Photosynthetic Pigments in Diatoms

Abstract

Photosynthetic pigments are bioactive compounds of great importance for the food, cosmetic, and pharmaceutical industries. They are not only responsible for capturing solar energy to carry out photosynthesis, but also play a role in photoprotective processes and display antioxidant activity, all of which contribute to effective biomass and oxygen production. Diatoms are organisms of a distinct pigment composition, substantially different from that present in plants. Apart from light-harvesting pigments such as chlorophyll a, chlorophyll c, and fucoxanthin, there is a group of photoprotective carotenoids which includes β-carotene and the xanthophylls, diatoxanthin, diadinoxanthin, violaxanthin, antheraxanthin, and zeaxanthin, which are engaged in the xanthophyll cycle. Additionally, some intermediate products of biosynthetic pathways have been identified in diatoms as well as unusual pigments, e.g., marennine. Marine algae have become widely recognized as a source of unique bioactive compounds for potential industrial, pharmaceutical, and medical applications. In this review, we summarize current knowledge on diatom photosynthetic pigments complemented by some new insights regarding their physico-chemical properties, biological role, and biosynthetic pathways, as well as the regulation of pigment level in the cell, methods of purification, and significance in industries.

Keywords: bioactive compounds; biosynthesis pathway; diatoms; photoprotection; photosynthesis; pigments.

Figure 1

Structural formula of photosynthetic pigments in diatoms including all-trans carotenoids: (A) diadinoxanthin; (B) diatoxanthin; (C) violaxanthin; (D) antheraxanthin; (E) zeaxanthin; (F) β-carotene; (G) fucoxanthin; and chlorophylls: (H) chlorophyll a; (I) chlorophyll c.

Figure 2

Biosynthetic pathway of photosynthetic carotenoids in the diatom Phaeodactylum tricornutum from lycopene to fucoxanthin and diatoxanthin.

Figure 3

Simplified model of diatom thylakoid membrane showing the localization of photosynthetic pigments within FCP, PS I, and those localized within an monogalactosyldiacylglycerol (MGDG) shield surrounding the FCP. See the text for more information. Based on Gundermann and Büchel [60].

Figure 4

Spectra composition of light depending on the depth.

Figure 5

The diadinoxanthin cycle: in high light, diadinoxanthin with one epoxy group is converted to epoxy-free diatoxanthin by violaxanthin de-epoxidase (VDE); the reverse reaction is observed in low light and dark and is catalyzed by zeaxanthin epoxidase (ZEP).

Figure 6

The violaxanthin cycle: under high light, violaxanthin (which is normally a precursor of fucoxanthin) is converted to zeaxanthin via the intermediate antheraxanthin and this reaction is catalyzed by violaxanthin de-epoxidase (VDE), whereas in low light and dark, two single steps of oxygenation catalyzed by zeaxanthin epoxidase (ZEP) lead to violaxanthin formation.

Figure 7

Examples of HPLC chromatograms of absorbance recorded at 430, 440, and 480 nm obtained with the method by Kraay and co-workers [117] for pigment extracts from the diatom P. tricornutum: (A) 1 h dark incubated cells growing in LL; (B) 1 h HL illuminated cells growing in LL; (C) 1 h HL illuminated cells growing in ML; (D) 48 h HL illuminated cells growing in LL. LL: white light with the intensity of 100 μmol m⁻²·s⁻¹ in a 16 h light/8 h dark photoperiod; ML: white light with the intensity of 700 μmol m⁻²·s⁻¹ in a 6 h light/18 h dark photoperiod; HL: white light with the intensity of 1250 μmol m⁻²·s⁻¹.

Figure 8

Absorption spectra of photosynthetic pigments recorded during HPLC-DAD analysis performed on extracts from the diatom P. tricornutum with the method by Kraay and co-workers [118]. The spectra were normalized at λ_max.