Abstract
Aquaculture releases organic matter to the water column through excretion, fecal pellets, and uneaten food, but also by the continuous release of fish epithelium mucus. The effect of the latter on natural bacterial assemblages was determined using ammonium amended experiments at Puyuhuapi fjord in Chilean Patagonia. Mucus was added to seawater coming from 2 and 100 m depth and ammonium, nitrite and nitrate, dissolved organic carbon (DOC), picoplankton abundance, and active composition (i‐tag 16S rRNA) were followed for 24 h. The results showed a significant response from the microbial community but only at surface depth after 2 and 6 h of incubation. A reduction of DOC and ammonium concentration and accumulation of nitrite and nitrate over time was observed, mainly at 100 m. Changes in the composition of active bacteria between treatments were observed at different taxonomic levels, associated with Alphaproteobacteria (Clade SAR11), Bacteroidetes (Polaribacter) and Gammaproteobacteria (Colwellia, Oceaniserpentilla) and other bacteria such as Nitrospina sp, a nitrite‐oxidizing bacteria at some hours during the incubation. Fish pathogens, such as Vibrio and Piscirickettsia were rare (<0.02%). Overall, our study suggests that fish mucus can cause rapid modifications in microbial assemblages and stimulate organic matter and nutrient cycling, including heterotrophic and autotrophic (nitrification) in areas influenced by aquaculture.
Highlights
Mucus is a matrix layer generated by fish epithelium with physical and chemical characteristics that provide the organisms with protection as well as ecological intra- and interspecific interactions through signaling, among other functions, recently reviewed (Reverter et al, 2018)
With ammonium, picoplankton abundance (Figure 1b) was characterized by higher values in surface waters (1,324 x 103 cell mL−1 at 2 m depth) compared with cell abundances observed at 100 m (257 x103 cell mL−1)
Ammonium and nitrate changes were observed through time in parallel with significant composition shifts studied through iTag 16S rRNA
Summary
Mucus is a matrix layer generated by fish epithelium with physical and chemical characteristics that provide the organisms with protection as well as ecological intra- and interspecific (even symbiotic) interactions through signaling, among other functions, recently reviewed (Reverter et al, 2018). Despite antimicrobial activities (Kumari et al, 2019), skin mucus is inhabited by many commensal/opportunistic microbes, reaching higher abundances ~4x108 cell (Bernadsky & Rosenberg, 1992; Minniti et al, 2017). This ecological interspecific interaction within the mucus is generated from early stages in teleost fishes such as Salmon due to microbial colonization from the surrounding ecosystem helping to develop the fish immune system (Kelly & Salinas, 2017). Despite its protective role and potential microbe-host beneficial interactions, fish mucus is vulnerable to the ability of certain pathogens to adhere and invade (e.g., Zuo et al, 2019)
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