123
7.3 N 2 Fixation Variables
There are several variables that define N 2 fixation in the surface oceans: the
numeric abundances and biomass of different species and their distributions, the
rate (both cellular and volumetric in bulk water) of N 2 fixation, the growth rate and
fate of diazotrophs, and the concentration and relative availability of key nutrients. Obtaining all of this information in a coordinated manner is an ongoing challange, particularly over the large spatial scales of the oceans. Nonetheless, data for
globally distributed measurements of rates and cell abundances are actively being
assembled into databases (Luo et al. 2012; Tang and Cassar 2019), and shows the
large number of measurements that have been made, and yet how poorly the
oceans are sampled overall in space and time. Abundances are one of the most
difficult measurements to make, as some species can easily be enumerated (e.g.
microscopy for Trichodesmium and diatom symbionts, flow cytometry or fluorescent microscopy for Crocosphaera), but others only by quantifying genes (qPCR)
or fluorescent in situ hybridization (FISH) for species that are not distinguishable
by microscopy or other means (e.g. FISH for UCYN-A or known N 2 -fixing heterotrophic bacteria for which there is an rRNA sequence) (see Chap. 5 for details).
Thus, mathematical modeling has proved extremely important in integrating different data streams and trying to obtain global views of the distributions of diazotrophs and their rates and dynamics over time (Dutkiewicz et al. 2012; Monteiro
et al. 2010; Tang and Cassar 2019) (Fig. 7.5).
-4
-3
-2
-1
0
1
2
3
4
Model Diazotroph Boimass (log 10 µgC m -3 )
80
60
40
20
-20
-40
-60
-80
0
Latitude
MAREDAT
Depth-Integrated
Biomass
4
6
(log 10 µgC m -2 )
(a)
(b)
Fig. 7.5 (a) Annual mean diazotroph biomass in μg C m
−3 derived from model of Dutkiewicz
et al. (2012) and compiled from qPCR data of Luo et al. (2012). (b) Latitudinal variation in
observed depth integrated (μg C m
−3 ) diazotroph biomass. (From Barton et al. (2013), with
permission)
7.4 N 2 Fixation Variables
7.3 N 2 Fixation Variables
There are several variables that define N 2 fixation in the surface oceans: the
numeric abundances and biomass of different species and their distributions, the
rate (both cellular and volumetric in bulk water) of N 2 fixation, the growth rate and
fate of diazotrophs, and the concentration and relative availability of key nutrients. Obtaining all of this information in a coordinated manner is an ongoing challange, particularly over the large spatial scales of the oceans. Nonetheless, data for
globally distributed measurements of rates and cell abundances are actively being
assembled into databases (Luo et al. 2012; Tang and Cassar 2019), and shows the
large number of measurements that have been made, and yet how poorly the
oceans are sampled overall in space and time. Abundances are one of the most
difficult measurements to make, as some species can easily be enumerated (e.g.
microscopy for Trichodesmium and diatom symbionts, flow cytometry or fluorescent microscopy for Crocosphaera), but others only by quantifying genes (qPCR)
or fluorescent in situ hybridization (FISH) for species that are not distinguishable
by microscopy or other means (e.g. FISH for UCYN-A or known N 2 -fixing heterotrophic bacteria for which there is an rRNA sequence) (see Chap. 5 for details).
Thus, mathematical modeling has proved extremely important in integrating different data streams and trying to obtain global views of the distributions of diazotrophs and their rates and dynamics over time (Dutkiewicz et al. 2012; Monteiro
et al. 2010; Tang and Cassar 2019) (Fig. 7.5).
-4
-3
-2
-1
0
1
2
3
4
Model Diazotroph Boimass (log 10 µgC m -3 )
80
60
40
20
-20
-40
-60
-80
0
Latitude
MAREDAT
Depth-Integrated
Biomass
4
6
(log 10 µgC m -2 )
(a)
(b)
Fig. 7.5 (a) Annual mean diazotroph biomass in μg C m
−3 derived from model of Dutkiewicz
et al. (2012) and compiled from qPCR data of Luo et al. (2012). (b) Latitudinal variation in
observed depth integrated (μg C m
−3 ) diazotroph biomass. (From Barton et al. (2013), with
permission)
7.4 N 2 Fixation Variables