78 Marine Microbiology 



nitrifying bacteria in media free of NaCl and in media containing 

 2.5 per cent NaCl. Terrestrial bacteria grew best in media free 

 of NaCl, whereas the marine forms generally fared better in 

 media containing 2.5 per cent NaCl. In media containing NaCl 

 both Nitrosomonas europaea and Nitrobacter agilis grew, but 

 generation time was prolonged from one to eight days and the 

 cultm'cs failed to divide after about five generations. Both failed 

 to survive after more than two serial transfers in the presence 

 of NaCl. Experiments with marine nitrifying bacteria were highly 

 variable. In some experiments the organisms grew equally well 

 in the presence or absence of NaCl, in others no growth occurred 

 in the absence of NaCl. It was impossible to maintain stock cul- 

 tures of marine nitrifying organisms for prolonged periods in the 

 absence of NaCl. 



Altliough both Nitrosomonas europaea and Nitrobacter agilis 

 were obtained in pure culture by streaking on Noble's agar en- 

 riched with ammonia or by making serial dilutions, the marine 

 organisms could not be isolated using similar techniques. Thus 

 the present studies were conducted with mixed cultrn-es of marine 

 microorganisms. However, the ammonia-oxidizing bacteria were 

 successfully separated from the nitrite-oxidizing bacteria. 



Several moi*phologically different microorganisms persisted 

 in the marine cultures, although ammonia or nitrite served as 

 the sole source of substrate. None morphologically resembled any 

 described species of Nitrosomonas or Nitrobacter. All were mark- 

 edly different from Nitrosomonas europaea and Nitrobacter agilis. 

 Nitrosomonas europaea is an ellipsoidal rod 0.6 - 1.9 by 0.7 - 1.0 

 /x and frequently occurs in pairs (Fig, 2a); Nitrobacter agilis is 

 a rod 0.6 - 1.0 by 2.0 /t (Fig. 2b). The most prevalent bacterium 

 in the marine cultures was a rod 0.3 by 1.0 - 2.0 ft (Fig. 2c). 

 There was also a protozoan-like organism (Fig. 2d) in all cultures 

 where ammonia was oxidized to nitrite where it constituted the 

 greatest cell volume and often the greatest cell number. The 

 cells were 5 - 6 fi in diameter immediately after division but 

 elongated to 10 - 12 /a prior to division (Fig. 2e). Often in old 

 cultures the cells hypertrophied to two to three times their 

 normal size (Fig. 2f). Although cells were motile, flagella were 

 not visible upon examination with a phase-contrast microscope. 



