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OPTIMIZATION OF GROWTH CONDITIONS FOR THE MARINE MICROALGA TETRASELMIS STRIATA - AN ALTERNATIVE RAW MATERIAL FOR FISH FEED

V. PATRINOU ¹, A. DASKALAKI ², C.N. ECONOMOU ³, I. KOTZAMANIS ⁴, D. BOKAS ⁵, G. AGGELIS ², D.V. VAYENAS ³, A.G. TEKERLEKOPOULOU ¹

¹ Department of Environmental Engineering, University of Patras, G. Seferi 2, Agrinio 30100, Greece (patrinou.v@upatras.gr)

² Department of Biology, University of Patras, Patras 26500, Greece

³ Department of Chemical Engineering, University of Patras, Patras 26500, Greece

⁴ Hellenic Centre for Marine Research / Institute of Marine Biology, Biotechnology and Aquaculture, Iera Odos 75, Athens 11855, Greece

⁵ PLAGTON S.A., Thesi Konaki Skentoy, Dimos Xiromerou, Aitoloakarnania, Greece

INTRODUCTION

The marine microalga Tetraselmis striata was cultivated in different drilling waters with natural salinities of 38 ‰ and 29 ‰ that were obtained from the facilities of the commercial fish farm Plagton S.A.,. The effects of salinity, pH, growth media and initial N and P concentration ratios on the specific growth rate and ability of the strain to biosynthesize proteins, lipids, polysaccharides and pigments were studied.

It was found that T. striata can grow well in 29 ‰ salinity with the commercial fertilizer Nutri-Leef 30-10-10 at pH 8. The biomass of T. striata produced under the above optimum growth conditions was rich in carbohydrates, proteins and pigments, while analysis of the fatty acids revealed high percentages of polyunsaturated eicosapentaenoic acid (EPA).

Acknowledgement: The project "Large-scale cultivation of microalgae and utilization of the biomass produced as alternative raw material in fish feed-AlgaFeed4Fish" (MIS 5045858, FK 80916), was co-funded by European (European Regional Development Fund) and National Resources (General Secretariat for Research and Technology) in the context of the Operational Program "Competitiveness, Entrepreneurship and Innovation (EPANEK, NSRF 2014-2020), specifically through the Action "Special Actions“ - "Aquacultures" - "Industrial Materials" - "Open Innovation in Culture".

EXPERIMENTAL PROCEDURE AND RESULTS

Laboratory-scale experiments were performed under steady (T = 26±2°C, continuous illumination of 56 μmol m⁻² s⁻¹, stirring with air pumps of flow rate 380 L h⁻¹) non-aseptic and suspended growth conditions. The drilling waters had no nutrient load and thus the supplementation of minerals was essential for Tetraselmis growth. The high salinity waters (38 ‰) were enriched only with N and P at two different initial nutrient ratios (N/P), 45 and 7.4, corresponding to experimental sets (exp. sets) A and B, respectively, while the growth medium with the ratio N/P of 7.4 was also evaluated with the addition of trace elements (exp. set C). The results revealed low biomass productivities ranging from 32.2 to 40.0 mg L^-1 d^-1 with specific growth rates varying from 0.06 to 0.1 d^-1 and lipid contents of 9.3-24.0% (Table 1, Figure 1). It was concluded that these low biomass efficiencies were probably due to high salinity and the chemical composition of the supplemented growth medias in the 38 ‰ waters. Consequently, growth of the microalgae was studied in less saline drilling waters (29 ‰) that were supplemented with different nutrient media. The waters were enriched with a modified F/2 substrate (exp. set D) or the commercial fertilizer Nutri-Leef (comprising 30% TN, 10% P, 10% K) with or with out the addition of an inorganic carbon source (NaHCO₃) (exp. sets E and F, respectively). The resulting maximum biomass productivities ranged from 69.3 to 85.0 mg L^-1 d^-1, while lipid content was 10.8-13.7% (Table 1, Figure 1).

Nutri-Leef 30-10-10 with the addition of NaHCO₃ produced the highest biomass yields and was therefore used as the matrix to study the effect of pH. Different pH values of 7 and 8 were tested (Figure 2) and pH 8 presented significant biomass productivity yields (79.81 mg L^-1 d^-1 with a specific growth rate of 0.16 d^-1) and a maximum oil content of 26.4% (Figure 3). The biomass of T. striata produced under the above optimum growth conditions was rich in carbohydrates, proteins and pigments (36.5%, 38.25% and 3.6%, respectively) (Table 2). Additionally, the lipids of the microalgal biomass contained high EPA contents ranging from 10 to 14%, which are among the highest values recorded in the relevant bibliography.

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CONCLUSIONS

Bibliography

Table 1. Results of the different growth conditions on T. striata biomass.

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Figure 1. Production of: a) biomass, and b) lipids from T. striata under different growth media.

Figure 2. T. striata growth and lipid accumulation in Nutri-Leef and NaHCO₃ at pH 7 and 8.

Table 2. Effect of pH values on the biomass and growth rates of T. striata at optimum growth media conditions.

According to the experimental results, under the optimized growth conditions the microalga Tetraselmis striata was able to produce high quality biomass rich in nutrients important for fish growth. Additionally, the lipids of the microalga biomass contained polyunsaturated fatty acids such as EPA that indicate its suitability for incorporation into conventional fish feed.

Imamoglu, E.; Demirel, Z.; Conk Dalay, M. Process optimization and modeling for the cultivation of Nannochloropsis sp. and Tetraselmis striata via response surface methodology. Journal of Phycology, 2015, 5, 442–453; DOI: org/10.1111/jpy.12286.

Figure 3. Effect of pH values on fatty acid composition.