Limited microbial degradation of elevated concentrations of  dissolved organic carbon in the deep ocean
Understanding the ocean's potential to store dissolved organic carbon (DOC) is essential for predicting its role in long-term carbon sequestration and climate regulation. This capacity depends on the behavior of DOC at elevated concentrations, a critical but unresolved question that has produced mixed results due to the narrow concentration ranges previously tested 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 2,000-meter-deep waters. Specific SPE-DOC compounds, such as 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, ranging from less than 3 percent in the control to a maximum of 9 percent in DOC-enriched groups. Degradation was predominantly confined to the initial 28 days, with negligible additional removal (0 to 2 percent) thereafter. Compound-specific analysis showed only moderate utilization (7 to 11 percent) of amino acid compounds within the first 3 days, indicating restricted microbial access even when these individual compounds were concentrated. These results indicate that deep-sea DOC molecules can persist for long periods at elevated concentrations. Our study demonstrates the ocean's substantial potential for DOC storage and suggests that modern oceans are capable of sustaining a larger DOC reservoir than is currently present.