Background: Fish skin microbiomes are rarely studied in inland water systems, in spite of their importance for fish health and ecology. This is mainly because fish species distribution often covaries with other biotic and abiotic factors, complicating the study design. We tackled this issue in the northern part of the Jordan River system, in which a few fish species geographically overlap, across steep gradients of water temperature and salinity. Results: Using 16S rRNA metabarcoding, we studied the water properties that shape the skin bacterial communities, and their interaction with fish taxonomy. To better characterise the indigenous skin community, we excluded bacteria that were equally abundant in the skin samples and in the water samples, from our analysis of the skin samples. With this in mind, we found alpha diversity of the skin communities to be stable across sites, but higher in benthic loaches, compared to other fish. Beta diversity was found to be different among sites and to weakly covary with the dissolved oxygen, when treated skin communities were considered. In contrast, water temperature and conductivity were strong factors explaining beta diversity in the untreated skin communities. Beta diversity differences between co-occurring fish species emerged only for the treated skin communities. Metagenomics predictions highlighted the microbiome functional implications of excluding the water community contamination from the fish skin communities. Finally, we found that human-induced eutrophication promotes dysbiosis of the fish skin community, with signatures relating to fish health. Conclusions: Consideration of the background water microbiome when studying fish skin microbiomes, across varying fish species and water properties, exposes patterns otherwise undetected and highlight among-fish-species differences. We suggest that sporadic nutrient pollution events, otherwise undetected, drive fish skin communities to dysbiosis. This finding is in line with a recent study, showing that biofilms capture sporadic pollution events, undetectable by interspersed water monitoring.

Fish skin microbiomes are rarely studied in inland water systems, in spite of their importance for fish health and ecology. This is mainly because fish species distribution often covaries with other biotic and abiotic factors, complicating the study-design. We tackled this issue in the northern part of the Jordan River system, in which a few fish species geographically overlap, across a steep gradients of water temperature and salinity. Using 16S rRNA metabarcoding, we studied the water properties that shape the skin bacterial communities, and their interaction with fish taxonomy. We found that considering the skin-community contamination by water microbial community is important, even when the water and skin communities are apparently different. With this in mind, we found alpha diversity of the skin-communities to be stable across sites, but higher in bentic loaches, compared to other fish.Beta diversity was found to be different among sites and to weakly covary with the dissolved oxygen, when treated skin-communities were considered. In contrast, water temperature and conductivity were strong factors explaining beta diversity in the untreated skin-communities. Beta diversity differences between co-occurring fish species emerged only for the treated skin-communities.Metagenomics predictions highlighted the microbiome functional implications of excluding the watercommunities contamination from the fish skin-communities. Finally, we found that human induced eutrophication promotes dysbiosis of the fish skin-community, with signatures relating to fish health. This finding was in line with recent studies, showing that biofilms capture sporadic pollution events, undetectable by interspersed water monitoring.

We anticipate that bi-functionality (increased rate of glucose uptake and augmented insulin secretion) will allow the lead compound to be a starting point for the development of a novel class of antidiabetic drugs.

Background: The welfare of farmed fish is influenced by numerous environmental and management factors. Fish skin is an important site for immunity and a major route by which infections are acquired. The objective of this study was to characterize bacterial composition variability on skin of healthy, diseased and recovered Gilthead Seabream (Sparus aurata) and Barramundi (Lates calcarifer). S. aurata, which are highly sensitive to gram-negative bacteria, were challenged with Vibrio harveyi. In addition, and to provide a wider range of infections, both fish species (S. aurata and L. calcarifer ) were infected with gram-positive Streptococcus iniae, to compare the response of the highly sensitive L. calcarifer to that of the more resistant S. aurata. All experiments also compared microbial communities found on skin of fish reared in UV (a general practice used in aquaculture) and non-UV treated water tanks. Results: Skin swab samples were taken from different areas of the fish (lateral lines, abdomen and gills) prior to controlled infection, and 24, 48 and 72 hours, 5 days, one week and 1 month post-infection. Fish skin microbial communities were determined using Illumina iSeq100 16S rDNA for bacterial sequencing. The results showed that naturally present bacterial composition is similar on all sampled fish skin sites prior to infection, but the controlled infections (T1 24 h post infection) altered the bacterial communities found on fish skin. Moreover, when the naturally occurring skin microbiome did not quickly recover, fish mortality was common following T1 (24 h post infection). We further confirmed the differences in bacterial communities found on skin and in the water of fish reared in non-UV and UV treated water under healthy and diseased conditions. Conclusions: Our experimental findings shed light on the fish skin microbiome in relation to fish survival (in diseased and healthy conditions). The results can be harnessed to provide management tools for commercial fish farmers; predicting and preventing fish diseases can increase fish health and welfare, and enhance commercial fish yields.

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