131
(Church et al. 2008; Messer et al. 2015a; Needoba et al. 2007) (Fig. 7.4). There have
been reports of Trichodesmium filaments or nifH genes in high latitude waters (Diez
et al. 2012; Rivero-Calle et al. 2016), although they might have been transported by
surface currents and not necessarily active. It was also tacitly assumed that Arctic Ocean
waters, being light-limited and for much of the year under ice-cover are not depleted
sufficiently in fixed N to select for N 2 fixation (Sipler et al. 2017). However, during the
summer ice-free season, fixed N availability and reduced N:P ratios can create N-limited
conditions that could be conducive to N 2 fixation (Blais et al. 2012; Mills et al. 2018;
Ortega-Retuerta et al. 2011; Tremblay et al. 2008; Yamamoto-Kawai et al. 2006). A
number of studies have reported nifH genes in Arctic regions (Blais et al. 2012; Diez
et al. 2008) and some have documented low rates of N 2 fixation (Blais et al. 2012; Sipler
et al. 2017). Primarily NCD nifH genes and some filamentous cyanobacterial nifH genes
were reported in the Canadian Arctic including the Beaufort Sea (Blais et al. 2012).
UCYN-A was recently found in the Beaufort and Chukchi Seas of the Arctic Ocean
(Harding et al. 2018; Shiozaki et al. 2018a). The finding in the Arctic coincided with the
reports of N 2 fixation rates in Arctic waters (Sipler et al. 2017). However, the cellular
abundances and bulk water rates are relatively low. Although some evidence suggests
UCYN-A are endemic populations, it is also likely that cells can be transported into the
Chukchi and Beaufort Seas through the Bering Strait (Harding et al. 2018). Recently,
similar findings have been reported for a transect in the South Pacific, with UCYN-A
found at latitudes greater than 60° south (Raes et al. 2020). However, in both northern
(and possibly southern) high latitudes, the most common diazotrophs based on nifH
gene sequences are UCYN-A and NCDs, and bulk N 2 fixation rates are generally low.
Interestingly, per cell UCYN-A rates in polar waters directly measured with nanoSIMS
are similar in magnitude to cells in warmer regions (Harding et al. 2018). N 2 fixation and
UCYN-A have now also been reported in the Southern Ocean near the ice covered coast
of Antarctica (Shiozaki et al. 2020). It is likely, however, that high latitude N 2 fixation
will yield yet more surprises.
7.10 Conclusions
Although much more is now known about the distribution of diazotrophs and N 2
fixation in the surface oceans, there is much yet to learn, and little is known about
distributions and factors controlling N 2 fixation in other habitats including sediments and the deep ocean. There are some general trends in distributions of diazotrophs, but data is sparse relative to the space and time scales of the oceans.
Trichodesmium, diatom symbionts, and UCYN-A are important in open ocean
gyres, whereas heterocyst-forming cyanobacteria are typically found in brackish
waters and estuaries. NCDs are widely distributed based on nifH gene sequences,
but their activity has not been directly quantified and is an important question for
future research. The symbiotic UCYN-A is more widely distributed than
Trichodesmium and the diatom symbionts, as are the NCDs. Regions not previously
recognized as N 2 -fixation habitats, including coastal waters, ODZs and polar waters
need further research. Analytical models are becoming better able to map current
7.10 Conclusions
(Church et al. 2008; Messer et al. 2015a; Needoba et al. 2007) (Fig. 7.4). There have
been reports of Trichodesmium filaments or nifH genes in high latitude waters (Diez
et al. 2012; Rivero-Calle et al. 2016), although they might have been transported by
surface currents and not necessarily active. It was also tacitly assumed that Arctic Ocean
waters, being light-limited and for much of the year under ice-cover are not depleted
sufficiently in fixed N to select for N 2 fixation (Sipler et al. 2017). However, during the
summer ice-free season, fixed N availability and reduced N:P ratios can create N-limited
conditions that could be conducive to N 2 fixation (Blais et al. 2012; Mills et al. 2018;
Ortega-Retuerta et al. 2011; Tremblay et al. 2008; Yamamoto-Kawai et al. 2006). A
number of studies have reported nifH genes in Arctic regions (Blais et al. 2012; Diez
et al. 2008) and some have documented low rates of N 2 fixation (Blais et al. 2012; Sipler
et al. 2017). Primarily NCD nifH genes and some filamentous cyanobacterial nifH genes
were reported in the Canadian Arctic including the Beaufort Sea (Blais et al. 2012).
UCYN-A was recently found in the Beaufort and Chukchi Seas of the Arctic Ocean
(Harding et al. 2018; Shiozaki et al. 2018a). The finding in the Arctic coincided with the
reports of N 2 fixation rates in Arctic waters (Sipler et al. 2017). However, the cellular
abundances and bulk water rates are relatively low. Although some evidence suggests
UCYN-A are endemic populations, it is also likely that cells can be transported into the
Chukchi and Beaufort Seas through the Bering Strait (Harding et al. 2018). Recently,
similar findings have been reported for a transect in the South Pacific, with UCYN-A
found at latitudes greater than 60° south (Raes et al. 2020). However, in both northern
(and possibly southern) high latitudes, the most common diazotrophs based on nifH
gene sequences are UCYN-A and NCDs, and bulk N 2 fixation rates are generally low.
Interestingly, per cell UCYN-A rates in polar waters directly measured with nanoSIMS
are similar in magnitude to cells in warmer regions (Harding et al. 2018). N 2 fixation and
UCYN-A have now also been reported in the Southern Ocean near the ice covered coast
of Antarctica (Shiozaki et al. 2020). It is likely, however, that high latitude N 2 fixation
will yield yet more surprises.
7.10 Conclusions
Although much more is now known about the distribution of diazotrophs and N 2
fixation in the surface oceans, there is much yet to learn, and little is known about
distributions and factors controlling N 2 fixation in other habitats including sediments and the deep ocean. There are some general trends in distributions of diazotrophs, but data is sparse relative to the space and time scales of the oceans.
Trichodesmium, diatom symbionts, and UCYN-A are important in open ocean
gyres, whereas heterocyst-forming cyanobacteria are typically found in brackish
waters and estuaries. NCDs are widely distributed based on nifH gene sequences,
but their activity has not been directly quantified and is an important question for
future research. The symbiotic UCYN-A is more widely distributed than
Trichodesmium and the diatom symbionts, as are the NCDs. Regions not previously
recognized as N 2 -fixation habitats, including coastal waters, ODZs and polar waters
need further research. Analytical models are becoming better able to map current
7.10 Conclusions