ZoBell 



Marine Microbiology 



and Carey (1935&). When they added 2.5 mgm, of glucose to a liter of 

 natural sea water, they found 2,005,000 bacteria per ml. of water after 

 24 hours incubation as compared with only 625,000 in the control. This 

 quantity of glucose increased the plate count to 3,850,000 when the sea 

 water was also enriched with a little ammonium sulfate. During the same 

 period the addition of 15 mgm. of glucose per liter of sea water increased 

 the plate count to 24,050,000 bacteria per ml. These observations were 

 confirmed by Waksman and Renn (1936) who concluded that organic 

 carbon is the principal factor which limits the bacterial population of sea 

 water. 



When 1.5 mgm. or more of glucose is added per hter of sea water, com- 

 bined nitrogen also becomes a limiting factor for the multiplication of bac- 

 teria. The nitrogen requirements of most marine bacteria are satisfied by 

 either ammonium salts or amino acids. All of the 15 ''representative 

 aerobic bacteria of marine origin" studied by Ostroff and Henry (1939) 

 were found to utilize organic nitrogen compounds, but only 5 were able to 

 utilize inorganic ammonium compounds as a source of nitrogen in a min- 

 eral medium enriched with glucose. Most cultures grew luxuriantly on 

 amino acids, which, as a class of compounds, were the best source of nitro- 

 gen and carbon. The number of cultures which grew in glucose media 

 utilizing different nitrogen compounds is shown in Table XXV. 



Table XXV. — Number of cultures of marine bacteria out of 15 tested which utilized dif- 

 ferent nitrogen compounds in glucose media {from Ostroff and Henry, iQjg): — 



It is generally claimed that the minimum concentration of organic 

 nutrients required for the multiplication of heterotrophs ranges from o.ooi 

 to o.oi per cent or 10 to 100 mgm. per hter. Curran (1931) reported that 

 250 mgm. of peptone per liter was the minimum quantity that supported 

 the germination of bacterial spores. Bacteria which live in the ocean or in 

 oHgotrophic lakes must be pecuHarly adapted to live in extremely dilute 

 media because the organic content of sea water and many fresh-water 

 lakes is less than 5 mgm. per liter. ZoBell and Grant (1943) found that 

 most marine bacteria multiplied in mineral media containing only o.i 

 mgm. of peptone or glucose per liter. However, the bacteria multipHed 

 very slowly in the presence of such low concentrations of organic nutri- 

 ents. Probably the solid surfaces of the walls of the culture receptacles 

 made it possible for the bacteria to utilize the small quantities of organic 

 nutrients. Supplementing the solid surface by the addition of glass beads, 

 glass tubes, or other inert solids promotes the growth of marine bacteria 

 in media containing less than 10 mgm. of organic nutrients per liter. 



The optimum concentration of readily utilizable organic matter such 



