Effect of dissolved organic matter on copper bioavailability to a coastal dinoflagellate at environmentally relevant concentrations.

The speciation and bioavailability of copper (Cu) in the marine environment are affected by the presence of dissolved organic matter (DOM). Previous studies conducted at dissolved Cu concentrations >100 nM confirmed that Cu bioavailability depends on the concentration of labile Cu, as measured by anodic stripping voltammetry (ASV), which aligns with the expectations of the biotic ligand model (BLM). However, ambient Cu concentrations in coastal waters are generally lower, ranging between 1 and 80 nM, and the effect of DOM on the bioavailability of Cu to marine organisms has not been tested within that range of Cu concentrations. The present study aims to assess the impact of two types of DOM, a commercially available fulvic acid, and marine DOM extracted by ultrafiltration, on Cu bioavailability to phytoplankton using short-term 65Cu internalisation by the marine dinoflagellate Prorocentrum micans. Results showed that Cu internalisation decreases with DOM additions as expected according to the BLM and in agreement with ASV measurements of labile Cu, at the highest tested Cu concentration (100 nM). On the contrary, at a lower Cu concentration (20 nM), organic complexes appear to be partially bioavailable, thereby challenging the general applicability of the BLM model at environmentally relevant concentrations in coastal areas.

Seasonal succession of small planktonic eukaryotes inhabiting surface waters of a coastal upwelling system.

Small eukaryotes (0.2-20 µm cell-size) represent a significant fraction of the microbial plankton community in shelf waters of NW-Spain. The community composition of small eukaryotes living at the surface and at the base of the photic zone was analysed by means of 18S rDNA high-throughput sequencing on a circa-monthly basis over a 23 months period. Ostreococcus was the most abundant taxon in surface waters, showing marked peaks in read abundance in spring and late summer, while Syndiniales dominated at the base of the photic zone. A well-defined seasonal pattern of community composition, linked to the succession of the dominant taxa, was found in surface waters. Seasonality was less apparent at the base of the euphotic zone. Temporal changes in abiotic factors significantly correlated with changes in community composition in surface (r = 0.71) and at the base of the photic zone (r = 0.38). Changes in community composition significantly correlated with changes in community function-related variables (including biomass, primary production and respiration) only in surface water (r = 0.36). Co-occurrence network analyses revealed 45 significant interspecies associations among the 50 most abundant taxa with highly connected OTUs belonging to cryptophyceans. The network topology, with small-world characteristics, suggests a stabilizing role of biotic interactions to environmental disturbance.