Microbial community dynamics during high CO2 acclimation and potential of microalgae isolates in mitigation systems

Microalgae are promising platforms for CO2 mitigation. However, high CO2 exposure can impose physiological and environmental constraints, such as acidification and disruption of cellular homeostasis, which impair sensitive strains. In this study, we enriched a sample collected from a cement plant under 25% CO2 (high carbon, HC) and different culture media (BG11 or MB3N), with parallel cultures under air (low carbon, LC). Over 15 years, metagenomics revealed simplified microalgae–bacterial consortia, in which CO2 levels and culture medium shaped the prokaryotic component, while eukaryotes exhibited progressive homogenization. At the genus level, Ralstonia and Paraburkholderia dominated the HC cultures, along with Desmodesmus microalgae and Sarocladium fungi. Consortia growth under 25% CO2 in both batch and continuous modes showed similar characteristics, but higher initial growth rates were observed in HC cultures, that suggest CO2 adaptation. Microalgae isolates from HC consortia were Desmodesmus species with up to 75–100% CO2 tolerance, which produced higher biomass than LC isolates. LC isolates were identified as unresolved species (Scenedesmus or Coelastrum), Desmodesmus, and Coelastrella with lower CO2 tolerance, except for the Desmodesmus strain, which tolerated up to 100% CO2. High protein content (>50% d.w.), moderate carbohydrate levels (40%), low lipid levels (8–16%), and high photosynthetic pigments characterize the isolates at tolerable CO2 levels. In addition, some strain-specific biomass traits were observed. Further studies should evaluate the role of prokaryotes in microalgae CO2 mitigation systems. Overall, long-term acclimation produced extremely high CO2 tolerant isolates with potential for CO2 mitigation and value-added byproduct generation.