The intensification of aquaculture has brought forth significant challenges, particularly with respect to disease outbreaks, water quality deterioration and the need for sustainable feeding practices. The integration of probiotics into aquaculture practices has emerged as a promising approach to enhance fish health, growth and overall farm productivity. In this context, probiotics have gained considerable attention as a natural and environmentally friendly strategy to improve the health and productivity of cultured fish species.
Probiotics are live microorganisms that, when administered in adequate amounts, confer health benefits to the host including improved growth performance, enhanced immunity and disease resistance (Verschuere et al., 2000). Probiotics in aquaculture are primarily utilized to modulate the microbial community in the aquatic environment, enhance fish growth, and improve resistance to diseases. The beneficial effects are largely attributed to the competitive exclusion of pathogenic microbes, production of antimicrobial compounds and enhancement of the host's immune responses (Merrifield & Blanchard, 2008). Traditional fish probiotics include Gram-negative bacteria (e.g., Acinetobacter, Aeromonas, Enterobacter, Pseudoalteromonas, Pseudomonas) alongside Gram-positive bacteria (e.g., Bacillus, Enterococcus, Lactobacillus, Lactococcus, Microbacterium, Micrococcus, Streptococcus, and Streptomyces) which have demonstrated positive impacts across different stages of fish development and diverse aquaculture systems (Gatesoupe et al., 2008, Hermosillo et al., 2012, Bunnoy, 2019).
Among the varied microorganisms used as probiotics in aquaculture, purple nonsulfur bacteria (PNSB) have emerged as promising candidates due to their unique metabolic capabilities, multifunctional properties and adaptability to various aquatic environments. PNSB are a group of photoheterotrophic bacteria that utilize light as an energy source and organic compounds as carbon sources, allowing them to thrive in a wide range of aquatic habitats, including the guts of wild fishes (Wu et al., 2021, Koga et al., 2022).
These bacteria, particularly Rhodobacter, Rhodospirillum and Rhodopseudomonas species, have demonstrated potential as probiotics in finfish culture due to their ability to enhance nutrient utilization, promote growth and protect against pathogens. PNSB additionally consume dissolved organic wastes, degrade pollutants and produce beneficial bioactive compounds (Reddy et al., 2012). The application of PNSB in aquaculture has shown particular promise in larval, juvenile and adult stages of fish development, offering a multifaceted approach to improving fish health and aquaculture sustainability.
Direct Nutrition
One of the primary benefits of PNSB in aquaculture is their role in providing direct nutritional supplementation to cultured fish. PNSB are known for their high protein content, which can range from 60% to 70% of their dry weight, as well as their rich profile of essential amino acids, vitamins (such as B-complex vitamins) and carotenoids (Noparatnaraporn & Nagai, 1990, Sang et al., 2012). These nutrients are critical for the growth and development of fish, particularly during early life stages where nutritional requirements are high.
Studies have demonstrated that the inclusion of PNSB in fish diets can significantly improve growth performance. For instance, the supplementation of Rhodobacter sphaeroides in the diets of Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio) resulted in increased growth rates, improved feed conversion ratios and enhanced survival rates (Lukwambe et al., 2015). Incorporation of PNSB into diets for Oncorhynchus mykiss (rainbow trout) led to enhanced growth performance and feed conversion ratios compared to control diets (Wu et al., 2016).
The ability of PNSB to enhance growth is attributed not only to their nutritional content but also to their capacity to produce bioactive compounds, such as polyhydroxyalkanoates (PHAs), which can stimulate growth and improve metabolic efficiency in fish (Shapawi et al., 2012).
Enhanced Digestion
PNSB positively influence the digestive processes in fish by producing enzymes that aid in the breakdown of complex feed components. This enzymatic activity facilitates better nutrient utilization and absorption. By promoting a healthy gut microbiome, PNSB can enhance the enzymatic breakdown of feed components, particularly in the earliest life stages of fish when digestive capabilities are still developing.
