The Microflora 289 



in decomposition. Sulfate-reducers are present, while sulfide-oxidizers 

 are rare. The wet, acidic nature of the soils similarly discourages hetero- 

 trophic nitrogen-fixing and nitrifying bacteria, while denitrifiers are 

 common, accentuating the importance of nitrogen fixation by algae. 

 Although all major classes of fungi are present, their diversity also is 

 low. Most fungi exhibit specificity for particular microtopographic units. 

 Generally, endemics are few and sterile forms make up more than 50*^0 

 of the isolates. In comparison to other Biomes, wood-rotting fungi are 

 rare, reflecting the low availabihty of suitable substrate, while mycor- 

 rhizal fungi are common in some areas. Aquatic fungi are present pri- 

 marily as algal parasites. The apparent dominance of basidiomycetes 

 among the fungi is associated with their role as mycorrhizal formers and 

 with their more versatile, dikaryotic hyphal system, which may facilitate 

 adaptation to an extreme environment. 



Average amounts of total microfloral biomass also differ signifi- 

 cantly among microtopographic units. Total biomass ranges from 12 to 

 20 g m"^ in the upper 7 cm. Biomass is generally highest in wet meadows 

 and polygon troughs, lower on rims of low-centered polygons, and low- 

 est in basins of low-centered polygons. The inhospitality exhibited by 

 basins is evident in fungal densities in standing dead vegetation and litter 

 as well as in the soil. Generally the eukaryotic decomposers, fungi and 

 yeasts, are much better represented in well aerated soils found on rims of 

 low-centered polygons and tops of high-centered polygons, while 

 bacteria become dominant in the wetter soils of polygon troughs and wet 

 meadows. Plate counts of bacteria are commonly a factor of ten lower 

 than values reported for soils of the temperate zone, while direct counts 

 are higher. Because of the smaller size of the bacteria found in the coastal 

 tundra at Barrow, total bacterial biomass is similar to that in temperate 

 regions. Fungal biomass is frequently much lower than values reported 

 for other Biomes and fungi-to-bacteria ratios approximate values for 

 temperate regions only in the well aerated soils. In wet soils decomposi- 

 tion is governed by bacteria to a greater degree than in most soils from 

 other Biomes. 



The broad relationships with moisture and aeration that govern rela- 

 tive distribution across microtopographic units also influence patterns 

 with depth. Densities of eukaryotic organisms decline rapidly with depth, 

 producing a shift from fungal to bacterial dominance. This shift reduces 

 the potential to degrade compounds of large molecular weight and total 

 carbon declines much less rapidly with depth than does root production. 

 Intra- and interseasonal differences in abundance also demonstrate the 

 influence of moisture in the tundra system, with moister periods or 

 moister microtopographic units generally showing depressed fungal bio- 

 mass or productivity. Both bacteria and fungi exhibit a similar seasonal 

 pattern of biomass with early- and late-season peaks. While the peak in 



