Abstract
Microalgae are of substantial biotechnological interest due their polyphyletic nature which grants them access to a wide array of high-value metabolites. The inherent genetic diversity of microalgae combined with their metabolic plasticity when grown using different trophic and illumination strategies necessitate the establishment of a reference knowledge base. In the present study we present a detailed characterisation of the combined effects of wavelength selection and trophic strategy on the growth kinetics and gene expression profile of the model microalgae Chlamydomonas reinhardtii grown under moderate to high light intensity (400 μmolph m−2·s−1). The aim is twofold: (a) to establish a list of reliable housekeeping genes valid for quantitative comparisons across several combinations of different wavelengths and trophic strategies and (b) to enable the investigation of the response of central carbon metabolic pathways under these process conditions. White, blue and red light emitting diodes (LEDs) were used to grow pH controlled photo- and mixo-trophic cultures over a period of 136 h in batch mode. A panel of 10 candidate genes, along with biomass growth rate and pigment content were dynamically monitored across all conditions. Statistical analysis identified genes (acetyl-CoA carboxylase subunit α and photosystem I reaction centre subunit II) with less variability observed in their expression levels across the entirety of conditions evaluated compared to housekeeping genes often referred to in literature (receptor of activated protein kinase C and ribosomal protein (large subunit) 19). Further analysis of gene expression profiles revealed substantial differences in response to changes in both wavelength selection (upregulation of ribulose bisphosphate carboxylase small subunit under red phototrophic growth) and trophic strategy (upregulation of malate synthase from phototrophic to mixotrophic conditions). The systematic approach used to establish reliable reference genes presented herein enables robust comparisons of cellular responses across different conditions to better understand algal metabolism and improve process performance.
Highlights
IntroductionFreshwater microalgae, like Chlamydomonas reinhardtii, have evolved an extensively versatile metabolism that increases their chances of survival in diverse environmental conditions in the presence (mixo trophy) or absence (autotrophy) of a readily available source of nutri ents [1]
Freshwater microalgae, like Chlamydomonas reinhardtii, have evolved an extensively versatile metabolism that increases their chances of survival in diverse environmental conditions in the presence or absence of a readily available source of nutri ents [1]
Given the biotechnological potential of triggering desired responses in a microalgal culture by utilising a tailored spectral composition, this study investigates the effects of wavelength selection and trophic strategy on gene expression in C. reinhardtii
Summary
Freshwater microalgae, like Chlamydomonas reinhardtii, have evolved an extensively versatile metabolism that increases their chances of survival in diverse environmental conditions in the presence (mixo trophy) or absence (autotrophy) of a readily available source of nutri ents [1]. Due to this metabolic plasticity, and despite significant research efforts, our knowledge of algal metabolism remains nascent, relative to other crops, and our control over biomass composition re mains rudimentary [2]. Photo trophic studies on biomass growth of C. reinhardtii have shown that below saturating light levels (25–100 μmolph m− 2 s− 1), illumination with red and red-blue light can achieve a higher biomass yield on
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