Effects of ethoxyquin on metabolism and composition of active marine microbial communities

Salmon culture has the potential to generate environmental impacts due to the release of organic matter and chemical products into the water column. Ethoxyquin (EQ) is a food additive widely used as an antioxidant in fish feed, but due to the scarce evidence on its safety for the environment it has been banned by the European Food Safety Authority. To evaluate and compare responses of marine microbial communities to EQ additions (10, 100 and 1000 μg L−1), we carried out 20 L seawater microcosm experiments in two distinct areas, Achao and Coliumo Bay, where the first is influenced by salmon farming. We estimated metabolic response (respiration, extracellular enzymatic activity and ECOplates) for two size fractions (< 1 μm and < 63 μm) and changes in the active microbial community diversity. The results revealed different responses associated with study sites and size fractions. For the < 1 μm fraction (bacterioplankton), the addition of EQ increased both oxygen consumption and the degradation of proteinaceus compounds, while for the entire community (< 63 μm), the addition of EQ decrease respiration and the capacity to degrade carbohydrates and lipids. The carbon substrates utilization was variable, decreasing in the 100 and 1000 μg EQ. L−1 treatments, suggesting that high EQ concentrations may limit the capacity to process different carbon sources, producing a preference for amino acids and carboxylic acids in the first hours of exposition in Achao. In the area without salmon farming (Coliumo Bay) substrates utilization, mainly carbohydrates and carboxylic acids, was stimulated by the addition of EQ. A shift was observed in the active microbial community between treatments at different taxonomic levels, especially associated with Alphaproteobacteria, which presented lower activity when exposed to EQ. Gammaproteobacteria (KI89A), Bacteroidea (NS10, NS3b clade) and MGII showed higher activity associated with EQ additions. Overall, our study indicates that EQ may generate both changes in the metabolism (respiration, enzymatic extracellular activity, carbon sources preferences) and composition of the active microbial community in the water column altering the degradation of organic matter.