Effect of different biofloc starters on ammonia, nitrate, and nitrite concentrations in the cultured tilapia Oreochromis niloticus system

Introduction

Water quality is a crucial factor in aquaculture systems. One important water quality parameter is nitrogen. In water, nitrogen can be found in the forms of ammonia, nitrite, and nitrate^1–5. Ammonia (NH₃-N) is produced from the breakdown of proteins from unconsumed feed, feces, and urine of fish. This compound will turn into nitrite (NO₂) when oxygen levels are poor⁶, which is toxic for fish⁷. By contrast, ammonia is changed into nitrate when the dissolved oxygen level is sufficient^8,9. Fish produces ammonia (inorganic N) through the osmoregulation process; feces and urine contribute about 10%–20% of total nitrogen¹⁰. The application of biofloc is one of the alternatives to overcome water quality problems especially in controlling total ammonia nitrogen in the aquaculture system^11–13.

Biofloc refers to the use of heterotrophs and autotrophs, which can convert organic waste into floc forms that can be utilized by fish as a food source^4,11,14–16. Biofloc technology is cheap, simple, and environmentally friendly^13,17. Several organisms, such as bacteria, plankton, fungi, and algae, and suspended particles exist in flocs^18,19. These organisms provide nutrition for cultured fish. However, the formation of biofloc needs a starter consisting of probiotics and a carbon source. Molasses, tapioca, and wheat flours are common starters in biofloc culture (African catfish, Nile tilapia, Litopenaeus vannamei)^16,19–23. Presently, limited information is available on the best starter for biofloc in the cultured system of red tilapia Oreochromis niloticus. Thus, the present study aimed to explore the best starter for biofloc in a red tilapia O. niloticus culture system.

Methods

Results

Over the six timepoints measured, the mean concentration of ammonia ranged from 0.02 mg L⁻¹ to 0.07 mg L⁻¹ (with an average value of 0.03 ± 0.02 mg L⁻¹), nitrite ranged from 0.20 mg L⁻¹ to 0.43 mg L⁻¹ (with an average value of 0.25 ± 0.12 mg L⁻¹), and nitrate ranged from 0.64 mgL⁻¹ to 3.20 mgL⁻¹ (with an average value of 1.42 ± 1.19 mg L⁻¹). The study revealed that the best concentrations of ammonia, nitrite, and nitrate were recorded in the biofloc starter of molasses. These values did not significantly differ from those obtained with the tapioca and sucrose starters, but they were significantly different from the control treatment without a starter (Table 1). Raw values for each replicate are available as Underlying data²⁸.

In the control treatment, the concentrations of ammonia, nitrite, and nitrate increased gradually with increasing experimental time. However, in the starter treatments, the ammonia concentration dropped during the first week of the experiment (day 8), became stagnant during the second and third weeks (day 16 to day 24), and increased again at day 32 of the experiment. However, the ammonia concentration decreased sharply at day 40 (Figure 1a). The nitrite concentration also decreased during the first week but increased slightly at day 16 and increased gradually until day 40 in treatments C (tapioca) and D (sucrose). By contrast, the nitrite concentration decreased at day 24, increased at day 32, and decreased at day 40 (Figure 1b). The nitrate concentration was relatively stagnant from day 1 to day 8. The nitrate concentration fluctuated during the experiment (Figure 1c). In general, the molasses starter (treatment B) yielded slightly better results than the other starters, but no significant different was found between the treatments except the control. The data of the growth performance of the red tilapia fish O. niloticus has been published separately²⁹.

Figure 1.

(a) Concentrations of ammonia, (b) nitrate, and (c) nitrate during 40 days of the experiment. A = control treatment, B = molasses starter, C = tapioca starter, and D = sucrose starter.

Discussion

The study revealed that the concentrations of ammonia, nitrite, and nitrate were significantly lower in the biofloc system using starters compared with those in the system without a starter (control). Biofloc has probiotic bacteria that can change ammonia to nontoxic materials (such as nitrate) that are useful for phytoplankton growth. Therefore, the ammonia and nitrate concentrations are low in the culture media^19,21,30. Biofloc does not necessarily only contain bacteria (for example, Bacillus), but is also composed of other useful microorganisms such as microalgae and zooplankton that are trapped by organic particles³¹. Algae and zooplankton can be used by cultured biota (tilapia) as natural food.

In general, the starters used in this study were carbohydrate compounds. However, the study showed that the molasses starter yielded slightly better results compared with the other starters. This finding indicated that molasses was the best carbon source for biofloc in the tilapia culture system. Molasses can provide a sufficient carbon level for heterotrophic bacteria that use this carbon as an energy source for growth^{6,19,21,23,32}. Molasses are a liquid byproduct from the sugar industry. This material has a total carbon content of around 37%²⁴. Therefore, molasses are rapidly soluble in water and can be quickly absorbed by heterotrophic bacteria³. In terms of chemical structure, molasses are classified as a simple carbohydrate containing six C atoms (monosaccharides), while sucrose (treatment D) is a combination of two monosaccharides that contain 12 C atoms (sucrose). Tapioca is classified as a complex carbohydrate (60,000 C atoms) and is more slowly digested by bacteria than molasses^25,33.

Conclusion

Carbon source from molasses is effective in reducing concentrations of ammonia, nitrite, and nitrate in red tilapia culture with biofloc technology.

Data availability