Tiaiisloiniatiou oi ArstMiic and Soleuiiini 243 



Transfohmahox of Ahsenic'and Selenium 



Arsenic is wideK' distributt'tl in natnre. It is iar('l\ lonnd, how- 

 ever, in toxic anionnts in tlie soil. Althongh arsenic has come into 

 Ueneral nse in \arions insecticides, only small amonnts find their way 

 into thc> soil. In some cases, however, as when arsenical dnst is 

 applied to combat certain insects, the amonnts left in the soil may 

 resnlt in the stnnting or dwarfing of the sncceeding crop. This is 

 trne also of soil in coniferons nnrseries where lead arsenate is nsed 

 to combat insect larvae. Microbial activities in the soil may be af- 

 fected nnless this arsenic is rendered inactive or insolnble. 



Certain soil fnngi have the capacity to volatilize arsenical snb- 

 stanees b\- redncing them to arsine. Cnltnres of snch fnngi readily 

 give off the odor of arsine from arsenic-containing media. Members 

 of the genns Scopiikiriopsis or PeniciUiiim brevicaule, certain asper- 

 gilli, notabl\- A. sydowi, A. fuinigatiis, and A. ochraceiis, species 

 of Fusariioii, and various dematiaceae are responsible for these 

 activities. 



The transformation of selenium by microorganisms in the soil is 

 of importance in connection with the selective absorption of this 

 element b)- crop plants. Certain geological formations contain se- 

 lenium. Plants grown in these areas accumulate the element in 

 their cell material. Various bacteria and fungi have the capacity of 

 volatilizing selenium, producing markedly offensive odors. Tliese are 

 readily detected in culture tubes and in pot experiments in the 

 greenliouse. When selenium-containing plants undergo decomposi- 

 tion, the activities of the various microorganisms result in the pro- 

 duction of strong odors. Microbiological activities in the soil render 

 the selenium-containing substances available to green plants under 

 conditions in which the plants are not otherwise able to obtain 

 selenium. The organisms concerned in the volatilization of selenium 

 include most of the arsenic fungi, especially the Scopiikiriopsis brevi- 

 caule group, and certain soil bacteria, notably Psetidomonos fiio- 

 rescens. 



Selenium compounds are subject to a variety of other bacterial 

 activities, in the reduction of selenates to selenites and in the oxi- 

 dation of elementarv selenium. 



