Recycling of microbial respiration-derived CO<sub>2</sub>(aq) plays a crucial role in regulating carbon sequestration and emissions in alkaline lakes, yet the extent to be assimilated by primary producers remains poorly constrained. In this study, a total of 35 surface sediments were collected across Lake Haixihai and organic molecular as well as carbon isotopic geochemical methods were employed to investigated the efficiency of diatom uptake of respiration-derived CO<sub>2</sub>(aq). The results show that organic matter in surface sediments is dominated by aquatic macrophytes and terrigenous inputs, with minor contributions from algae and bacteria. The diatom-sourced C<sub>25 </sub>highly branched isoprenoids (HBIs) were detected in all sediments with C<sub>25:2</sub> HBI as the most abundant HBI. The δ<sup>13</sup>C values of C<sub>25:2</sub> HBI strongly correlates with δ<sup>13</sup>C<sub>org</sub> (<em>r</em> = 0.946, <em>p</em> < 0.001) and the extent of microbial reworking as indicated by concentrations of C<sub>30</sub>ββ-hopane + hopenes and lignin-derived P/(V+S) ratios (<em>r</em> = -0.92 ~ -0.93, <em>p</em> < 0.001), suggesting that diatoms can substantially assimilate respiration-derived CO<sub>2</sub>(aq). A two-endmember mixing model reveals that the respiration-derived CO<sub>2</sub>(aq) utilized by diatoms accounts for an estimated 31 % ± 8 % of diatom carbon uptake. Comparisons across oceans, lakes, and wetlands show that inland waters sustain substantial recycling of respiration-derived CO<sub>2</sub>(aq), which is efficiently assimilated by diatoms and reintegrated into the biological carbon pump. These findings suggest that lakes should not be recognized easily as conduits for terrestrial carbon loss. They are active systems in which biological re-fixation of respiration-derived CO<sub>2</sub>(aq) can substantially enhance long-term carbon sequestration and mitigate CO<sub>2</sub> emissions.