Flavins in Coastal Marine Sediments: New Perspectives on Diagenetic Electron Transfer

Coastal marine sediments play a critical role in the global cycling of metals and nutrients, many of which undergo diagenetic alteration. Central to these transformations are redox reactions where electron-rich organic matter is oxidized via transfer to terminal electron acceptors (NO₃^-, MnOx, FeOx, SO₄^2-). The flavins (flavin adenine dinucleotide [FAD], flavin mononucleotide [FMN], and riboflavin [B₂]) are microbially synthesized organic coenzymes that perform both single and double electron transfer and are known to mediate reduction of insoluble metal oxides. Culture experiments have found high rates of flavin excretion in metal-reducing Shewanella and Geobacter species, however environmental measurements of these highly labile molecules have not been previously reported. Here we present porewater measurements of FAD, FMN, and B₂ from San Pedro Basin. This California Borderland basin is silled, suboxic, ~900 m deep, and experiences high sedimentation. Flavin concentrations ranged from pico- (FAD: 0– 60 pM; B₂: 40 – 90 pM) to nanomolar (FMN: 0.4 - 1.2 nM). The concentration cascade of FMN>B₂>FAD fits well within culture experiments. Interestingly, profiles of all three flavins show a near linear increase with depth from 0-30 cm and a relatively steady concentration from 30-45 cm, supporting likely in situ production. Additionally, the flavins showed a negative correlation with dissolved Fe (R² = 0.7 for FMN, 0.8 for FAD, and 0.9 for B₂), which decreased linearly with depth from 160µM to 65µM. We discuss hypothesized mechanisms controlling flavin concentrations based on a suite of sediment geochemical parameters (dissolved Fe, Mn, TCO₂, δ¹³C, NH₃, DOM, and SO₄^2-) as well as implications for microbial redox syntrophy. These data provide a critical link between the extensive culture-based mechanistic understanding of flavin function and the sedimentary environment. Furthermore, these results demonstrate that flavins likely serve as a significant electron transfer intermediaries in the marine sediment carbon cycle.