Gene expression profiling of astaxanthin and fatty acid pathways in Haematococcus pluvialis in response to different LED lighting conditions

Selectively biorefining astaxanthin and triacylglycerol co-products from microalgae with supercritical carbon dioxide extraction

Clustering behavior of supercritical carbon dioxide, triacylglycerides, and astaxanthin solutes were analyzed using Kamlet-Taft solvatochromic properties of dipolarity/polarizability, π^∗, and hydrogen-bond acceptance, β. Both parameters were decreased for supercritical carbon dioxide with TAG at low densities and with astaxanthin at high densities. These results indicated supercritical carbon dioxide could selectively extract triacylglycerides at low densities followed by astaxanthin at higher densities from microalgae. Accordingly, Haematococcus pluvialis microalgae were subject to a two-stage continuous extraction scheme where a density of 642 mg/ml was employed to extract triacylglycerides followed by a density of 971 mg/ml, by an increase in pressure, to extract astaxanthin. The first, lower density extract yielded over 78% of the total triacylglycerides and was composed of less than 1% astaxanthin. The sequential, higher density extract yielded less than 5% of the total microalgae triacylglycerides, over 70% of the total astaxanthin, and was composed of 60-76% astaxanthin by mass.

Variation of fatty acid desaturation in response to different nitrate levels in Auxenochlorella pyrenoidosa

Microalgae are promising feedstocks for biodiesel, where the high proportion of monounsaturated fatty acid such as oleic acid (C18:1) is preferred. To regulate fatty acid desaturation in microalgae, the relationship among nitrate concentration, fatty acid composition and the expression levels of desaturase genes was explored. Dynamic variations of fatty acid profiles suggested nitrate could induce desaturation of C18 fatty acids. The content of C18:1 in Auxenochlorella pyrenoidosa was 30.88% at 0 g l^-1 nitrate concentration compared with 0.48% at 1.5 g l^-1. The expressions of relative delta-9, 12 and 15 fatty acid desaturase genes (Δ9, Δ12 and Δ15FADs) were further investigated. The 330% upregulated expression of Δ9FAD in logarithmic phase at 0 g l^-1 resulted in C18:1 accumulation. Moreover, nitrate replenishment caused a sharp reduction of C18:1 from 34.79% to 0.22% and downregulation of Δ9FAD expression to 1% of the nitrate absence level, indicating the pivotal role of Δ9FAD in C18:1 accumulation. Finally, overexpression of Δ9FAD in Escherichia coli and Saccharomyces cerevisiae resulted in an increase of C18:1, confirming its ability of desaturating C18:0. The results could provide a new approach and scientific guidance for the improvement of biodiesel quality and industrialization of high-valued chemicals by means of metabolic engineering.

Melatonin: A Multifunctional Molecule That Triggers Defense Responses against High Light and Nitrogen Starvation Stress in Haematococcus pluvialis

Melatonin (MLT), a ubiquitously distributed small molecule, functions in plant responses to various biotic and abiotic stresses. However, the interactions between melatonin and other important molecules in Haematococcus pluvialis response stresses are largely unknown. In the present study, exogenous melatonin improved H. pluvialis resistance to nitrogen starvation and high light. We concluded that exogenous melatonin treatment prevented the reactive oxygen species (ROS) burst and limited cell damage induced by abiotic stress through activation of antioxidant enzymes and antioxidants. Astaxanthin, a major antioxidant in H. pluvialis cells, exhibited a 2.25-fold increase in content after treatment with melatonin. The maximal astaxanthin content was 32.4 mg g^-1. The functional roles of the nitric oxide (NO)-mediated mitogen activated protein kinase (MAPK) signaling pathway and cyclic adenosine monophosphate (cAMP) signaling pathway induced by melatonin were also evaluated. The results clearly indicate that cAMP signaling pathways are positively associated with microalgal astaxanthin biosynthesis. Additionally, the NO-dependent MAPK signaling cascade is activated in response to astaxanthin accumulation induced by melatonin, confirming that MAPK is a target of NO action in physiological processes. This work is the first to use H. pluvialis as in vivo model and documents the influence of melatonin on the physiological response to abiotic stress in this microalgae.