208 BACTERIA IN THE DEEP SEA 



grass Standing in the pasture, but more continues to grow. In a somewhat 

 similar manner, marine animals are believed to graze off the bacteria 

 until the standing crop is reduced to a few million per millimetre (cc). 



Protozoans, worms, sponges, filter feeders, and mud-eaters are among 

 the predatory animals known to ingest and digest bacteria as a source of 

 food. What percentage of the food of deep-sea animals comes either 

 directly or indirectly from bacteria remains problematical, but it may be 

 appreciable. About the only other sources of food for deep-sea animals are 

 the bodies or parts of animals and plants raining down from the photo- 

 synthetic zone. The Galathea Expedition obtained ample evidence that 

 particulate organic matter from the photosynthetic zone does reach the 

 deep-sea floor, but whether it is enough to nourish the animals there is 

 questionable. 



The thinking reader will also wonder how the deep-sea bacteria obtain 

 their food or nourishment. In the first place, bacteria are more versatile 

 than animals in the kinds and varieties of carbon compounds that they 

 can utilize for food. Besides being able to utilize virtually all classes of 

 organic matter regardless of complexity, many bacterial species can ob- 

 tain all of their nutrient necessities from inorganic substances. Then, unlike 

 most marine animals, there are bacteria which thrive on organic sub- 

 stances dissolved in sea water. Dissolved organic matter carried into oceanic 

 abysses by circulating water is believed to be the principal source of nutri- 

 ments for deep-sea bacteria. 



Samples of sea water, filtered to remove all particulate matter, provide 

 for the growth and reproduction of bacteria until the population may 

 reach several hundred million per millilitre, since no animals are present 

 in the filtered water to consume the bacteria. The utilization of dissolved 

 organic matter by bacteria may be partly responsible for water samples 

 from great depths sometimes containing less than one part per million, 

 whereas samples from the photosynthetic zone may contain ten times as 

 much organic matter in solution or suspension. 



In addition to serving as a source of food for certain marine animals, 

 bacteria may aid animals in the digestion of their food in much the same 

 manner as the bacteria in the rumen of cattle or sheep aid these ruminants 

 in the digestion of their food. Particles of cellulose, chitin, keratin, lignin, 

 and other complex compounds swallowed by animals may be digested 

 only with the help of bacteria residing in the animal's alimentary tract. 



Accessory growth factors or vitamins synthesized by bacteria may con- 

 tribute to the nutrition of animals in the deep sea. Vitamins of the B com- 

 plex are produced by many marine bacteria. Some species produce caro- 



