The Effect of Two Dietary Protein Sources on Water Quality and the Aquatic Microbial Communities in Marron (Cherax cainii) Culture

Replacing starch with resistant starch (Laminaria japonica) improves water quality, nitrogen and phosphorus budget and microbial community in hybrid snakehead (Channa maculata ♀ × Channa argus ♂)

It is essential to increase the use of carbohydrates as an energy source and improve protein synthesis and utilization to reduce ammonia nitrogen emissions. A 60-day cultural experiment was conducted to assess the impact of resistant starch (kelp meal, Laminaria japonica) replacing starch on water quality, nitrogen and phosphorus budget and microbial community of hybrid snakehead. Approximately 1350 experimental fish (11.4 ± 0.15 g) were randomly divided into control group (C, 20% starch) and four resistant starch groups: low replacement group (LR, 15% starch), medium replacement group (MR, 10% starch), high replacement group (HR, 5% starch) and full replacement group (FR, 0% starch). The crude protein and crude fat content of hybrid snakehead fish fed with the FR diet had the most significant improvement (P < 0.05). However, resistant starch also increased the effectiveness of nitrogen and phosphorus utilization in hybrid snakeheads, which decreased the proportion of total nitrogen and total phosphorus in tail water. The minimum nitrogen and phosphorus emission rate was when the starch level was 6.1%. Denitrifying microbes including Gemmobacter, Rhodobacter, Emticicia and Bosea have become much more prevalent in group FR (P < 0.05). In general, replacing starch with resistant starch can enhance the rate at which nitrogen and phosphorus are used in feeding, lessening water pollution and altering environmental microbial composition. PRACTITIONER POINTS: Resistant starch (RS) improves whole fish nutritional content. Resistant starch improves dietary nitrogen and phosphorus utilization. Resistant starch acts as a carbon source and encourages the colonization of denitrifying bacteria in water.

Red mark syndrome: Is the aquaculture water microbiome a keystone for understanding the disease aetiology?

Aquaculture significantly contributes to the growing demand for food worldwide. However, diseases associated with intensive aquaculture conditions, especially the skin related syndromes, may have significant implications on fish health and industry. In farmed rainbow trout, red mark syndrome (RMS), which consists of multiple skin lesions, currently lacks recognized aetiological agents, and increased efforts are needed to elucidate the onset of these conditions. Most of the past studies were focused on analyzing skin lesions, but no study focused on water, a medium constantly interacting with fish. Indeed, water tanks are environmental niches colonized by microbial communities, which may be implicated in the onset of the disease. Here, we present the results of water and sediment microbiome analyses performed in an RMS-affected aquaculture facility, bringing new knowledge about the environmental microbiomes harbored under these conditions. On the whole, no significant differences in the bacterial community structure were reported in RMS-affected tanks compared to the RMS-free ones. However, we highlighted significant differences in microbiome composition when analyzing different samples source (i.e., water and sediments). Looking at the finer scale, we measured significant changes in the relative abundances of specific taxa in RMS-affected tanks, especially when analyzing water samples. Our results provide worthwhile insight into a mostly uncharacterized ecological scenario, aiding future studies on the aquaculture built environment for disease prevention and monitoring.

Gut Microbiome as a Potential Biomarker in Fish: Dietary Exposure to Petroleum Hydrocarbons and Metals, Metabolic Functions and Cytokine Expression in Juvenile Lates calcarifer

The gut microbiome of fish contains core taxa whose relative abundances are modulated in response to diet, environmental factors, and exposure to toxicogenic chemicals, influencing the health of the host fish. Recent advances in genomics and metabolomics have suggested the potential of microbiome analysis as a biomarker for exposure to toxicogenic compounds. In this 35-day laboratory study, 16S RNA sequencing and multivariate analysis were used to explore changes in the gut microbiome of juvenile Lates calcarifer exposed to dietary sub-lethal doses of three metals: vanadium (20 mg/kg), nickel (480 mg/kg), and iron (470 mg/kg), and to two oils: bunker C heavy fuel oil (HFO) (1% w/w) and Montara, a typical Australian medium crude oil (ACO) (1% w/w). Diversity of the gut microbiome was significantly reduced compared to negative controls in fish exposed to metals, but not petroleum hydrocarbons. The core taxa in the microbiome of negative control fish comprised phyla Proteobacteria (62%), Firmicutes (7%), Planctomycetes (3%), Actinobacteria (2%), Bacteroidetes (1%), and others (25%). Differences in the relative abundances of bacterial phyla of metal-exposed fish were pronounced, with the microbiome of Ni-, V-, and Fe-exposed fish dominated by Proteobacteria (81%), Firmicutes (68%), and Bacteroidetes (48%), respectively. The genus Photobacterium was enriched proportionally to the concentration of polycyclic aromatic hydrocarbons (PAHs) in oil-exposed fish. The probiotic lactic acid bacterium Lactobacillus was significantly reduced in the microbiota of fish exposed to metals. Transcription of cytokines IL-1, IL-10, and TNF-a was significantly upregulated in fish exposed to metals but unchanged in oil-exposed fish compared to negative controls. However, IL-7 was significantly downregulated in fish exposed to V, Ni, Fe, and HFOs. Fish gut microbiome exhibits distinctive changes in response to specific toxicants and shows potential for use as biomarkers of exposure to V, Ni, Fe, and to PAHs present in crude oil.

Sources of protein diet differentially stimulate the gut and water microbiota under freshwater crayfish, marron (Cherax cainii, Austin 2002) culture

To reduce the reliance on fishmeal (FM), other protein sources have been evaluated on cultured animals. In a 60-days feeding trial, marrons (Cherax cainii) were fed a FM diet and five test diets containing 100% of plant-based protein sources such as soybean, lupin and valorised animal-based proteins such as poultry-by-product, black soldier fly and tuna hydrolysate. At the end of the trial, DNA samples from marron gut and rearing water were investigated through DNA-based 16S rRNA gene sequencing. Plant-based diets increased abundance for Aeromonas, Flavobacterium and Vogesella, whereas animal and insect proteins influenced diverse bacterial groups in the gut linked to various metabolic activities. Insect meal in the water favoured the growth of Firmicutes and lactic acid bacteria, beneficial for the marron health. Aeromonas richness in the gut and reared water signified the ubiquitous nature of the genus in the environment. The higher bacterial diversity in the gut and water with PBP and BSF was further supported by qPCR quantification of the bacterial single-copy gene, rpoB. The overall results suggested that PBP and BSF can exhibit positive and influential effects on the gut and water microbial communities, hence can be used as sustainable ingredients for the crayfish aquaculture.