# Parametric Studies and Semi-Continuous Harvesting Strategies for Enhancing CO2 Bio-Fixation Rate and High-Density Biomass Production Using Adaptive Laboratory-Evolved Chlorella vulgaris

## TL;DR

Researchers improved CO2 capture and biomass production using a specially adapted green algae strain, finding optimal conditions and a new harvesting method for better efficiency.

## Contribution

The study introduces a semi-continuous harvesting strategy and identifies optimal cultivation parameters for a CO2-tolerant Chlorella vulgaris strain to maximize CO2 bio-fixation and biomass yield.

## Key findings

- Optimal conditions for ALE-Cv achieved 7.03 g/L biomass and 13.4 g/L CO2 bio-fixation in batch cultivation.
- Semi-continuous harvesting without nutrient addition reached 9.91 g/L biomass and 18.16 g/L CO2 bio-fixation over 15 days.
- Adaptive evolution of Chlorella vulgaris significantly enhanced CO2 tolerance and biomass productivity compared to non-adapted strains.

## Abstract

This study adopts a biochemical approach to sequester CO2 while producing biomass rich in protein and lipids, using an adapted strain of Chlorella vulgaris (ALE-Cv), which had previously evolved to tolerate a gas mixture containing 10% CO2 and 90% air. The research studied the operating parameters of the batch photobioreactor for ALE-Cv to evaluate the effects of inoculum size, photoperiod, light intensity, pH of culture, and CO2 supply rate on biomass productivity and CO2 bio-fixation rate. The optimal conditions were identified as 16:8 h light–dark cycles, 5000 lux, pH 7, 20 mL of 10 g/L inoculum, and 0.6 VVM; the system achieved a maximum total biomass production of 7.03 ± 0.21 g/L with a specific growth rate of 0.712 day−1, corresponding to a CO2 bio-fixation of 13.4 ± 0.45 g/L in batch cultivation. While the pre-adapted strain of Chlorella vulgaris under the same operating conditions, except for the gas supply, which was air, achieved a maximum total biomass production of 0.52 ± 0.008 g/L, and the total CO2 bio-fixation was 1.036 ± 0.021 g/L during 7-day cultivation. A novel semi-continuous harvesting process, with and without nutrient addition, was also investigated to maximise biomass yield and enable water recycling for culture media. The maximum biomass production in semi-continuous harvesting process with and without nutrition added was 5.29 ± 0.09 and 9.91 ± 0.11 g/L, while the total corresponding CO2 bio-fixation was 9.70 ± 0.13 and 18.16 ± 0.11 g/L, respectively, during 15-day cultivation. The findings provide critical insights into enhancing CO2 bio-fixation through adaptive evolution of ALE-Cv and offer optimal operational parameters for future large-scale microalgae cultivation. This research also links microalgae-based CO2 sequestration to green technologies and the bioeconomy, highlighting its potential contribution to climate change mitigation while supporting environmental sustainability, food security, and ecosystem resilience.