Environmental Engineering Reference
In-Depth Information
4.1 Heterotrophs
In the oxygenated oceans, diazotrophic bacteria are likely to be largely
aerobic, utilizing either photoautotrophic or aerobic heterotrophic metabolism.
Representatives of diverse aerobic proteobacteria lineages fix N
2
; however,
even within N
2
-fixing genera, not all species appear capable of diazotrophy
[136]. For example, the genera
Vibrio
and
Klebsiella
contain many species that
do not appear to be able to fix N
2
.
Azotobacter
species are obligate aerobic diazotrophic chemoheterotrophs
that fix N
2
. They have been isolated from salt marshes or sediments [27]. Other
known aerobic chemoheterotrophs include
Rhizobia, Bradyrhizobia
and
Bei-
jerinckia
, but they have not been isolated from the marine environment. Aerobic
bacteria of a wide variety of morphologies (rods, cocci) have been isolated from
seawater samples, but acetylene reduction by yeasts cultivated by these methods
were also reported [135]. Part of the problem of isolating N
2
-fixing microor-
ganisms is the contamination of media reagents with trace nitrogen compounds
that can serve as nitrogen sources for oligotrophs. Kawai and Sugahara [64]
isolated aerobic N
2
fixing microorganisms from seawater and sediments. These
authors reported abundances much lower than total heterotrophs, but again, due
to the problems of removing trace nitrogen compounds, it is not clear whether
they enumerated oligotrophs or N
2
-fixers. Martinez et al. [78] described N
2
-
fixing bacteria in association with free-floating mats of diatoms. These bacteria
are probably aerobes, since the mats are in aerobic waters, but could be fac-
ultatively anaerobic. Anaerobic microzones can occur in aggregates in aerobic
water columns [93].
Methane oxidizers and methylotrophs are obligate aerobes that include a
variety of diazotrophs. A number of methanotrophs including
Methylocystis
sp.,
Methylosinus
sp., and
Methylobacter
sp. contain
nif
genes and are capable
of N
2
fixation [3]. The methane oxidizers may be important in aquatic systems,
particularly at oxic/anoxic interfaces.
Representatives of diazotrophs are found among all groups of proteobacteria,
the green and purple sulfur bacteria, purple nonsulfur bacteria, methanogens,
sulfate reducers and clostridia. The anaerobic and microaerophilic diazotrophs
include members of both the Archaea and Eubacteria. Diverse physiological
capabilities characterize anaerobic N
2
fixers, with organisms deriving energy
from both phototrophy and chemotrophy. A suite of electron acceptors can be
utilized for anaerobic respiration including nitrate (denitrifiers such as
Brady-
rhizobium
), sulfate (sulfate reducers such as
Desulfovibrio
), CO
2
(methanogens
such as
Methanosarcina
), Mn (manganese reducers such as
Arthrobacter
), Fe
(iron reducers such as
Geobacter
), and organics (fermentation such as
Kleb-
siella
). Facultative diazotrophs include the photoheterotrophic purple bacteria.
Rhodospirillum
and
Rhodobacter
are aerobic heterotrophs which can grow
