Photochemical performance of the acidophilic red alga Cyanidium sp. in a pH gradient.

Abstract

The acidophilic red alga Cyanidium sp. is one of the dominant mat-forming species in the highly acidic waters of Río Tinto, Spain. The culture of Cyanidium sp., isolated from a microbial mat sample collected at Río Tinto, was exposed to 9 different pH conditions in a gradient from 0.5 to 5 for 24h and its physiological status evaluated by variable chlorophyll a fluorescence kinetics measurements. Maximum quantum yield was determined after 30min, 1h, 2h, 4h, 6h and 24h of exposure after 15min dark adaptation. The effect of pH on photochemical activity of Cyanidium sp. was observable as early as 30min after exposure and the pattern remained stable or with only minor modifications for 24h. The optimum pH ranged from 1.5 to 2.5. A steep decrease of the photochemical activity was observed at pH below 1 even after 30min of exposure. Although the alga had tolerated the exposure to pH = 1 for at least 6h, longer (24h) exposure resulted in reduction of the photochemical activity. At pH above 2.5, the decline was more moderate and its negative effect on photochemistry was less severe. According to the fluorescence measurements, the red alga Cyanidium sp. is well-adapted to prevailing pH at its original locality at Río Tinto, i.e. pH of 1 to 3. The short-term survival in pH < 1.5 may be adaptation to rare exposures to such low pH in the field. The tolerance of pH above 3 could be caused by adaptation to the microenvironment of the inner parts of microbial mats in which Cyanidium sp. usually dominates and where higher pH could occur due to photosynthetic oxygen production.