Introduction
Innovative ingredients in fish diets are used to improve fish growth performance indicators and the ecological footprint of aquaculture activities with the benefits of improving fish health. Thus, the replacement of fishmeal and fish oil in fish diets with novel ingredients that can be sustainable produced is of much interest. It is widely accepted that under experimental rearing or aquaculture farming conditions, the fish gut microbiome can be manipulated by changes in the fish diet. Gut microbiota may play significant role in fish nutrition, producing enzymes (Ray et al. 2012) that promote host growth and health. However, the extent to which microbial function varies with host demand and the underlying mechanisms are largely unknown. The aim of this study was to investigate the effect of dietary inclusion of insect meal, tunicate meal and algae meal on Sparus aurata midgut microbiota.
Materials and methods
The experimental trial took place at the Department of Ichthyology and Aquatic Environment, University of Thessaly, Greece. Briefly, 1080 individuals S. aurata (initial mean weight 6.52±0.03g) were distributed randomly to eighteen 250l tanks. Six experimental diets were formulated to be isolipidic. The Control (26.55% fishmeal- FM), the 0%FMFO (total replacement of FM and fish oil with algae meal, Schizochytrium limacinum and Phaeodactylum tricornutum, insect meal Hermetia illucens and tunicate meal Ciona intestinalis), the IM (68.09% replacement of FM with insect meal H. illucens), the TM (45.91% replacement of FM with tunicate meal C. intestinalis), the PA ( 18.41% replacement of FM with P. tricornutum), and HA, ( 100% replacement of FO with S. limacinum) diet. Each diet was assigned to triplicate groups of 60 fish per group. The trial lasted 45 days, after 15 days of acclimatization. The fish fed three times per day at libitum. At the end of experiment, fish were starved for 24 hours and aseptically the midgut was dissected. After DNA extraction bacterial diversity was assessed by amplification of the V3-V4 region of the bacterial 16S rDNA gene using IlluminMiSeq with universal bacterial primers. Raw sequence reads were processed and analyzed, using the MOTHUR software (v. 1.45.3) and SILVA database was used to classify the operational taxonomic units (OTUs).
Results
Several bacterial Families were identified with different distribution in the midgut of fish fed with the different diets (Figure 1). The most abundant Family was LWQ8 in fish fed the control and IM dietary treatment and Burkholderiales was in fish fed the 0%FMFO dietary treatment. In the midgut of fish fed the TM and HA diets Rhodobacteraceae had an increased relative abundance and in midgut of those fed PA diet Bacillaceae was most abundant.
Discussion
Fish fed the TM diet had increased their bacterial Taxa in their midgut, an indicator of fish health, implying a normal gut physiology. Saccharimonadales, (LWQ8 bacteria) were found in all dietary treatments. It is associated with the degradation of sugars and glucose (Albertsen et al. 2013). Increased abundance of Burkholderiales in fish fed the 0%FMFO diet was related to immune and stress response (Sehnal et al. 2021). Rhodobacteraceae were a common family among the dietary treatments and were found to assist in the fermentation of complex polysaccharides (Gupta et al. 2019). Bacillaceae produce amylase, cellulase, phytase, protease, lipase, and chitinase (Askarian et al. 2012), enzymes that could contribute to better fish growth, performance. Bacterial Families that were beneficial to the host were found in the midgut of fish fed the six experimental diets associated with better growth performance.
Acknowledgement
This project has received funding from the European Union’s Horizon 2020 Innovation Action, FutureEUAqua, under grant agreement No 817737.This output reflects the views only of the author(s), and the European Union cannot be held responsible for any use which may be made of the information contained therein.
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