76 Marine Microbiology 



buffer until less than 1.25 /xM NH4^-N or NO2 -N remained. The 

 cell paste was resuspended in sterile medium and diluted to give 

 oxygen uptake values of about 5 ml/L/hr. 



The respiration chamber was a 250-ml graduated cylinder 

 shortened to 150 ml. The mouth was sealed off tightly with a 

 rubber stopper; care was taken to prevent the inclusion of air 

 bubbles. The oxygen electrode, pH electrodes, a thermometer, 

 and a capillary tube extended into the chamber through the 

 stopper. The culture was stirred by a Teflon-coated magnetic 

 bar. The pH was monitored with a Zeromatic pH meter, and the 

 oxygen consumption recorded continuously on a Leeds and North- 

 throp recorder using a 1 mv sensitivity. The substrate concentra- 

 tion could be altered by adding (NHj)2SOi or the pH read- 

 justed by injecting 1 N HCl or NaOH via the capillary tube. 

 The temperature was held at 24.8 C ± 0.01° and the pH was 

 controlled to within 0.05 units. 



EXPERIMENTAL RESULTS 



Ammonia was oxidized in cultures inoculated with sediment 

 from nine out of the fourteen stations less than 100 m deep; ni- 

 trite was oxidized in all bottom samples from the seven shallow 

 stations. Nitrification was not demonstrated in mud collected 

 from stations over 100 m deep. Nitrification did occur in fifteen 

 out of the 32 water samples collected in 5- to 10-liter quantities, 

 but not in any of the 100-ml samples. The fifteen samples were 

 from 10 m over the continental shelf, from slope waters at depths 

 of 10, 50, 300, 1300, and 1500 m, and from the Sargasso Sea at 

 a depth of 10 m. 



In the samples enriched with ammonia, nitrification began 

 after a long lag of 20 to 40 days ( Fig. 1 ) . Once nitrification had 

 started, nitrites and nitrates increased almost simultaneously until 

 each reached concentrations of 20 to 60 mM. 



Subcultures of ammonia- and nitrite-oxidizing microorgan- 

 isms were obtained while nitrites and nitrates were increasing ex- 

 ponentially. Both physiological types of microorganisms have 

 been maintained in the laboratory for the last eighteen months 

 by subculturing them periodically. 



Media with CaCO:: were superior to completely soluble 



