Beneficial Microbes for the Sustainable Management of Shrimp Aquaculture

Aquaculture production has grown enormously in recent years and among that Penaeid shrimps are one of the most important cultured species worldwide especially in Asia due to their high economic value and export. Shrimps are infected by numerous pathogenic microbes which caused high mortality. The excess feed and fecal matter in grow out ponds accumulating in the impoundments may result in bacterial decomposition of organic matter in the sediment and produce excess of toxic compounds like ammonia and also pollute the environment. Very limited research has been carried out on the culture, growth performance and disease management in aquaculture using beneficial microbes. Moreover, in the beginning years of aquaculture the antibiotics were employed to kill the microbial pathogens and to enhance the growth of the shrimps in intensive and semi-intensive types of culture. The excess use of antibiotics has created the problem of residue buildup in tissue of shrimps, resistance of culture species to the consistent use of antibiotics and rejection of export consignments etc. As a remediation of these unpleasant and unwanted problems in aquaculture there is a need of beneficial microbes in shrimp culture through feed and/or water to prevent the aquaculture ponds from undergoing eutrophication and to control the microbial diseases in shrimps and to enhance their production in an ecofriendly ambience without the use antibiotics thus, resulting in quality assurance, quality control and above all environmental safety. 

Introduction

Aquaculture is an emerging branch of biosciences boosting food production. The marine fisheries stocks have declined worldwide in general and provided an impetus for rapid development in fish and shellfish farming. In general, shrimp ponds are enclosed cultivation systems, subject to periodic water renewal to compensate for volume changes (due to evaporation) and salinity changes (evaporation, precipitation) and to maintain water quality. The production process in shrimp aquaculture is determined by biological, technological, economical and environmental factors (Yuvaraj and Karthik, 2015 b). Especially, the microbial diseases are commonly implicated in episodes of mortality, so that our Indian farmers prefer to use commercial antibiotics available in markets. However, the use of antibiotics has recently become a major public concern because their use can lead to the development of drug resistant bacteria, thereby reducing drug efficacy. Moreover, the accumulation of antibiotics both in the environment and in shrimp tissues can be potentially risky to consumers and the environment. Another most important problem faced by our Indian shrimp culture farmer is excess of Ammonia in pond water and sediment which is caused by excess feed, fecal matter and dead algae deposited in the bottom of the pond. Due to this, shrimps are exposed to toxic gases like NH3, NO2 and H2S further leads to eutrophication in culture system and cause stress to the animal and ultimately end with microbial diseases and high mortality occurs (Yuvaraj et al., 2015 a). 

Though shrimp culture has undergone rapid development in India, successful production is increasingly hampered especially by diseases in addition to environmental pollution and poor management practices. Of all the infectious diseases, bacterial infections cause serious diseases like vibriosis, White spot, ulcer, tail rot, necrosis etc. Currently, antibiotics have been widely used in shrimp aquaculture to control bacterial infections. However, the use of antibiotics has recently become a major public concern because their use can lead to the development of drug resistant bacteria, thereby reducing drug efficacy. Moreover, the accumulation of antibiotics both in the environment and in shrimp tissues can be potentially risky to consumers and the environment. Such adverse effects have prompted scientists to search for alternatives to replace antibiotics in controlling diseases in shrimp farms (Karthik et al., 2013 & 2014).

Role of beneficial microorganisms (BMs) in aquatic ecosystem

Nowadays, beneficial microorganisms (BMs) are used in aquaculture mainly for two aspects: one is to regulate microdysbiosis, to maintain microeubiosis, to raise the health level of hosts, and to promote the proliferation of BMs and their metabolic products in microecology. The other is to decompose organic matter (feces, wastes, remnants or leftovers of feed), to maintain dynamic ecological balance among organisms from the five kingdoms in nature and microbial ecological equilibrium in water and sediments, and to create a friendly environment for fish and shrimp growth (Zhou et al., 2009). Microbes include microalgae, bacteria, fungi and virus. Microalgae mainly photosynthesize to absorb CO2 and to supply oxygen to aquatic animals. The main role of bacteria and fungi is to decompose organic matter in sediments so as to keep the water clean. However, due to the lack of oxygen there might be anaerobic decomposition or fermentation in sediments, producing harmful gases such as hydrogen sulphide and methane. There are two food chains in aquatic ecosystem: detritus food chain and prey–predatory food chain. 

