Microplastics are pollutants of concern in the freshwater and marine environments. These microparticles carry biofilm communities unique from the surrounding water.

Atlantic salmon smolts (approx. 20-months old) were fed experimental diets with different combinations of omega-6:omega-3 fatty acids (FAs) (high-ω6, high-ω3, or balanced) and eicosapentaenoic acid plus docosahexaenoic acid (EPA + DHA) levels (0.3, 1.0 or 1.4%) for 12 weeks. Muscle FA (% total FA) reflected dietary C 18 -polyunsaturated FA; however, muscle EPA per cent and content (mg g −1 ) were not different in salmon fed high-ω3 or balanced diets. Muscle DHA per cent was similar among treatments, while DHA content increased in fish fed 1.4% EPA + DHA, compared with those fed 0.3–1.0% EPA + DHA combined with high-ω6 FA. Muscle 20:3 ω 6 (DGLA) content was highest in those fed high-ω6 with 0.3% EPA + DHA. Quantitative polymerase chain reaction analyses on liver RNA showed that the monounsaturated FA synthesis-related gene, scdb, was upregulated in fish fed 1.0% EPA + DHA with high-ω6 compared to those fed 0.3% EPA + DHA. In high-ω3-fed salmon, liver elovl2 transcript levels were higher with 0.3% EPA + DHA than with 1.0% EPA + DHA. In high-ω6-fed fish, elovl2 did not vary with EPA + DHA levels, but it was positively correlated with muscle ARA, 22:4 ω 3 and DGLA. These results suggest dietary 18:3 ω 3 elongation contributed to maintaining muscle EPA + DHA levels despite a two- to threefold change in dietary proportions, while 18:2 ω 6 with 0.3% EPA + DHA increased muscle DGLA more than arachidonic acid (ARA). Positive correlations between hepatic elovl2 and fabp10a with muscle ω 6: ω 3 and EPA + DHA + ARA, respectively, were confirmed by reanalysing data from a previous salmon trial with lower variations in dietary EPA + DHA and ω 6: ω 3 ratios. This article is part of the theme issue ‘The next horizons for lipids as ‘trophic biomarkers’: evidence and significance of consumer modification of dietary fatty acids’.

Microplastics (MP) have been proposed as a vector for pathogenic microorganisms in the freshwater environment. The objectives of this study were (a) to compare the fecal indicator growth in biofilms on MP and material control microparticles incubated in different wastewater fractions and (b) to compare MP biofilm, natural microparticle biofilm, and planktonic cell susceptibility to disinfection by peracetic acid (PAA). Biofilms were grown on high‐density polyethylene, low‐density polyethylene, polypropylene MP, or wood chips (as a material control) and incubated in either wastewater influent or pre‐disinfection secondary effluent. Reactors were disinfected with PAA, biofilms were dislodged, and total coliform and Escherichia coli were cultivated. Fecal indicators were quantifiable in both MP and wood biofilms incubated in the wastewater influent but only on the wood biofilms incubated in secondary wastewater effluent. More total coliform grew in the wood biofilms than MP biofilms, and the biofilms grown on MP and woodchips were more resistant to disinfection than planktonic bacteria. Thus, it may be possible to refer to the disinfection literature for fecal indicators in biofilm on other particles to predict behavior on MP. Treatments that remove particles in general would help reduce the potential for fecal indicator bypass of disinfection. Practitioner points MP biofilm had lower concentrations of fecal indicators than wood biofilm Biofilm on MP was not more resistant to disinfection than wood biofilm Biofilms, regardless of substrate, were more resistant to disinfection than planktonic organisms

Moritella viscosa is a Gram-negative pathogen that causes large, chronic ulcers, known as winter-ulcer disease, in the skin of several fish species including Atlantic salmon. We used a bath challenge approach to profile the transcriptome responses of M. viscosa-infected Atlantic salmon skin at the lesion (Mv-At) and away from the lesion (Mv-Aw) sites. M. viscosa infection was confirmed through RNA-based qPCR assays. RNA-Seq identified 5212 and 2911 transcripts differentially expressed in the Mv-At compared to no-infection control and Mv-Aw groups, respectively. Also, there were 563 differentially expressed transcripts when comparing the Mv-Aw to control samples. Our results suggest that M. viscosa caused massive and strong, but largely infection site-focused, transcriptome dysregulations in Atlantic salmon skin, and its effects beyond the skin lesion site were comparably subtle. The M. viscosa-induced transcripts of Atlantic salmon were mainly involved in innate and adaptive immune response-related pathways, whereas the suppressed transcripts by this pathogen were largely connected to developmental and cellular processes. As validated by qPCR, M. viscosa dysregulated transcripts encoding receptors, signal transducers, transcription factors and immune effectors playing roles in TLR- and IFN-dependent pathways as well as immunoregulation, antigen presentation and T-cell development. This study broadened the current understanding of molecular pathways underlying M. viscosa-triggered responses of Atlantic salmon, and identified biomarkers that may assist to diagnose and combat this pathogen.

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