dekas et al. 2009, 2014,
2016), as well as at “whale-falls” (deep environments with the decaying carcass of
a whale (Dekas et al. 2018a; Kapili et al. 2019). Diazotrophy in the organic and
energy rich hydrocarbon seep environments appears to be mainly associated with
the methanotroph partner of the broadly distributed anaerobic methane oxidizing
(ANME) consortia of a methanotroph and sulfate-respiring bacteria (in which the
oxidation of methane occurs by reverse methanogenesis by the methanotroph partner) (Boetius et al. 2000; Orphan et al. 2001). This dependency was not observed in
the ‘whale-fall” environments suggesting other metabolic pathways support
observed diazotrophy.
More recent studies have considered N 2 fixation in deep-sea sediments outside of
seeps/vents/whale falls (Dang et al. 2009; Dekas et al. 2018b; Kapili et al. 2020).
Sulfur-oxidizing bacteria symbiotic with deep vent animals have also been implicated in diazotrophy, but with only indirect evidence to date (Rau 1981).
4.5 Conclusions
Our knowledge of the range of habitats, diversity and physiologies of marine diazotrophs has expanded greatly over the last several decades. We now know that active
diazotrophy occurs in cold waters of the deep sea as well as at higher latitudes in the
Arctic. New marine niches for N 2 fixation are bound to be identified in the future.
4.4 The Benthos
benthic Sargassum (Raut et al. 2018) as well as the giant kelp, Macrocystis pyrifera
(Hamersley et al. 2015).
While one earlier study reported diazotrophic isolates from a saltmarsh that
apparently utilized alternative nitrogenases based on growth in the absence of Mo,
and the conversion of acetylene to ethane (a diagnostic test for some alternative
nitrogenases, see Chap. 5) (Tibbles and Rawlings 1994), a more recent study has
more definitively identified genes for alternative nitrogenases in Everglades sediments and Sippewisett marsh sediments (McRose et al. 2017).
Deep Sea Benthos
The earliest study considering deep sea sediments was that of Hartwig and Stanley
(1978) who retrieved deep sea sediments and performed C 2 H 2 reduction assays on
deck and finding very low to unmeasureable rates. The potential for N 2 fixation in
the deep benthos was renewed by the detection of nif genes at hydrothermal vents
(Mehta et al. 2003) followed by cultivation of a thermophilic isolate (Mehta and
Baross 2006). Diazotrophy was found to occur in the unique methanogen-sulfate
respiring associations discovered at a range of deep hydrocarbon seeps (Dang et al.
2009; Miyazaki et al. 2009). N 2 fixation was demonstrated in these habitats using
direct isotope uptake methods in tandem with direct visualization of uptake with
nanoscale secondary ion mass spectrometry (nanoSIMS) (Dekas et al. 2009, 2014,
2016), as well as at “whale-falls” (deep environments with the decaying carcass of
a whale (Dekas et al. 2018a; Kapili et al. 2019). Diazotrophy in the organic and
energy rich hydrocarbon seep environments appears to be mainly associated with
the methanotroph partner of the broadly distributed anaerobic methane oxidizing
(ANME) consortia of a methanotroph and sulfate-respiring bacteria (in which the
oxidation of methane occurs by reverse methanogenesis by the methanotroph partner) (Boetius et al. 2000; Orphan et al. 2001). This dependency was not observed in
the ‘whale-fall” environments suggesting other metabolic pathways support
observed diazotrophy.
More recent studies have considered N 2 fixation in deep-sea sediments outside of
seeps/vents/whale falls (Dang et al. 2009; Dekas et al. 2018b; Kapili et al. 2020).
Sulfur-oxidizing bacteria symbiotic with deep vent animals have also been implicated in diazotrophy, but with only indirect evidence to date (Rau 1981).
4.5 Conclusions
Our knowledge of the range of habitats, diversity and physiologies of marine diazotrophs has expanded greatly over the last several decades. We now know that active
diazotrophy occurs in cold waters of the deep sea as well as at higher latitudes in the
Arctic. New marine niches for N 2 fixation are bound to be identified in the future.
4.4 The Benthos