Abstract
As a consequence of global climate change, lakes are increasingly receiving terrestrial dissolved organic carbon (DOC), which serves as a key substrate for microbial metabolism and fuels bacterial production (BP). However, bacteria in aquatic systems play a dual role in the carbon cycle by not only incorporating DOC into their biomass but also respiring it as CO(2) into the atmosphere (bacterial respiration, BR). As such, the estimation of bacterial growth efficiency (BGE), defined as BP/(BP + BR), is critical for understanding lake carbon dynamics and bacterial carbon processing. To investigate the effects of terrestrial organic carbon on bacterial carbon processing in lakes, we conducted a (13)C-labeling experiment utilizing three microcosms, each filled with 0.22 μm filtered lake water inoculated with a microbial inoculum and set as follows: no extra DOC addition as a control, adding phytoplankton-derived DOC, and adding a mixture of phytoplankton-derived and terrestrial DOC. Our findings revealed that the addition of terrestrial DOC significantly enhanced both overall BGE (40.0%) and specific BGE based on phytoplankton-DOC (62.3%) and indigenous lake DOC (27.0%). Furthermore, terrestrial DOC inputs also altered bacterial carbon consumption pathways, as indicated by isotopic evidence. These results suggest that the input of terrestrial DOC may significantly affect lake DOC processing by changing the way bacteria process phytoplankton-DOC and indigenous lake DOC. This study highlights the profound influence of terrestrial DOC on lake carbon processing and suggests that terrestrial-aquatic cross-ecosystem interactions are critical for understanding lake carbon dynamics under changing climatic conditions.