High temperature favors sedimentary organic carbon fixation potential via chemoautotrophy and iron fraction transformation in urban artificial lakes.

Urban artificial lake sediments play crucial role in organic carbon (OC) fixation for cities. However, the regulatory mechanism on sedimentary OC fixation by water temperature (WT) changes is still unclear. This study investigated the water qualities, organic carbon components, the microbial metabolism and iron (Fe) fractions of five lakes in summer and winter. Results revealed that high temperature group (HT) exhibited 1.5-2.5 times higher average contents of total organic carbon (TOC), particulate organic carbon (POC), mineral-associated organic carbon (MAOC) and Fe-bound organic carbon (Fe-OC), comparing to the winter low-temperature (LT) group. The average MAOC/TOC ratio in the HT group and LT group was 65 % and 46 %, respectively, indicating that WT played a dominant role in stable OC formation. Analysis of metabolomics indicated predominant microbial carbon dioxide fixation pathway was transformed from photosynthetic autotrophy to chemoautotrophy, accompanied by an increased relative abundance of the associated regulatory enzymes as WT rose. Furthermore, elevated temperatures promoted a significant increase in the Fe reduction rate, activation degree, and complexation degree of sediments, thereby also facilitating the formation of stable OC. In conclusion, high temperature triggered the "microbe-iron-carbon" cascade by increasing chlorophyll-a concentrations and reducing dissolved oxygen and transparency, which may enhance the sedimentary carbon fixation potential. These findings provide a theoretical basis for optimizing the ecological design and management strategies to enhance carbon sink of urban artificial lakes.