Salinity Stress Driven Organic Matter Preservation in Carbonates Induced by the Cyanobacterium Leptolyngbya boryana

Microbialites formed under saline conditions usually have relatively higher hydrocarbon generation potential, although the mechanisms remain insufficiently understood. In this study, microbial mineralization experiments were conducted using a filamentous cyanobacterium, Leptolyngbya boryana, under varying salinity and Mg/Ca molar ratios. The results show that elevated salinity slightly inhibits the growth of Leptolyngbya boryana, while the final pH and alkalinity of the medium remain largely unchanged. Under saline conditions, extracellular polymeric substance (EPS) production increases, with a marked shift in composition characterized by a significant enrichment in polysaccharides and a relative decrease in proteins. The enhanced EPS production promotes the precipitation of Ca-Mg carbonates, resulting in increased formation of calcite and low-Mg calcite. Interactions between cyanobacterial organic matter and minerals likely involve multiple stages, including surface adsorption, intercrystalline entrapment, and cellular encapsulation. Thermogravimetric analysis reveals that under the same Mg/Ca ratio, the amount of organic matter preserved biominerals increases from ∼3.1 wt% to ∼5.52 wt% with rising salinity. Organic matter characterization further indicates that both polysaccharide and protein contents in biominerals increase under saline conditions, with selective preservation of Glc-UA monosaccharides, Asp and Glu. This study provides new insights into the interplay between microbial EPS, carbonate mineralization, and organic matter preservation in saline environments.