Culture Conditions Affect Antioxidant Production, Metabolism and Related Biomarkers of the Microalgae Phaeodactylum tricornutum

Abstract

Phaeodactylum tricornutum (Bacillariophyta) is a worldwide-distributed diatom with the ability to adapt and survive in different environmental habitats and nutrient-limited conditions. In this research, we investigated the growth performance, the total lipids productivity, the major categories of fatty acids, and the antioxidant content in P. tricornutum subjected for 15 days to nitrogen deprivation (N-) compared to standard culture conditions (N+). Furthermore, genes and pathways related to lipid biosynthesis (i.e., glucose-6-phosphate dehydrogenase, acetyl-coenzyme A carboxylase, citrate synthase, and isocitrate dehydrogenase) and photosynthetic activity (i.e., ribulose-1,5-bisphospate carboxylase/oxygenase and fucoxanthin-chlorophyll a/c binding protein B) were investigated through molecular approaches. P. tricornutum grown under starvation condition (N-) increased lipids production (42.5 ± 0.19 g/100 g) and decreased secondary metabolites productivity (phenolic content: 3.071 ± 0.17 mg GAE g^-1; carotenoids: 0.35 ± 0.01 mg g^-1) when compared to standard culture conditions (N+). Moreover, N deprivation led to an increase in the expression of genes involved in fatty acid biosynthesis and a decrease in genes related to photosynthesis. These results could be used as indicators of nitrogen limitation for environmental or industrial monitoring of P. tricornutum.

Keywords: Phaeodactylum tricornutum; antioxidant activity; gene expression; lipid biosynthesis; nitrogen stress; photosynthesis.

Figure 1

Representative diagram of the enzymes (in red) related to lipid biosynthesis and photosynthesis pathways in Phaeodactylum tricornutum. Cytosol: glucose-6-phosphate dehydrogenase (G6PDH); chloroplast: acetyl-coenzyme A carboxylase (ACCase); citrate synthase (Cit syn); isocitrate dehydrogenase (isocit DH); mitochondria: ribulose-1,5-bisphospate carboxylase/oxygenase (RBCL); fucoxanthin-chlorophyll a/c binding protein B (FCP B).

Figure 2

Growth performance of P. tricornutum cultivated for 18 days under standard (N+, blue line) and starvation condition (N−, green line). Values are presented as the mean ± SEM (n = 3) with standard deviations. Different letters within the same day indicate significant differences (p < 0.05) within the two treatment conditions (i.e., N+ and N−).

Figure 3

Total lipids (g/100 g) of P. tricornutum cultivated for 15 days under initial culture conditions (T0, white bar), under standard conditions (N+, blue bar), and under starvation conditions (N−, green bar). Values are expressed as the mean ± SEM (n = 3) with standard deviations. Different letters indicate statistical differences (p < 0.05) between groups.

Figure 4

Fatty acids class composition (%, w/w) of P. tricornutum cultivated for 15 days under initial culture condition (T0, white bar), under standard conditions (N+, blue bar), and under starvation conditions (N−, green bar). Values are expressed as the mean ± SEM (n = 3) with standard deviations. Different letters indicate statistical differences (p < 0.05) between groups.

Figure 5

Total polyphenol content (mg GAE/g DW) in P. tricornutum extracts cultivated for 15 days under initial culture conditions (T0, white bar), under standard conditions (N+, blue bar), and under starvation conditions (N−, green bar). Values are reported as the means (n = 3), and error bars report the standard deviations. Treatments that do not share the same letter were significantly different from each other (p < 0.05).

Figure 6

Carotenoid content (mg/g DW) in P. tricornutum extracts cultivated for 15 days under initial culture conditions (T0, white bar), under standard conditions (N+, blue bar), and under starvation conditions (N−, green bar). Values are reported as the means (n = 3), and error bars report the standard deviations. Treatments that do not share the same letter were significantly different from each other (p < 0.05).

Figure 7

DPPH radical scavenging activity (IC 50, mg DW/mL) in P. tricornutum extracts cultivated for 15 days under initial culture conditions (T0, white bar), under standard conditions (N+, blue bar), and under starvation conditions (N−, green bar). Values are reported as the means (n = 3), and error bars report the standard deviations. Treatments that do not share the same letter were significantly different from each other (p < 0.05).

Figure 8

Reducing power (EC50, mg DW/mL) in P. tricornutum extracts cultivated for 15 days under initial culture conditions (T0, white bar), under standard conditions (N+, blue bar), and under starvation conditions (N−, green bar). Values are reported as the means (n = 3), and error bars report the standard deviations. Treatments that do not share the same letter were significantly different from each other (p < 0.05).

Figure 9

Relative gene expression of genes related to lipid biosynthesis (glucose-6-phosphate dehydrogenase (G6PDH), acetyl-coenzyme A carboxylase (ACCase), citrate synthase (Cit syn), and isocitrate dehydrogenase (isocit DH)) and photosynthetic activity (ribulose-1,5-bisphospate carboxylase/oxygenase (large subunit) (rbcL) and fucoxanthin-chlorophyll a/c binding protein B (FCP B)) in P. tricornutum under initial culture conditions (T0, white bars) and after 15 days of cultivation under standard conditions (N+, blue bars) and starvation conditions (N−, green bars). Values are the mean ± SEM (n = 3) with standard deviations. Statistical differences (p < 0.05) between groups are indicated by different letters.