The molecular geography of dissolved organic matter in the Atlantic Ocean can largely be explained by a simple two-source mixing model.

Marine dissolved organic matter (DOM) is one of the largest active carbon reservoirs on Earth, similar in size to atmospheric CO₂. The primary source for marine DOM is photosynthesis, but a wide variety of biotic and abiotic processes shape the DOM pool over time. With ultrahigh-resolution analytical techniques such as Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) it is possible to characterize DOM on the molecular level in unprecedented detail. Despite the geographical diversity of primary producers, microbial community structures, variety of abiotic conditions and ecosystem types, marine molecular DOM composition is exceptionally similar across the globe, even when analyzed at highest analytical resolution. Molecular dissimilarities of DOM occur mainly between the surface mixed layer and the deep sea. Here we hypothesize that this observed dissimilarity can largely be explained by a simple two-source mixing model of freshly produced and old background DOM. Furthermore, we hypothesize that the two endmembers in this model each carry a universal molecular signature. To test these hypotheses, we mixed deep sea DOM with increasing proportions of DOM produced in the laboratory by a marine microbial community over the course of two years. The molecular composition of these artificial mixtures was compared via FT-ICR-MS with >300 natural samples across the entire Atlantic Ocean. In support of our hypotheses, the addition of laboratory-created DOM made deep sea DOM molecularly more similar to surface DOM. We conclude that much of the observed molecular differences across water masses and between surface and deep sea is simply explained by mixing of two endmembers, i. e. freshly produced DOM and aged background DOM. The two endmembers have characteristic and universal molecular features. Our results also show that additional processes at the sea surface, such as photochemistry, influence the molecular composition of DOM beyond simple mixing.