Accumulation of recalcitrant dissolved organic carbon during cyanobacterial blooms in Meiliang Bay, Lake Taihu: insights into the microbial carbon pump

Introduction: Cyanobacterial blooms are increasing in frequency, intensity, and duration in both freshwater and marine environments, potentially enhancing carbon sequestration by producing recalcitrant dissolved organic carbon (RDOC). Methods: We conducted monthly analyses of dissolved organic matter (DOM) composition and bacterial community dynamics in Lake Taihu (Meiliang Bay), China, integrating fluorescence DOM and ^¹H NMR to quantify carboxyl-rich alicyclic molecules (CRAM) as a molecular proxy for RDOC. Results: Estimated CRAM increased from 51.86 ± 11.22 μM C in the non-bloom period to 60.80 ± 8.21 μM C during blooms (~17% higher). The annual average RDOC was 62.93 ± 10.66 μM C, accounting for ~16% of the total DOC. Bacterial community analysis revealed that labile DOC was actively metabolized and transformed into more recalcitrant compounds through microbial carbon pump mechanisms. Specifically, the CL500-29 marine group and Sphaerotilus contributed to the degradation of protein-like DOM, while the CL500-29 and hgc1 clades played key roles in CRAM formation. Discussion: The pronounced RDOC enrichment in eutrophic lakes compared to non-eutrophic lakes, rivers, and marine systems underscores the potential of eutrophic lakes to function as significant carbon sinks, highlighting the necessity of integrating bloom-driven RDOC accumulation into carbon budget frameworks to reassess the long-term carbon sequestration potential of these systems.