Chapter XII — 161 — Sulfur Bacteria 



this ability is an unsolved problem. Certainly most microorganisms are 

 capable of obtaining their sulfur requirements from sulfate by a process 

 which Beijekinck (1895) called ''non-specific" sulfate reduction. For 

 such organisms, however, sulfate reduction is not an essential part of their 

 oxidative metabolism as it appears to be for the Desulfovibrio. 



Desulfovibrio aestuarii can utilize not only sulfate, but also sulfite, 

 thiosulfate, and sulfur as hydrogen acceptors. Ordinarily H2S is the 

 chief end product, although there is evidence that sulfur may accumulate 

 under certain conditions. Many marine microorganisms are able to re- 

 duce sulfite and thiosulfate to H2S. 



The oxidation of sulfur compounds : — Sulfur bacteria probably de- 

 pend largely upon the activities of sulfate-reducing bacteria for H2S, 

 thiosulfate, and other sulfur compounds which they oxidize. Several 

 physiological types of bacteria, including both autotrophs and sapro- 

 phytes, which oxidize sulfur or its compounds, occur in the sea. Some of 

 the autotrophs obtain their energy chemosynthetically from the oxidation 

 of H2S, sulfur, thiosulfate, or tetrathionate, and some use photic energy. 



The photosynthetic autotrophs can oxidize H2S in an anaerobic en- 

 vironment, whereas most of the chemosynthetic autotrophs are strict 

 aerobes or microaerophiles. This greatly restricts the localities in which 

 sulfur bacteria can be functional in the sea. The penetration of sunlight 

 is limited to the topmost hundred or so meters of water, and light pene- 

 trates bottom deposits to a negligible extent. The presence of H2S in 

 aerobic environments is limited largely to localities where it is being con- 

 tinuously produced. Several physiological types of sulfur bacteria occur 

 in shallow water where these conditions persist. Such conditions may also 

 occur in growth zones or lamina at greater depths, where H2S from below 

 difi'uses upward into overlying oxygenated waters. Such lamina have 

 been reported in the Black Sea, Caspian Sea, Lake Ritom, and elsewhere. 

 The question of the bacterial activity in this thin layer or lamina in the 

 Black Sea is elaborated by Ravich-Sherbo (1930). The term "bacterial 

 plate" or "bacterial fountain" has been applied to localized laminae 

 (Ellis, 1932). 



The phylogenetic position of the recognized genera of sulfur-oxidizing 

 bacteria is outlined in Table XXXIV on page 1 24. They can be grouped 

 for the convenience of this discussion into the following categories accord- 

 ing to their pigmentation and sulfur metabolism: 



I. Achromic sulfur bacteria, which have neither bacteriopurpurin nor 

 bacterioverdin and consist of (a) the Thiohacilhis group, species of which 

 oxidize various sulfur compounds, usually to sulfate, although sulfur may 

 be deposited extracellularly and {h) Leucothiohacteria belonging to genera 

 of Achromatiaceae and Beggiatoaceae, which deposit sulfur intracellu- 

 larly. 



II. Purple sulfur bacteria, or RJwdothiobacteria, which contain bac- 

 teriopurpurin and consist of (a) the Thiorhodaceae or Chromotioidaceae, 

 the cells of which contain sulfur granules and {b) the Athiorhodaceae or 

 Rhodobacterioidaceae, which do not deposit sulfur intracellularly. The 

 green sulfur bacteria, or Chlorobacteria, are generally considered in this 

 category, although they contain bacteriochlorin instead of bacterio- 

 purpurin. 



The foregoing is not proposed as a new system of classification. It is a 

 hybrid system designed only to expedite the discussion of bacteria which 



