Nutrient management on Spirulina cultivation

Introduction

Spirulina platensis is microalgae belongs to cyanobacteria that can carry out photosynthesis to meet its own food [1] . Spirulina platensis is distributed widely in all ecosystems, including terrestrial and aquatic ecosystems (fresh water, brackish water, and sea water). Spirulina platensis can grow with condition of pH 8, 5-11, temperature of 18-40 °C with low to high light intensity at range of 500350, 000 lux [2] . As a single cell protein with high protein and micronutrients, Spirulina is compatible as functional and healthy food [3] . Spirulina contains 60-71% protein, 8% fat, 16% carbohydrates, 17% carotene, 1, 6% chlorophyll-a, 18% phycocyanin, and 20-30% linoleic acid from the total fatty acids [4] . Spirulina platensis is easy to digest because of its thin membrane composed of sugar group. Furthermore, the advantage of culturing Spirulina is in the easiness of biomass harvesting, as the biomass can be separated from the media by filtration sized at 20-30 μm [5,6] . The quality of Spirulina platensis depends on complex nutrients inside the culture medium. Nutrition is essential for the growth and biochemical composition of microalgae. Nutrient content necessities for Spirulina are macronutrients (N, P, K, S, Ca, Mg, Na), micronutrients (Cu, Zn, Bo, Co, Mo), and additional nutrients (H, C, O). Nitrogen is an element needed in the growth of Spirulina platensis [7] . In sea water, nitrogen formed as organic and inorganic nitrogen. Organic nitrogen is in the form of protein, amino acids, and urea. Inorganic nitrogen such as nitrate (NO 3 ), nitrite (NO 2 ), ammonium (NH 4 ), ammonia (NH 3 ), and molecular nitrogen (N 2 ) in gaseous form. Nutrient management is the cornerstone of successful Spirulina cultivation. Nitrogen is the most critical element, as it is the primary building block of amino acids and chlorophyll. While nitrate-based fertilizers are traditional, Urea (CO(NH 2 ) 2 ) is an attractive, low-cost alternative. However, Urea must be managed carefully; at high concentrations, it can release ammonia, which is toxic to the culture and can inhibit growth. Simultaneously, Potassium plays a fundamental role in osmotic regulation and enzyme activation within the cell. The interaction between nitrogen and potassium determines the metabolic efficiency of the culture. In Kakinada's tropical climate, where light intensity and temperature are often favorable for cyanobacterial growth, the optimization of these specific nutrients can significantly reduce the "doubling time" of the biomass. Previous studies have shown that excessive nutrient loading does not always translate to higher yields and can lead to medium instability. Therefore, finding the "Goldilocks zone" the precise concentration where growth is maximized without wasting resources is essential. This study aims to fill a gap in local knowledge by testing specific ppm (parts per million) ranges of Urea and Potassium. By focusing on 80-150 ppm for Urea and 100-200 ppm for Potassium, we seek to establish a protocol that is both scientifically sound and economically viable for the Department of Food Science. The following sections detail the experimental setup, the observations made regarding biomass density, and the nutritional implications of these varied nutrient regimes.

Materials and Methods

Cultivation Conditions

• Vessels: 1-liter glass flasks containing a modified Zarrouk's base (minus standard N/K sources). • Light: Artificial illumination provided by cool white fluorescent lamps (12:12 light/dark cycle). • Agitation: Manual stirring was performed three times daily to ensure uniform nutrient distribution and prevent clumping.

• pH Management: The pH was maintained between 9.0 and 10.5, which is optimal for Spirulina.

Data Collection: Biomass growth was measured over a 15day period using optical density (OD) at 560 nm and dry weight analysis (g/L).

Results and Discussion

The experimental data showed a clear preference for specific nutrient ratios. The 100 ppm Urea + 150 ppm Potassium combination emerged as the superior treatment. At this level, the nitrogen was sufficient to support rapid chlorophyll synthesis without reaching the ammoniatoxicity threshold, while the 150 ppm Potassium provided the necessary ionic balance for cell expansion. Interestingly, the 80 ppm Urea + 100 ppm Potassium treatment was found to be at par with the 100/150 combination. This is a significant finding for the "Food Science and Nutrition" perspective, as it suggests that a 20% reduction in urea and a 33% reduction in potassium does not lead to a statistically significant loss in biomass. Conversely, the 150 ppm Urea and 200 ppm Potassium treatments showed a slight decline in growth rate, likely due to increased salinity stress or localized ammonia accumulation. Spirulina contains 60-71% of protein [4] , or 55-70% based on [11] compared to other sources such as dry soybeans (35%), grains (8-10%) or peanuts (25%) protein. These values make Spirulina essential for functional diet. Protein is needed for cell metabolism processes to support growth, which affects the synthesis process and accumulation of cell contents such as carbohydrates, amino acids, nucleic acids, and fats. Type and concentration of nitrogen source affect the variations in protein, carbohydrate and lipid contents, thus the increase in nitrogen will increase protein biosynthesis in Spirulina [8] . Nitrogen and phosphorus are necessary to compound protein elements in cells, therefore sufficiency of these sources should be maintained [9,10] . The growth of Spirulina is affected by the nutrition in the medium, the higher growth rate so the higher mass density and the faster harvest time. Nitrogen content in potassium nitrate could fill in the nitrogen loss and toxic excess that resulted from urea addition. The concurrent use of urea and potassium nitrate will provide sufficient nitrogen needs for cells growth.

Conclusion

The study concludes that a nutrient regime of 100 ppm Urea and 150 ppm Potassium provides the optimal environment for Spirulina cultivation at the Ideal College laboratory. However, for cost-sensitive applications, the 80 ppm Urea and 100 ppm Potassium formula is highly recommended as it performs at a comparable level. These results emphasize that "more is not always better" in nutrient management. Future research should focus on the protein quality and phycocyanin content of the biomass produced under these specific optimized conditions.

References

1. Christawardana M, Hadiyanto MMA N. Jurnal Aplikasi Teknologi Pangan. 2013;2(1):1-4.
2. Kabinawa K. Spirulina, Ganggang Penggempur Semua Penyakit. Jakarta: Agro Media Pustaka; 2006. p. 92. https://books.google.co.id/
3. Becker EW. Microalgae Biotechnology and Microbiology. New York: Cambridge University Press; 1995. p. 196-250.
4. Jongkon P, Siripen T, Richard DL. Maejo Int J Sci Technol. 2008;2(2):159-171.
5. Desmorieux H, Decaen N. J Food Eng. 2006;77:64-70.
6. Richmond A. Spirulina. In: Borowitzka MA, editor. Micro-algal Biotechnology. UK: Cambridge University Press; 1988. p. 85-121.
7. Borowitzka MA, Borowitzka LJ. Algal media and sources of algal culture. In: Microalgal Biotechnology. Cambridge University Press; 1988. p. 456-465.
8. Madkour FF, Kamil AEW, Nasr HS. Egypt J Aquat Res. 2012;38:51-57. https://doi.org/10.1016/j.ejar.2012.09.003
9. Rodrigues MS, Ferreira LS, Converti A, Sato S, Carvalho JCM. Bioresour Technol. 2010;101:44914498. https://doi.org/10.1016/j.biortech.2010.01.054
10. Sassano CEN, Carvalho JCM, Gioielli LA, Sato S, Torre P, Converti A. Appl Biochem Biotechnol. 2004;112:143-150.
11. Jung F, Krüger-Genge A, Waldeck P, Küpper JH. J Cell Biotechnol. 2019;5:43-54. https://doi.org/10.3233/JCB-189012