Optimization of culture medium performance for growth of microalga Trachydiscus sp. LCR-Awa9/2

Abstract

The native New Zealand microalga Trachydiscus sp. LCR-Awa9/2 has potential as a commercial eicosapentaenoic acid (EPA) producer. The aim of this study was to optimize a culture medium to produce a high cell density culture of the microalga in shake flasks incubated at 25˚C. Bold Basal medium (BBM) was used as the basis culture medium for culture media study. The role of major nutrients and ions such as nitrogen, phosphate, chlorine, sulphate, potassium, magnesium and calcium, and trace nutrients such as cobalt, copper, iron, EDTA, citrate, manganese, molybdenum, zinc, selenium, and vitamins was observed. The trace elements study resulted in removing cobalt and boron from the BBM. Supplementing the culture medium with selenium or vitamins did not improve the microalga growth. Additionally, using different sources of iron and chelating agent did not cause significant change in both biomass production and fatty acid profile.

The microalga was able to utilize all nitrogen compounds tested such as sodium nitrate, urea, and ammonium chloride. However, the ammonia culture medium did not produce as high biomass as nitrate and urea culture media. The nitrate culture medium was the most favourable nitrogen source for the microalga in terms of its production of biomass, fatty acid, and EPA.

A dynamic model was developed to determine the nitrogen, phosphorous, and light consumption for the alga population growth. It was found that light limitation occurred in 250 μmol m⁻² s⁻¹ light intensity, therefore the light intensity needed to be increased to 530 μmol m⁻² s⁻¹. The microalgal productivity was improved 100% when the light intensity was increased to the high light level. The model fitted the data impressively and some unknown parameters were simulated such as maximum growth rate (μmax), consumption ratio (Y), and Monod constant (K). The amount of nitrogen and phosphate needed for the microalga were 0.1113 and 0.0043 g nutrient/g biomass, respectively.

The starvation of nitrogen or phosphorous, and complete starvation (the microalga was cultivated in distilled water) increased the fatty acid production but they stimulated more elaidic acid than EPA.

The final prototype culture medium is called Bold Basal Modified Awarua medium (BBMA). The use of the modified culture medium led to 100% higher biomass production than the media previously used. The highest biomass and EPA production achieved 4.7 g/L dry biomass, with 12% fatty acid of total dry mass and 33.8% EPA as a factor of total fatty acid.