N85-32737
CHAPTER II
SULFUR CYCLING AND METABOLISH OF PHOmTROPHIC
AND FILAMENTOUS SULFUR BACTERIA
Prof. R. Guerrero
D. Brune
R. Poplawski
T.n. Schnidt
Introduction
Sulfur, an abundant eleeent in the biosphere, is rarely a
limiting nutrient -for organisns. Its proportion in living
material has been estimated to be 1 atom of S for IS atoms of N
and luO atoms of C. The element sulfur exists in oxidation states
from -2 to ■•-6 in organic and inorganic 'Molecules. Microorganisms
catalyse the oxidation and reduction of different forms of sulfur,
establishing a cycle that involves sulfur incorporation into
organic matter (anabolic, structural, sIom cycling) or the use of
different sulfur compounds as acceptors or donors of electrons
(catabolir, energetic, rapid cycle). Nonassimilatory sulfur
metabolism coupled with the carbon cycle may represent the olde!i:^t
energy cycle in the biosphere, one used by the earliest
autotrophic prokaryotes to obtain energy (Clark, 1981).
Hydrogen sulfide is a highly reactive, extremely toxic
compound subject to both biological and nonbiological oxidation.
Sulfide can be o>:idized to sulfur and sulfate by bacteria under
aerobic as well as anaerobic conditions. Soa« bacteria oxidize
sulfide aerobically to generate energy. Bcggiatoa and
Thiothriji. for instance, are filamentous, microaerophilic
bacteria capable of oxidizing sulfide, and depositing sulfur
globules within the cells:
2 HzS *• Oa — » 2S + 2 H^O
In the absence of sulfide, the sulfur globules are oxidized
further to sulfate. These are typical "gradient organisms,"
positioning themselves in the interfaces of anaerobic environments
<sul fate-reducing sediment or sul fide-rich layers of water) with
over lying, partially oxygenated waters.
Hydrogen sulfide is also subject to biological photooxidation
m anaerobic environments. Phototrophic sulfur bacteria
<Chromatiaceae and Chlorobi aceae) are able to photoreduce carbon
dioxide while oxidizing sulfide, first to elemental sulfur and
later to sulfate (CHaO symbolizes photosynthate) :
CO, + 2 HsS > CHaC + 2S + HaO
3 COa + 2S + 5 HaO — ♦ 3 CHaO + 2 HaSO^
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Chromatiaceae are also capable c-f accumulating internal sulfur
globules <the genus Ectothiorhodospira is the only exception,
and accordingly will be separated into a new fanily). Some
ChroAatiaceae ca>i tolerate low concentrations o-f oxygen aiid thus
are also considered "gradient organisms."
Although o;<idation o-f sul-fide to sulfate by different
microorganisms is well known, tde use of internal sulfur globules
as electron acceptors to oxidize or derive energy from storage
compounds such as glycogen in the absence of an external source of
energy (endogenixis metabolism) was hypothesized a long time ago
(Oawes and Ribbons, 1964; van Gemerden, 1968) but not definitively
demonstrated.
To investigate different aspects of the ecophysiology of
purple and green bacteria the following studies were performed:
1. Phototrophic sulfur bacteria Letken from different
habitats (Alum Rock State Park, Palo Alto salt marsh, and
Big Soda Lake) were grown on selective media,
characterized by morphological and pigment analysis, and
compared with bacteria maintained in pure culture.
2. A study was made of the anaerobic reduction of
intracellular sulfur globules by a phototrophic sulfu<^
bacterium (Chromatiam vinos am) and a filamentous
aerobic sulfur bacterium iBegjiatoa ai£>a).
3. Buoyant densities of different bacteria were measured
in Percoll gradients. This method was also used to
separate different chlorobia in mixed cultures and to
assess the relative homogeneity of cultures taken directly
or enriched from natural samples (including the purple
bcscterial layer found at a depth of 20 meters at Big Soda
Lake. )
4. Interactions between sul fide-oxidizing bacteria were
studied. Pairs of sulfi de-oxidizing species competed for
electrons (sulfide was the only available electron donor
in the medium common to a purple sulfur bacterium
(Chromatium vinosum) ^ a green sul-.ur bacterium
(Chi orobjum phaeobacteroide-s) and a cyanobacterium
iOsciJ I atoria limnetica)). These bacteria, selected
because of their sulfide requirements and the tact that
they can co— exist in aquatic enviro«nents ^here intense
gradients occur, were handled pairwise by placement in a
common medium separated by a membrane filter. Competition
between two of these species at a time was measured under
conditions where metabolites and toxins (but not cells)
passed easily through the common culture medium.
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References
Clark, B.C., 1981. Sulfur: Fountainhead of life in the
universe? In Life in the Universe (J. Bill Ingham,
ed.), pp. A7-60, MIT Press, Cambridge.
Dawes, E.A. and Ribbons, O.M. , 1964. Some aspects o-f the
endogenous metabolisn of bacteria, Bacteriol. Rev.,
28:126-149.
van 6eraeden, H. , 1968. On the ATP generation of
Chroaatium in darkness, A.^ch. Mikrobiol.,
64:118-124.
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