Limited microbial degradation of elevated concentrations of dissolved organic carbon in the deep ocean

Abstract Understanding the ocean's capacity potential to store dissolved organic carbon (DOC) is essential for predicting its role in long‐term carbon sequestration and climate regulation. This capacity hinges on the behavior of DOC at elevated concentrations, a critical yet unresolved question that has produced mixed results due to narrow concentration ranges tested previously and limited molecular insights. This study addresses these gaps by investigating microbial degradation of DOC across a broad concentration range (2‐ to 55‐fold) in year‐long bioassay experiments using solid‐phase extracted DOC (SPE‐DOC) from 2000‐m‐deep waters. Specific SPE‐DOC compounds (combined amino acids) were analyzed to provide a molecular‐level understanding of DOC reactivity at varying concentrations. Our results show that microbial communities rapidly proliferated and became more uniform following SPE‐DOC amendments, with Nitrosococcales , Flavobacteriales , and Alteromonadales dominating. Despite these shifts, microbial utilization of SPE‐DOC was constrained, exhibiting a nonlinear relationship with concentration, from < 3% in the control to a maximum of 9% in DOC‐enriched groups. Degradation was predominantly confined to the initial 28 d, with negligible additional removal (0–2%) thereafter. Compound‐specific analysis showed only moderate utilization (7–11%) of amino acid compounds within the first 3 d, indicating restricted microbial access even when these individual compounds were concentrated. These results indicate that a fraction of deep‐sea DOC molecules can persist for long at elevated concentrations. Our study demonstrates the ocean's substantial potential for DOC storage and suggests that modern ocean is capable of accommodating a larger DOC reservoir than is currently present.