Nitrogen availability modulates CO2 tolerance in a symbiotic chlorophyte

Abstract We studied acclimation to nitrogen starvation, a very high (20%) CO 2 level, and combination thereof in a symbiotic chlorophyte Desmodesmus sp. 3Dp86E-1 to reveal potential mechanisms of its outstanding CO 2 tolerance. The changes in growth rate, photosynthetic activity, cell pigment (chlorophyll and carotenoid), and elemental N and C contents were followed with special attention to rearrangement of the cell ultrastructure in the cultures grown with sparging by air or 20% CO 2 in complete or N-free medium. Transcriptome of the microalga was screened for the presence of putative components of carbon concentrating mechanisms (CCM). The elevated CO 2 was beneficial for the microalga whereas N-starvation impaired the buffer capacity, growth, and photosynthetic performance of the culture and promoted the accumulation of C-rich reserve compounds, degradation of the photosynthetic apparatus and pyrenoid. These effects were exacerbated under 20% CO 2 in comparison with the air-grown cells. It is concluded that the exceptional CO 2 tolerance in Desmodesmus sp. 3Dp86E-1 might have been acquired during co-evolution with the animal host. This trait is obviously conveyed to the symbiotic alga by concerted and swift responses of different mechanisms (CCM, photosynthetic apparatus, biosynthesis of reserve compounds) maintaining the pH homeostasis and carbon fixation and sink balance in the cell.