The Photoinactivation of Micro-Algal Ribulose Bisphosphate Carboxylase; its Physiological and Ecological Significance

Light-activated heterotrophic growth of the cyanobacterium Synechocystis sp. strain PCC 6803: a blue-light-requiring process

A glucose-tolerant strain of Synechocystis sp. strain 6803 will not grow on glucose under complete darkness unless given a daily pulse of white light, typically 5 min of 40 mumol m-2 s-1 (light-pulsed conditions). The light pulse is insufficient for photoautotrophy, as glucose is required and growth yield is dependent on glucose concentration. Growth rate is independent of fluence, but growth yield is dependent on fluence, saturating at 40 to 75 mumol m-2 s-1. A Synechocystis strain 6803 psbA mutant strain grows under light-pulsed conditions at rates similar to those for the glucose-tolerant strain, indicating that photosystem II is not required for growth. The relative spectral sensitivity of the growth of light-pulsed cultures (growth only in blue light, 400 to 500 nm, maximum at 450 nm) precludes energetic contribution from cyclic electron transport around photosystem I. Pulses of long-wavelength light (i.e., 550 and 650 nm) did not support the growth of Synechocystis strain 6803 and, when supplied before or after a blue-light pulse, did not inhibit blue-light-stimulated growth of Synechocystis strain 6803. We conclude that the required blue-light pulse does not support growth via photosynthetic electron transport but appears instead to function as an environmental signal regulating heterotrophic metabolism, cell division, or other photomorphogenic processes. We have termed the growth of Synechocystis strain 6803 pulsed with light and kept otherwise in complete darkness light-activated heterotrophic growth. This observation of a blue-light requirement for the growth of Synechocystis strain 6803 represents a novel blue light effect on the growth of a cyanobacterium.

Photoinhibition of photosynthesis in the cyanobacterium Microcystis aeruginosa

We have examined characteristics of the photoinhibition of photosynthesis which occur in the unicellular cyanobacterium Microcystis aeruginosa, following exposure to photon fluence rates in excess of those required for growth. Photoinhibition occurs following exposure of cells to a photon fluence rate of 1,000 μmol m(-2) s(-1), which is manifested as a decrease in either light-limited CO2 fixation or light-saturated CO2-dependent O2 evolution. The extent and rapidity of this photoinhibition is greatly enhanced under CO2-depleted conditions. Experiments in which cultures were sparged with different gases indicate that photoinhibition is not an obvious consequence of elevated O2 tensions, unlike the photooxidative bleaching of photosynthetic pigments. Comparative studies on the photoinactivation of CO2-dependent O2 evolution and of the methyl viologen-dependent Mehler reaction, in whole cells, indicate that a primary site of light damage is within the photosynthetic electron-transport reactions and that carbon fixation is initially unaffected.