52
shallow and deep coral habitats. These include specific symbiotic associations, or
inclusion of N 2 -fixing species within the microbiome of organisms (Fiore et al.
2010; Ribes et al. 2015). Some hermatypic corals were shown to have diazotrophic
cyanobacteria contributing to their nitrogenous nutrition (Lesser et al. 2007;
Meunier et al. 2019). N 2 fixation associated with deep water corals was recently
reported as well (Middelburg et al. 2015). Some soft corals apparently host symbiotic diazotrophs as well (Bednarz et al. 2015). Indeed, comprehensive studies of the
coral microbiome (or holobiont) routinely reveal a robust diazotrophic population
(Benavides et al. 2017; Hernandez-Agreda et al. 2017).
Marine Plant Communities
As noted in Chap. 3, the highly productive seagrass and salt marsh ecosystems of
shallow coastal waters received considerable attention during the early modern era
evaluating the importance of N 2 fixation in diverse marine ecosystems. Populations
of N 2 -fixers and active N 2 fixation were found to occur on the leaves (phyllosphere
and in the root zone (rhizosphere) of tropical seagrasses such as Thalassia testudinum (Capone 1983b, 1988), which occur as extensive meadows in shallow tropical
environments. N 2 fixation could account for an appreciable fraction of the N
demanded of primary production by these marine angiosperms (Capone et al. 1979).
N 2 fixation was also detected in the root zone of the temperate counterpart, Zostera
marina (Capone 1982). The importance of N 2 fixation to primary production in the
rhizophere of Zostera noltii has been demonstrated in the Bay of Arcachon, France
(Welsh 2000) and in the seagrass Halophilia stipulacea in the Red Sea (Cardini
et al. 2018).
Sulfur oxidizers can be active N 2 -fixers in symbiosis with lucinid clams in seagrass communities (Cardini et al. 2019). Hence, while the range of benthic habitats
with diazotrophic inhabitants has expanded substantially there are more yet to be
explored.
A number of recent benthic studies have implicated sulfate-respiring bacteria as
important in N 2 fixation in many shallow organic rich sediments (Bertics et al. 2010,
2013; Brown and Jenkins 2014) as well as in the deep sea sediments (Dekas et al.
2018a; Gier et al. 2016, 2017), confirming earlier observations (Capone 1982;
Nedwell and Azni bin Abdul Aziz 1980) of their contribution to N 2 fixation in seagrass (Zostera marina) and unvegetated sediments.
Marine macroalgae, particularly in coral reef environments, have been long recognized to host diazotrophic cyanobacterial epi-phytes (Capone et al. 1977; Goldner
1980; Shashar et al. 1994). Diazotrophs have also been reported in association with
the green macroalga, Codium (Head and Carpenter 1975) and pelagic Sargassum
(Carpenter 1972). More recent studies have found N 2 fixation to be present on juvenile forms of species of benthic Sargassum as well (Raut et al. 2018). Heterotrophic
N 2 fixation appears to also play a role during decomposition of macroalgae such as
4 Microorganisms and Habitats
shallow and deep coral habitats. These include specific symbiotic associations, or
inclusion of N 2 -fixing species within the microbiome of organisms (Fiore et al.
2010; Ribes et al. 2015). Some hermatypic corals were shown to have diazotrophic
cyanobacteria contributing to their nitrogenous nutrition (Lesser et al. 2007;
Meunier et al. 2019). N 2 fixation associated with deep water corals was recently
reported as well (Middelburg et al. 2015). Some soft corals apparently host symbiotic diazotrophs as well (Bednarz et al. 2015). Indeed, comprehensive studies of the
coral microbiome (or holobiont) routinely reveal a robust diazotrophic population
(Benavides et al. 2017; Hernandez-Agreda et al. 2017).
Marine Plant Communities
As noted in Chap. 3, the highly productive seagrass and salt marsh ecosystems of
shallow coastal waters received considerable attention during the early modern era
evaluating the importance of N 2 fixation in diverse marine ecosystems. Populations
of N 2 -fixers and active N 2 fixation were found to occur on the leaves (phyllosphere
and in the root zone (rhizosphere) of tropical seagrasses such as Thalassia testudinum (Capone 1983b, 1988), which occur as extensive meadows in shallow tropical
environments. N 2 fixation could account for an appreciable fraction of the N
demanded of primary production by these marine angiosperms (Capone et al. 1979).
N 2 fixation was also detected in the root zone of the temperate counterpart, Zostera
marina (Capone 1982). The importance of N 2 fixation to primary production in the
rhizophere of Zostera noltii has been demonstrated in the Bay of Arcachon, France
(Welsh 2000) and in the seagrass Halophilia stipulacea in the Red Sea (Cardini
et al. 2018).
Sulfur oxidizers can be active N 2 -fixers in symbiosis with lucinid clams in seagrass communities (Cardini et al. 2019). Hence, while the range of benthic habitats
with diazotrophic inhabitants has expanded substantially there are more yet to be
explored.
A number of recent benthic studies have implicated sulfate-respiring bacteria as
important in N 2 fixation in many shallow organic rich sediments (Bertics et al. 2010,
2013; Brown and Jenkins 2014) as well as in the deep sea sediments (Dekas et al.
2018a; Gier et al. 2016, 2017), confirming earlier observations (Capone 1982;
Nedwell and Azni bin Abdul Aziz 1980) of their contribution to N 2 fixation in seagrass (Zostera marina) and unvegetated sediments.
Marine macroalgae, particularly in coral reef environments, have been long recognized to host diazotrophic cyanobacterial epi-phytes (Capone et al. 1977; Goldner
1980; Shashar et al. 1994). Diazotrophs have also been reported in association with
the green macroalga, Codium (Head and Carpenter 1975) and pelagic Sargassum
(Carpenter 1972). More recent studies have found N 2 fixation to be present on juvenile forms of species of benthic Sargassum as well (Raut et al. 2018). Heterotrophic
N 2 fixation appears to also play a role during decomposition of macroalgae such as
4 Microorganisms and Habitats