Selection and Ultrastructural Observation of a High-CO₂-requiring Mutant of Cyanobacterium Synechococcus sp. PCC7942

A high-CO₂-requiring mutant of Synechococcus sp. PCC7942 has been isolated after chemical mutagenesis of ethyl methane sulphonate (EMS). It was able to grow at 4% CO₂, but not under ambient CO₂. The initial screening of the mutant showed that the genetic reversion rate was about 10^-7 and death occurred 2 -3 days after being transferred from 4% CO² to the ambient air. Its photosynthetic dependence on external dissolved inorganic carbon was higher than that of the wild type cells, but its carbonic anhydrase activity was comparatively low. In the ultrastructural level, various types of aberrant carboxysomes appeared in the mutant cells: rod-shaped carboxysomes, irregular carboxysomes and the "empty-inclusion carboxysomes" with increasing number of glycogen granules surrounding the thylakoids. All these alterations indicated that the mutant was defective in utilizing the external CO₂. The induction of carboxysomes by lower levels of CO₂ and the biogenesis of carboxysomes are herein discussed.