Biomass, lipid accumulation kinetics, and the transcriptome of heterotrophic oleaginous microalga Tetradesmus bernardii under different carbon and nitrogen sources

Baoyan Gao,Chengwu Zhang,Yuming Zhong,Hui Liu,Feifei Wang,Luodong Huang

doi:10.1186/s13068-020-01868-9

Abstract

BackgroundHeterotrophic cultivation of microalgae has been proposed as a viable alternative method for novel high-value biomolecules, enriched biomass, and biofuel production because of their allowance of high cell density levels, as well as simple production technology. Tetradesmus bernardii, a newly isolated high-yielding oleaginous microalga under photoautotrophic conditions, is able to grow heterotrophically, meaning that it can consume organic carbon sources in dark condition. We investigated the effect of different carbon/nitrogen (C/N) ratios on the growth and lipid accumulation of T. bernardii in heterotrophic batch culture under two nitrogen sources (NaNO3 and CO(NH2)2). In addition, we conducted time-resolved transcriptome analysis to reveal the metabolic mechanism of T. bernardii in heterotrophic culture.ResultsT. bernardii can accumulate high biomass concentrations in heterotrophic batch culture where the highest biomass of 46.09 g/L was achieved at 100 g/L glucose concentration. The rate of glucose to biomass exceeded 55% when the glucose concentration was less than 80 g/L, and the C/N ratio was 44 at urea treatment. The culture was beneficial to lipid accumulation at a C/N ratio between 110 and 130. NaNO3 used as a nitrogen source enhanced the lipid content more than urea, and the highest lipid content was 45% of dry weight. We performed RNA-seq to analyze the time-resolved transcriptome of T. bernardii. As the nitrogen was consumed in the medium, nitrogen metabolism-related genes were significantly up-regulated to speed up the N metabolic cycle. As chloroplasts were destroyed in the dark, the metabolism of cells was transferred from chloroplasts to cytoplasm. However, storage of carbohydrate in chloroplast remained active, mainly the synthesis of starch, and the precursor of starch synthesis in heterotrophic culture may largely come from the absorption of organic carbon source (glucose). With regard to lipid metabolism, the related genes of fatty acid synthesis in low nitrogen concentration increased gradually with the extension of cultivation time.ConclusionT. bernardii exhibited rapid growth and high lipid accumulation in heterotrophic culture. It may be a potential candidate for biomass and biofuel production. Transcriptome analysis showed that multilevel regulation ensured the conversion from carbon to the synthesis of carbohydrate and lipid.

Highlights

Microalgae are generally photoautotrophic organisms, but several can grow heterotrophically, exhibiting considerable metabolic flexibility and versatility [1].Gao et al Biotechnol Biofuels (2021) 14:4Heterotrophically growing microalgae absorb exogenous organic carbon as the source of energy instead of light
In the culture with NaNO3 used as nitrogen source and glucose concentration from 10 to 60 g/L, the biomass of T. bernardii increased with increasing initial glucose concentrations, and the highest biomass achieved was 21.55 g/L
When urea was used as nitrogen source, the biomass of T. bernardii increased with increasing glucose concentrations from 10 to 40 g/L

Summary

Introduction

Microalgae are generally photoautotrophic organisms, but several can grow heterotrophically, exhibiting considerable metabolic flexibility and versatility [1].Gao et al Biotechnol Biofuels (2021) 14:4Heterotrophically growing microalgae absorb exogenous organic carbon as the source of energy instead of light. Heterotrophic cultivation can significantly increase growth rate and biomass productivity, eliminate the light dependence, and be easier to control monoculture and scale up [2]. Due to these advantages, hetertrophic production of valuable products, including pigments, fatty acids, pharmaceuticals, and biofuels, has received substantially increasing interest [3,4,5]. Lipid accumulation in oleaginous microalgae depends on diverse factors, such as the strain in use, nutritional imbalances of medium components, the available carbon (C) and nitrogen (N) source, trophic mode (autotrophy, mixotrophy, or heterotrophy), cultivation conditions, and culture time [1, 11]. We conducted time-resolved transcriptome analysis to reveal the metabolic mechanism of T. bernardii in heterotrophic culture

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