Microalgal Systems for Efficient CO\(_2\) Utilisation and Carbon Sequestration: A Review

Abstract

Global anthropogenic carbon dioxide (CO₂) emissions have risen steeply since the Industrial Revolution, primarily due to fossil fuel combustion and deforestation. Microalgae are emerging as potent biological platforms for efficient carbon dioxide (CO₂) sequestration and renewable biomass production. Due to their rapid growth, high photosynthetic efficiency (6–8%), and capacity to tolerate elevated CO₂ concentrations (up to 15–20%), microalgae outperform terrestrial plants in carbon fixation per unit area. The taxonomy, physiology, and engineering of microalgal systems for CO₂ capture, highlighting key genera such as Chlorella, Spirulina, and Nannochloropsis. The mechanisms underlying CO₂ assimilation as carbon concentrating mechanisms (CCMs), RuBisCO compartmentalisation in carboxysomes or pyrenoids, and light-harvesting regulation examined in detail. Cultivation approaches, including open raceway ponds, closed photobioreactors (PBRs), and hybrid systems (e.g., algal turf scrubbers and membrane-integrated units), are compared based on productivity, CO₂ transfer rates, and energy inputs. Sources of CO₂, such as industrial flue gas, power plant exhaust, biogas digesters, and direct air capture (DAC), are evaluated for their integration potential. The role of CRISPR/Cas and synthetic biology in enhancing RuBisCO activity, lipid accumulation, and CCM expression is assessed. Life cycle assessment (LCA) indicates that optimised systems can achieve CO₂ removal efficiencies exceeding 70%, particularly when coupled with wastewater treatment and industrial symbiosis. Algal biomass can be valorised into biodiesel, bioethanol, biogas, bioplastics, animal feed, and long-lived carbon-storing materials such as biochar and construction composites. This multifunctional approach supports a circular bioeconomy by transforming waste CO₂ and nutrients into valuable bioproducts while contributing to climate mitigation. Despite challenges in scalability, energy consumption, and economic viability, innovations in reactor design, metabolic engineering, and integrated biorefineries strengthen the feasibility of algae-based carbon capture systems. Microalgal systems offer a sustainable and efficient platform for carbon dioxide (CO₂) sequestration and biomass valorisation. With the capacity to fix 1.8 kg of CO₂ per kilogram of biomass, species such as Chlorella, Nannochloropsis, and Scenedesmus exhibit high photosynthetic efficiencies and adaptability to industrial flue gases, biogas digesters, and DAC systems. The scientific advancements and technological pathways required to position microalgae as a pivotal component of global carbon neutrality and sustainable bioindustrial development.