Microbial dynamics in shrimp ponds

Understanding the microbial ecological process in ponds and in the shrimp intestinal tract is vital to maintain and optimize productivity without prophylactic use of hazardous substances. The maintenance of good water quality and the control of disease are closely linked to managing the communities of microbes and phytoplankton.

Healthy shrimp in a pond will be responding to variables that include interactions, with each other, interactions of shrimp and the pond physico-chemical environment (especially oxygen concentration) and interactions of the microbial community, in particular the phytoplankton and the bacteria. As ponds are shallow, water chemistry is infl uenced primarily by the chemistry of the soil and sediment (material settled on to the pond bottom, including dead algae, waste feed and faeces.) These are governed by bacteria activities.

Most organic matter is decomposed by aerobic heterotrophic bacteria in the water column and sediment surface. The remaining organic matter is broken down in the anoxic zone of the sediment mostly by fermentative bacteria, of which clostridia are important constituents, but it also includes vibrios. Short chain fatty acids are the fi nal products of fermentation by these bacteria and they are then used as nutrients by the sulphate-reducing bacteria and respired to CO2. Thus organic matter in the sediment that is not oxidised in aerobic and other respiratory processes will fi nally be converted to carbon dioxide by these bacteria. The toxic hydrogen sulphate is the other main product of this process, and thus it is important that fermentation be minimised in a shrimp pond. The strategy that can used to achieve this is discussed below.

Probotics, which are live, natural, benefi cial bacteria, are now accepted and widely used in shrimp aquaculture. They are designed to modify the microbial communities in the alimentary tract of animals and in their aquatic environment and thus also compete with and displace pathogens.

In this article, we report on fi eld data from Potiporã (Queiroz Galvão Group), a major aquaculture company located in RN, Brazil. Their operations at the company include a hatchery unit with an overall production capacity of 150-200 million post larvae per month, an outgrowing farm with 900 ha total pond area, capable of producing up to 6,000 tonnes of shrimp per year from ponds stocked at 18-30/m2, and a processing/packaging plant with a daily capacity of 45 tonnes of shrimp. The company has access to modern techniques (molecular biology, pathology, microbiology) that are used as part of the disease prevention and shrimp genetic programs. 

In 2007, Potiporã started using a range of probotics in their larviculture and ongrowing operations. These were a combination of Sanolife PROFMC (mixture of Bacillus strains, supplied as dry spores, coated on feed by the feed-mill) and Sanolife PRO-W (mixture of Bacillus strains, supplied as dry spores, which was applied directly to the pond water. As part of their quality control analyses, Potipora confi rmed the abundance of viable spores and the composition of the commercial products. For example, their Real Time PCR analyses confi rmed the absence of Bacillus cereus contaminants from these products.

The combination of Bacillus strains products in feed and water improved the farm productivity, i.e. biomass, survival and profi ts. As part of their investigation, Potiporã also analyzed numerous water and soil samples in order to understand the microbial dynamics better. These data indicate that the Bacillus strains were abundant in the sediment of the pond (Figure 1). 

Figure 1. Abundance of Bacillus and Vibrio (cfu/g soil wet weight) in the sediment of ponds.

 

Difference in colour in the outlet of Sanolife-treated ponds (light-colour, in the middle of the picture) and control ponds (black water, right side of the picture). Source: Indonesia.

The reasons for higher numbers of the Bacillus in the sediment than in the water column were:

As documented in previous publications, these Bacillus species were selected for their ability to inhibit pathogens directly (Figure 2). Furthermore, they inhibit pathogens indirectly by competitive exclusion (ie competition for nutrients and attachment sites in the gut or an algal particles and waste material in ponds.)

These data also confi rm the scientifi c publications suggesting that (1) Gram-positive bacteria comprise a relatively large proportion of the bacterial communities in habiting marine sediments (Gontang et al. 2007, Moriarty and Hayward, 1982);) (2) the dominant species of Bacillus reported from Brazilian marine sediments include B. subtilis, B. licheniformis and B. pumilus (Miranda et al, 2008). The range of probotic includes strains of B. subtilis, B. licheniformis and B. pumilis. 

Figure 2. Inhibition of Vibrio strains (Brazilian isolates, horizontal streaks) by a strain of Bacillus (vertical streak).

These data confi rm the benfi t of using specifi cally selected strains of Bacillus for their ability to degrade waste organic matter, improve water quality (Figure 4) and out compete Vibrio on the upper sediment layer, the area of the pond where shrimp spend most of their time: