FRESH WATER ECOLOGY; SOIL ALGAE 381 



(6) If desiccation is rapid most of the cells will die but some will 

 plasmolyse and retain their vitahty in that state for weeks or 

 months. In spite of the death of the bulk of the cells no 

 species disappears from the flora during a rapid onset of 

 drought. 



(7) If desiccation continues, the number of hving resting cells 

 will remain constant for several years. 



(8) During a very long drought the resting cells of algae below 

 the surface will still survive. 



Apart from the moisture relations there are also other factors 

 that may be involved. Diatoms can survive very low temperatures, 



- 80° C. for 8 days or - 192° C. for 13 hours, whilst dry spores of 

 Nostoc sp. and Oscillatoria hrevis can survive - 80° C, though if 

 they are moist a temperature lower than - 16° C. will kill them. 

 As the vegetative filaments of Nostoc die after four days at - 2 to 



- 8° C. this genus must survive severe winters in the form of 

 spores. So far as the algae of tropical soils are concerned the dry 

 spores of Nostoc sp. and O. brevis can tolerate 2 min. at loo' C, 

 the wet spores 20 min. at 60-70° C, and the vegetative filaments 

 10 min. at 40° C, this latter being a temperature that is frequently 

 reached on open ground in such regions. Acidity and alkalinity do 

 not appear to be of any great importance, although members of the 

 Chlorophyceae usually thrive better on basic soils. 



It has been demonstrated that Anabaena and Nostoc can fix 

 nitrogen from the air in the presence of light, but other soil algae 

 apparently do not possess this power unless they occur in combina- 

 tion with bacteria, and even then the actual fixation is probably 

 carried out by the bacteria. It has been found by De (1939) that 

 Anabaena will only fix nitrogen from the air so long as nitrate is 

 absent from the soil. The combination of bacteria and algae fix 

 nitrogen better than the bacteria do alone, so that the algae must 

 act as a kind of catalytic agent, and it has been suggested that they 

 (a) provide carbohydrate, and hence energy, for the bacteria, or {b) 

 remove the waste nitrogen compounds, since it has been sho^\^l 

 that if these accimiulate bacterial activity is reduced. In some cases 

 the algae play a part in aeration because of the oxygen they produce 

 during photosynthesis, and in this connexion it may be mentioned 

 that unless certain species are present in the soil of rice fields during 

 the period they are waterlogged the aeration deteriorates and the 

 rice becomes much more susceptible to disease. Rice is also capable 



