14 Marine Microbiology 



matter for food, the well-being or survival of heterotrophic 

 bacteria is influenced by predators, solid surfaces, and numerous 

 other factors. 



Unless othenvise stated, data on the abundance of bacteria 

 are based upon the number which form visible colonies under 

 laboratory conditions of cultivation. Since no one medium or com- 

 bination of cultural conditions ( temperature, oxygen tension, pH, 

 etc. ) pro\'ide for colony formation by all bacteria, colony counts 

 generally represent only a small fraction of the total number of 

 \iable cells. Direct microscropic methods of observation often 

 reveal from 10 to 100 times more bacteria than do cultural pro- 

 cedures (20). 



In the littoral zone, where most obsei'vations have been made 

 and where biological activity is usually most intense, colony 

 counts commonly indicate the presence of from 10^ to 10^ bacteria 

 per ml of water; the extremes range from <1 to 10^ bacteria per 

 ml. The scantest bacterial populations occur in the open ocean par- 

 ticularly at depths exceeding 1000 meters. So scarce are bacteria 

 in many pelagic areas that from 50 to 1000 ml of water must be 

 analyzed (either by concentration on membrane ultrafilters or 

 by enrichment with nutrients ) in order to demonstrate the pres- 

 ence of viable bacteria. 



During the 1957-58 Vitiaz expedition Kriss (25) examined 

 1039 water samples collected at depths to 9,000 meters in the 

 central regions of the Pacific Ocean along the 172nd meridian 

 east longitude and 174th meridian west longitude between 37° 

 north latitude and 41° south latitude. Following filtration of 50 

 ml of water through membrane ultrafilters, the filters were super- 

 imposed on meat or fish peptone agar in Petri dishes. After three 

 to four days, incubation at 25 to 30 C the colonies were counted. 

 The number of colonies developing from 50 ml of water ranged 

 from nil to a few hundred. There was a high order of variability 

 in the abundance of bacteria vertically as well as horizontally. 

 Great differences in the abimdance of bacteria found in different 

 areas were attributed to the concentration of easily assimilable 

 organic matter. Considerable variation in the abundance of bac- 

 teria was also noted bv Kriss (25) in the Indian Ocean where 