Microbes are the predominant photosynthetic organisms in most aquatic environments. In aerobic conditions (e.g. shallow water), algae, diatoms and Cyanobacteria predominate. In anaerobic conditions (polluted or eutrophic waters), other photosynthetic bacteria are dominant. There is a big difference between natural aquatic ecosystem and man-made aquatic ecosystem, especially in intensive culture with high-stocking density and high input. The water body in a high-stocking density fishpond often in a small area has a limited self-purification capability. Therefore, there is a need to add BMs in stages to improve water quality to reduce the prevalence of fish diseases. When BM population in water body grows to a certain extent forming a specific population, BMs would hinder the growth of harmful ones during the competition for survival, thus the pond water is in a balanced and stable microbial ecological condition conducive to the growth of aquatic animals (Karthik et al., 2015 a-d).

Beneficial role of bacteria 

In normal, diseases in aquaculture practices are mostly caused by luminous bacteria Vibrio harveyi, and it has been referred as the largest economic loss in the shrimp aquaculture due to mass mortalities (Sivakumar et al., 2014). To control the pathogens, the use of beneficial microbes in aquaculture is increasing demand for its more environment friendly aquaculture practices (Petlu Nitya et al., 2013). Nowadays, the use of beneficial microbes in aquaculture might represent a valuable mechanism to increase shrimp growth and survival rate. In general, the gastro intestinal tract (GIT) of the aquatic animal is mainly composed of gram negative bacteria (Vine et al., 2006). Researchers also have demonstrated about the use of beneficial bacteria in aquaculture to improve the water quality and immune system by balancing bacterial flora in water and reducing pathogenic bacterial load (Kesarcodi-Watson et al., 2008). 

Recently the researchers have been attempting to isolate beneficial bacteria from various sources like soil, water and animal gut to control disease causing pathogens in aquaculture systems (Austin and Day 1990; Munro et al., 1995; Gomez-Gil et al., 2000; Ahmed et al., 2005; Skrodenyte- Arbaciauskiene et al., 2006; Kim et al., 2007 and Bestha Lakshmi et al., 2013). Beneficial microbes that currently used in aquaculture industry include a wide range of taxa – from Lactobacillus, Bifidobacterium, Pediococcus, Lake Streptococcus sp and Carnobacterium sp. Bacillus, Flavobacterium, Cytophaga, Pseudomonas, Alteromonas, Aeromonas, Enterococcus, Nitrosomonas, Nitrobacter and Vibrio spp., and yeast Saccharomyces, Debaryomyc (Irianto and Austin 2002; Sahu et al., 2008 and Hemaiswarya et al., 2013). While using some beneficial bacteria for fish, some might be highly pathogenic Vibrio alginolyticus and lead to destructive effect in the aquaculture systems. Therefore, it is necessary to take care of the choice of beneficial microbes before administration. The best known beneficial bacteria such as Bifidobacteria species, Lactobacillus sp and Streptococcus thermophilus are employed as the dietary supplementation with feed in aquaculture industry and increased the efficiency and sustainability of aquaculture production (Kim et al., 2007).  Beneficial microbes thus fulfill the concept of sustainable growth and production of shrimps by minimizing or de-promoting the growth of the disease causing harmful pathogens. 

