Abstract
While photosynthetic microalgae, such as Chlorella, serve as feedstocks for nutritional oils and biofuels, heterotrophic cultivation can augment growth rates, support high cell densities, and increase triacylglycerol (TAG) lipid content. However, these species differ significantly in their photoautotrophic and heterotrophic characteristics. In this study, the phylogeny of thirty Chlorella strains was determined in order to inform bioprospecting efforts and detailed physiological assessment of three species. The growth kinetics and lipid biochemistry of C. protothecoides UTEX 411, C. vulgaris UTEX 265, and C. sorokiniana UTEX 1230 were quantified during photoautotrophy in Bold's basal medium (BBM) and heterotrophy in BBM supplemented with glucose (10 g L−1). Heterotrophic growth rates of UTEX 411, 265, and 1230 were found to be 1.5-, 3.7-, and 5-fold higher than their respective autotrophic rates. With a rapid nine-hour heterotrophic doubling time, Chlorella sorokiniana UTEX 1230 maximally accumulated 39% total lipids by dry weight during heterotrophy compared to 18% autotrophically. Furthermore, the discrete fatty acid composition of each strain was examined in order to elucidate lipid accumulation patterns under the two trophic conditions. In both modes of growth, UTEX 411 and 265 produced 18∶1 as the principal fatty acid while UTEX 1230 exhibited a 2.5-fold enrichment in 18∶2 relative to 18∶1. Although the total lipid content was highest in UTEX 411 during heterotrophy, UTEX 1230 demonstrated a two-fold increase in its heterotrophic TAG fraction at a rate of 28.9 mg L−1 d−1 to reach 22% of the biomass, corresponding to as much as 90% of its total lipids. Interestingly, UTEX 1230 growth was restricted during mixotrophy and its TAG production rate was suppressed to 18.2 mg L−1 d−1. This constraint on carbon flow raises intriguing questions about the impact of sugar and light on the metabolic regulation of microalgal lipid biosynthesis.
Highlights
While many benefits of microalgae production are inherent to photosynthetic carbon dioxide assimilation, heterotrophic growth can circumvent certain limitations of photoautotrophic cultivation, such as ineffective light transfer in saturated cultures and low photosynthetic efficiencies [1,2]
For our initial species selection, a ‘‘genetic fingerprint’’ based on the 18S ribosomal RNA’s internal transcribed spacer (ITS) regions was established for each isolate from a collection of over thirty Chlorella strains and used to construct a phylogenetic tree (Figure 1)
We encountered some Chlorella strains that had been designated as related species (e.g., UTEX 29, 2714) and some strains that group within a separate genus entirely (UTEX 2248, 252)
Summary
While many benefits of microalgae production are inherent to photosynthetic carbon dioxide assimilation, heterotrophic growth can circumvent certain limitations of photoautotrophic cultivation, such as ineffective light transfer in saturated cultures and low photosynthetic efficiencies [1,2]. The presence of a fixed carbon source (i.e., sugar) can promote rapid growth, support high cell densities, and augment lipid accumulation [3]. As such, this mode of growth has been exploited for the industrial production of polyunsaturated fatty acids and bioactive pigments to serve nutritional markets [4,5]. If heterotrophic substrates can be sustainably sourced for relatively low cost (e.g., derived from cellulosic biomass, wastewater, or directly produced by other photoautotrophs) [11], the bioconversion of organic compounds to lipids may be a viable model for algal biofuel production [12,13]
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