RESUMO
Diffuse and point sources of dissolved organic matter (DOM) in streams influence its composition, interactions and fate in the aquatic ecosystem. These inputs can be very numerous at the scale of a watershed, and their identification remains a challenge, especially for diffuse sources related to land use. The complexity of the transfer mechanisms and the reactivity of DOM throughout the soil-water column continuum raise questions about the sampling of diffuse sources in watercourses. To answer this issue, we compared the characteristics of soil-extracted DOM influenced by a particular land use (homogenous sub-catchment of forest and vineyard) and DOM collected from the watercourse adjacent to the soil samples. A 28-day incubation experiment of soil extracts was designed to remove the labile fraction of DOM. During the first 3 days, between 40 and 70 % of the DOC mass was lost for both types of soils. A set of optical indicators (UV-Visible, EEM fluorescence and HPSEC/UV-fluorescence) showed that the labile fraction was mostly composed by low (<1 kDa) and high (>10 kDa) protein-like molecules. At the end of the incubation, soil-extracted DOM was mainly composed of medium molecules (1-10 kDa) associated to terrigenous humic-like compounds. Its optical and size molecular signature tended towards that in the adjacent watercourses and was specific to land use. However, the characteristics of DOM in watercourses was also influenced by the hydrological conditions, probably due to a transfer of top soil DOM during high water periods and both deep soil and autochthonous DOM during low water periods. These results were obtained by a set of indicators, including novel ones derived from HPSEC/UV-fluorescence. Finally, this study demonstrated that it is possible to sample the DOM representative of a land use directly in the river downstream of the homogeneous sub-basin by multiplying the samples during contrasting hydrological conditions.
RESUMO
The adsorption of trace metals on microplastics (MPs) is affected by the presence of surficial biofilms but their interactions are poorly understood. Here, we present the influence of Cu levels in real seawater (Toulon Bay, NW Mediterranean Sea) on microbial communities and Cu content of the resulting biofilms grown during incubation experiments on high density polyethylene. Two sets of incubation experiments were run with seawater supplied with MPs, sampled in two sites with contrasting Cu levels: Pt12 (most contaminated site) and Pt41P (less contaminated site). For each incubation experiment, 5 treatments were considered differing in Cu concentrations, ranging between 30 and 400 nM and between 6 and 60 nM, for Pt12 and Pt41p, respectively. A control experiment (filtered at 0.2 µm) was run in parallel for each incubation experiment. We observed that, at the time scale of the incubation period, both prokaryotic and eukaryotic richness and diversity were higher in the biofilms formed from the most contaminated site. In addition, we showed that Cu levels are shaping biofilm communities, evidencing co-occurrence patterns between prokaryotes and eukaryotes with diatoms playing a central role. These differences in biofilm formation were reflected in the amount of bioaccumulated Cu per dry weight of MPs, exhibiting higher values in the most contaminated site. Within this site, the increase of Cu seawater content enhanced its bioaccumulation onto MPs until reaching saturation. This study strongly suggests a striking link between seawater copper content, biofilm community shaping and the resulting Cu bioaccumulation onto MPs.