IDENTIFICATION AND INDUCED BIOSYNTHESIS OF NATURAL HEALTH COMPOUNDS IN MICROALGAE

Abstract

Microalgae have attracted the attention of scientists and researchers for their ability to produce high-value bioactive compounds for human and livestock consumption. In the last few decades, microalgae-derived fatty acids, carotenoids and antimicrobials, have been of particular interest due to nutritional benefits and their purported capacity to prevent numerous diseases, such as cancer, cardiovascular disease, as well as degenerative and neurological disorders. Therefore, microalgae have been extensively studied to examine their potential in producing bioactive compounds at scale, with a view to introducing them into the diets of both humans and livestock. The importance of microalgae–microbe interactions in aquatic environments is increasingly being recognized, which has led to the first hypothesis of this study that microalgae may produce antimicrobial compounds that control the microbial population in their environment. Additionally, the commercial exploitation of microalgae species to produce bioactive compounds for human health has led to the second hypothesis that different species have a different response to environmental cues, which may result in the elevated production of certain metabolites, such as the carotenoids, lutein, β-carotene and violaxanthin. The aims of this thesis, therefore, were as follows: (1) to evaluate different microalgal species for their potential to produce antimicrobial compounds; (2) to isolate and structurally elucidate those antimicrobial compounds with a view producing them commercially; (3) to investigate the effect of plant hormones and UV-C on carotenoid biosynthesis and (4) to perform transcriptomic analysis in order to reveal cross-talk between plant hormone signaling and carotenoid biosynthesis, which should improve our knowledge of the active pathway components of plant hormone-induced carotenogenesis. First, screening was conducted on various microalgal species collected from brackish and marine waters for antimicrobial activity, including plant food and human pathogens. Different extraction approaches were carried out to ensure that most of the compounds were efficiently extracted. Both intracellular and extracellular metabolites were examined for their microbial inhibition activity. Thereafter, antimicrobial assays were performed using different approaches to reveal the active compounds with different chemical properties. The crude extracts of three out of fifteen strains showed promising minimum inhibitory concentrations (MICs) and so were selected for detailed isolation and chromatographic purification complemented by nuclear magnetic resonance (NMR) to reveal the active metabolites. As a result of this extensive exercise long chain fatty acids, namely linoleic acid, oleic acid and docosahexaenoic acid (DHA), were found to be the dominant metabolites in bioactive microalgae responsible for the observed antimicrobial effect. The other hypothesis tested in this body of work involved induction and accumulation of carotenoids by plant hormones and UV-C in green microalgae, going on to investigate whether this resulted in longer term changes in the carotenoid biosynthesis pathway within the microalgae. Initially, induction was conducted on three microalgal strains— Chlorella sp. BR2, Dunaliella salina and Tetraselmis sp. M8—using different dosages of four plant hormones (indole-3-acetic acid, methyl jasmonate, salicylic acid and abscisic acid) and UV-C radiation. Interestingly, indole‐ 3‐ acetic acid was found to be the only hormone with an inductive effect on carotenoid accumulation. As such, transcriptome-wide changes following auxin treatment were profiled Chlorella sp. BR2 using RNA- Seq and expressed sequences were reconstructed by de novo assembly. This revealed the active pathway components of auxin-induced carotenogenesis. Data analysis specified the differentially expre