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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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