Effect of prolonged dark incubation on pigments and photosynthesis of the cave-dwelling cyanobacterium Phormidium autumnale (Oscillatoriales, Cyanobacteria).

The effect of prolonged darkness on the photosynthetic apparatus of the cyanobacterium Phormidium autumnale was investigated under nonheterotrophic conditions. The purpose of this study was to better understand the processes that allow this organism to rapidly resume growth after prolonged exposure to darkness, as it is often the case in show caves, where periods of darkness are applied to reduce algal proliferation. Phormidium autumnale was subjected to 4 weeks in the dark; this treatment elicited a large decrease in the cells' photosynthetic capacity (Pmax), while the photosynthetic affinity for photons (α) was affected to a smaller degree. Chlorophyll a and carotenoid contents were approximately constant over the 4 weeks of dark incubation, whereas the amount of phycobilins (phycocyanin, allophycocyanin and phycoerythrin) per cell decreased by 50% between the first and the last week of incubation. The relative abundance of the three phycobilins did not vary throughout the treatment, suggesting that the decrease in the number of phycobilisomes was not paralleled by changes in their relative composition. Upon re-exposure to light, cells promptly resumed photosynthesis and were able to cope with high irradiance up to 1600 μmol photons m−2 s−1. The resilience of the photosynthetic apparatus of P. autumnale in the dark may be one of the features that determine the prevalence of this organism in the cave flora at the expenses of more dark-sensitive photolithotrophs (i.e. organisms whose photosynthetic apparatus is substantially reduced or disassembled in the dark).