|Title||Carotenoids are the likely precursor of a significant fraction of marine dissolved organic matter|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Arakawa N, Aluwihare LI, Simpson AJ, Soong R, Stephens BM, Lane-Coplen D|
The ocean’s biota sequester atmospheric carbon dioxide (CO2) in part by producing dissolved organic matter (DOM) that persists in the ocean for millennia. This long-term accumulation of carbon may be facilitated by abiotic and biotic production of chemical structures that resist degradation, consequently contributing disproportionately to refractory DOM. Compounds that are selectively preserved in seawater were identified in solid-phase extracted DOM (PPL-DOM) using comprehensive gas chromatography (GC) coupled to mass spectrometry (MS). These molecules contained cyclic head groups that were linked to isoprenoid tails, and their overall structures closely resembled carotenoid degradation products (CDP). The origin of these compounds in PPL-DOM was further confirmed with an in vitro β-carotene photooxidation experiment that generated water-soluble CDP with similar structural characteristics. The molecular-level identification linked at least 10% of PPL-DOM carbon, and thus 4% of total DOM carbon, to CDP. Nuclear magnetic resonance spectra of experimental CDP and environmental PPL-DOM overlapped considerably, which indicated that even a greater proportion of PPL-DOM was likely composed of CDP. The CDP-rich DOM fraction was depleted in radiocarbon (14C age > 1500 years), a finding that supports the possible long-term accumulation of CDP in seawater. By linking a specific class of widespread biochemicals to refractory DOM, this work provides a foundation for future studies that aim to examine how persistent DOM forms in the ocean.
This study identified carotenoids as widespread biosynthetic precursors to a significant component of refractory dissolved organic matter (DOM). Carotenoids are a specific class of terpenoids and play numerous roles in marine and terrestrial environments, which is consistent with the similar chemical features identified in DOM from both types of aquatic environments... Without better radiocarbon data, it is not yet possible to determine the role carotenoids play in the very long-term cycling of DOC. Uncertainties surrounding this calculation demonstrate why the results of this study are so valuable: By assigning carotenoids as a specific biosynthetic precursor for some of the DOM that accumulates in the ocean, the study enables testable hypotheses to be designed to identify important processes that shape refractory DOM and control its removal from the ocean