Optical properties and molecular differences in dissolved organic matter at the Bermuda Atlantic and Hawai'i ALOHA time-series stations†

Abstract

Optically active components, namely marine chromophoric DOM (CDOM) and fluorescent DOM (FDOM), have been used as proxies for refractory DOM (RDOM) in the world's oceans, and numerous studies using ultrahigh resolution mass spectrometry (HRMS) approaches have supplied a tremendous amount of data on the chemical complexity and diversity of DOM. Here, we collected and analyzed high-resolution depth profiles of DOM throughout the water column in the North Atlantic Gyre at the Bermuda Atlantic Time-series Study (BATS) station in August 2019 and in the North Pacific Gyre at station ALOHA (A Long-term Oligotrophic Habitat Assessment) used by the Hawaii Ocean Time-series (HOT) in July 2021. Water samples were collected at 200 m depth intervals from 4530 m at BATS and 4700 m at ALOHA up to the surface and DOM was isolated by solid-phase extraction (SPE). Parallel factor analysis modeled EEM fluorescence revealed changes of “humic-like” and “protein-like” FDOM (FDOM_H and FDOM_P, respectively) in SPE-DOM throughout the water column with higher fluorescence intensities present at ALOHA. Dissolved organic phosphorous (DOP) and dissolved organic sulfur (DOS) concentrations were always higher in SPE-DOM at BATS than at ALOHA, except for DOP in the surface at ALOHA. Negative mode electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) data also revealed fundamental differences between BATS and ALOHA. A novel machine learning algorithm (SOFAR) was implemented and revealed much higher overall oxygen to carbon (O/C) ratio molecular signatures at BATS as the major difference and also much more DOS signatures at BATS when compared to ALOHA. Furthermore, we extracted for the first-time weak anion exchange (WAX) amendable DOM, and the results also showed drastic differences between the two stations. The optical and FT-ICR MS data, converged and supported the idea that DOM in the Atlantic and Pacific basins are fundamentally different when looked at through these analytical windows, at least at these long-term monitoring stations. This finding suggests that the marine DOM is likely turning over at different rates at ALOHA versus BATS and that geographical differences in DOM composition are likely a compounding factor in DOM reactivity.