DARK HETEROTROPHIC GROWTH-CONDITIONS RESULT IN AN INCREASE IN THE CONTENT OF PHOTOSYSTEM-II UNITS IN THE FILAMENTOUS CYANOBACTERIUM ANABAENA-VARIABILIS ATCC-29413

The filamentous nitrogen-fixing cyanobacterium Anabaena variabilis ATCC 29413 is capable of heterotrophic growth in complete darkness. After 6 months of continuous dark growth, both the autotrophic and heterotrophic cultures were found to have the same doubling time of 14 h. On a cellular basis, the chlorophyll content remained the same and the phycobilin content showed an increase in the dark-grown cultures. Fluorescence emission spectra at 77 K of dark-grown cells indicated that the phycobilisomes are functionally associated with photosystem II (PSII). Moreover, upon transfer to light, the dark-grown cells readily evolved oxygen. Although photosystem I (PSI) and whole chain-mediated electron transfer rates were comparable in both types or cultures, the rate of PSII-mediated electron transfer was found to be 20% higher in dark-grown cells. The PSI to PSII ratio changed from 6:1 in autotrophic cultures to 4:1 in the dark-grown cells. These changes in the rate of PSII electron transfer and in the stoichiometry between the two photosystems under dark, heterotrophic growth conditions were brought about by a preferential increase in the number of PSII units while the number of PSI units remained unchanged. The advantages of using this organism in the selection of PSI-deficient mutants are discussed.