177
buoyant, accumulates in near surface waters. More needs to be known about the
depth distributions and activities of other diazotrophs including the NCDs.
Although temperature affects all enzyme kinetics, as an environmental factor it
does not directly or intrinsically determine the distribution or activity of diazotrophs
in the marine environment. N 2 fixation is found at temperature extremes from hydrothermal vents to polar regions. Trichodesmium generally occurs in waters above
20–25 °C, and relationships between temperature and abundances have been found
for other oceanic diazotrophs, such as Crocosphaera and UCYN-A.
Salinity is a general factor that affects diazotroph species distributions although
it does not directly constrain N 2 fixation itself. Increased or decreased salinity can
affect the geographic distributions of individual species, however.
pH is not generally thought to be an important controlling factor in ocean biology since seawater is so well-buffered by the carbonate system. However, the drop
of about 0.3 pH units in some areas of the surface ocean over the last several decades
represents a 30-fold increase in H
+
ion concentration and recent evidence has implicated it as well as a factor that can affect diazotrophy.
P and Fe availability are critical nutrients for the growth of marine diazotrophs
and either may be in short supply relative to growth needs. Fe and P have distinct
sources and sinks, and organically complexed forms can be particularly important.
Diazotrophs differ in their abilities to use different P and Fe compounds, such as
organic-P compounds or by producing siderophores which solubilize Fe, respectively, thus providing for ecological differences that affect their broad scale distributions (see below).
In general, the presence of high levels of fixed inorganic N inhibits N 2 fixation.
However, there are an increasing number of examples from the field where diazotrophs are present and active when nitrate is present, such as the finding of UCYN-A
in coastal and polar waters. N 2 fixation, albeit at low rates, is also found in sediments, which often have high concentrations of ammonium.
Other nutrients and metals may also be important, such as Mo and V availability
and their effect on conventional and alternative nitrogenases, and Ni which is used
in hydrogenase.
It is clear that the environmental factors affecting and controlling the distribution
of marine diazotrophs and their rates of N 2 fixation are complex, interactive and
vary across environments. They remain to be fully understood and should be the
focus of future research.
10.7 Biogeography
The distributions of temperature, inorganic nutrients, trace elements and salinity all
contribute to distinct distribution patterns of N 2 fixation and of individual diazotrophic species. The oceans are large and dynamic, and ocean circulation, solar
insolation, and patterns of atmospheric inputs lead to non-homogenous distributions
of light, temperature and nutrients. The warm subtropical ocean gyres are stratified
10.7 Biogeography
buoyant, accumulates in near surface waters. More needs to be known about the
depth distributions and activities of other diazotrophs including the NCDs.
Although temperature affects all enzyme kinetics, as an environmental factor it
does not directly or intrinsically determine the distribution or activity of diazotrophs
in the marine environment. N 2 fixation is found at temperature extremes from hydrothermal vents to polar regions. Trichodesmium generally occurs in waters above
20–25 °C, and relationships between temperature and abundances have been found
for other oceanic diazotrophs, such as Crocosphaera and UCYN-A.
Salinity is a general factor that affects diazotroph species distributions although
it does not directly constrain N 2 fixation itself. Increased or decreased salinity can
affect the geographic distributions of individual species, however.
pH is not generally thought to be an important controlling factor in ocean biology since seawater is so well-buffered by the carbonate system. However, the drop
of about 0.3 pH units in some areas of the surface ocean over the last several decades
represents a 30-fold increase in H
+
ion concentration and recent evidence has implicated it as well as a factor that can affect diazotrophy.
P and Fe availability are critical nutrients for the growth of marine diazotrophs
and either may be in short supply relative to growth needs. Fe and P have distinct
sources and sinks, and organically complexed forms can be particularly important.
Diazotrophs differ in their abilities to use different P and Fe compounds, such as
organic-P compounds or by producing siderophores which solubilize Fe, respectively, thus providing for ecological differences that affect their broad scale distributions (see below).
In general, the presence of high levels of fixed inorganic N inhibits N 2 fixation.
However, there are an increasing number of examples from the field where diazotrophs are present and active when nitrate is present, such as the finding of UCYN-A
in coastal and polar waters. N 2 fixation, albeit at low rates, is also found in sediments, which often have high concentrations of ammonium.
Other nutrients and metals may also be important, such as Mo and V availability
and their effect on conventional and alternative nitrogenases, and Ni which is used
in hydrogenase.
It is clear that the environmental factors affecting and controlling the distribution
of marine diazotrophs and their rates of N 2 fixation are complex, interactive and
vary across environments. They remain to be fully understood and should be the
focus of future research.
10.7 Biogeography
The distributions of temperature, inorganic nutrients, trace elements and salinity all
contribute to distinct distribution patterns of N 2 fixation and of individual diazotrophic species. The oceans are large and dynamic, and ocean circulation, solar
insolation, and patterns of atmospheric inputs lead to non-homogenous distributions
of light, temperature and nutrients. The warm subtropical ocean gyres are stratified
10.7 Biogeography