Linking Source, Fate/Transport and Chemical Composition of CDOM in the North Atlantic Subtropical Gyre

The oceanic reservoir of inorganic carbon is substantially larger than that of dissolved organic carbon (DOC) (ca. 38,000 vs. 660 Pg C). However, DOC plays an important role in carbon cycling in the ocean, and as such, efforts to constrain this pool of carbon are invaluable to our understanding of the global carbon cycle. A fraction of dissolved organic matter is chromophoric (CDOM) and accounts for approximately 50% of blue light absorption in the ocean. It also absorbs light in the visible portion of the spectrum and therefore regulates light available for photosynthesis. Furthermore, CDOM in the surface ocean can be observed in satellite measurements of ocean color, and in turn influences algorithmic predictions of chlorophyll and primary production. Despite its importance in ocean biogeochemistry and remote sensing, the sources, fate and composition of CDOM remain unresolved. This is especially true in the pelagic ocean, in regions such as the North Atlantic subtropical gyre. In this study, we investigated the sources and cycling of CDOM in the North Atlantic, using a decade-long time series of biogeochemical samples, as well as in vitro incubations of seawater collected from the Bermuda Atlantic Time Series in the Sargasso Sea (31° 40’ N, 64° 10’ W). We found that autochthonous processes contribute greatly to the oceanic CDOM pool. Both the heterotrophic production and microbial breakdown of CDOM appear to be taxon-specific, with two genera of marine archaeota demonstrating the ability to alter portions of lignin (a component of terrigenous CDOM) on a timescale of days. This reveals a linked cycle of terrigenous and autochthonous CDOM, in which breakdown of one component, can lead to the production of new CDOM. Additionally, we investigated the role of marine autotrophs and found a significant correlation (R = 0.58, p < 0.01) between CDOM and Prochlorococcus cell abundance at the depth of the CDOM maximum. Both were correlated with virioplankton abundance at the same depths (R = 0.65, p < 0.01). As such, we posit a scenario whereby CDOM is produced by the viral lysis of Prochlorococcus. As Prochlorococcus is the most abundant photosynthetic organism on Earth and its abundance is predicted to increase by 29% by 2100, this could have a significant effect on the global CDOM pool. Finally, we created a model to investigate the sources of CDOM in the bathypelagic ocean. Although it is thought that the majority of deep CDOM in the North Atlantic is transported via the North Atlantic Deep Water, Prochlorococcus abundance in the euphotic zone accounted for ~30% of the variance in our model, suggesting that particulate matter containing Prochlorococcus lysate or cells may be transported to the deep ocean, where it leaches CDOM. The results of our study highlight the influence of autochthonous processes in open ocean CDOM cycling, and suggest that the roles of Prochlorococcus and archaea may be especially important.