51
Microbial Mats
Microbial mats, vertically compressed, layered microbial ecosystems, have long
been a subject of research in microbial ecology (Cohen and Rosenberg 1989)
including their capacity to fix N 2 (Steppe et al. 1996). Cyanobacteria are often a
major component of the diazotrophic communities of microbial mats in photic environments (Stal 1988) although anoxygenic photosynthetic and heterotrophic bacteria are also members of the mat biota (Omoregie et al. 2004; Paerl et al. 1989; Zehr
et al. 1995).
Species of the bacteria Beggiatoa and Thioploca are also known to form large
mats in benthic environments ranging from salt marshes to shelf sediments beneath
highly productive upwelling zones (Gallardo and Espinoza 2007; Teske and Nelson
2006). Their physiologies can range from heterotrophic to autotrophic, and they
often exploit reduced inorganic sulfur species as electron donors. Both genera have
known diazotrophic representatives (Nelson et al. 1982; Teske and Nelson 2006),
although their role as diazotrophs in these large shelf mat communities has not been
explored to the best of our knowledge.
Shallow Sediments
Shallow and deep sediments are major biomes inhabited by diverse, N 2 -fixing
microorganisms (Carpenter and Capone 2008). Recent work in Narragansett Bay
has found that at times, N 2 fixation in shallow sediment environments can dominate
N transformations, exceeding N losses through denitrification and anammox
(Fulweiler et al. 2013; Fulweiler et al. 2007). Identification of diverse heterotrophic
diazotrophs in these sediments by gene abundance (Newell et al. 2016) and expression (Brown and Jenkins 2014) corroborate these observations. As earlier mentioned, a new group of marine hydrocarbon degrading gammaproteobacteria has
been found in shallow sediments exposed to petroleum hydrocarbons globally
(Karthikeyan et al. 2019) (see also Chap. 9).
Two other deep sediment studies have also documented substantial N 2 fixation in
shelf sediments off the coast of Peru and under the ODZ (Gier et al. 2016) and off
the coast of NW Africa (Gier et al. 2017).
Corals and Coral Reefs
As mentioned earlier (Chap. 3), coral reefs have been a long-standing subject of N 2
fixation studies. Stony (or hermatypic) corals have long been known to have evolved
highly integrated symbioses with their dinoflagellate partners (Davy et al. 2012).
Recent studies have now uncovered novel associations of diazotrophs in both
4.4 The Benthos
Microbial Mats
Microbial mats, vertically compressed, layered microbial ecosystems, have long
been a subject of research in microbial ecology (Cohen and Rosenberg 1989)
including their capacity to fix N 2 (Steppe et al. 1996). Cyanobacteria are often a
major component of the diazotrophic communities of microbial mats in photic environments (Stal 1988) although anoxygenic photosynthetic and heterotrophic bacteria are also members of the mat biota (Omoregie et al. 2004; Paerl et al. 1989; Zehr
et al. 1995).
Species of the bacteria Beggiatoa and Thioploca are also known to form large
mats in benthic environments ranging from salt marshes to shelf sediments beneath
highly productive upwelling zones (Gallardo and Espinoza 2007; Teske and Nelson
2006). Their physiologies can range from heterotrophic to autotrophic, and they
often exploit reduced inorganic sulfur species as electron donors. Both genera have
known diazotrophic representatives (Nelson et al. 1982; Teske and Nelson 2006),
although their role as diazotrophs in these large shelf mat communities has not been
explored to the best of our knowledge.
Shallow Sediments
Shallow and deep sediments are major biomes inhabited by diverse, N 2 -fixing
microorganisms (Carpenter and Capone 2008). Recent work in Narragansett Bay
has found that at times, N 2 fixation in shallow sediment environments can dominate
N transformations, exceeding N losses through denitrification and anammox
(Fulweiler et al. 2013; Fulweiler et al. 2007). Identification of diverse heterotrophic
diazotrophs in these sediments by gene abundance (Newell et al. 2016) and expression (Brown and Jenkins 2014) corroborate these observations. As earlier mentioned, a new group of marine hydrocarbon degrading gammaproteobacteria has
been found in shallow sediments exposed to petroleum hydrocarbons globally
(Karthikeyan et al. 2019) (see also Chap. 9).
Two other deep sediment studies have also documented substantial N 2 fixation in
shelf sediments off the coast of Peru and under the ODZ (Gier et al. 2016) and off
the coast of NW Africa (Gier et al. 2017).
Corals and Coral Reefs
As mentioned earlier (Chap. 3), coral reefs have been a long-standing subject of N 2
fixation studies. Stony (or hermatypic) corals have long been known to have evolved
highly integrated symbioses with their dinoflagellate partners (Davy et al. 2012).
Recent studies have now uncovered novel associations of diazotrophs in both
4.4 The Benthos