174
N 2 fixation in the water column is also found within O 2 deficient zones (ODZs),
although generally at low rates. Few cyanobacteria have been detected in surface
waters of ODZ regions, and the low rates within the ODZs are probably due to noncyanobacterial diazotrophs (NCDs). N 2 fixation has been reported for deep subsurface waters, again at low rates and presumably due to NCDs. However, the rates are
near the limit of detection and although the integrated rates over the large scales of
the ocean basins would be significant, these findings are the subject of some controversy and an important area for future research.
Diazotrophs in the benthos are found in shallow sediments, microbial mats, coral
reefs, sea grass sediments and at hydrothermal vents and cold seeps. Shallow illuminated benthic environments can have substantial populations of diazotrophic cyanobacteria. Prokaryotic diazotrophs occur in sediments and can include heterotrophic
and chemolithotrophic forms. Sulfate-respiring diazotrophs have been found in a
broad range of marine sediments. Chemolithotrophic diazotrophs in sediments can
use H 2 S, CH 4 , Mn
+2
, and Fe
+2
, which are formed in these habitats, as electron donors.
These chemical species do not typically occur in the water column, except in ODZs.
Deep sea environments, including hydrocarbon seeps and hydrothermal vents, have
diverse diazotrophs often using chemolithoautotrophic metabolisms, including the
anaerobic methane oxidizers.
10.5 Measurements
There are a wide variety of approaches for measuring N 2 fixation in the marine
environment. Challenges in the ocean environment are the large spatial scales (horizontal and vertical), and temporal variability, including daily and seasonal cycles.
Many marine diazotrophs have yet to be cultivated such as UCYN-A or representative NCDs, although a few important species such as Trichodesmium and
Crocosphaera have been in unialgal culture for many years.
N 2 fixation rate measurements have been made routinely in water or sediment
samples, using reduction of the substrate analogue acetylene (acetylene reduction)
to ethylene or the uptake of enriched
15
N 2 . Both methods have their strengths and
weaknesses, including differing detection limits. The acetylene reduction method is
easy to use but requires incubation with a headspace for gas addition, and generally
the limit of detection is not as low as with
15
N 2 in bulk water applications. These
approaches, based on relatively small samples, have provided much information on
the distribution of N 2 fixation in a range of habitats, by specific microorganisms, and
the factors such as nutrient limitation that control marine N 2 fixation.
Although the
15
N 2 uptake method has been routinely used for years, potential
problems have been identified for application in seawater because of the equilibration
time of
15
N 2 gas in seawater. Moreover, some commercial stocks of
15
N 2 have been
found to be contaminated with traces of combined N. Nonetheless,
15
N 2 rate measurements have provided the most data on surface ocean N 2 fixation rates globally.
10 Summary and Conclusions
N 2 fixation in the water column is also found within O 2 deficient zones (ODZs),
although generally at low rates. Few cyanobacteria have been detected in surface
waters of ODZ regions, and the low rates within the ODZs are probably due to noncyanobacterial diazotrophs (NCDs). N 2 fixation has been reported for deep subsurface waters, again at low rates and presumably due to NCDs. However, the rates are
near the limit of detection and although the integrated rates over the large scales of
the ocean basins would be significant, these findings are the subject of some controversy and an important area for future research.
Diazotrophs in the benthos are found in shallow sediments, microbial mats, coral
reefs, sea grass sediments and at hydrothermal vents and cold seeps. Shallow illuminated benthic environments can have substantial populations of diazotrophic cyanobacteria. Prokaryotic diazotrophs occur in sediments and can include heterotrophic
and chemolithotrophic forms. Sulfate-respiring diazotrophs have been found in a
broad range of marine sediments. Chemolithotrophic diazotrophs in sediments can
use H 2 S, CH 4 , Mn
+2
, and Fe
+2
, which are formed in these habitats, as electron donors.
These chemical species do not typically occur in the water column, except in ODZs.
Deep sea environments, including hydrocarbon seeps and hydrothermal vents, have
diverse diazotrophs often using chemolithoautotrophic metabolisms, including the
anaerobic methane oxidizers.
10.5 Measurements
There are a wide variety of approaches for measuring N 2 fixation in the marine
environment. Challenges in the ocean environment are the large spatial scales (horizontal and vertical), and temporal variability, including daily and seasonal cycles.
Many marine diazotrophs have yet to be cultivated such as UCYN-A or representative NCDs, although a few important species such as Trichodesmium and
Crocosphaera have been in unialgal culture for many years.
N 2 fixation rate measurements have been made routinely in water or sediment
samples, using reduction of the substrate analogue acetylene (acetylene reduction)
to ethylene or the uptake of enriched
15
N 2 . Both methods have their strengths and
weaknesses, including differing detection limits. The acetylene reduction method is
easy to use but requires incubation with a headspace for gas addition, and generally
the limit of detection is not as low as with
15
N 2 in bulk water applications. These
approaches, based on relatively small samples, have provided much information on
the distribution of N 2 fixation in a range of habitats, by specific microorganisms, and
the factors such as nutrient limitation that control marine N 2 fixation.
Although the
15
N 2 uptake method has been routinely used for years, potential
problems have been identified for application in seawater because of the equilibration
time of
15
N 2 gas in seawater. Moreover, some commercial stocks of
15
N 2 have been
found to be contaminated with traces of combined N. Nonetheless,
15
N 2 rate measurements have provided the most data on surface ocean N 2 fixation rates globally.
10 Summary and Conclusions