Environmental Engineering Reference
In-Depth Information
4. DIVERSITY OF AEROBIC AND ANAEROBIC
N
2
-FIXING MICROORGANISMS
Although N
2
fixation is a property found only in prokaryotes, the phyloge-
netic and physiological diversity of N
2
-fixing microorganisms spans most of the
prokaryotic kingdom [136]. N
2
fixation is found in a wide diversity of anaero-
bic, aerobic, and facultative microorganisms, with equally diverse physiological
strategies including anoxygenic and oxygenic phototrophy, photoheterotrophy,
chemoheterotrophy, and chemolithotrophy. Archaea and Eubacteria contain
diazotrophic representatives, and within the Eubacteria various members of
the proteobacteria (alpha, beta, gamma and delta), cyanobacteria, clostridia,
green and purple sulfur bacteria all appear to have the genetic potential for
N
2
fixation. There are selected phylogenetic groups within the Eubacteria and
Archaea that do not contain N
2
-fixing representatives, including
Planctomyces
and
Flavobacter
, but this may reflect a lack of information on the genomic
diversity of these groups rather than evolutionary patterns. Relatively few N
2
-
fixing microorganisms have yet been targeted by genome sequencing projects,
and our knowledge of most of the prokaryote N
2
fixers remains quite poor.
Characterizing the diversity of N
2
fixing microorganisms in marine environ-
ments continues to be hampered by the difficulties of cultivating microorgan-
isms from oligotrophic environments. Many isolates have been obtained from
organic matter-rich environments, such as salt marshes and sediments [5]. Di-
azotrophic microorganisms were plated from the oligotrophic oceans in early
studies that predate characterizations of microbes by rRNA sequence analysis
[64]. Efforts to cultivate diazotrophs from oceanic environments is an area ripe
for progress in future studies, as new high throughput cultivation techniques
have recently been developed [58, 103]. Here we review some of the diversity
of cultivated microorganisms that fix N
2
, in order to provide context for the
observed physiological diversity in marine diazotrophic communities.
The American Type Culture Collection contains over 650 bacterial isolates
from the marine environment. Genera that are included in this collection that
could be diazotrophic are
Vibrio
,
Bacillus
,
Alcaligenes
,
Rhodopseudomonas
,
Clostridium
,
Spirochaeta
,
Methanococcus
,
Desulfobacter
and
Desulfonema
.
Paerl and Zehr [95] provide a list of marine cultivated microorganisms that
are available from culture collections. Various microorganisms have been cul-
tivated from salt marshes such as vibrios, rhizobia, spirilli and pseudomonads
[5]. Many of these organisms are facultative or obligate anaerobes and are typ-
ically found at oxic/anoxic interfaces and could also be present in anoxic water
columns. These collections would serve as important sources of microorgan-
isms in investigations of diversity and activity of N
2
fixing microorganisms in
anoxic marine environments.
