BIOGRAPHICAL MEMORANDA 



desulfuricans has also been reinvestigated elsewhere, and it has been 

 found that strains of varied origin display unpredictable differences in 

 behaviour in this respect. Some strains appear to be readily adapt- 

 able, others not at all [Littlewood and Postgate, 1957]. 



While thus the variability of some physiological properties of the 

 sulphate reducing bacteria appears to be less general or pronounced 

 than was claimed in 1930, the studies of Kluyver and Manten [1942] 

 with a culture of an hydrogen-oxidizing bacterium, identified as Hydro- 

 genomonas flava, provided an example of another sort of physiological 

 variability. By means of manometric experiments it was established 

 that suspensions of this organism grown in mineral media at the 

 expense of hydrogen oxidation, can oxidize hydrogen gas as well as 

 organic substrates, and that such oxidations can even proceed simul- 

 taneously; whereas cultures grown in media with organic compounds 

 in the absence of hydrogen completely failed to oxidize the latter. This 

 indicated that 'obviously the formation of the special hydrogen oxi- 

 dizing system only takes place when the bacteria are grown under 

 autotrophic conditions'. Such a behaviour is reminiscent of that dis- 

 played by certain yeasts with respect to their ability to ferment galac- 

 tose, and, as mentioned earlier, Kluyver was fully cognizant of its 

 significance. 



The experiments with H. flava revealed furthermore that, after 

 prolonged cultivation on organic media in the absence of gaseous 

 hydrogen, its ability to grow autotrophically was irretrievably lost. A 

 fully satisfactory explanation of this phenomenon, previously also ob- 

 served by Grohmann, has not yet been proposed. 



This investigation of H. flava, together with renewed studies in 

 Kluyver's institute on bacterial denitrification, eventually led to a 

 search for naturally occurring micro-organisms that can oxidize hy- 

 drogen with nitrate as the sole oxidant. The existence of such organ- 

 isms was probable on theoretical grounds, but had never been con- 

 clusively demonstrated. By means of appropriate elective cultures the 

 presence of such specialists in soil samples was readily ascertained, 

 although it appeared that the bacteria that can carry out this con- 

 version can grow in an anaerobic environment with hydrogen and 

 nitrate only if a small amount of yeast extract is present in the medium. 



A careful examination of such cultures showed that they contained 

 only one predominant bacterial type which closely resembled the 



