Photosynthetic response of Chara braunii towards different bicarbonate concentrations

A variety of inorganic carbon acquisition modes have been proposed in Characean algae, however, the specific inorganic carbon uptake mechanism is unknown for the genus Chara. In the present study, we analyzed if C. braunii can efficiently use HCO3- as a carbon source for photosynthesis. For this purpose, C. braunii was 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 (, the initial slope of the ETR), and the light saturation point of photosynthesis (Ek) were evaluated. Additionally, pigment contents (chlorophyll a, chlorophyll b, and carotenoids) were determined. Bicarbonate addition positively affected ETRmax after direct HCO3- application of both PSII and PSI, but this effect decreased after 1 h and 24 h. Similar trends were seen for Ek, but no significant effect was observed for . 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 ETRmax was calculated but did not show a distinctive trend. These results suggest that C. braunii can utilize NaHCO3- in short time periods as a carbon source but relies on other carbon acquisition mechanisms over prolonged time periods. These observations differentiate C. braunii from other examined Chara spp. and suggest a minor direct role of HCO3- as a carbon source for photosynthesis in this alga.