The acclimating mechanisms of Nannochloropsis sp. In response to long-term high CO2 stress-the important role of active cytoplasmic pH regulation.

Microalgae-based biological carbon sequestration presents the sustainable and environmentally friendly approach to CO2 capture, in which the selection of high CO2 tolerant microalgal strains acting as the most significant issue. In this study, the important marine resource microalgae Nannochloropsis sp. were selected as target organisms. The high CO2 tolerant strains (LAMB106 and 100) were screened among 12 strains, and tolerant mechanisms were elucidated based on 20 days' semi-continuous culture under the upper limit of CO2 concentration in the flue gas (20%). The results indicated LAMB106 showed better growth and photosynthetic performance than LAMB100 under high CO2 stress and exhibited the trend of acclimation, especially after 8 days of high CO2 culture. Physiological-biochemical results showed active cytoplasmic pH regulation mechanisms in LAMB106, that were shown as the fast response of plasma proton pump (P-ATPase) within 4 h, and later maintenance by enhanced vacuole proton pump (H+-PPase), intracellular carbonic anhydrase (iCA) and RuBisCO activities, coupled with the accumulation of various organic acids (especially malate), in order to enhance the fixation of CO2 and provide energy for cellular metabolisms. Further transcriptomic results were consistent with the physiological-biochemical changes (appeared from 8 days), with the gene expression responses appeared from 4 days. This study successfully screened the high CO2 tolerant Nannochloropsis sp. strain LAMB106 and comprehensively clarified its acclimating mechanisms, proved the possibility of applying LAMB106 on flue CO2 sequestration, with important theoretical and applied significance.