Fast media optimization for mixotrophic cultivation of Chlorella vulgaris

Valerie C A Ward,Lars Rehmann

doi:10.1038/s41598-019-55870-9

Valerie C A Ward, Lars Rehmann

Open Access

https://doi.org/10.1038/s41598-019-55870-9

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Journal: Scientific Reports	Publication Date: Dec 1, 2019
Citations: 17	License type: open-access

Affiliation: Western University, University of Waterloo

Abstract

Microalgae can accumulate large proportions of their dry cell weight as storage lipids when grown under appropriate nutrient limiting conditions. While a high ratio of carbon to nitrogen is often cited as the primary mode of triggering lipid accumulation in microalgae, fast optimization strategies to increase lipid production for mixotrophic cultivation have been difficult to developed due to the low cell densities of algal cultures, and consequently the limited amount of biomass available for compositional analysis. Response surface methodologies provide a power tool for assessing complex relationships such as the interaction between the carbon source and nitrogen source. A 15 run Box-Behnken design performed in shaker flasks was effective in studying the effect of carbon, nitrogen, and magnesium on the growth rate, maximum cell density, lipid accumulation rate, and glucose consumption rate. Using end-point dry cell weight and total lipid content as assessed by direct transesterification to FAME, numerical optimization resulted in a significant increase in lipid content from 18.5 ± 0.76% to 37.6 ± 0.12% and a cell density of 5.3 ± 0.1 g/L to 6.1 ± 0.1 g/L between the centre point of the design and the optimized culture conditions. The presented optimization process required less than 2 weeks to complete, was simple, and resulted in an overall lipid productivity of 383 mg/L·d.

Highlights

Microalgae can accumulate large proportions of their dry cell weight as storage lipids when grown under appropriate nutrient limiting conditions
In order to reduce the number of runs, have all points within the design space, and improve prediction in the extremes for the remaining factors, glucose, nitrate, and magnesium, a Box-Behnken design (BBD) was chosen to gain greater insight into the interactions and effects of these factors on mixotrophic growth performance and culture productivity
Lipid accumulation was measured using the Nile Red lipid assay in shaker flasks during the cultivation period as it uses small sample sizes and the end point total lipid content was confirmed using an National Renewable Energy Laboratories (NREL) Laboratory Analytical Procedure[29]

Summary

Introduction

Microalgae can accumulate large proportions of their dry cell weight as storage lipids when grown under appropriate nutrient limiting conditions. While a high ratio of carbon to nitrogen is often cited as the primary mode of triggering lipid accumulation in microalgae, fast optimization strategies to increase lipid production for mixotrophic cultivation have been difficult to developed due to the low cell densities of algal cultures, and the limited amount of biomass available for compositional analysis. Almost all microbial species are capable of production of lipids, those capable of accumulating over 20% of their dry weight as lipids are designated oleaginous microorganisms[12] Those species which accumulate such significant portions generally use these lipids as an alternative energy storage molecule in the form of triacylglycerides (TAGs)[7]. While a high ratio of carbon to nitrogen (C/N) is often cited as the main method for triggering lipid production, limitation of nitrogen limits the overall growth of the culture[8,24]

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Conclusion