126
by qPCR and can be as abundant as the cyanobacterial UCYN-A symbiosis (a few
cells per milliliter) (Moisander et al. 2014). Metagenomic analysis predicted a new
phylotype related to Planctomycetes to be very abundant at some locations, but was
not verified by quantitative methods (Delmont et al. 2018). Heterotrophic bacterial
sequences have been reported in the Atlantic and Pacific Oceans, the Indian Ocean,
and the Mediterranean Sea (Farnelid et al. 2011; Man- Aharonovich et al. 2007)
including aphotic zone waters (Benavides et al. 2016a). Recently, about 26 metagenomes have been reconstructed from the TARA dataset (Salazar et al. 2019). NCDs
are often present even though N 2 fixation rates are negligible or below detection limits
(Gradoville et al. 2020). NCDs might be most important in N 2 fixation on particles in
oligotrophic oceans, as particles might be key sources of organic matter for energy,
and allow for development of low O 2 microzones that facilitate N 2 fixation (Bombar
et al. 2016; Farnelid et al. 2018; Moisander et al. 2017).
As field data continue to accumulate, some distinct patterns of interbasin biogeography are emerging that may be related to the distribution of availability of the primary
limiting nutrients, P and Fe (Fig. 7.4). Presumably, the spatial trends reflect the relative
availability of phosphate and Fe among basins (severe Fe limitation in the S. Atlantic,
N. and S. Pacific gyres; predominant P limitation in the N. Atlantic) (Sohm et al.
2011c) and the ability of the different diazotrophic groups to cope with nutrient limitations. There are extremely low P concentrations in the North Atlantic, suggesting that
its availability limits Trichodesmium (Sanudo-Wilhelmy et al. 2001), whereas phosphate concentrations in the ultra-oligotrophic South Pacific Ocean are relatively high
(~100 nmol L
−1
) and yet diazotrophs are relatively rare (Bonnet et al. 2008, 2009).
As mentioned above, Trichodesmium has the capacity to use organic P compounds
such as the phosphonates that are not readily used by all microorganisms. The small
cell size and relatively high surface to volume ratio of the UCYN-A symbiosis (dictated by the host haptophyte size) may confer a competitive advantage to them at very
low Fe concentrations. Trichodesmium can be limited by P, Fe (Küpper et al. 2008;
Paerl et al. 1994) or both (Held et al. 2020). Molecular assays for nutrient limitation
corroborate that P, rather than Fe, was likely more important in regulating N 2 fixation
in the Atlantic (Webb et al. 2007), although Fe is known to also be potentially limiting
(Paerl et al. 1994). A more recent metatranscriptomic study found that Trichodesmium
may be more Fe stressed in the North Pacific and P stressed in the Atlantic based on
relative transcript abundances for Fe (idiA, feoB) and P (pstS, phoX, phnD) stress
response genes (Rouco et al. 2016).
N 2 -fixers were suggested to be co-limited by P and Fe in areas of the eastern
tropical Atlantic (Mills et al. 2004). Responses of N 2 fixation to P and Fe additions
at Station ALOHA off of the island of Oahu are variable (Grabowski et al. 2008),
but long time-scale oscillations of phosphate concentrations in the North Pacific
likely results in shifts from Fe to P limitation of N 2 fixation (Letelier et al. 2019).
Surface P concentrations are very low in the ultra-oligotrophic eastern Mediterranean
Sea (Béthoux and Copin-Montégut 1986).
As noted above, some diazotrophs and N 2 fixation vary at spatial scales much
smaller than ocean basins (Davis and McGillicuddy 2006; Fong et al. 2008), which
can be due to heterogeneity in nutrient fluxes caused by mesoscale physical processes.
High resolution sampling with the Environmental Sample Processor suggested that
7 Biogeography of N 2 Fixation in the Surface Ocean
by qPCR and can be as abundant as the cyanobacterial UCYN-A symbiosis (a few
cells per milliliter) (Moisander et al. 2014). Metagenomic analysis predicted a new
phylotype related to Planctomycetes to be very abundant at some locations, but was
not verified by quantitative methods (Delmont et al. 2018). Heterotrophic bacterial
sequences have been reported in the Atlantic and Pacific Oceans, the Indian Ocean,
and the Mediterranean Sea (Farnelid et al. 2011; Man- Aharonovich et al. 2007)
including aphotic zone waters (Benavides et al. 2016a). Recently, about 26 metagenomes have been reconstructed from the TARA dataset (Salazar et al. 2019). NCDs
are often present even though N 2 fixation rates are negligible or below detection limits
(Gradoville et al. 2020). NCDs might be most important in N 2 fixation on particles in
oligotrophic oceans, as particles might be key sources of organic matter for energy,
and allow for development of low O 2 microzones that facilitate N 2 fixation (Bombar
et al. 2016; Farnelid et al. 2018; Moisander et al. 2017).
As field data continue to accumulate, some distinct patterns of interbasin biogeography are emerging that may be related to the distribution of availability of the primary
limiting nutrients, P and Fe (Fig. 7.4). Presumably, the spatial trends reflect the relative
availability of phosphate and Fe among basins (severe Fe limitation in the S. Atlantic,
N. and S. Pacific gyres; predominant P limitation in the N. Atlantic) (Sohm et al.
2011c) and the ability of the different diazotrophic groups to cope with nutrient limitations. There are extremely low P concentrations in the North Atlantic, suggesting that
its availability limits Trichodesmium (Sanudo-Wilhelmy et al. 2001), whereas phosphate concentrations in the ultra-oligotrophic South Pacific Ocean are relatively high
(~100 nmol L
−1
) and yet diazotrophs are relatively rare (Bonnet et al. 2008, 2009).
As mentioned above, Trichodesmium has the capacity to use organic P compounds
such as the phosphonates that are not readily used by all microorganisms. The small
cell size and relatively high surface to volume ratio of the UCYN-A symbiosis (dictated by the host haptophyte size) may confer a competitive advantage to them at very
low Fe concentrations. Trichodesmium can be limited by P, Fe (Küpper et al. 2008;
Paerl et al. 1994) or both (Held et al. 2020). Molecular assays for nutrient limitation
corroborate that P, rather than Fe, was likely more important in regulating N 2 fixation
in the Atlantic (Webb et al. 2007), although Fe is known to also be potentially limiting
(Paerl et al. 1994). A more recent metatranscriptomic study found that Trichodesmium
may be more Fe stressed in the North Pacific and P stressed in the Atlantic based on
relative transcript abundances for Fe (idiA, feoB) and P (pstS, phoX, phnD) stress
response genes (Rouco et al. 2016).
N 2 -fixers were suggested to be co-limited by P and Fe in areas of the eastern
tropical Atlantic (Mills et al. 2004). Responses of N 2 fixation to P and Fe additions
at Station ALOHA off of the island of Oahu are variable (Grabowski et al. 2008),
but long time-scale oscillations of phosphate concentrations in the North Pacific
likely results in shifts from Fe to P limitation of N 2 fixation (Letelier et al. 2019).
Surface P concentrations are very low in the ultra-oligotrophic eastern Mediterranean
Sea (Béthoux and Copin-Montégut 1986).
As noted above, some diazotrophs and N 2 fixation vary at spatial scales much
smaller than ocean basins (Davis and McGillicuddy 2006; Fong et al. 2008), which
can be due to heterogeneity in nutrient fluxes caused by mesoscale physical processes.
High resolution sampling with the Environmental Sample Processor suggested that
7 Biogeography of N 2 Fixation in the Surface Ocean