138 



were indeed, as claimed by their disco verer, chemo-autotrophic, i.e., 

 that they were able to reduce carbon dioxide with the aid of the 

 energy derived from the oxidation of the sulphur compound. This led 

 Beijerinck to the bold idea that there might also be bacteria which 

 could derive the energy necessary for their maintenance and proli- 

 feration from an analogous process in which the sulphur compound 

 was oxidized, not with the aid of free oxygen, but with the aid of the 

 oxygen available in nitrates. A further simplification led to the pre- 

 paration of a fully inorganic medium of which the chief constituents 

 were finely divided sulphur, chalk and nitrate. Herewith an enrich- 

 ment culture was started in complete absence of free oxygen, and the 

 startling result was obtained that there exist indeed forms of life 

 which can adapt themselves to these extremely primitive conditions. 



Beijerinck once more returned to this subject in a paper which is 

 the "swan song" of his academie career »). 



Here many details regarding this remarkable process and its 

 causative organism Thiohacillus denitrificans are given. Especially 

 striking is the demonstration that, in this inorganic medium, the 

 f ormation of organic matter — mostly in the form of bacterial slime — 

 attains such dimensions that it can be demonstrated by the carbonisa- 

 tion reaction which occurs on addition of concentrated sulphuric acid. 

 The paper is concluded by a section in which Beijerinck expressed 

 the opinion that Thiohacillus denitrificans may well be an auto- 

 trophic form of an ordinary heterotrophic denitrifying bacterium like 

 Bacterium Stutzeri. 



n. On nitrogen fixation by free-living micro-organisms. 



All students'of general and agricultural microbiology are familiar 

 with the association of Beijerinck's name with the important subject 

 of nitrogen fixation by free-living micro-organisms. The isolation of 

 Azotobacter chroococcum Beijerinck is nowadays a part of thebeginner's 

 curriculum in soil microbiology. 



A survey of the history of the discovery of this highly remarkable 

 micro-organism is particularly interesting, because it shows clearly 

 that minor factors may largely influence the course of scientific 

 development. 



It was Berthelot who in 1885 for the first time experimentally 

 proved that the gain in nitrogen which can be nearly always as- 

 certained in fallow land is due to the action of living agents present in 

 the soil. So one can easily understand that, from that time on, several 

 attempts were made to become acquainted with the particular type or 

 types of micro-organisms endowed with the faculty of fixing gaseous 

 nitrogen. 



1) Proc. Kon. Akad. v. Wet. Amsterdam 22, 899, 1920. 



