Acclimation of photosynthetic pigments and photosynthesis of the cyanobacterium Nostoc sp. strain UAM206 to combined fluctuations of irradiance, pH, and inorganic carbon availability

We analysed the combined effect of pH, irradiance, and inorganic carbon availability on growth and pigment composition of the cyanobacterium Nostoc sp. strain UAM206, isolated from rice fields. This cyanobacterium contains phycoerythrin in its phycobilisomes and can show chromatic acclimation. Under inorganic carbon limitation, the growth rate of Nostoc sp. strain UAM206 was affected by pH, but not by irradiance. Chlorophyll a phycoerythrin (PE), and phycocyanin (PC) contents were inversely correlated to irradiance. Chlorophyll a (Chla) content was not affected by pH; however, with increasing pH, phycocyanin, phycoerythrin, and allophycocyanin (APC) content increased. Inorganic carbon availability masked or decreased some of the effects of pH under inorganic carbon limitation; however, the significant effect of pH on the allophycocyanin contents was clearly independent of the inorganic carbon availability. Analysis of our results indicates that elevation of external pH and available inorganic carbon results in an increase in the number of phycobilisomes along with a decrease in their size (PC+PE/APC ratio), while irradiance mostly affects the size of the phycobilisome. Likewise, increasing irradiance, pH, and available inorganic carbon results in an increase in the PSII/PSI ratio (APC/Chla ratio). Finally, Nostoc sp. strain UAM206 seems able to acclimate its photosynthetic apparatus to variations of the three studied environmental factors that are known to occur in rice fields.