Sulphate-limited growth in the N₂ -fixing unicellular cyanobacterium Gloeothece (Nägeli) sp. PCC 6909

The unicellular diazotrophic cyanobacterium Gloeothece PCC 6909 was grown in batch and continuous culture, It was found that the organism scavenged sulphate from the medium and incorporated it in the polysaccharide sheath. As a result batch cultures became sulphur-depleted whilst continuous cultures were sulphur-limited. This was observed regardless of the nitrogen source of the culture. Sulphur-depletion was the reason for the low nitrogenase activities usually observed in batch cultures of Gloeothece. Sulphate-amended cultures showed sustained high nitrogenase activity in the light. Sulphate uptake from the medium was light-dependent. Earlier work on N₂ fixation in Gloeothece is discussed in the light of possible sulphur limitation.