125
Atlantic (Moore et al. 2009; Tyrrell et al. 2003) North Pacific (Church et al. 2008;
Shiozaki et al. 2017; Sohm et al. 2011c) and South Pacific gyres (Bonnet et al. 2007).
In the North Pacific gyre, observations include Station ALOHA (Böttjer et al.
2017), the western Pacific and Kuroshio current region (Cheung et al. 2017; Shiozaki
et al. 2018b) and throughout the gyre (Church et al. 2008; Kitajima et al. 2009) to a
sharp boundary near 35
o
N latitude at the northern edge (Gradoville et al. 2020;
Shiozaki et al. 2017). Cyanobacterial N 2 -fixers have been reported in the southern
hemisphere as well, in the South Atlantic (Ribeiro et al. 2018; Sohm et al. 2011b)
and South Pacific Oceans (Bonnet et al. 2008; Halm et al. 2012; Moisander et al.
2010; Raes et al. 2020), including the western S. Pacific (Bonnet et al. 2009;
Campbell et al. 2006) and Eastern Australian current (Messer et al. 2015b). The
global surveys, including TARA and MALASPINA have reported UCYN-A and its
partner at numerous locations in the Atlantic and Pacific Oceans (Cabello et al.
2016; Martinez-Perez et al. 2016; Salazar et al. 2019).
Apart from considerable work in the Arabian Sea (Ahmed et al. 2017; Capone
et al. 1998; Krishnan et al. 2007; Mazard et al. 2004) studies in the Indian Ocean
proper have been much more limited (Carpenter 1983; Mulholland and Capone
2009; Sournia 1968). However, two recent studies found proteobacterial diazotrophs to predominate in the near surface waters of the equatorial, southern (Shiozaki
et al. 2014) and eastern waters (Wu et al. 2019) of the Indian Ocean proper. Poulton
et al. (2009) found substantial populations of Trichodesmium and diatom diazotroph
symbioses in the southwest Indian Ocean proximal to Madagascar. Shiozaki et al.
(2014) reported considerably lower rates of N 2 fixation when compared to their
observations in the Arabian Sea. Interestingly, while Loscher et al. (2020) found the
presence of diverse non-photorophic diazotrophs throughout the water column to
560 m in the hypoxic waters of the Bay of Bengal, they could not detect N 2 fixation
and suggested that N 2 fixation was micronutrient limited.
Cyanobacterial diazotrophs have also been reported in smaller seas, such as the
oligotrophic Caribbean (Carpenter and Price 1977; Goebel et al. 2010), South China
(Zhang et al. 2011), and Red Seas (Ehrenberg 1830) including the Gulf of Aqaba
(Foster et al. 2009). N 2 fixation has been suggested to be important in the
Mediterranean Sea because of the low N:P ratios in discharges to the basin and
despite high N:P ratios in deep water (Béthoux and Copin-Montégut 1986), particularly in the western basin (Bonnet et al. 2011). Cyanobacterial N 2 -fixers identified
in the Mediterranean include the diatom symbionts as well as UCYN-A (Le Moal
et al. 2011; Man-Aharonovich et al. 2007; Zeev et al. 2008). Trichodesmium,
although not often observed in the greater Mediterranean, has been documented to
occur in a few locations (Margalef 1969; Sabeur et al. 2016). Geological evidence
of large surface populations of diatom-diazotroph associations have also been found
in deep organic deposits (sapropels) of the eastern Mediterranean basin (Sachs and
Repeta 1999; Sachs et al. 1999).
Diverse non-cyanobacterial diazotroph (NCD) bacterial nifH sequences have been
reported at most if not all sites studied using nifH amplification or metagenomic
approaches (Delmont et al. 2018; Farnelid et al. 2011). However, total abundances are
not really known and most are likely quite low. A few phylotypes have been quantified
7.4 Oligotrophic Oceans
Atlantic (Moore et al. 2009; Tyrrell et al. 2003) North Pacific (Church et al. 2008;
Shiozaki et al. 2017; Sohm et al. 2011c) and South Pacific gyres (Bonnet et al. 2007).
In the North Pacific gyre, observations include Station ALOHA (Böttjer et al.
2017), the western Pacific and Kuroshio current region (Cheung et al. 2017; Shiozaki
et al. 2018b) and throughout the gyre (Church et al. 2008; Kitajima et al. 2009) to a
sharp boundary near 35
o
N latitude at the northern edge (Gradoville et al. 2020;
Shiozaki et al. 2017). Cyanobacterial N 2 -fixers have been reported in the southern
hemisphere as well, in the South Atlantic (Ribeiro et al. 2018; Sohm et al. 2011b)
and South Pacific Oceans (Bonnet et al. 2008; Halm et al. 2012; Moisander et al.
2010; Raes et al. 2020), including the western S. Pacific (Bonnet et al. 2009;
Campbell et al. 2006) and Eastern Australian current (Messer et al. 2015b). The
global surveys, including TARA and MALASPINA have reported UCYN-A and its
partner at numerous locations in the Atlantic and Pacific Oceans (Cabello et al.
2016; Martinez-Perez et al. 2016; Salazar et al. 2019).
Apart from considerable work in the Arabian Sea (Ahmed et al. 2017; Capone
et al. 1998; Krishnan et al. 2007; Mazard et al. 2004) studies in the Indian Ocean
proper have been much more limited (Carpenter 1983; Mulholland and Capone
2009; Sournia 1968). However, two recent studies found proteobacterial diazotrophs to predominate in the near surface waters of the equatorial, southern (Shiozaki
et al. 2014) and eastern waters (Wu et al. 2019) of the Indian Ocean proper. Poulton
et al. (2009) found substantial populations of Trichodesmium and diatom diazotroph
symbioses in the southwest Indian Ocean proximal to Madagascar. Shiozaki et al.
(2014) reported considerably lower rates of N 2 fixation when compared to their
observations in the Arabian Sea. Interestingly, while Loscher et al. (2020) found the
presence of diverse non-photorophic diazotrophs throughout the water column to
560 m in the hypoxic waters of the Bay of Bengal, they could not detect N 2 fixation
and suggested that N 2 fixation was micronutrient limited.
Cyanobacterial diazotrophs have also been reported in smaller seas, such as the
oligotrophic Caribbean (Carpenter and Price 1977; Goebel et al. 2010), South China
(Zhang et al. 2011), and Red Seas (Ehrenberg 1830) including the Gulf of Aqaba
(Foster et al. 2009). N 2 fixation has been suggested to be important in the
Mediterranean Sea because of the low N:P ratios in discharges to the basin and
despite high N:P ratios in deep water (Béthoux and Copin-Montégut 1986), particularly in the western basin (Bonnet et al. 2011). Cyanobacterial N 2 -fixers identified
in the Mediterranean include the diatom symbionts as well as UCYN-A (Le Moal
et al. 2011; Man-Aharonovich et al. 2007; Zeev et al. 2008). Trichodesmium,
although not often observed in the greater Mediterranean, has been documented to
occur in a few locations (Margalef 1969; Sabeur et al. 2016). Geological evidence
of large surface populations of diatom-diazotroph associations have also been found
in deep organic deposits (sapropels) of the eastern Mediterranean basin (Sachs and
Repeta 1999; Sachs et al. 1999).
Diverse non-cyanobacterial diazotroph (NCD) bacterial nifH sequences have been
reported at most if not all sites studied using nifH amplification or metagenomic
approaches (Delmont et al. 2018; Farnelid et al. 2011). However, total abundances are
not really known and most are likely quite low. A few phylotypes have been quantified
7.4 Oligotrophic Oceans