Environmental Engineering Reference
In-Depth Information
photoheterotropically in the light under anoxic conditions. Spirochaetes are
commonly found in marine environments and are obligately anaerobic or fac-
ultatively anaerobic.
The recently characterized oceanic aerobic anoxygenic photosynthetic bac-
teria (
Roseobacter
and
Erythrobacter
) have not yet been shown to be N
2
fixers,
although it seems likely that some representatives of this group may be dia-
zotrophic.
Most of the sulfate reducers (mostly delta proteobacteria) appear to fix N
2
,
as do many of the purple sulfur (mostly gamma proteobacteria) bacteria [136].
Numerous genera of green and purple sulfur bacteria fix N
2
, and both of these
groups are often found at the interface between oxic and anoxic layers in the
water column, as well as in microbial mats [120].
The observation that members of Archaea contain
nif
genes and can fix N
2
provides additional evidence that nitrogenases likely evolved early in prokary-
ote evolution. Among the anaerobic Archaea, N
2
fixation has been observed in
three orders of methanogenic
Euryarchaeota
:
Methanococcales
,
Methanomi-
crobiales
and
Methanobacteriales
[76]. To date, none of the
Crenarchaeota
that have been genetically characterized appear to have the capacity for N
2
fixation [76].
4.2 Phototrophs
The cyanobacteria include five major different morphologies that have
formed the basis of cyanobacterial taxonomy [106]. Diverse cyanobacteria
are N
2
fixers including representatives of all five morphological groups [129].
Many of these have been isolated from marine environments, although most
have been isolated from benthic environments rather than pelagic habitats.
Cyanobacterial blooms in lakes and estuaries are often composed of fila-
mentous heterocystous diazotrophs. The blooms of
Nodularia
are well known
in the Baltic Sea and around the world. Heterocystous free-living cyanobac-
teria, although not abundant, have been observed in the open ocean as well
[22]. One of the most abundant heterocystous species in the open ocean is the
cyanobacterial symbiont
Richelia
, which forms a symbiotic association with
the diatom
Rhizosolenia
. There are other similar symbioses observed between
marine diatoms and heterocystous cyanobacteria, although the taxonomic and
phylogenetic relationship of this filamentous group is not yet known.
Filamentous nonheterocystous cyanobacterial species are found in many
marine microbial mats and in the open ocean.
Trichodesmium
is one of the
most abundant diazotrophs in oligotrophic subtropical and tropical waters.
Other nonheterocystous species have been observed, usually in association
with
Trichodesmium
aggregates [117]. Diverse unicellular cyanobacteria also
fix N
2
, including representatives of the genera
Synechocystis
, and
Cyanothece
.
