• Energy Research

There is a particularly high interest to derive carotenoids such as β-carotene and lutein from higher plants and algae for the global market. It is well known that β-carotene can be overproduced in the green microalga Dunaliella salina in response to stressful light conditions. However, little is known about the effects of light quality on carotenoid metabolism, e.g., narrow spectrum red light. In this study, we present UPLC-UV-MS data from D. salina consistent with the pathway proposed for carotenoid metabolism in the green microalga Chlamydomonas reinhardtii. We have studied the effect of red light-emitting diode (LED) lighting on growth rate and biomass yield and identified the optimal photon flux for D. salina growth. We found that the major carotenoids changed in parallel to the chlorophyll b content and that red light photon stress alone at high level was not capable of upregulating carotenoid accumulation presumably due to serious photodamage. We have found that combining red LED (75 %) with blue LED (25 %) allowed growth at a higher total photon flux. Additional blue light instead of red light led to increased β-carotene and lutein accumulation, and the application of long-term iterative stress (adaptive laboratory evolution) yielded strains of D. salina with increased accumulation of carotenoids under combined blue and red light.

Marine diatoms have recently gained much attention as they are expected to be a promising resource for sustainable production of bioactive compounds such as carotenoids and biofuels as a future clean energy solution. To develop photosynthetic cell factories, it is important to improve diatoms for value-added products. In this study, we utilized UVC radiation to induce mutations in the marine diatom Phaeodactylum tricornutum and screened strains with enhanced accumulation of neutral lipids and carotenoids. Adaptive laboratory evolution (ALE) was also used in parallel to develop altered phenotypic and biological functions in P. tricornutum and it was reported for the first time that ALE was successfully applied on diatoms for the enhancement of growth performance and productivity of value-added carotenoids to date. Liquid chromatography-mass spectrometry (LC-MS) was utilized to study the composition of major pigments in the wild type P. tricornutum, UV mutants and ALE strains. UVC radiated strains exhibited higher accumulation of fucoxanthin as well as neutral lipids compared to their wild type counterpart. In addition to UV mutagenesis, P. tricornutum strains developed by ALE also yielded enhanced biomass production and fucoxanthin accumulation under combined red and blue light. In short, both UV mutagenesis and ALE appeared as an effective approach to developing desired phenotypes in the marine diatoms via electromagnetic radiation-induced oxidative stress.

Green microalgae have recently drawn attention as promising organisms for biofuel production; however, the question is whether they can grow sufficient biomass relative to limiting input factors to be economically feasible. We have explored this question by determining how much biomass the green microalga Chlorella vulgaris can produce in photobioreactors based on highly efficient light-emitting diodes (LEDs). First, growth results were improved under the less expensive light of 660 nm LEDs, developing them in the laboratory to meet the performance levels of the traditional but more expensive 680 nm LEDs by adaptive laboratory evolution (ALE). We then optimized several other key parameters, including input superficial gas velocity, CO(2) concentration, light distribution, and growth media in reference to nutrient stoichiometry. Biomass density thereby rose to approximately 20 g dry-cell-weight (gDCW) per liter (L). Since the light supply was recognized as a limiting factor, illumination was augmented by optimization at systematic level, providing for a biomass productivity of up to 2.11 gDCW/L/day, with a light yield of 0.81 gDCW/Einstein. These figures, which represent the best results ever reported, point to new dimensions in the photoautotrophic performance of microalgal cultures.

In order to produce natural pigments with competitive prices, algal strains employed in industrial production need to be improved for increasing the productivity of valuable metabolites, thereby reducing the overall production cost. Adaptive laboratory evolution (ALE) is a traditional method for strain improvement, which has been effectively utilized in bacteria and fungi. With the growing interest in algal biotechnology, attempts have recently been put forward to improve microalgal strains with ALE approach. This chapter describes a stepwise adaptive evolution strategy that enhances carotenoid yield from microalgae.