In recent years, we have noted an interest and a growing use of recirculation systems in aquaculture. As the technological developments in aquaculture progress towards better systems, the understanding of the biological process will become increasingly important. The most important metabolic rejection of aquatic organisms is ammonia (NH3). Since ammonia is toxic to aquatic organisms, the most important method of the closed circuit is the removal of ammonia and sediments. Ammonia removal is done under bacterial action in the nitrification cycle, so ammonia is converted to nitrite (NO2) and then to nitrate (NO3) much less toxic.
The AQUAMERIK company has been distributing nitrifying bacteria for more than eight years. In the interests of aquaculture and for the benefit of our customers, we present to you the following information on nitrifying bacteria in order to help understand their roles in an aquatic system and to provide the information necessary for an efficient management of closed circuits.
FACTORS AFFECTING BACTERIAL GROWTH
Factors affecting bacterial growth and their ability to oxidize ammonia and nitrite under aquaculture conditions are as follows:
- DISSOLVED OXYGEN
- MEDICATIONS AND DISINFECTANTS
- STARTING THE SYSTEM
- LIGHTING AND MAINTENANCE
1 – SUBSTRATE
Nitrifying bacteria are dependent on food. With a growth rate that increases under high concentrations of ammonia and nitrite. A false belief is that high concentrations of ammonia affect Nitrobacter and that this high concentration of nitrite affects Nitrosomonas. In bacterial cultures, we found that concentrations up to 10 ppm did not affect the bacterium.
The major reasons for food dependence are: A) Filters that are left with ammonia and nitrite concentrations stronger than their concentrations of operation, will go more easily and with a greater number of bacteria; B) Systems with low ammonia and nitrite concentration requirements (broodstock, larval production, etc.) will require a smaller bio-filter than those in high-concentration operations.
2 – TEMPERATURE
Nitrifying bacteria are eurithermal and can easily adapt to temperature changes. Maximum growth is reached at 25° – 300°C (77° – 860°F), at 180°C (64.40°F) The growth rate is reduced by 50% and at temperatures of 8° – 100°C (46.4° – 500°F) The growth rate is 30% of the maximum growth conditions. There is virtually no bacterial activity at -40°C (-39.20°F). We found that the bacteria are not affected by the sudden changes in temperature. In order to boot the cold water systems, it would be better to start the bacterial process at temperatures approaching the ideal. Once the bacteria is stabilized, lower the temperature. Nitrobacter tolerates less well low temperatures, so it is necessary to check the accumulations of nitrite in cold water systems.
3 – SALINITY
Freshwater bacteria species are different from those for salt water. The freshwater species that we distribute tolerate salinity up to 6 ppt. Saltwater species can easily withstand a salinity rate of 6 to 44 ppt without problems. Adaptation to varying degrees of salinity is relatively fast, normally 1 to 3 days, and bacteria are not affected by drastic changes in salinity. Once the bacterium is adapted to a different salinity, its growth is not affected.
4 – pH
Nitrifying bacteria are quite sensitive to pH changes. Nitrosomonas species prefer a pH of 7.8 – 8.0 while Nitrobacter prefer a pH of 7.3 – 7.5. In marine systems operating at pH above 8.0, nitrite accumulation is more important than ammonia accumulation. Systems with a pH less than 7.0 will tend to have more problems with ammonia accumulation. The process of nitrification is an acidic process due to the production of hydrogen ions in the transformation of nitrite and nitrate. These acids are quickly neutralized by the carbonates present in the water. In order not to reduce the pH value too much in a closed system, 7.14 grams of calcium carbonate (CaCO3) must be added for each gram of oxidized ammonia. This represents approximately 0.1428 grams of calcium carbonate (CaCO3) for each gram of dry food given to organisms in the system.
5 – DISSOLVED OXYGEN
6 – NUTRIMENTS
Apart from ammonia and nitrite, there are several other components necessary for optimal bacterial growth. The most critical element is phosphorus. Phosphorus in sea water, in the form of phosphate, reacts with calcium and precipitates in the form of calcium phosphate. It is normal that at the beginning of the start of a sea-water system that nitrites are high, this is caused by the lack of phosphate available. The addition of 0.1 ppm phosphate should solve the problem. In systems in operation for some time, bacteria are able to absorb organic phosphorus from organic waste, so phosphate is not a problem.
7 – MEDICATIONS AND DISINFECTANTS
The majority of bacterial pathogens of aquatic organisms are Gram (-), which means all antibiotic used to control bacterial diseases will affect the nitrifying bacteria. Disinfectants like chlorine, iodine, ozone, quaternary ammonium, potassium permanganate and formaldehyde will also affect bacteria. Other chemicals like copper, malachite green and methylene blue are not recommended as well.
8 – STARTING THE SYSTEM
Since nitrifying bacteria are dependent on a substrate, the ideal place to introduce bacteria is the biofilter. Nitrifying bacteria are really small (less than 2 microns) and at the time of the introduction of the product there will be bacteria throughout the system. At the time of start-up, all sterilization equipment (U.V. and others) must be disconnected for a period of 7 days in order to allow enough time for bacteria to have a good contact surface with its bacterial support. The following equipment is considered to be potentially harmful to nitrifying bacteria: Sterilizers Ozone generators, ultraviolet Disinfectants Chemicals , Protein skimmers Low, and mechanical filters Micronage. After a period of 7 days, the use of this type of equipment is necessary for closed circuits in order to protect the biological filter of organic materials.
9 – LIGHTING AND MAINTENANCE
Nitrifying bacteria are fragile to light, so it’s best to keep them in the dark. Our biofilters do not require regular maintenance, however when cleaning the entire biofilter should not be washed, which would destroy the active bacterial mass and an increase in ammonia would follow. Proceed by cleaning only 1/3 of the bacterial supports at the same time.
Do not hesitate to contact us for more information.