Oxygenic photosynthesis may have evolved as early as 3.8–3.5 Gya (Rosing and Frei, 2004; Tice and Lowe, 2004; Oliver et al., 2021) and has shaped the Earth more than any other physiological process. Light-driven water splitting has not only resulted in the enrichment of the atmosphere with molecular oxygen (Luo et al., 2016) but also affected the geological record through, e.g., oxidation of ocean-water-dissolved iron, resulting in large-scale deposition of banded iron formations (Thompson et al., 2019). Beyond that, aquatic oxygenic photosynthesis is associated with the precipitation of carbonate minerals such as dolomite (MgCa(CO₃)₂) and calcite or aragonite (both CaCO₃) in a process called “biomineralization” (Merz, 1992; Riding, 1992). Biomineralization has given rise to most extant carbonate rocks (Vasconcelos et al., 1995), which consist of >50% carbonate minerals and make up for 20%–25% of all sedimentary rocks and as much as 10% of all rocks exposed at the Earth’s surface (Parker, 1967). Such dolostones (dolomite) and limestones (aragonite and calcite) are estimated to store over 80% of the Earth’s carbon (Falkowski et al., 2000), but limestone is being extensively sourced as raw material for industry and agriculture. Upon mining, limestone is commonly converted into quicklime (CaO) through thermal decarboxylation (Niu et al., 2022; Comes et al., 2024), with CaO extraction for cement production alone causing around 7% of global CO₂ emissions (Durastanti and Moretti, 2024). As less than half of this CO₂ is subsequently re-sequestered through cement carbonation (Xi et al., 2016), CaO production contributes significantly to atmospheric CO₂ enrichment and anthropogenic climate change (Callendar, 1938; Jones et al., 2023). Mitigating the latter through reduction of net CO₂ emissions and opening up CaCO₃ as a sustainable resource could be achieved by coupling CaCO₃ thermolysis with microbial biomineralization that re-precipitates released CO₂ and abundant ocean-water-dissolved Ca²⁺ into CaCO₃, thus paving the way towards a more sustainable carbonate economy. While cyanobacterial biomineralization has been discussed as a potential means of cost-efficient CO₂ capture and sequestration (CCS) for more than a decade (Jansson and Northen, 2010; Kamennaya et al., 2012) and some inherently productive calcifying species could be identified (Lee et al., 2004; Liang et al., 2013) little practical progress has been made in this field. In this perspective, we suggest a new approach to reason-guided enhancement of light-driven, cell-surface catalysed CaCO₃ precipitation in planktonic cyanobacteria, allowing to harness this mechanism for future biotechnological applications.

Cyanobacteria are photolithoautotrophic prokaryotes and the only recent bacteria known to perform oxygenic photosynthesis. Cyanobacterial photosynthetic activity is assumed to have given rise to significant limestone sediments (Kaźmierczak et al., 1996; Altermann et al., 2006; Banerjee et al., 2006) such as stromatolites (i.e., lithified laminated organosedimentary deposits) and micritic mudstones (Kaźmierczak et al., 1996; Suosaari et al., 2016). While in some cyanobacteria intracellular formation of CaCO₃ granules has been documented (Benzerara et al., 2014; Moreira et al., 2017), extracellular CaCO₃ precipitation is more commonplace and an arguably much more promising engineering target for light-driven biomineralization. This may technically allow to uncouple cell-surface catalysed carbonate precipitation from biomass production on which most approaches discussed for cyanobacterial CCS rely (Chen et al., 2012; Victoria et al., 2024).

