A novel CO2-tolerant symbiotic Desmodesmus (Chlorophyceae, Desmodesmaceae): Acclimation to and performance at a high carbon dioxide level

Abstract

We studied the acclimation, growth, and photosynthetic performance of a novel symbiotic chlorophyte Desmodesmus sp. 3Dp86E-1 from a White Sea benthic hydroid Dynamena pumila cultivated at a very high (20% by volume) CO2 level relevant for biomitigation applications. Under the elevated CO2 conditions, a two-fold increase in the rate of the culture growth, which remained balanced in terms of carbon and nitrogen assimilation, was recorded. The cultivation at 20% CO2 did not disturb the ultrastructure or functioning of the photosynthetic apparatus of the microalgae. By contrast, the increase in saturating light intensity, quantum yield and the rates of oxygen evolution and CO2 fixation were recorded in comparison with cultivation at the atmospheric CO2. The analysis of photosystem I (PS I) reaction center, P700, reduction revealed an increase in cyclic electron flow over PS I, a putative source of the extra ATP for the maintenance of cell pH homeostasis. This suggestion was supported by a characteristic expansion of the thylakoid lumen observed on electron micrographs. Carbonic anhydrase inhibitors blocked the capability of the microalga to maintain a favorable culture pH at 20% CO2 suggesting the involvement of these enzymes in CO2 tolerance. It is emphasized that the robustness of photosynthetic carbon assimilation together with the efficient channeling of the photosynthates to the biosynthesis of the structural cell components is crucial for acclimation to and efficient growth under very high carbon dioxide levels. These capabilities make the symbiotic microalgae particularly suitable for employment in the advanced CO2 biomitigation technologies.