Mechanisms of CO2 fixation and production driven by dissolved organic matter from different algae based on FT-ICR MS and metagenomics analysis

To explore the transformation mechanism of dissolved organic matter (DOM) and carbon dioxide (CO 2 ), as well as its carbon sequestration principle within the algal-bacterial symbiotic system, a three-month microcosm algal culture experiment was conducted. By combining FT-ICR MS with metagenomics, the carbon cycling process, along with the transformation of DOM molecules and the emission and fixation of CO 2 during the whole growth and decline phase was analyzed. Algal respiration and DOM degradation were the principal pathways for CO 2 production. Specifically, carbohydrates, peptide-likes and lipids, which were easily degraded, contributed significantly to CO 2 production. Additionally, a small quantity of CO 2 was generated through the degradation of certain degradation-resistant DOM molecules like lignin and unsaturated hydrocarbons. Within the algal-bacterial symbiotic system, DOM molecules progressively became more stabilized. The stabilization of DOM molecules serves to prevent the re-release of the fixed CO 2 and determines the ultimate fate of CO 2 . Microcystis was capable of sequestering more carbon than other algal species, which may be attributed to the greater resistance of DOM derived from its decay and degradation. Apart from algal photosynthesis, autotrophic bacterial reduction may also play a significant role in CO 2 fixation.