BACTERIA IN THE DEEP SEA 203 



about two-thirds of the carbon in organic compounds decomposed by 

 bacteria is oxidized to carbon dioxide; the other one-third is used to 

 build new bacterial cells. This may be important in the carbon economy 

 or food cycles in the sea. Besides producing carbon dioxide, bacteria 

 catalyze the con\'crsion of waste organic materials into other plant nutri- 

 ents, including ammonium, phosphates, and nitrates. 



Most species of marine bacteria require organic compounds for their 

 nutrition and growth. A few species are unique, however, in being able 

 to synthesize their protoplasm entirely from inorganic substances, much 

 as green plants do. Instead of utilizing sunlight as a source of energy, 

 though, as do the plants, these unique bacteria, called chemosynthetic 

 autotrophs, oxidize either molecular hydrogen, hydrogen sulphide, methane, 

 ammonium, nitrite, or other inorganic substances as an energy source. 

 Such bacteria are probably the only primary producers of organic matter 

 below the photosynthetic zone, the topmost layer of a few hundred metres 

 of sea water penetrated by sunlight. 



By virtue of their action on both organic and inorganic constituents, 

 bacteria are believed to affect the chemical composition of sea water in 

 many ways. In a like manner they may affect geochemical and geological 

 conditions in marine bottom deposits. 



It has been known for a long time that bacteria occur abundantly in 

 shallow seas. There has been little opportunity, however, to look for bac- 

 teria in the deep sea. The Galathea Expedition was the first to afford 

 facilities for collecting and examining material from the greatest known 

 depths of the ocean, some samples coming from bottom deposits more 

 than 10,000 meters below sea level. This nearly doubled the depths pre- 

 viously examined for the presence of living bacteria. 



A feeling of intense excitement and expectation inspired every officer, sea- 

 man, and scientist on the Galathea as the first sample of mud from 

 record-breaking depths was hauled aboard on July 15, 195 1. This 

 sample collected at Station No. 413 in the Philippine Trench came from 

 a depth of 10,060 metres. A glimpse at material from such great depths 

 was an exciting privilege. 



Many questions raced through the minds or were on the lips of the 

 fortunate few who first saw this mud sample: What is its chemical com- 

 position? What would it reveal regarding the nature of the deep sea 

 floor? Is there any evidence of life in the bottom mud? Many authorities 

 had questioned the existence of living organisms in oceanic abysses owing 

 to the high hydrostatic pressure, low temperature, alleged lack of food, 

 and other adverse environmental conditions. 



