Cell Surface-Dependent Mineralization Induced by Cyanobacterium Anabaena sp. PCC7120: Implications for Fossilization and Carbon Sequestration

Cyanobacteria, as primordial earliest oxygenic photosynthetic microorganisms, have profoundly influenced microbialite formation throughout Earth’s sedimentary history. However, the mechanisms underlying cyanobacterial calcification and morphological preservation remain unresolved. With morphological ambiguity and biomarker alteration posing challenges for biogenicity assessment. This study investigates the mineralization processes of the filamentous cyanobacterium Anabaena sp. PCC7120 under varying Mg/Ca molar ratios (0, 2, 4, 6, 8) in controlled culture systems. Experimental results reveal that Anabaena sp. PCC7120 growth modulates medium pH, which governs precipitate mineralogy and crystallinity. Notably, Mg/Ca ratios critically influence mineral composition, with a ratio of 2 inducing intense surface-dependent mineralization. Under this condition, cell-shaped mineralized envelopes were formed, preserving intricate morphological details of cyanobacterial cells. Microscopic analyses demonstrate heterogeneous coprecipitation of Ca²⁺ and Mg²⁺ on cell surfaces. Heat treatments demonstrated mineralized envelopes inhibit organic degradation during burial by forming stable organo-mineral composites. We propose that OH- enrichment within cyanobacterial sheaths drives surface mineralization, while sheath composition and Mg²⁺-mediated inhibit calcite crystallization to form aragonite. This surface-dependent mineralization critically preserves filamentous cyanobacterial structures and organic carbon. Our findings establish a mechanistic framework of cyanobacterial mineralized envelopes, with implications for microbial fossil identification and organic matter preservation in geological and environmental contexts.