Photosynthetic response of Chara braunii towards different bicarbonate concentrations

A variety of inorganic carbon acquisition modes have been proposed in Characean algae, however, a broadly applicable 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 (-) at different time scales. The photosynthetic electron transport through photosystem I (PSI) and II (PSII), the maximum electron transport rate (ETRmax ), the efficiency of the electron transport rate (alpha, the initial slope of the ETR), and the light saturation point of photosynthesis (E-k ) 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 seems to decrease after 1 h and 24 h. Similar trends were seen for E-k , but no significant effect was observed for alpha. 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-term periods as a carbon source but could rely on other carbon acquisition mechanisms over prolonged time periods. These observations suggest that the minor role of HCO3 (-) as a carbon source for photosynthesis in this alga might differentiate C. braunii from other examined Chara spp.