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Abstract
Microalgae convert inorganic carbon into biomass and other carbon-containing products, while also supporting nutrient recovery and low-carbon bioproduct generation. This systematic review synthesizes evidence on microalgal carbon sequestration, with emphasis on monitoring indicators, environmental controls, biological mechanisms, cultivation systems, and applications in wastewater treatment, bioenergy, and circular resource use. Following screening, 109 studies were retained as the core evidence set because they explicitly addressed microalgae-related carbon sinks and reported methods or results for carbon monitoring, calculation, modelling, or performance assessment. Across these studies, carbon fixation and storage were shaped by species traits and cultivation context, particularly light, temperature, pH, nutrient supply, CO2 availability, salinity, pollutant exposure, and reactor or ecosystem design. Common monitoring variables included biomass productivity, CO2 removal, carbon-fixation rate, dissolved inorganic carbon, particulate organic carbon, photosynthetic efficiency, nutrient removal, and life-cycle or techno-economic metrics. Recent work has moved from single-species laboratory cultivation toward integrated systems, including photobioreactors, algal-bacterial consortia, wastewater-linked cultivation, flue-gas utilization, natural phytoplankton communities, and carbonate chemistry-based carbon management. The value of these systems depends on the fate of the fixed carbon - whether it remains in harvested biomass, enters products, or is released during downstream processing. When cultivation is paired with wastewater treatment and biomass valorization, fixed carbon can be routed into fuels, bioproducts, residual biomass, or other defined product streams. Durable carbon-sink contributions remain context-dependent and require full-boundary assessment of energy inputs, biomass fate, downstream processing, and long-term carbon storage. The evidence indicates that microalgal carbon-sink performance cannot be judged from CO2 uptake alone. It also depends on whether fixed carbon is retained in biomass, transferred to durable products, mineralized, or rapidly returned to the atmosphere through downstream processing. Carbon-fixation rate, biomass fate, energy inputs, and product allocation should be reported within the same accounting boundary.