Website design By BotEap.comThe probiotic bacteria cultures added to shrimp ponds are usually composed primarily of heterotrophic bacteria or a mixture of heterotrophic and autotrophic nitrifying bacteria. Heterotrophic bacteria are those that obtain their nutrition mainly from organic sources. The main source of carbon for these bacteria is carbohydrates. Nitrogen is typically obtained from proteins in organic material consumed by bacteria. Like shrimp, heterotrophic bacteria excrete ammonia as a by-product of the metabolism of the proteins they consume. However, some heterotrophic bacteria can use ammonia directly as an alternative source of nitrogen.

Website design By BotEap.comWhat does all this have to do with C: N relationships? Shrimp feeds used in intensive shrimp ponds are typically at least 35% protein. These foods do not contain many carbohydrates. The C: N ratios in these feeds are typically around 9: 1. Bacteria require about 20 carbon units per assimilated nitrogen unit. With such a low C: N ratio in food, carbon is the limiting nutrient for heterotrophic bacteria populations. The bacterial population will not expand beyond a certain point due to limited carbon availability. Protein in organic debris supplies most of the nitrogen requirement for heterotrophic bacteria under these circumstances, and inorganic ammonia is largely not used as a nitrogen source.

Website design By BotEap.comIf the C: N ratio is increased, either by feeding lower protein foods with a higher percentage of carbohydrates, or by adding a carbohydrate source such as molasses in addition to regular feed, the higher carbon availability allows the heterotrophic bacterial population to consume a greater percentage of protein in organic material. This results in a complete digestion of the organic matter in the pond by the heterotrophic bacteria. As the C: N ratio increases, heterotrophic bacteria increasingly turn to ammonia metabolism to meet their nitrogen needs. As C: N ratios increase further, a point is reached where nitrogen, rather than carbon, becomes the limiting nutrient. At this point, the ammonia concentrations should be close to 0 mg / L in the pond.

Website design By BotEap.comIt should be noted that keeping feed protein constant and supplementing with pure carbohydrates will result in much higher bacterial counts in the pond. The oxygen required to maintain this additional bacterial biomass will increase proportionally with the increase in the bacterial population. It will also increase the production of CO2, which will lower the pH. If you are considering carbohydrate supplementation to increase C: N ratios, make sure your pond is well aerated and circulating to keep organic debris suspended in the water column where there is enough oxygen for heterotrophs. Also, once you develop a dense heterotroph population through carbohydrate supplementation, do not stop carbohydrate supplementation suddenly. This will starve the bacteria of carbon, a death will occur, and you will get an ammonia spike.

Website design By BotEap.comAnother point to consider before improving C: N ratios in P. monodon ponds. P. monodon does not utilize organic debris and associated bacterial protein as effectively as a food source as P. vannamei does. With vannamei, C: N ratios can be improved by reducing the overall protein levels of the food and using foods that are high in carbohydrates. Because vannamei feeds on organic flocs and uses bacterial protein efficiently, growth rates are unaffected and protein utilization efficiencies are drastically improved. With monodon, feeding low-protein, high-carbohydrate diets will likely result in lower growth rates. Therefore, it may be necessary to rely more on pure carbohydrate supplementation to increase C: N ratios. But this will result in more bacterial biomass, more BOD, and more CO2. This makes it somewhat questionable, in my opinion, whether it is worth the risk of managing a monodon pond with high C: N ratios.

Website design By BotEap.comThe most common genera of heterotrophic bacteria used in probiotic formulations are Bacillus and Lactobacillus, both gram-positive. However, it is not necessary to inoculate a pond with commercial probiotics to handle a heterotrophic production system. This can be achieved simply by maintaining a C: N ratio greater than 12: 1 and providing adequate aeration. Bacteria are already present in all ponds. By removing the carbon (and perhaps oxygen) constraint, they will proliferate.

Website design By BotEap.comNatural bacteria counts are in the several thousand per milliliter, so a one hectare pond contains astronomical amounts of bacteria. It would be very difficult to add enough bacteria to a pond to significantly change its bacterial composition.

Website design By BotEap.comIn addition, it would be expected that the species of natural bacteria are the best adapted to the conditions of the pond. There is no guarantee that the bacteria in the probiotic culture will adapt well to pond conditions, much less that they will compete with natural bacteria species. Even if enough bacteria were added to have an effect on the bacterial composition at any one time, it would probably be necessary to re-inoculate the bacteria periodically to maintain the dominance of the probiotic species. I admit there have been studies that seem to show survival benefits in probiotic treated ponds. But there are also many studies that fail to find any measurable impact on the composition of bacterial species. Perhaps something is happening that allows probiotic bacteria to positively influence survival even when they are not the predominant species.

Website design By BotEap.comBacillus and Lactobacillus are common genera of heterotrophic bacteria used in probiotic formulas. What genera of heterotrophic bacteria are already found in ponds, but not in commercial probiotic products?

Website design By BotEap.comSeafloor sediments contain naturally beneficial bacteria such as Bacillus subtillis, B. circulans, B. megaterium, B. polymyxa, and B. licheniformis. They are purified and multiplied in fermenters and then processed as spray-dried liquids or powders for commercialization in vegetative or spore form).

Website design By BotEap.comAlso, what is the best way to measure the C: N ratio in a pond?

Website design By BotEap.comMeasuring C / N is only part of the story. If you measure TOC (total organic carbon), some of that carbon may be refractory and not help bacteria grow or absorb ammonia. Measurement of TOC and BOD (biological oxygen demand, with and without inhibition of ammonia oxidation) together with TKN (total Kjeldahl nitrogen) will provide useful management information. For these systems to work, you must also breed a species that can utilize the single-celled protein that is produced in the pond. If not, all you are doing is turning ammonia into unusable biomass using a significant amount of carbohydrates and oxygen. You have to discharge that biomass or oxidize it at the bottom of the pond when it drains. If it remains in the system, it will be metabolized back into ammonia and CO2.

Website design By BotEap.comThe only difference between a photosynthetic system (algae in a pond) and a heterotrophic system (carbohydrates and oxygen) is the energy supply for the waste treatment function. Sunlight limits its energy density per unit area in algae-based systems, limiting its foraging area. With heterotrophic systems, the energy density is not limited; it is volumetric.

Website design By BotEap.comThe real trick is to convert the biomass from these waste systems into a usable animal as quickly and efficiently as possible so that you don’t waste energy remaking the ammonia over and over again as the biomass (or algae) you produced with your energy input. It decomposes.

Website design By BotEap.comRemember: all closed aquaculture is polyculture. The only question is how many salable species you have and what are your energy flows. The job of an aquaculturist is to control that microbiological ecology so that the energy flows and the treatment biomass get to where you want them.