Beneficial role of microalgae 

Diatoms are always dominant in ponds of good quality. As organic pollution increases, Pyrrophyta sp, Cyanophyta sp and Cryptophyta sp dominate instead of diatoms. Literature has revealed that, diatoms secrete anti-bacterial substances and good food for post larvae of Penaeus shrimp. Some diatoms such as Chaetoceros sp can rapidly reduce ammonia nitrogen. In order to maintain the good quality of pond water, some scholars have suggested that inoculation of diatoms to predominate the probiotic system would be a prudent approach. However, it is not easy to inoculate diatoms for they need severe growth conditions. The kind of ecological environment that is required to promote the growth of diatoms has not been reported so far (Xu, 2006). Austin et al., (1992) reported a kind of microalgae (Tetraselmis suecica), which can inhibit pathogenic bacteria of fish. Teraselmis suecica was observed to inhibit Aeromonos hydrophila, A. salmonicida, Serrstia liquefaciens, Vibrio anguillaram, V. salmonicida and Yersnia ruckeri type I. When used as a food supplement, the algal cells inhibited laboratory induced infection in Atlantic salmon. When used therapeutically, the algal cells and their extracts reduced mortalities caused by A. salmonicida, Serrstia liquefaciens, V. anguillaram,                    V. salmonicida and Yersnia ruckeri type I. They suggested that there may be some bioactive compounds in the algal cells, and there appears to be a significant role for Tetraselmis in the control of fish diseases (Ramalingam et al., 2015 a&b).

Bioremediation in aquaculture pond

Biodeterioration in water bodies can be alleviated by bioremediation, using individual or combined organisms to minimize the effluents from aquaculture or any other industries.

Major advantages of bioremediation are:

1) It can be done on site and the process does not lead to any site disruption.

2) There is every possibility for permanent waste elimination.

3) Being a biological process it will be comparatively too inexpensive.

Bioremediation of aquaculture pond waste using microbes

Water quality in aquaculture systems are controlled through microorganisms by microbial degradation of organic matter (Anderson et al., 1987; Rheinheimer, 1992 and Avnimelech et al., 1995).  The dissolved and suspended organic matter contains mainly carbon chains and is highly available to microbes and algae and it helps the microbes to multiply rapidly and have good enzymatic capability. The bacteria such as, Bacillus subtilis, B.licheniformis, B.cereus, B.coagulans were used as suitable for bioremediation of organic detritus (Sharma, 1999 and Singh et al., 2001). Lactobacillus sp are also used along with Bacillus sp to break down the organic detritus. These bacteria produce a variety of enzymes that break down proteins and starch to small molecules, which are then taken up as energy sources by other organisms. The removal of large organic compounds reduces water turbidity (Haung, 2003). Many examples in which both singular bacterial strains and microbial systems have been successfully utilized to reduce and or transform selected pollutants to nontoxic molecule in laboratory conditions (Eschenhagen et al., 2003; Gallizia et al., 2003; Gallizia et al., 2005 and Bhaarathi Rajan et al., 2015). 

MECHANISM OF BENEFICIAL MICROBES (ENDOCOMMENSALS/ NONSPECIFIC)

SYSTEM BIOLOGICAL APPROACH TO AQUACULTURE 

Conclusion:

Shrimp aquaculture is burdened with environmental problems that arise from the consumption of resources, such as land, water, seed, feed, their transformation into products valued by society. In addition after the culture period, effluents from aquaculture ponds contain living and dead particulate organic matter, dissolved organic matter, ammonia, nitrite, nitrate, phosphate, suspended soil particles, and other substances that can be considered potential pollutants. Hence, application of beneficial microbes in aquaculture systems has been rising over the years with increasing demand for more environment friendly aquaculture practices. The use of beneficial microbes as beneficial microbes in the previous research studies revealed significant reduction in the CFU of the pathogenic bacterial species. This findings is of interest since the pathogenic forms of bacterial species will have quarom sensing mechanisms which is mainly distributed to their cytopathogenicity in the host forms. By containing their population buildup the beneficial microbes also contain or repners their quorum sensing genes operation. Thus the efficacy of beneficial microbes is proved by several researchers. Hence, it could be concluded that the extrapolation of beneficial microbes as feed and/or water beneficial microbes in shrimp culture will definitely prevent the aquaculture ponds from undergoing organic matter accumulation, ammonification, eutrophication and prevent the environment from pollution and also control the microbial diseases to the shrimps and enhance the productivity of the farms to the benefit of local economies in an ecofriendly ambience without antibiotics. 

Dr. Karthik Ramachandran, Ph.D 

Product Manager –Aquaculture 

Guybro Chemicals Pvt Ltd, Corporate Office, 

201, Maruti Chamber, Fun Republic Lane, Veera Desai Road, Andheri (West),

Mumbai – 400 053, Maharashtra, India