Digging deeper: Exploring the impact of soil probiotics in aquaculture

The use of soil probiotics in aquaculture can lead to improved production efficiency, reduced disease and mortality rates, and a more sustainable approach to aquaculture. However, it is important for farmers to carefully evaluate the available probiotic products and choose those that have been scientifically tested and shown to be effective for their specific needs.

Dr. Amit Ranjan
Assistant Professor, Department of Fish Nutrition & Feed
Technology
TNJFU-Institute of Fisheries Post Graduate Studies

Soil probiotics are beneficial microorganisms that live in the soil and play important roles in nutrient cycling and disease suppression. These microorganisms can also have positive effects on aquatic organisms when added in culture systems. In recent years, there has been growing interest in the use of soil probiotics in aquaculture as a natural and sustainable way to improve water quality, enhance the health of aquatic organisms, and increase productivity.

The build-up of waste compounds in aquaculture ponds can create an environment that encourages the growth of harmful microorganisms and adversely affects the performance of farmed species. To ensure high production and a healthy crop, effective pond management is crucial. Understanding the various interactions within ponds is of great importance, as they are complex and depend on factors such as the pond environment, stocked biomass, nutrient input, and management practices. Organic waste accumulation and degradation in the pond leads to increased consumption of oxygen (O2) and production of waste compounds like ammonia (NH3), nitrites (NO2-), and hydrogen sulphide (H2S), resulting in a phytoplankton bloom. This massive growth of phytoplankton can further deplete oxygen levels during the night and cause a crash in the bloom. All these factors contribute to water and soil contamination, creating favorable conditions for pathogen growth and affecting the condition of shrimp. Under such unfavorable conditions, shrimp face higher stress levels and increased susceptibility to diseases, ultimately resulting in poor growth or crop failure due to disease outbreaks. Hence, maintaining optimal pond conditions through efficient management practices is essential for successful aquaculture production.

For many years, managing water quality has been recognized as a critical aspect of pond aquaculture, while pond bottom soil quality management has received relatively less attention. However, growing evidence indicates that the state of pond bottoms and the transfer of substances between soil and water play a vital role in determining water quality.

In the aquaculture industry, the control of pathogens by beneficial bacteria (probiotics) through competitive processes is gaining popularity as a more effective approach than the use of antibiotics for pathogen control. Probiotics typically comprise microbiota such as Lactobacillus, Bacillus, Nitrobacter, Nitrosomonas, Photosynthetic bacteria, Rhodobacter, and Yeast, among others, which are applied to enhance the properties of the host’s indigenous microflora. This results in improved animal health by reducing the population density of pathogens and by promoting major rapid degradation of organic waste matter, thus improving water quality.

PROBIOTICS
Probiotics are increasingly recognized as a critical input for the future of aquaculture. In aquaculture probiotics are increasingly utilized to enhance pond soil and water quality, promote survival, and boost the growth of cultured species. Probiotic products are diverse and can be of microbial or botanical origin and comprises of bacteria, cyanobacteria, microalgae, fungi, and others. Those used to enhance bottom soil and water quality typically consist of cultures of living bacteria, enzyme preparations, or a combination of the two.

Probiotic bacteria, which improve water quality and inhibit pathogens, are often referred to as “probiotics,” “probionts,” “probiotic bacteria,” or “beneficial bacteria.” The term “probiotics” derives from the Greek words “pro” and “bios,” which refer to beneficial microbes. Although live microbes have long been believed to be effective as probiotics, recent studies have confirmed the immunostimulatory role of inactivated bacteria in fish. Therefore, probiotics may consist not only of living bacterial cells but also metabolites and peptides derived from bacteria or dead cells that ultimately impart beneficial effects on the host animal. Based on their components, probiotics may be categorized into different subgroups or categories. Many microbiologists endorse the use of probiotics to improve soil and water quality.

WHAT ARE SOIL-BASED ORGANISMS (SBOS)?
Soil-based organisms refer to a group of more than 100 microbial species that are typically present in soil, and some of these have been identified and utilized as probiotics. The majority of SBOs used in soil-based probiotics are capable of forming spores, which are protected by a tough outer layer, allowing them to resist heat and acid during replication.

SOIL PROBIOTICS
Soil-based probiotics are bacteria that occur naturally in the soil, with one of the most commonly used types of soil-based probiotics being bacillus, which has been used to ferment foods for centuries. These probiotics are known for their durability and longer shelf life, and can improve digestion, boost the immune system, and promote a healthy gut microbiome. Unlike some other types of probiotics, soil-based probiotics do not require refrigeration. Various products are available to enhance beneficial chemical and biological processes and improve soil quality, such as living bacterial cultures, enzyme preparations, plant extracts, and other mixtures. While there is no evidence to support the claim that these products improve soil quality, they are considered safe for the aquaculture industry and do not pose a risk to workers or the surrounding environment, nor do they affect the quality of aquaculture products.