Research has indicated that the administration of Rhodopseudomonas palustris in the diets of Japanese flounder (Paralichthys olivaceus) larvae improved digestive enzyme activities, including protease, amylase, and lipase, resulting in better growth performance and higher survival rates (Wang et al., 2014). The enhancement of digestive enzyme activity is likely due to the production of extracellular enzymes by PNSB, which complement the endogenous enzyme activities of the fish, thereby improving feed efficiency and nutrient uptake. In larval stages of Pagrus major (red sea bream), PNSB supplementation has been associated with improved digestive enzyme activity, leading to better growth rates and higher survival rates (Kang et al., 2015). Additionally, PNSB can improve the intestinal microbiota balance, which supports more efficient digestion and assimilation of nutrients (Nagai et al., 2018).
Enhanced Resistance to Disease
The ability of PNSB to enhance disease resistance in cultured fish is well-documented and is one of their most valuable attributes as probiotics. PNSB exert their protective effects through several mechanisms, including competitive exclusion of pathogenic bacteria, modulation of the immune system and production of antimicrobial compounds.
In addition to direct antimicrobial activity, PNSB can also modulate the immune response of fish, leading to enhanced resistance to infections. For example, administration of PNSB to Oreochromis niloticus (Nile tilapia) has been shown to increase the fish’s resistance to Aeromonas hydrophila infections, likely due to enhanced immune responses and competitive inhibition of pathogens (Yang et al., 2019). Rhodobacter sphaeroides has been shown to inhibit the growth of pathogenic bacteria such as Vibrio spp. and Aeromonas hydrophila through competitive exclusion and the production of bacteriocins (Li et al., 2020). Studies on Asian seabass (Lates calcarifer) have demonstrated that the administration of Rhodopseudomonas palustris significantly increased the expression of immune-related genes, including those involved in the production of pro-inflammatory cytokines and antimicrobial peptides, resulting in improved survival following pathogen challenge (Li et al., 2018).
Enhanced Tolerance to Environmental Stress
In addition to improving nutrition and disease resistance, PNSB have also been reported to improve the tolerance of fish to various environmental stressors such as fluctuating water quality and temperature extremes. Their metabolic activities contribute to the stabilization of water quality parameters, including reduction of toxic metabolites and regulation of pH levels. In Scophthalmus maximus (turbot) aquaculture, PNSB supplementation has demonstrated beneficial effects in maintaining water quality and supporting fish health under stress conditions (Sato et al., 2017).
For instance, the addition of Rhodopseudomonas palustris to the rearing water of tilapia has been shown to reduce ammonia levels and improve the overall water quality, leading to reduced stress and improved growth performance (Lukwambe et al., 2015). The ability of PNSB to mitigate environmental stress is particularly valuable in intensive aquaculture systems, where high stocking densities and suboptimal water conditions can lead to chronic stress and increased susceptibility to diseases.
Future Developments
The potential of PNSB as probiotics in aquaculture is still being explored; future developments could further enhance their application in fish farming. Research into the genetic and metabolic pathways of PNSB may lead to the identification of specific strains with enhanced probiotic properties, such as increased production of bioactive compounds or greater resilience to environmental fluctuations. Additionally, the use of PNSB in combination with prebiotics, immunostimulants other probiotics (including other PNSB) could offer synergistic benefits, further improving the health and productivity of cultured fish.
The application of PNSB in ornamental fish culture also presents a promising avenue for future research within the aquarium industry. Ornamental fish are often reared in recirculating aquaculture systems, where water quality and disease control are critical. The use of PNSB to improve water quality and enhance the immune response of ornamental fish could lead to more sustainable and disease-resistant production practices in this sector.
Conclusion
Purple non-sulfur bacteria represent a well-studied and high-value class of probiotics in aquaculture, offering a range of benefits that include enhanced nutrition, improved digestion, increased disease resistance and greater tolerance to environmental stress. The multifunctional properties of PNSB, coupled with their adaptability to various aquatic environments, make them valuable tools for improving the health and productivity of cultured fish species at all life stages. As research into PNSB continues, the development of more effective and tailored probiotic formulations could further revolutionize their use in aquaculture, contributing to more sustainable and resilient aquarium fish husbandry practices.
For additional information on adding PNSB to convention aquarium diets, please see:
References
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