Effects of filter-feeding fish faeces on microbial driving mechanism of lake sediment carbon transformation

Silver carp (Hypophthalmichthys molitrix) can filter the carbon in the food taken up by phytoplankton and plays an important role in carbon fixation. In this study, the faeces of silver carp, the dominant fish species in Qiandao Lake, China, were collected and subjected to a closed incubation and transformation experiment for three months. The physical and chemical indices of water and sediment mixture, carbon metabolic enzyme activity, and microbial sequences were analyzed to identify the key microbial strains that affect carbon transformation as well as the main factors influencing carbon transformation. The results showed maximum CO₂ and CH₄ emission fluxes on day 15 of fish faeces and sediment interaction. In the faeces addition group, the contents of soluble organic carbon, soluble inorganic carbon, SO₄²⁻, and PO₄³⁻ were significantly increased, while the dissolved oxygen content was significantly decreased. Furthermore, the pH, total carbon content, volatile suspended solids content, and activities of four carbon-metabolizing enzymes were significantly increased in the faeces addition group. The 16sRNA analysis of methanogenic and methane-oxidizing bacteria showed that Euryarchaea and Pseudomonas accounted for the highest proportion respectively. The most significant differences expression were found for Methylbacterium in the methanogenic bacteria and Methylobacter in the methane oxidizing bacteria. Structural variance model showed that interaction of fish faeces and sediments mainly caused changes in sulfate content, leading to variations in methanogens and methanotrophs and promotion of CH₄ emission. The results of this study can provide a theoretical reference for the mechanism of carbon reduction and emission reduction of lake filter-feeding fish.