Award: OCE-1459294

The origin and mechanisms driving the formation of fluorescent dissolved organic matter (FDOM) in the open ocean remain unclear. One hypothesis is that oceanic FDOM, derives from heterotrophic bacterial transformation of primary productivity (Nelson and Siegel 2013, and references therein), while a second interpretation is that the FDOM appears terrestrial because its optical properties arise from intramolecular electronic interactions of charge transfer between electron-rich donors and electron-poor acceptors in molecules such as lignin that are more abundant in terrestrial DOM (formulated as the EI model) (Del Vecchio and Blough 2004; Andrew et al. 2013). Here, we investigated the transformations in oceanic FDOM/FPOM using a biomarker approach in a controlled growth and degradation experiment. In this experiment, a natural assemblage of phytoplankton was collected off the coast of North Carolina and incubated within roller bottles containing 0.2 µm-filtered North Atlantic surface water amended with f/2 nutrients. Samples were collected at the beginning (day 0), during exponential growth (day 13), stationary (day 20), and dark degradation (day 62) phases of the phytoplankton incubation. Amino acids of the DOM and POM were measured in conjunction with enzyme assays and bacterial counts to track shifts in OM composition as FDOM and FPOM formed and was then transformed throughout the experiment. In the FDOM, a strong protein-like peak dominated during the initial and mid-exponential phases of the phytoplankton and increased in ?humic-like? peaks during the stationary and degradation phases (Fig. 1). The POM and DOM were significantly altered as the incubation experiment progressed, and they remained distinct from each other with regards to amino acid and fluorescent composition (Fig. 2). The aromatic amino acids were significantly correlated to both protein-like peaks B and T (Fig. 3). Tryptophan was significantly correlated for the POM only (Fig. 4). This could be due to dissolved tryptophan remaining near the limit-of-detection, or the presence of other indole-containing compounds. Particulate serine and threonine increased in relative abundance throughout the incubation and was significantly correlated to all humic-like FDOM peaks (Fig.5). We hypothesize that the degradation of POM is driving many of the changes in DOM fluorescence. Last Modified: 04/07/2018 Submitted by: Thomas S Bianchi