238 P. L. Gersper et al. 



The strong inverse relationship between oxygen tension and nitrogenase 

 activity (Figure 7-9) indicates the variation could be a very significant 

 factor influencing rates of nitrogen fixation by algae associated with 

 moss. Similar relationships have been described for a mire site in Sweden 

 (Granhall and Selander 1973). The relationship between nitrogen fixa- 

 tion in lichens and oxygen is complex, particularly because there appears 

 to be a strong interaction between light and oxygen requirements and a 

 conflicting influence between the inhibitory effects of oxygen on the ni- 

 trogenase enzyme and the need for photosynthetically produced sources 

 of energy. 



A simple model, nfixr, was developed that integrates the available 

 laboratory measures and permits evaluation of their general applicability 

 against field observations (Bunnell and Alexander, unpubl.). The model 

 assumes that the influences of temperature, moisture and oxygen interact 

 in a multiplicative fashion. Thus fixation is reduced as any single envir- 

 onmental control departs from the optimal range, even though other 

 conditions may not be Umiting. Seasonal courses of nitrogen fixation for 

 specific genera can be predicted from measured environmental variables 

 and compared with observed fixation rates (Figure 7-10). 



Although actual magnitudes differ, the observed seasonal courses of 

 nitrogen fixation in polygonal terrain at both the Biome research area 

 and in a birch site at Kevo, Finland, are similar to those predicted by the 

 model. Apparently, the measured relationships are broadly applicable to 

 lichens and algae inhabiting a variety of sites. The inaccurate prediction 

 of the magnitude of rates of fixation apparently is largely due to the dif- 

 ficulties in estimating biomass of the fixing organisms, particularly 

 algae. Blue-green algae are relatively more important at Barrow than at 

 Kevo, and the predictions for Barrow are therefore less accurate. 



In light of the recent observation that non-heterocystous blue-green 

 algae also contain the enzyme nitrogenase and are capable of nitrogen 

 fixation under conditions of low oxygen (Kenyon et al. 1972, Stewart 

 1973), special interest centers on the ecology of these moss-associated 

 algae. Present findings suggest that the majority of blue-green algal 

 forms found in the moss layer may contribute to nitrogen fixation, and 

 that estimates of nitrogen-fixing biomass based only on heterocystous 

 algae may be greatly in error both in the wet, mossy layer and in soils. 



There is no marked adaptation by the major nitrogen fixer, blue- 

 green algae, to the arctic environment. The predominant nitrogen-fixing 

 form, Nostoc commune, is found in the Antarctic and in all circumpolar 

 tundra regions. Its temperature optimum is not greatly different from 

 temperature optima for blue-green algae from temperate and tropical 

 regions. Arctic lichens, however, appear to be rather well adapted. Nitro- 

 genase activity of lichens recovers after freezing, with the rate of recovery 

 depending on the length of time the lichens were kept frozen and the tem- 