Cyanobacterial CaCO₃ precipitation is widely considered a passive byproduct of light-driven metabolic activity (Obst et al., 2009), with CaCO₃ crystal formation being largely determined by alkaline conditions in the aqueous media, availability of Ca²⁺ cations, and presence of heterogenous crystallisation nuclei (Jroundi et al., 2022). Cyanobacteria in particular provide all these conditions in the microenvironment around their cells due to (i) media alkalization in the wake of photosynthetic carbon assimilation of CO₂ from HCO₃ ⁻ releasing hydroxide ions (OH⁻) and thus increasing the extracellular pH to up to 10.5 (de Brito et al., 2022), and (ii) production of cell-surface components such as acidic exopolysaccharides (EPS) (Kamennaya et al., 2018; de Brito et al., 2022; Martinho De Brito et al., 2023) and negatively-charged surface-layer (S-layer) proteins (Schultze-Lam et al., 1992) attracting Ca²⁺ and nucleating CaCO₃ crystallisation. Further contributing to (bi-) carbonate ion availability, some cyanobacteria produce active extracellular carbonic anhydrase (eCA) enzymes which catalyse the hydration of water-dissolved CO₂ into HCO₃ ⁻/H⁺, presumably as a means of re-capturing CO₂ leaving the cell by diffusion (Soltes-Rak et al., 1997; Trimborn et al., 2009). While likely fostering extracellular CaCO₃ mineralisation (Kupriyanova et al., 2007; Hazarika and Yadav, 2023), no direct benefits of cyanobacterial eCA for carbon assimilation have been documented in so far (Kupriyanova et al., 2024), rendering its physiological relevance elusive. Light-driven processes underlying passive cell-surface catalyzed biomineralization (Obst et al., 2009; Görgen et al., 2021) and its intersection with anthropogenic biogeochemical carbon cycle contributions (Friedlingstein et al., 2025) are schematically summarized in Figure 1. With all relevant components being mechanistically understood, cyanobacteria are uniquely suited as synthetic biology chassis for engineered CaCO₃ production.

Large-scale precipitation of water-dissolved CO₂ as CaCO₃ will require considerable amounts of Ca²⁺. With ocean water containing approximately 10 mM Ca²⁺ (Millero et al., 2008; Emmanuel et al., 2012) and total ocean volume ranging around 1.35 × 10⁹ km³ (Charette and Smith, 2010) some 1.35 × 10²⁰ mols of dissolved Ca²⁺ are available for CaCO₃ precipitation, corresponding to approximately 1.351 × 10¹⁹ kg of CaCO₃ or 4.985 × 10⁶ km³ of limestone (density ∼2.71 g cm⁻³). For reference, precipitation of all anthropogenically emitted CO₂ since the industrial revolution (i.e., ∼1.5 × 10¹⁵ kg) (Ritchie, 2019) as CaCO₃ would result in 1,260 km³ of limestone equivalents, rendering ocean-water-dissolved Ca²⁺ a non-critical resource. Extracellular precipitation of seawater Ca²⁺ as CaCO₃ using marine or euryhaline cyanobacterial cell surfaces may thus provide a powerful tool for light-driven re-capture of CO₂ from CaCO₃ thermolysis or other industrial processes. Since most modern cyanobacteria do not precipitate relevant amounts of CaCO₃ for various reasons (Riding, 2006; Kamennaya et al., 2018), engineering their cell surface properties is likely required.

Omitting the seemingly impractical engineering of cyanobacterial EPS, the previous considerations culminate in two promising strategies towards reason-guided enhancement of light-driven biomineralization. A schematic overview of engineered strains following both strategies, relying on modified S-layers and free eCA on the one hand, or synthetic Ca²⁺-enriching polypeptides and surface-immobilized eCA on the other hand, is provided in Figure 2.

Storing CO₂ in the form of stable carbonate minerals faces fewer engineering and environmental challenges than other sequestration approaches like deep sea storage or deep ground injection, while maintaining minimal leakage risk as demonstrated by its natural counterpart (Ma et al., 2024). In natural systems, carbonate rocks are predominantly formed through biological and biologically induced processes, commonly summarized in the notion that carbonates are born, not made (James and Jones, 2016). Nowadays, photosynthetic plankton constitutes the most prolific carbonate factory, with recently evolved coccolithophore haptophyte algae producing around 50% of Holocene marine CaCO₃ sedimentation (Broecker and Clark, 2009). Due to their large contribution to pelagic CaCO₃ production in situ (Ziveri et al., 2023), these algae are being discussed as promising biotechnological CCS platforms. As these organisms remain hardly accessible to genetic engineering tools (Flavin and Chatterjee, 2024), any application remains largely limited to preexisting strains and their maximum productivity, however. A pronounced sensitivity to changes in carbonate chemistry and water acidification in most species (Beaufort et al., 2011; Meyer and Riebesell, 2015; Vázquez et al., 2023) and a general preference for growth temperatures below 30 °C (Gafar and Schulz, 2018; von Dassow et al., 2021) furthermore limit coccolithophore utility and application potential for CO₂ capture from, e.g., hot cement industry flue gasses commonly containing 10%–20% CO₂ (Camargo and Lombardi, 2018). Lastly, coccolithophore formation involves active transport and Ca²⁺ concentration within the cell (Sviben et al., 2016), rendering it more energetically taxing than passive extracellular CaCO₃ precipitation. Cyanobacteria meanwhile are highly accessible to genetic engineering and tailoring to harsh growth conditions through adaptive laboratory evolution (Tillich et al., 2012; Dann et al., 2021), their photosynthetic activity results in more pronounced media alkalization than that of eukaryotic algae (Touloupakis et al., 2016; Zepernick et al., 2021; de Brito et al., 2022), and their smaller cells provide a favourable surface-to-volume ratio for cell-surface catalysed CaCO₃ precipitation. This renders engineered cyanobacteria a likely superior platform for any future work on light-driven CaCO₃ precipitation, and marks cyanobacterial cell-surface engineering a promising pathway towards flexible and scalable biotechnological CaCO₃ recovery.

