50
4.3 Mesopelagic and Deep Ocean
N 2 -fixing phylotypes have been reported in deep waters to over 2500 m depth in the
N.E. Pacific (Mehta et al. 2005) and down to almost 6000 m in the deep N. Atlantic
Ocean (Hewson et al. 2007). Measurable rates of
15
N 2 uptake have been documented
below 1000 m in the deep Mediterranean Sea (Benavides et al. 2016) and South
Pacific Ocean (Benavides et al. 2015) and studies of N 2 -fixers in the deep sea have
been reviewed (Benavides et al. 2018; Moisander et al. 2017). The role of heterotrophic N 2 -fixing bacteria in the low-energy, oxic waters of the deep sea is currently of
great research interest and a conundrum which needs to be resolved. Indeed, phylotypes of novel diazotrophs have been found at hydrothermal sites (Mehta et al.
2003) and an hyperthermal diazotrophic archeon has been isolated from fluids from
a deep hydrothermal vent environment (Mehta and Baross 2006).
4.4 The Benthos
The benthic environment has a range of known and potential habitats for N 2 -fixers.
Benthic marine environments have long been a focus of N 2 fixation research and
several previous reviews cover much of the earlier work (Capone 1983a, 1988;
Carpenter and Capone 2008; Howarth et al. 1988).
N 2 -fixing bacteria are diverse in terrestrial environments, as free-living, associated or endosymbiotic with other organisms (Davies-Barnard and Friedlingstein
2020; Valentine et al. 2018). Similarly, in benthic marine environments, there are
many Bacteria and Archaea that have nitrogenase genes (Brown and Jenkins 2014;
Burns et al. 2002; Dang et al. 2009). Phylotypes of putative diazotrophs have been
also noted deep in shelf sediments (e.g. (D’Hondt et al. 2004). However, the marine
benthos is very different than soil environments, often containing high concentrations of interstitial dissolved inorganic nitrogen in the form of ammonium and N 2
fixation rates are often low and at times have been questioned (Capone 1988;
Howarth et al. 1988).
Heterotrophic N 2 -fixers in sediments can draw on often rich sources of dissolved
and particulate organic matter. Diverse cyanobacteria can dominate shallow sediment surfaces as part of microbial mats. Chemoautotrophs are primarily found in
sediments or localized environments such as hydrothermal vents and cold seeps that
have rich sources of reduced compounds such as H 2 S, H 2 , NH 4
+
, CH 4 , Mn
2+
, and
Fe
2+
. Sulfide oxidizers, aerobic and anaerobic methane oxidizers, and Mn and Fe
oxidizers all have representative N 2 -fixing taxa. Diazotrophic bacteria and cyanobacteria have been reported to be associated with corals.
4 Microorganisms and Habitats
4.3 Mesopelagic and Deep Ocean
N 2 -fixing phylotypes have been reported in deep waters to over 2500 m depth in the
N.E. Pacific (Mehta et al. 2005) and down to almost 6000 m in the deep N. Atlantic
Ocean (Hewson et al. 2007). Measurable rates of
15
N 2 uptake have been documented
below 1000 m in the deep Mediterranean Sea (Benavides et al. 2016) and South
Pacific Ocean (Benavides et al. 2015) and studies of N 2 -fixers in the deep sea have
been reviewed (Benavides et al. 2018; Moisander et al. 2017). The role of heterotrophic N 2 -fixing bacteria in the low-energy, oxic waters of the deep sea is currently of
great research interest and a conundrum which needs to be resolved. Indeed, phylotypes of novel diazotrophs have been found at hydrothermal sites (Mehta et al.
2003) and an hyperthermal diazotrophic archeon has been isolated from fluids from
a deep hydrothermal vent environment (Mehta and Baross 2006).
4.4 The Benthos
The benthic environment has a range of known and potential habitats for N 2 -fixers.
Benthic marine environments have long been a focus of N 2 fixation research and
several previous reviews cover much of the earlier work (Capone 1983a, 1988;
Carpenter and Capone 2008; Howarth et al. 1988).
N 2 -fixing bacteria are diverse in terrestrial environments, as free-living, associated or endosymbiotic with other organisms (Davies-Barnard and Friedlingstein
2020; Valentine et al. 2018). Similarly, in benthic marine environments, there are
many Bacteria and Archaea that have nitrogenase genes (Brown and Jenkins 2014;
Burns et al. 2002; Dang et al. 2009). Phylotypes of putative diazotrophs have been
also noted deep in shelf sediments (e.g. (D’Hondt et al. 2004). However, the marine
benthos is very different than soil environments, often containing high concentrations of interstitial dissolved inorganic nitrogen in the form of ammonium and N 2
fixation rates are often low and at times have been questioned (Capone 1988;
Howarth et al. 1988).
Heterotrophic N 2 -fixers in sediments can draw on often rich sources of dissolved
and particulate organic matter. Diverse cyanobacteria can dominate shallow sediment surfaces as part of microbial mats. Chemoautotrophs are primarily found in
sediments or localized environments such as hydrothermal vents and cold seeps that
have rich sources of reduced compounds such as H 2 S, H 2 , NH 4
+
, CH 4 , Mn
2+
, and
Fe
2+
. Sulfide oxidizers, aerobic and anaerobic methane oxidizers, and Mn and Fe
oxidizers all have representative N 2 -fixing taxa. Diazotrophic bacteria and cyanobacteria have been reported to be associated with corals.
4 Microorganisms and Habitats