Distribution of Extracellular Flavins in a Coastal Marine Basin and Their Relationship to Redox Gradients and Microbial Community Members

The
flavins (including flavin mononucleotide (FMN) and riboflavin
(RF)) are a class of organic compounds synthesized by organisms to
assist in critical redox reactions. While known to be secreted extracellularly
by some species in laboratory-based cultures, flavin concentrations
are largely unreported in the natural environment. Here, we present
pore water and water column profiles of extracellular flavins (FMN
and RF) and two degradation products (lumiflavin and lumichrome) from
a coastal marine basin in the Southern California Bight alongside
ancillary geochemical and 16S rRNA microbial community data. Flavins
were detectable at picomolar concentrations in the water column (93–300
pM FMN, 14–40 pM RF) and low nanomolar concentrations in pore
waters (250–2070 pM FMN, 11–210 pM RF). Elevated pore
water flavin concentrations displayed an increasing trend with sediment
depth and were significantly correlated with the total dissolved Fe
(negative) and Mn (positive) concentrations. Network analysis revealed
a positive relationship between flavins and the relative abundance
of Dehalococcoidia and the MSBL9 clade of Planctomycetes, indicating
possible secretion by members of these lineages. These results suggest
that flavins are a common component of the so-called shared extracellular
metabolite pool, especially in anoxic marine sediments where they
exist at physiologically relevant concentrations for metal oxide reduction.