ZoBell — 148 — Marine Microbiology 



of the Dead Sea, Elazari-Volcani (1943) obtained enrichment cultures 

 in mineral media containing only kerosene or petroleum as the carbon 

 source. 



Rubber, both natural and synthetic, is another type of hydrocarbon 

 which is attacked by microorganisms (ZoBell and Beckwith, 1944). 

 Although the decomposition of rubber in the sea is of little consequence 

 in the carbon cycle, it is a problem of economic importance. 



Marine humus: — There is evidence that more than 99 per cent of 

 the organic matter produced in the sea undergoes complete decomposition 

 or mineralization. The remaining fraction of one per cent is buried to be- 

 come an integral part of old marine sediments. This fraction, which is 

 highly resistant to decomposition, is known as humus. It consists of the 

 residues of plants, animals, and bacteria which have been subjected to 

 the enzymes of animals and bacteria in the marine environment. Some 

 marine humus originates from terrigenous deposition, but in the open 

 ocean pelagic organisms, primarily plankton, contribute most of the 

 organic residues. Nearer shore, Zostera and sessile algae may be impor- 

 tant sources of humus in bottom deposits (Jensen, 191 5). 



The chemical composition of humus depends primarily upon the or- 

 ganic matter from which it was derived and the transformation of the 

 organic matter during and after its deposition. The relative roles played 

 by autolytic enzymes, animal digestion, and bacterial activity constitute 

 an unsolved problem, but it is generally agreed that bacterial activity is 

 of prime importance. Waksman (1933) states that only a small part of 

 the organic matter built up by marine plants passes through the animal 

 body, and that the major portion is destroyed post mortem through direct 

 bacterial action. 



The quality and numbers of transformations caused by bacterial activ- 

 ities depends to a great extent upon the kinds of microorganisms present 

 and the environmental conditions. For example, certain constituents of 

 buried humus in bottom deposits which are relatively stable in an anaer- 

 obic environment may be subject to further decomposition by bacteria 

 in the presence of free oxygen. The oxidation-reduction potential and 

 the hydrogen-ion concentration are important factors which influence the 

 state and composition of humus. 



Waksman (1933) finds that marine humus consists predominantly of 

 a lignoprotein complex and a hemicellulose-protein complex. Uronic 

 acid, ether-soluble extractives, and alcohol-soluble extractives were also 

 found. The chemical composition as well as relative amounts of the differ- 

 ent fractions vary in different mud samples. 



Although humus is defined as being organic matter which is resistant 

 to further decomposition by bacteria, there is evidence that humus is not 

 absolutely resistant. Gradual decreases in the organic content of sedi- 

 ments with core depth show that over a period of thousands of years of 

 burial the organic matter is undergoing gradual decomposition, a process 

 which appears to be progressively slower with age. 



Waksman (1933) found that, in the presence of sufficient oxygen, 

 marine humus undergoes a process of slow bacterial decomposition, as 

 shown by the continuous liberation of CO2 and ammonia. He points out 

 that humus imparts certain characteristic properties to the marine bot- 

 tom, making it a more favorable medium for the growth of bacteria and 

 animals. In summary, the composition and amounts of humus both affect 

 and are affected by the activities of marine microflora and fauna. 



