physiology; symbiosis 



413 



mechanism in the algae differs materially from the mechanism in 

 higher green plants. From the work that has been carried out it 

 seems that the rate of photosynthesis in marine algae such as fucoids 

 is lower than that of land plants. 



Certain algae are capable of photo-reduction using elementary 

 hydrogen after a period of adaptation under anaerobic conditions. 

 The marine algae require a long period of adaptation but Chlamy- 

 domonas moewusii appears capable of almost an immediate change. 



Table 22 



Algae which have been tested for ability to carry out 

 photo-reduction using hydrogen 



Class 

 Chlorophyceae : 



Bacillariophyceae 

 Phaeophyceae : 

 Rhodophyceae : 



Myxophyceae : 



Species adapted 

 Scenedesmus obliquus 

 Ankistrodesmus sp. 

 Chlamydomonas moewusii 

 Ulva lactuca 



Ascophyllum nodosum 

 Porphyra umbilicalis 

 Porphyridium cnientum 

 Synechoccus elongatus 

 Synechocystis sp. 



Species not adapted 

 Chlorelia pyrenoidosa 



Nitschia spp. 



Oscillatoria sp. 

 Nostoc musconim 

 Cylindrospermum sp. 



This type of carbon fixation probably does not occur in algae to 

 any great extent. There is also evidence that some algae can utilize 

 hydrogen sulphide as the hydrogen donor in place of water. In this 

 respect they are Hke certain bacteria. Oscillatoria^^ Pinnularia (di- 

 atom), Scenedesmus and Synechoccus can all utihze HgS, though in 

 the two last examples molecular hydrogen is used preferentially. 



Nitrogen fixation 



The estabhshment of pure bacteria-free cultures has enabled 

 workers to show that some twenty or more species of Myxophy- 

 ceae are capable of 'fixing' atmospheric nitrogen. The genera in- 

 volved include Nostoc, Anabaena, Cylindrospermum, Calothrix, 

 Tolypothrix, Anabaeniopsis and Mastigocladus. So far no complete 

 proof of nitrogen fixation by Chlorophyceae has come to hand. 

 Fixation of atmospheric nitrogen by Myxophyceae only takes place 