In accordance with their large contribution to the geological record, the utilization of cyanobacterial biomineralization for CCS was suggested before (Jansson and Northen, 2010), but no significant upscaling or commercial application has been achieved so far. Owing to a focus on EPS and the practical inaccessibility of complex anionic EPS biosynthesis to genetic and metabolic engineering, previous studies have near-exclusively focused on the identification of inherently productive calcifying species (Lee et al., 2004; Liang et al., 2013) and conductive cultivation methods (McCutcheon et al., 2014). A single genetic engineering attempt to increase calcification capacities was limited to the knockout of cax1 (Ca²⁺/H⁺ antiporter) in the mesophilic freshwater model species Synechocystis sp. PCC 6803, resulting in enhanced BCT1 (Ca²⁺-dependent HCO₃ ⁻ transporter) activity, increased CCM activity, and thus increased CaCO₃ precipitation (Jiang et al., 2013). Despite these first successes, biomineralization yields remain insufficient for large-scale applications. With documented rates of cyanobacterial Ca²⁺ precipitation from saltwater media corresponding to approximately 120–240 mg of CaCO₃ per liter of batch culture over a 2-week cultivation cycle (Lee et al., 2004; Yang et al., 2023), precipitation of 1 metric ton (t) of CaCO₃ would require the equivalent of two Olympic swimming pools (i.e., ∼5*10⁶ L). This corresponds to approximately 5.8 t of CO₂ sequestration capacity per Olympic swimming pool equivalent per year, valued around 430 € worth of CO₂ certificates at current EU Emissions Trading System pricing. Hence, an increase in biomineralization capacity by several orders of magnitude is likely required to attain economic viability. Although no comprehensive understanding of the modulation of CaCO₃ crystallization through biological agents has been achieved to date, mechanic deformation of heterogenous nucleation sites alone has been reported to increase CaCO₃ nucleation rate by one order of magnitude (Taylor et al., 2020). Meanwhile, calcite and aragonite nuclei were found the only nuclei capable of markedly catalyzing CaCO₃ precipitation in natural surface seawater (Pan et al., 2021), highlighting the crucial importance of crystallization nuclei surface properties for efficient CaCO₃ mineralization. As crystallization rates in more complex biogenic systems such as supersaturated lysozyme solution have been found to increase by 8-10 orders of magnitude upon exposure to suitable heterogenous nuclei (Filobelo et al., 2005), ample room for major improvement of bio-mediated CaCO₃ precipitation appears conceivable.

As opposed to previous attempts, engineering cyanobacterial cell surface properties through the introduction of modified or synthetic protein components and simultaneous genetic removal of obstructive features can be expected to allow for enhanced cell-surface catalysis of CaCO₃ precipitation. Especially recent breakthroughs in protein structure prediction and engineering (Watson et al., 2023) render this new approach worth pursuing. Here, a two-pronged empirical approach of introducing a re-engineered S-layer or disorganized synthetic cell-surface peptides with Ca²⁺ binding capacity appears a reasonable choice to determine the most conductive strategy, while EPS engineering remains prohibitively complex and porin/OMP engineering likely too functionally constrained for large-scale Ca²⁺ attraction and nucleation site provision. Finally, utilization of thermophilic chassis strains accessible for genetic engineering such as Thermosynechococcus elongatus BP-1 which strives at cultivation temperatures as high as 55 °C (Yamaoka et al., 1978; Iwai et al., 2004), or adaptive laboratory evolution for enhanced thermotolerance of mesophilic strains (Tillich et al., 2012) can likely enhance CaCO₃ precipitation efficiency due to reduced solubility of aragonite and calcite in warmer solutions, specifically enabling CO₂ capture from hot thermolysis or flue gasses. This should eventually allow for efficient light-driven re-routing of CO₂ emissions into carbonate resource production and lithospheric carbon storage.