Nitrogen fixation and the heterocyst in blue-green algae.

of the relatively small number of substrates that can feed into the oxidative pentose phosphate cycle, is the most probable explanation for the widespread occurrence of obligate photoautotrophy among cyanobacteria. However, certain cyanobacteria that are facultative photoheterotrophs, able to grow'in the light in the presence of 3-(3,4dichloropheny1)-1 ,I-dimethylurea at the expense of glucose, seem wholly incapable of dark growth with glucose. This phenomenon obviously cannot be accounted for by postulating a permeability barrier; its explanation remains unknown.