The Microflora 259 



where nitrogen-fixing hchens or blue-green algae were absent or did not 

 seem to be present in sufficient numbers to account for this fixation. 

 Thus, heterotrophic, free-living, nitrogen-fixing bacteria may be active 

 in some areas. In most other tundra locations nitrifying bacteria are simi- 

 larly rare or absent; soils are generally too wet and acid. At Barrow, wet 

 meadow soils are relatively rich in ammonia and low in nitrates. Nitrify- 

 ing bacteria are present in these soils in extremely low numbers but the 

 presence of some nitrate provides indirect evidence that nitrification is 

 taking place. It is unlikely that rates of nitrification are low solely be- 

 cause of low temperatures (Alexander 1971) or the total lack of nitrifying 

 bacteria propagules. 



In contrast to nitrifiers, the presence of denitrifying bacteria in tun- 

 dra is widely reported (Dunican and Rosswall 1974). Studies in the Biome 

 research areas at Barrow and Eagle Summit suggest that denitrifying bac- 

 teria constitute 5% of the total aerobic population and are almost exclu- 

 sively of the genus Pseudomonas. The two most prominent types, repre- 

 senting 65% of the isolates, were closely related to Pseudomonas denitri- 

 ficans. Estimated rates of nitrogen transformation are presented in 

 Chapter 7. 



The role of sulfur may be especially important for two reasons. 

 First, concentrations of inorganic sulfur in soils are very low, generally 

 less than 1 ppm, within the range of possible sulfur deficiency for vascu- 

 lar plants and well below the values of 50 to 100 ppm reported for tem- 

 perate regions (Starkey 1950, Walker 1957). Second, the wet, organic en- 

 vironment may favor accumulation of hydrogen sulfide as a terminal 

 step in anaerobic mineralization of proteins. Hydrogen sulfide is toxic to 

 many aerobic microorganisms and the precipitation of metallic sulfides 

 could influence the mineral nutrition of vascular plants. 



The low levels observed suggest that most of the sulfur in the system 

 is resident in the protein of live cells. Boyd (1967) reported that chemo- 

 autotrophic, sulfur-oxidizing bacteria were absent in arctic and antarctic 

 soils, while Cannon et al. (1970) documented their presence in low num- 

 bers in soils of blanket peat. Using enrichment cultures of pH 2 and 7, we 

 found sulfur oxidizers to be rare or absent at Barrow. These observations 

 are not surprising, for such organisms are found in low numbers even in 

 soils of the temperate zone, except where enrichments of sulfur exist, as 

 in mine tailings and pyrites (Starkey 1966). Photosynthetic sulfur 

 bacteria have not been found in the soils at Barrow and are only reported 

 from one subarctic site (Rosswall and Svensson 1974). Sulfate-reducing 

 bacteria have been reported previously from both arctic (Russell et al. 

 1940, Dunican and Rosswall 1974) and antarctic soils (Barghoorn and 

 Nichols 1961, lizuka et al. 1969). They were isolated in low numbers in 

 enrichment cultures from some of the wettest soils of the Biome research 

 area and appear sufficiently rare that accumulation of hydrogen sulfide 



