Algal-bacterial consortium promotes carbon sink formation in saline environment

The level of atmospheric CO₂ has continuously been increasing and the resulting greenhouse effects are receiving attention globally. Carbon removal from the atmosphere occurs naturally in various ecosystems. Among them, saline environments contribute significantly to the global carbon cycle. Carbonate deposits in the sediments of salt lakes are omnipresent, and the biological effects, especially driven by halophilic microalgae and bacteria, on carbonate formation remain to be elucidated.

The present study aims to characterize the carbonates formed in saline environments and demonstrate the mechanisms underlying biological-driven CO₂ removal via microalgal-bacterial consortium.

The carbonates naturally formed in saline environments were collected and analyzed. Two saline representative organisms, the photosynthetic microalga Dunaliella salina and its mutualistic halophilic bacteria Nesterenkonia sp. were isolated from the inhabiting saline environment and co-cultivated to study their biological effects on carbonates precipitation and isotopic composition. During this process, electrochemical parameters and Ca²⁺ flux, and expression of genes related to CaCO₃ formation were analyzed. Genome sequencing and metagenomic analysis were conducted to provide molecular evidence.

The results showed that natural saline sediments are enriched with CaCO₃ and enrichment of genes related to photosynthesis and ureolysis. The co-cultivation stimulated 54.54% increase in CaCO₃ precipitation and significantly promoted the absorption of external CO₂ by 49.63%. A pH gradient was formed between the bacteria and algae culture, creating 150.22 mV of electronic potential, which might promote Ca²⁺ movement toward D. salina cells. Based on the results of lab-scale induction and ¹³C analysis, a theoretical calculation indicates a non-negligible amount of 0.16 and 2.3 Tg C/year carbon sequestration in China and global saline lakes, respectively.

The combined effects of these two typical representative species have contributed to the carbon sequestration in saline environments, by promoting Ca²⁺ influx and increase of pH via microalgal and bacterial metabolic processes.