152
spectrometry (MIMS), at times coupled with
15
N tracers, has been used in several
studies to examine the net fluxes of N 2 from shallow coastal sediments (Newell et al.
2016a, b and studies cited therein). The fluxes vary substantially over diel and seasonal time scales, but N 2 influx, presumptively driven by N 2 fixation, can often equal
or exceed its efflux (from denitrification/ anammox) thereby indicating that N 2 fixation and microbial pathways producing N 2 (denitrification/anammox) in these shallow coastal systems are occurring at comparable levels. Fulweiler et al. (2014) also
considered the relationship of N 2 fixation and denitrification in these systems. As
noted in Chap. 4, nifH sequences and transcripts have been shown to co-occur with
net influxes (Brown and Jenkins 2014). Inclusion of enriched
15
N tracers (
15
NO 3 in
several and
30
N 2 ) has provided corroborating evidence for the validity of the inferences made regarding the relative balance of N 2 fixation and denitrification determining the sign of the fluxes.
The importance of N 2 fixation in seagrass meadows of the tropical seagrass,
Zostera noltii, was considered in the Bassin d’Arcachon where it can account for a
major fraction of the N demand of the seagrass during the summer and fall. More
recent work on seagrasses has been summarized (Welsh 2000). Approximately 20%
of the demand for N by Halophilia stipulacea in the northern Red Sea could be
provided by epiphytic N 2 fixation on the seagrass leaves (Cardini et al. 2016).
Much of the research on diazotrophy in coral reef ecosystems has also been
recently summarized (Benavides et al. 2017). Primary production and N 2 fixation in
a reef system in the Gulf of Aqaba in the northern Red Sea was studied and reported
a relatively high seasonality in N 2 fixation relative to primary production whereas
seasonality was more damped (Cardini et al. 2016). Areal N 2 fixation was highest in
the reef flat and reef crest relative to other zones and could account for supplying as
much as 20% of the N demand of net primary production. Similarly, N 2 fixation
could supply on average about 11% of the photosynthetic N demand of the coral
Stylophora pistillata across a depth range of habitats in the northern Red Sea ranging from 5 to 20 m depth.
8.3 Conclusions
Our understanding of the biogeochemical importance of N 2 fixation in the oceans
has increased markedly over the last several decades, particularly in the marine
water column (Zehr and Capone 2020). However, there are still important outstanding questions and gaps in our understanding. Most obvious is the issue of the importance of diazotrophic heterotrophs in the euphotic and sub-euphotic zone. However,
if the reported rates in the sub-euphotic zone are robust (Benavides et al. 2018a;
Moisander et al. 2017), the integrated rates of N 2 fixation at the basin-scale could be
significant. However, the spatial and temporal variability of this activity is unknown
and difficult to assess in the vast waters of the deep oceans and should be intrinsic
to the geochemically-derived estimates and would not increase the total contribution of N 2 fixation (see below). Rates, when reported, in these sub-euphotic zone
8 N 2 Fixation in Ocean Basins
spectrometry (MIMS), at times coupled with
15
N tracers, has been used in several
studies to examine the net fluxes of N 2 from shallow coastal sediments (Newell et al.
2016a, b and studies cited therein). The fluxes vary substantially over diel and seasonal time scales, but N 2 influx, presumptively driven by N 2 fixation, can often equal
or exceed its efflux (from denitrification/ anammox) thereby indicating that N 2 fixation and microbial pathways producing N 2 (denitrification/anammox) in these shallow coastal systems are occurring at comparable levels. Fulweiler et al. (2014) also
considered the relationship of N 2 fixation and denitrification in these systems. As
noted in Chap. 4, nifH sequences and transcripts have been shown to co-occur with
net influxes (Brown and Jenkins 2014). Inclusion of enriched
15
N tracers (
15
NO 3 in
several and
30
N 2 ) has provided corroborating evidence for the validity of the inferences made regarding the relative balance of N 2 fixation and denitrification determining the sign of the fluxes.
The importance of N 2 fixation in seagrass meadows of the tropical seagrass,
Zostera noltii, was considered in the Bassin d’Arcachon where it can account for a
major fraction of the N demand of the seagrass during the summer and fall. More
recent work on seagrasses has been summarized (Welsh 2000). Approximately 20%
of the demand for N by Halophilia stipulacea in the northern Red Sea could be
provided by epiphytic N 2 fixation on the seagrass leaves (Cardini et al. 2016).
Much of the research on diazotrophy in coral reef ecosystems has also been
recently summarized (Benavides et al. 2017). Primary production and N 2 fixation in
a reef system in the Gulf of Aqaba in the northern Red Sea was studied and reported
a relatively high seasonality in N 2 fixation relative to primary production whereas
seasonality was more damped (Cardini et al. 2016). Areal N 2 fixation was highest in
the reef flat and reef crest relative to other zones and could account for supplying as
much as 20% of the N demand of net primary production. Similarly, N 2 fixation
could supply on average about 11% of the photosynthetic N demand of the coral
Stylophora pistillata across a depth range of habitats in the northern Red Sea ranging from 5 to 20 m depth.
8.3 Conclusions
Our understanding of the biogeochemical importance of N 2 fixation in the oceans
has increased markedly over the last several decades, particularly in the marine
water column (Zehr and Capone 2020). However, there are still important outstanding questions and gaps in our understanding. Most obvious is the issue of the importance of diazotrophic heterotrophs in the euphotic and sub-euphotic zone. However,
if the reported rates in the sub-euphotic zone are robust (Benavides et al. 2018a;
Moisander et al. 2017), the integrated rates of N 2 fixation at the basin-scale could be
significant. However, the spatial and temporal variability of this activity is unknown
and difficult to assess in the vast waters of the deep oceans and should be intrinsic
to the geochemically-derived estimates and would not increase the total contribution of N 2 fixation (see below). Rates, when reported, in these sub-euphotic zone
8 N 2 Fixation in Ocean Basins