176
of N 2 fixation or N loss over large spatial scales. Since N 2 fixation does not discriminate between N isotopes, the natural abundance of key N compounds, such as
nitrate, can also provide a biogeochemical proxy for N 2 fixation. These nutrient and
isotopic analyses are integrative over larger space and time scales.
More indirect approaches for obtaining large scale estimates are based on properties that can be measured by aircraft or satellites (remote sensing). Although these
methods provide large scale information on specific broader N 2 -fixers, they rely on
a variety of assumptions, and optical properties such as reflectance and absorption
that can only be detected near the surface of the ocean. There has been some development of instrumentation that can be deployed to increase the sampling resolution
of diazotrophic populations and their activity in the ocean, such as the Environmental
Sample Processor.
Analytical models have been developed to understand processes controlling N 2
fixation at the cell scale (such as the effect of O 2 ), for predicting the distribution of
N 2 fixation from relationships to biogeochemical data (i.e. from biogeochemical
proxies), and for predicting the distribution of diazotroph species types (such as size
classes). Models coupled with biogeochemical proxies mentioned above which are
integrative over space and time provide some of our best current estimates of ocean
basin or global N 2 fixation. Although still needing validation, such modeling
approaches provide the data needed to assess current and help predict future N 2 fixation scenarios at the basin and global scale.
10.6 Factors
N 2 fixation is affected by a variety of environmental factors in the marine environment. O 2 concentration is a primary consideration since nitrogenase is sensitive to
O 2 inactivation. Microorganisms have developed a suite of cellular and physiological adaptations to avoid O 2 inactivation. Some diazotrophs are anaerobes and occupy
habitats with low O 2 , such as sediments, particles, ODZs and in invertebrate guts.
Others modify the cell wall and have high respiration rates to maintain low O 2 concentrations when fixing N 2 .
Cyanobacteria separate photosynthetic O 2 production from O 2 -sensitive N 2 fixation spatially or temporally. Heterocyst-forming cyanobacteria compartmentalize
nitrogenase into specialized cells lacking oxygenic photosynthesis that can fix N 2
during the day, whereas unicellular cyanobacteria fix N 2 primarily at night. The
symbiotic unicellular UCYN-A fixes N 2 during the day, but has lost photosystem II
and does not evolve O 2 or fix C. Trichodesmium is unusual in that it does not form
heterocysts, and fixes N 2 during the day, which is not yet entirely understood.
Since N 2 fixation is an energy requiring process, light is a major factor controlling N 2 fixation for phototrophic microorganisms. Light decreases with depth in the
ocean (and over small scales in microbial mats and sediments) and there are distinct
patterns of depth distribution and activity of diazotrophs. Trichodesmium, being
10 Summary and Conclusions
of N 2 fixation or N loss over large spatial scales. Since N 2 fixation does not discriminate between N isotopes, the natural abundance of key N compounds, such as
nitrate, can also provide a biogeochemical proxy for N 2 fixation. These nutrient and
isotopic analyses are integrative over larger space and time scales.
More indirect approaches for obtaining large scale estimates are based on properties that can be measured by aircraft or satellites (remote sensing). Although these
methods provide large scale information on specific broader N 2 -fixers, they rely on
a variety of assumptions, and optical properties such as reflectance and absorption
that can only be detected near the surface of the ocean. There has been some development of instrumentation that can be deployed to increase the sampling resolution
of diazotrophic populations and their activity in the ocean, such as the Environmental
Sample Processor.
Analytical models have been developed to understand processes controlling N 2
fixation at the cell scale (such as the effect of O 2 ), for predicting the distribution of
N 2 fixation from relationships to biogeochemical data (i.e. from biogeochemical
proxies), and for predicting the distribution of diazotroph species types (such as size
classes). Models coupled with biogeochemical proxies mentioned above which are
integrative over space and time provide some of our best current estimates of ocean
basin or global N 2 fixation. Although still needing validation, such modeling
approaches provide the data needed to assess current and help predict future N 2 fixation scenarios at the basin and global scale.
10.6 Factors
N 2 fixation is affected by a variety of environmental factors in the marine environment. O 2 concentration is a primary consideration since nitrogenase is sensitive to
O 2 inactivation. Microorganisms have developed a suite of cellular and physiological adaptations to avoid O 2 inactivation. Some diazotrophs are anaerobes and occupy
habitats with low O 2 , such as sediments, particles, ODZs and in invertebrate guts.
Others modify the cell wall and have high respiration rates to maintain low O 2 concentrations when fixing N 2 .
Cyanobacteria separate photosynthetic O 2 production from O 2 -sensitive N 2 fixation spatially or temporally. Heterocyst-forming cyanobacteria compartmentalize
nitrogenase into specialized cells lacking oxygenic photosynthesis that can fix N 2
during the day, whereas unicellular cyanobacteria fix N 2 primarily at night. The
symbiotic unicellular UCYN-A fixes N 2 during the day, but has lost photosystem II
and does not evolve O 2 or fix C. Trichodesmium is unusual in that it does not form
heterocysts, and fixes N 2 during the day, which is not yet entirely understood.
Since N 2 fixation is an energy requiring process, light is a major factor controlling N 2 fixation for phototrophic microorganisms. Light decreases with depth in the
ocean (and over small scales in microbial mats and sediments) and there are distinct
patterns of depth distribution and activity of diazotrophs. Trichodesmium, being
10 Summary and Conclusions