118
Temperature can set theoretical limits on some N 2 -fixers because of the relationships of temperature with gas diffusion of N 2 and O 2 and respiration rates (Stal
2009) as well as enzymatic rates in general (Hochachka and Somero 2002).
However, in the open ocean, temperature correlates with many other factors (Sohm
et al. 2011c), including concentrations of nutrients (Goes et al. 1999), since solar
irradiance heats surface waters, leads to stratification and supports photosynthesis
which consumes nutrients. Thus, temperature alone probably does not control N 2
fixation in surface waters. Tang et al. (2020) have recently concluded that temperature is a primary determinant of diazotroph community structure in the coastal
waters of the eastern US, but that P availability controls rates of N 2 fixation.
Concentrations of nutrients largely reflect circulation and temperature. There are
distinct general distributions of inorganic nutrients and trace elements, including N,
P and Fe throughout the oceans (Fig. 7.1). Fixed N is present in relatively high concentrations in the form of nitrate (a few to tens of micromolar) in coastal regions and
at higher latitudes as a result of runoff, atmospheric deposition and sometimes
upwelling. Inorganic P availability in the form of phosphate, usually follows these
general basin-scale patterns as well. Equatorial waters contain moderate levels of
nutrients because of weak upwelling driven by convergence of the northern and
southern gyres. Low to extremely low but variable concentrations occur in surface
waters of the major gyres (Martiny et al. 2019) where the major form of N used by
phytoplankton is ammonium recycled from organic matter within the upper
water column.
In well-lit waters, photosynthesis rapidly depletes nitrate and phosphate to
extremely low concentrations (nanomolar), creating distinct vertical distributions
(Fig. 7.2). The low concentrations (nanomolar) in surface waters (< 200 m depth)
are depleted by phytoplankton uptake and assimilation into the food web which
contrasts with the much higher concentrations (micromolar) at depth (> 200 m) that
result from the remineralization of sinking detrital organic matter. In addition to
large-scale ocean basin and gyre patterns and vertical distributions, mesoscale features such as ocean eddies are important in causing small-scale (km to 100 s of km)
variation in nutrient concentrations and distributions of microorganisms.
Dinitrogen is dissolved in seawater at much higher concentrations than inorganic
fixed N (~400–800 micromolar), as a function of temperature and salinity, in equilibrium with the atmosphere (Gruber 2008). Dissolved organic compounds containing
N and P are diverse and poorly characterized, but are present at higher concentrations
and are used by some microorganisms including diazotrophs (Benavides et al. 2017).
Beneath some biologically productive areas (and in sediments), the consumption of
organic matter reduces O 2 concentrations and reductive N transformations of canonical denitrification and anammox lead to removal of combined N forming N 2 and
N 2 O. These oxygen deficient zones (ODZs, previously called oxygen minimum
zones) have been hypothesized to also be sites of N 2 fixation (Fernandez et al. 2011).
The distribution of iron (Fe) concentrations differs from those of N and P
(Fig. 7.1c) because sources, sinks and chemistry differ. There are multiple sources of
Fe, including ocean margins and hydrothermal vents, but atmospheric deposition of
aeolian dust is a major input, especially to the surface open ocean (Jickells et al.
7 Biogeography of N 2 Fixation in the Surface Ocean
Temperature can set theoretical limits on some N 2 -fixers because of the relationships of temperature with gas diffusion of N 2 and O 2 and respiration rates (Stal
2009) as well as enzymatic rates in general (Hochachka and Somero 2002).
However, in the open ocean, temperature correlates with many other factors (Sohm
et al. 2011c), including concentrations of nutrients (Goes et al. 1999), since solar
irradiance heats surface waters, leads to stratification and supports photosynthesis
which consumes nutrients. Thus, temperature alone probably does not control N 2
fixation in surface waters. Tang et al. (2020) have recently concluded that temperature is a primary determinant of diazotroph community structure in the coastal
waters of the eastern US, but that P availability controls rates of N 2 fixation.
Concentrations of nutrients largely reflect circulation and temperature. There are
distinct general distributions of inorganic nutrients and trace elements, including N,
P and Fe throughout the oceans (Fig. 7.1). Fixed N is present in relatively high concentrations in the form of nitrate (a few to tens of micromolar) in coastal regions and
at higher latitudes as a result of runoff, atmospheric deposition and sometimes
upwelling. Inorganic P availability in the form of phosphate, usually follows these
general basin-scale patterns as well. Equatorial waters contain moderate levels of
nutrients because of weak upwelling driven by convergence of the northern and
southern gyres. Low to extremely low but variable concentrations occur in surface
waters of the major gyres (Martiny et al. 2019) where the major form of N used by
phytoplankton is ammonium recycled from organic matter within the upper
water column.
In well-lit waters, photosynthesis rapidly depletes nitrate and phosphate to
extremely low concentrations (nanomolar), creating distinct vertical distributions
(Fig. 7.2). The low concentrations (nanomolar) in surface waters (< 200 m depth)
are depleted by phytoplankton uptake and assimilation into the food web which
contrasts with the much higher concentrations (micromolar) at depth (> 200 m) that
result from the remineralization of sinking detrital organic matter. In addition to
large-scale ocean basin and gyre patterns and vertical distributions, mesoscale features such as ocean eddies are important in causing small-scale (km to 100 s of km)
variation in nutrient concentrations and distributions of microorganisms.
Dinitrogen is dissolved in seawater at much higher concentrations than inorganic
fixed N (~400–800 micromolar), as a function of temperature and salinity, in equilibrium with the atmosphere (Gruber 2008). Dissolved organic compounds containing
N and P are diverse and poorly characterized, but are present at higher concentrations
and are used by some microorganisms including diazotrophs (Benavides et al. 2017).
Beneath some biologically productive areas (and in sediments), the consumption of
organic matter reduces O 2 concentrations and reductive N transformations of canonical denitrification and anammox lead to removal of combined N forming N 2 and
N 2 O. These oxygen deficient zones (ODZs, previously called oxygen minimum
zones) have been hypothesized to also be sites of N 2 fixation (Fernandez et al. 2011).
The distribution of iron (Fe) concentrations differs from those of N and P
(Fig. 7.1c) because sources, sinks and chemistry differ. There are multiple sources of
Fe, including ocean margins and hydrothermal vents, but atmospheric deposition of
aeolian dust is a major input, especially to the surface open ocean (Jickells et al.
7 Biogeography of N 2 Fixation in the Surface Ocean