Do microbes contribute to the FDOM signature in the ocean?

Abstract

Samples from the MALASPINA circumnavigation expedition (2010‐2011) were collected to study the influence of microbial abundances in the distribution of the fluorescent dissolved organic matter (FDOM). The FDOM excitation‐emission matrix (EEM) data, obtained using a Fluoromax‐4 spectrofluorimeter, were examined with Parallel Factor Analysis (PARAFAC). The PARAFAC analysis identified four components, two of them associated with humic‐like substances (C1 and C2) and the other two with protein‐like compounds (C3 and C4). In the specific context of this study, we will only refer to the protein‐like components: C3 corresponds to classic peak‐T, which is related to the essential aminoacid tryptophan (excitation‐emission 290/340 nm); and C4, associated to classic peak‐B, is related to the non‐essential aminoacid tyrosine (excitation‐emission 270/310 nm). We study the relationships between these two FDOM compounds and viruses and bacteria abundances compiled in the Malaspina database which includes values from 5 different oceanic basins (North Atlantic, South Atlantic, Indian, North Pacific and South Pacific). Samples were collected from 0 m to 4000 m depth, distinguishing three different layers (epipelagic 0‐200 m, mesopelagic 200‐1000 m, and bathypelagic 1000‐4000 m). Our aim was to determine if the dynamics of these FDOM components followed the evolution of the bacteria and/or viruses abundances. To achieve this objective we applied residual analyses to exclude the variability due to physicochemical parameters (temperature and salinity). When the whole database was considered, these parameters accounted for a high percentage (~60%) of both virus and prokaryotic variability. The residual analyses shown, in general, that the C3 component was significantly correlated to virus abundance. In contrast, only a weak C3‐prokaryotic abundance relationship was found in the mesopelagic zone of the North Pacific basin. On the other hand, C4 shown, in general, no clear relationship neither with prokaryotes nor viruses. Finally, we will discuss these results in the context of the usage of FDOM components as tracers of abundances and interactions of marine microbes