Photosynthetic response ofChara brauniitowards different bicarbonate concentrations

Abstract

AbstractA variety of inorganic carbon acquisition modes have been proposed in Characean algae, however, the specific inorganic carbon uptake mechanism is unknown for the genusChara. In the present study, we analyzed ifC. brauniican efficiently use HCO3-as a carbon source for photosynthesis. For this purpose,C. brauniiwas exposed to different concentrations of NaHCO3-for different time scales. The photosynthetic electron transport through photosystem I (PSI) and II (PSII), the maximal electron transport rate (ETRmax), the efficiency of the electron transport rate (α, the initial slope of the ETR), and the light saturation point of photosynthesis (Ek) were evaluated. Additionally, pigment contents (chlorophylla, chlorophyllb, and carotenoids) were determined. Bicarbonate addition positively affected ETRmaxafter direct HCO3-application of both PSII and PSI, but this effect decreased after 1 h and 24 h. Similar trends were seen forEk, but no significant effect was observed for α. Pigment contents showed no significant changes in relation to different HCO3-concentrations. To evaluate if cyclic electron flow around PSI was involved in active HCO3-uptake, the ratio of PSI ETRmax/PSII ETRmaxwas calculated but did not show a distinctive trend. These results suggest thatC. brauniican utilize NaHCO3-in short time periods as a carbon source but relies on other carbon acquisition mechanisms over prolonged time periods. These observations differentiateC. brauniifrom other examinedCharaspp. and suggest a minor direct role of HCO3-as a carbon source for photosynthesis in this alga.