The impacts of chemical composition on bacterial processing of dissolved organic matter in the deep Arctic Ocean

Abstract

The cycling of dissolved organic matter (DOM) in the ocean plays a central role in carbon dynamics, influencing the ocean's ability to sequester carbon and regulate climate. However, the microbial processes that govern the transformation and persistence of DOM across different water masses remain poorly understood. In this study, we explore how a single bacterial community processes DOM from four distinct water masses (i.e, Pacific winter water, Atlantic halocline, Atlantic water - Barents Sea Strait branch, and deep temperature minimum) in the Canada Basin of the Arctic Ocean, using a combination of optical and molecular techniques to reveal the microbial mechanisms behind DOM reactivity and persistence. Although all water masses contained the same fluorescent components, each underwent unique processing by the microbial community after 20 days of laboratory-controlled incubation. This suggests that the composition of specific fluorescent components likely varies between different water layers, with an increase in these features by the end of the incubation. The number of common molecular features measured with a trapped ion mobility spectrometer coupled with a time-of-flight mass spectrometer (TIMS-TOF) and an electrospray ionization source (positive ionization mode) was higher in contiguous water masses, likely due to water mass mixing resulting in the formation of the Atlantic halocline. Spearman's rank correlations between fluorescence characteristics and TIMS-TOF features established connections between fluorescent characteristics and molecular formulas. Twenty-two distinct molecular features were related to the four humic-like. None of them were associated with tyrosine-like, revealing the distinct molecular-optical linkages between humic- and protein-like DOM. Overall, our findings indicate that the DOM reactivity depends on its optical and molecular composition. The mechanistic understanding of the factors that control the persistence of DOM in the ocean is essential for predicting the ocean's role in the global carbon cycle.