Chapter VI — 93 — Bottom Deposits 



bacteria in Pennsylvania anthracite at a depth of 1800 feet, but Farrell 

 and Turner (1932) have questioned the significance of bacteria found in 

 anthracite coal. Issatchenko (1940) avers that bacteria, which he found 

 in oil to a depth of 2000 meters, are indigenous species from near the lower 

 limits of the biosphere. If his contention is substantiated, then we might 

 expect temperature, organic matter, and water to be the chief factors 

 which limit the depth to which living microorganisms may occur in sedi- 

 ments. While there is no reason to believe that living bacteria are widely 

 distributed in ancient sediments at great depths, accumulating field and 

 laboratory data indicate that bacteria are alive and active in certain 

 deeply buried deposits containing organic matter. 



The occurrence of viable bacteria at great depths in sediments leads 

 one to wonder whether they have been reproducing throughout the thou- 

 sands of years that they have been buried or if they are passively resting 

 cells in a state of suspended animation. ZoBell and Anderson (19366) 

 have pointed out that conditions in the bottom deposits of the sea, 

 namely low temperature and absence of free oxygen, have proven most 

 conducive to great longevity of bacteria in a dormant state. Anaerobic 

 bacteria may be physiologically active in such an environment, but some 

 of the strict aerobes which have been found at a depth of several feet 

 either have been carried there by burrowing organisms or other agencies 

 or else they have been buried in a dormant state for many centuries. Since 

 we have no reasons for believing that strict aerobes have been carried deep 

 into the bottom deposits and since the aerobes in question are mostly 

 sporogenous Bacillus species, these bacteria may have survived in a dor- 

 mant state for many centuries. Of course, there is a possibility that the 

 so-called "strict aerobes" are capable of reproducing in a highly reducing 

 environment by physiological mechanisms still unknown to microbiolo- 

 gists. 



While a few strict aerobes are found in bottom deposits at appreciable 

 depths, their abundance decreases sharply from the surface downward 

 (ZoBell, 1938a). Curves depicting the vertical distribution of aerobes 

 are reminiscent of logarithmic curves for the order of death of bacteria 

 when the survivors are plotted against time. Henrici and McCoy (1938) 

 found that profile series of bacteria from lake bottom deposits give typical 

 logarithmic death curves, suggesting that aerobes are slowly dying oflf. 

 However, that many of the bacteria (probably anaerobes) in bottom 

 deposits are physiologically active to great depths is indicated by the 

 changes which they cause in the chemical composition and physico- 

 chemical properties of the bottom deposits, as is elaborated in Chapter 

 VII. 



Factors influencing abundance of bacteria in mud : — Neither distance 

 from the mainland nor depth of the overlying water seems to have any 

 direct influence upon the bacterial population of bottom deposits. This 

 conclusion is based upon a study of several hundred mud samples, the 

 bacterial content of which has been reported by various workers. In the 

 Gulf of Naples, Russell (1892) observed a decreasing abundance of bac- 

 teria in mud samples with distance from shore, but in light of recent infor- 

 mation this horizontal distribution appears to be more closely related to 

 the organic content of the mud than to its distance from land or to the 

 depth of the overlying water. This also apphes to Reuszer's (1933) ob- 

 servation that the bacterial numbers on the continental shelf in the vicin- 

 ity of Woods Hole decrease regularly with distance from land. 



