Chapter VI — 95 — Bottom Deposits 



Anderson (1940) has studied some factors which influence the bacterial 

 oxidation of the organic matter in marine sediments. 



In emphasizing the importance of organic matter raining down from 

 above, the presence of autotrophic bacteria in well aerated bottoms is 

 not being overlooked, but in most marine bottoms, autotrophs occur only 

 in insignificant numbers as compared with heterotrophic organisms. 



The dissolved oxygen content of the overlying water influences the 

 rate and kind of bacterial activity in bottom deposits, but there is no evi- 

 dence that oxygen tension has a marked effect on the total numbers of 

 bacteria found in mud. An abundance of free oxygen favors the rapid 

 multiplication of aerobes and, consequently, a rapid destruction of organic 

 matter. In the absencfe of free oxygen, bacteria may reproduce more 

 slowly, under which conditions the organic content is not rapidly depleted. 

 This, together with the reducing conditions characteristic of anaerobic 

 sediments, favors the prolonged survival of bacteria. Therefore, when 

 equilibrium is established, it is found that the replenishment of organic 

 matter from the overlying water is a more important factor than oxygen 

 tension in controlling the numbers of bacteria in bottom deposits. 



The oxygen tension influences the kinds of microorganisms present. 

 With very few exceptions, anaerobic bacteria are the only organisms 

 which are capable of normal activity in the absence of free oxygen. A few 

 anaerobic protozoans have been described, and some animals are able to 

 make temporary excursions into anaerobic environments. Most marine 

 yeasts, molds, actinomycetes, and chemo-autotropic bacteria require free 

 oxygen. 



Due to the rapidity with which oxygen is consumed by bacteria (Zo- 

 Bell, 1940a, b) in bottom deposits and the slowness with which it is re- 

 placed by diffusion or oceanic circulation, there is little or no free oxygen 

 in many bottom deposits. As far as is known, conditions are completely 

 anaerobic below the topmost few inches of sedimentary material. In the 

 mud at the bottom of stagnant bodies of water including land-locked 

 fjords, isolated ocean basins, and certain inland seas such as the Black 

 Sea, for example, bacteria appear to be the only forms of life (Str0m, 

 1939). According to Allgeier et al. (1932), the organic matter in lake 

 deposits is subject to anaerobic fermentation. These workers demon- 

 strated by plating procedures the presence of as many as 450,000 bacteria 

 per ml. of anaerobic material. 



Biocoenosis and bacteria in bottom deposits : — Due to the effects of 

 bacteria upon environmental conditions in bottom deposits, ecologists 

 (MacGinitie, 1935; Mare, 1942; ZoBell and Feltham, 1942; Pearse 

 et al., 1942) recognize that bacterial activities must be taken into account 

 in explaining the distribution of plants and animals. Similarly plants and 

 animals in bottom deposits have a pronounced effect upon the bacterial 

 population. 



Photos>Tithetic plants provide the principal source of organic matter. 

 In deep water, phytoplankton organisms or their remains settle to the sea 

 floor where they are buried or decomposed by microorganisms. In shal- 

 low waters penetrated to the floor by sunlight, both planktonic and 

 benthonic plants may be found growing at the bottom. Bes'des providing 

 food for bacteria and animals, such plants produce or consume oxygen and 

 carbon dioxide and otherwise influence the chemical composition of the 

 mud and water. 



