ZoBell — 112 — Marine Microbiology 



Besides their ability to produce and transform petroleum hydrocar- 

 bons (Tausson and Alioschina, 1932), sulfate-reducing bacteria may 

 help to liberate adsorbed oil by producing CO2 which decreases the viscos- 

 ity of oil. Likewise, sulfate reducers may dissolve limestone or dolomite, 

 thereby liberating the adsorbed oil, or the latter may be set free by 

 microbially produced surface-active or detergent substances. Other 

 anaerobes besides sulfate reducers may also contribute to these processes. 



The low Eh, or highly reducing conditions, created by bacteria in bot- 

 tom deposits tend to favor the formation and preservation of petroleum 

 hydrocarbons. Trask and Patnode (1942) point out that source beds of 

 petroleum are usually highly reducing. In so far as is known, bacteria are 

 the principal agents responsible for these reducing conditions. 



While there are several ways in which bacterial activities may con- 

 tribute to petroleum formation, under certain conditions hydrocarbon- 

 oxidizing bacteria may prevent the accumulation of oil. The literature 

 on this subject has been reviewed by ZoBell et al. (1943). Further in- 

 formation on the relation of bacteria to petroleum formation is given by 

 ZoBell (1943a). 



Sulfur deposition : — There are two main types of elementary sulfur 

 occurring in nature. The solfataric or volcanic type is formed from H2S 

 and SO2 in volcanic gases. The gypsum type is believed to result from the 

 reduction of calcium sulfate. The gypsum type of sulfur deposits are as- 

 sociated with marine sediments. There is evidence that g3^sum or cal- 

 cium sulfate is being reduced to sulfur at the expense of the buried organic 

 matter which serves as a source of energy for the reaction. Although con- 

 clusive proof is still lacking, it is generally believed by geologists and 

 microbiologists that anaerobic bacteria are responsible for sulfur deposits 

 of the gypsum type. Some of these sulfur deposits in Louisiana and Texas 

 are a hundred or more feet thick. Unique sulfate-reducing bacteria, which 

 appear to be indigenous species, have been isolated from sulfur-limestone- 

 anhydrite formations from a depth of 1550 feet. Hunt (191 5) attributed 

 the origin of the sulfur deposits of Sicily to the bacterial reduction of 

 sulfates in ancient shallow marine seas resembling present conditions in 

 the Black Sea. 



Sulfur is also deposited either intracellularly or extracellularly by vari- 

 ous autotrophic bacteria. The activities of autotrophic sulfur bacteria as 

 well as those of heterotrophic sulfate-reducing bacteria are discussed in 

 Chapter XII. As geological agents which deposit sulfur, the sulfate re- 

 ducers are probably by far the more important of the two categories of 

 microorganisms. 



Particle-binding action of microorganisms : — Besides their effects 

 on the chemical and physico-chemical conditions of bottom deposits, there 

 are several ways in which microorganisms may promote mechanically the 

 diagenesis, lithification, or consolidation of particles of sand, silt, clay, 

 and colloids which constitute marine sediments. 



The calcareous materials which are precipitated either directly or in- 

 directly by bacteria in recent sediments tend to cement together solid 

 particles such as clay, silt, or sand. CaCOs is one of the commonest 

 cementing materials in sedimentary rocks. MgCOs is also important. 



Certain bacteria produce mucilaginous exudates or capsular materials 

 which adhere to particles of sediments or which cause the particles to 