The commercial soil probiotic ‘Super-PS’ is composed of a combination of Rhodobacter and Rhodococcus species and is used to enhance water and soil quality and control bacterial infections. Rhodobacter and Rhodococcus species in Super-PS are live microbial cells that, when consumed in sufficient quantities, positively impact the health and growth of the host by improving the balance of intestinal microflora. This soil probiotic is utilized to enhance the pond bottom condition, diminish harmful bacteria, and maintain a favorable environment for aquaculture. It contains primarily 109 CFU mL−1 of Rhodobacter and Rhodococcus species.

Rhodococcus is obtained through the process of bioconversion, which involves using biological systems to transform inexpensive starting materials into more valuable compounds. On the other hand, Rhodobacter is a type of gram-negative bacteria that can be found in various environments, such as freshwater, marine, and hypersaline habitats. It possesses a broad range of metabolic abilities, including photosynthesis, lithotrophy, and both aerobic and anaerobic respiration. Additionally, Rhodobacter can fix nitrogen, which enables it to thrive in diverse habitats.

Commonly used soil-based species as probiotics:
The following are among the most frequently utilized soil-based microorganisms:
• Bacillus coagulans
• Bacillus subtilis
• Bacillus clausii
• Bacillus licheniformis
• Clostridium butyricum

Dosages and application of soil probiotics:
In the process of preparing the pond, after removing the sediments, it is recommended to spray 50-100 liters of soil probiotics per hectare over the soil to promote the utilization of organic matter and establishment of a beneficial bacterial community. The amount of soil probiotics to be applied per hectare ranges from 10 to 20 liters, and may vary depending on the pond’s condition, stocking density, and culture duration.

Benefits of soil probiotics:
Soil probiotics play a crucial role in improving various aspects of aquaculture. These include:
• Accelerating the decomposition of organic waste in the pond.
• Reducing the occurrence of black soil problems by oxidizing organic matter.
• Preventing the formation of toxic gases such as ammonia and hydrogen sulfide.
• Promoting the development and stabilization of a healthy algal bloom.
• Stabilizing pH levels, which helps to balance the pond’s ecosystem.
• Inhibiting the growth of harmful bacteria through substrate competition.
• Reducing the occurrence of white faeces and white gut problems.
• Creating a stress-free environment for aquatic species, which ultimately leads to an increase in production.

Potential drawbacks of soil probiotics:
There is limited scientific evidence on the potential disadvantages of using soil probiotics in aquaculture. However, some concerns have been raised, including:
• Possible introduction of non-native microorganisms into the aquatic environment, which could potentially alter the existing ecosystem and affect the native species.
• Risk of contamination of the aquaculture products with pathogenic microorganisms if the probiotics are not properly managed.
• Overuse of probiotics can lead to microbial imbalances and resistance to antibiotics, which could be harmful to the aquatic animals and human health.
• High cost of some commercial soil probiotics, which may not be feasible for small-scale aquaculture operations.

CONCLUSION
Soil probiotics play an important role in aquaculture by improving water and soil quality, enhancing survival rates, and promoting growth of cultured species. Fish and shrimp farmers must have knowledge of the available probiotics in the market and make a well-informed decision based on available evidence before making a purchase. Overall, the use of soil probiotics in aquaculture can lead to more sustainable and profitable operations, while also reducing the environmental impact of aquaculture activities.

References:
1. Boyd, C. E., & Gross, A. (1998). Use of probiotics for improving soil and water quality in aquaculture ponds. Advances in Shrimp Biotechnology, 101-105.
2. Boyd, C. E., Wood, C. W., & Thunjai, T. (2002). Aquaculture pond bottom soil quality management. Pond Dynamics/Aquaculture Collaborative Research Support Program, Oregon State University.
3. Gatesoupe, F. J. (1999). The use of probiotics in aquaculture. Aquaculture, 180(1-2), 147-165.
4. Ninawe, A. S., & Selvin, J. (2009). Probiotics in shrimp aquaculture: avenues and challenges. Critical Reviews in Microbiology, 35(1), 43-66.
5. Pantjara, B., & Kristanto, A. H. (2020). Pond bottom management and probiotic application in extensive Tiger prawn (Penaeus monodon) culture on acid sulfate soil. AACL Bioflux, 13(2).
6. Qi, Z., Zhang, X. H., Boon, N., & Bossier, P. (2009). Probiotics in aquaculture of China—current state, problems and prospect. Aquaculture, 290(1-2), 15-21.

About Dr. Amit Ranjan
Dr. Amit Ranjan is an Assistant Professor in the Department of Fish Nutrition & Feed Technology at the Institute of Fisheries Post Graduate Studies of Tamil Nadu Dr. J. Jayalalithaa Fisheries University (TNJFU), India. His research focus is on fish and shrimp nutrition, where he conducts both strategic and applied research. With his extensive experience in commercial culture of shrimp and freshwater fish, he has published several research papers in international peer-reviewed journals and serves as a reviewer for over 30 international peer-reviewed journals.