Physiological adaptations of Nannochloropsis sp., under elevated pCO2conditions: Evidence from controlled culture experiments

Abstract: Understanding the nutrient assimilation and carbon fixation in microalgae found crucial for analysing the role inmarine productivity and potential applications in carbon sequestration. In the present study, controlled culture experimentsunder varying pCO2 conditions to assess nutrient assimilation, pigment synthesis and carbon fixation. The secondary peak insilicate, 189.04 μM (Day 9 - 11) likely attributed to the frustule dissolution from dead cells in the medium. Carbon fixationby Nannochloropsis sp. ranged between 8.64 and 10.78 g L−1 d−1, lower than reported in previous studies, reflecting theinfluence of culture conditions and sensitivity to acidification. Elevated CO2 was found to hinder RuBisCO activity andreduce bicarbonate availability, thereby limiting fixation efficiency. Pearson correlation analysis revealed strong negativerelationships between chlorophyll-a and both inorganic phosphate (-0.77) and nitrate (-0.52), indicating their critical role inpigment production. A positive correlation between nitrate, nitrite, and IP suggested consistent nutrient uptake patternsthroughout the culture. Collectively, the observed results highlight the adaptations of Nannochloropsis sp.under stressconditions, highlighting the importance of nutrient equilibrium and optimal carbonate chemistry for sustaining productivity.