LAUSR

Abstract Charophytes of the genus Chara can form a bottom-dwelling calcified vegetation of high density in nutrient-poor hardwater lakes and ponds, whereas they are rare compared to tall canopy-forming vascular plants (elodeids) in nutrient-rich waters. Based on the preferred habitat and appearance, we hypothesized that Chara species may use bicarbonate efficiently for photosynthesis accompanied by calcification. We measured photosynthesis and calcification in incubation experiments of 3–18 h duration with seven species of Chara by quantifying changes in dissolved inorganic carbon (DIC), oxygen, alkalinity and calcium. All Chara-species used bicarbonate efficiently according to: 1) an unaltered photosynthetic rate between pH 7.5 and 8.5 and a modest decline at pH 9.5, and 2) their ability to drive pH above 10 and deplete CO2 below 0.1 μM. Calcification measured by the parallel loss of bicarbonate and dissolved calcium to precipitated calcium carbonate increased markedly with rising pH, resulting in additional consumption of DIC and a falling molar quotient of oxygen release to DIC consumption from 1.61 at pH 7.5 to 0.68 at pH 9.5. The stoichiometry of alkalinity and calcium loss relative to oxygen production at pH 9.5 was in accordance with simultaneous calcification and photosynthetic assimilation, which buffers pH rise. Our findings support that Chara species are efficient bicarbonate users and calcifying organisms. Through calcification, Chara species maintain photosynthesis at high pH and can form dense stands in oligotrophic hardwater lakes and ponds.