Unusual phenolic compounds contribute to ecophysiological performance in the purple-colored green alga zygogonium ericetorum (zygnematophyceae, streptophyta) from a high-alpine habitat

Abstract

The filamentous green alga Zygogonium ericetorum (Zygnematophyceae, Streptophyta) was collected in a high-alpine rivulet in Tyrol, Austria. Two different morphotypes of this alga were found: a purple morph with a visible purple vacuolar content and a green morph lacking this coloration. These morphotypes were compared with respect to their secondary metabolites, ultrastructure, and ecophysiological properties. Colorimetric tests with aqueous extracts of the purple morph indicated the presence of soluble compounds such as phenolics and hydrolyzable tannins. High-performance liquid chromatography-screening showed that Z. ericetorum contained several large phenolic peaks with absorption maxima at ∼280 nm and sometimes with minor maxima at ∼380 nm. Such compounds are uncommon for freshwater green microalgae, and could contribute to protect the organism against increased UV and visible (VIS) irradiation. The purple Z. ericetorum contained larger amounts (per dry weight) of the putative phenolic substances than the green morph; exposure to irradiation may be a key factor for accumulation of these phenolic compounds. Transmission electron microscopy of the purple morph showed massive vacuolization with homogenous medium electron-dense content in the cell periphery, which possibly contains the secondary compounds. In contrast, the green morph had smaller, electron-translucent vacuoles. The ecophysiological data on photosynthesis and desiccation tolerance indicated that increasing photon fluence densities led to much higher relative electron transport rates (rETR) in the purple than in the green morph. These data suggest that the secondary metabolites in the purple morph are important for light acclimation in high-alpine habitats. However, the green morph recovered better after 4 d of rehydration following desiccation stress.

Keywords: UV acclimation; desiccation; photoprotection; pigment; tannins; ultrastructure.

FIG. 1

Zygogonium ericetorum. Habitat (a), macroscopic appearance (b and d) and photomicrographs (c and e) of; (b–c) purple morph, mostly collected in the radiation-exposed upper layer, (d–e) green morph, collected in lower layers. Scale bars, (b, d) 1 cm, (c, e) 20 μm.

FIG. 2

Transmission electron micrographs of the purple morph of Zygogonium ericetorum. (a) at low magnification, different electron densities of the filaments are visible, with a central cytoplasmic portion surrounded by large vacuoles, (b) detail of the cell center containing the nucleus, the chloroplasts are surrounded by electron-dense particles, and the cell periphery contains large vacuoles, (c) central area of the cell; the peroxisome is located directly adjacent to the nucleus, (d) individual peroxisome, nucleus, and Golgi body, (e) chloroplast is surrounded by compartments with high electron-density, (f) electron-dense compartments might contain crystals that appear electron-translucent. Abbreviations: Chl chloroplast, Cry crystal, CW cell wall, G Golgi body, N nucleus, P peroxisome, V vacuole. Scale bars, (a) 10 μm, (b) 2 μm, (c–f) 1 μm.

FIG. 3

Transmission electron micrographs of the green morph of Zygogonium ericetorum. The chloroplasts contain pyrenoids. (a) overview of central area with nucleus and chloroplasts, (b) chloroplast is surrounded by globular structures with partially electron-dense content (arrows) and vacuoles, (c) compartmented vacuoles in the cell periphery, mostly electron-translucent. Abbreviations: Chl chloroplast, N nucleus, V vacuole. Scale bars, (a) 2 μm, (b, c) 1 μm.

FIG. 4

HPL-Chromatogram (280 nm) of the hydrophilic extracts (20% methanol) of the purple morph (solid line, above) and the green morph (dotted line, below) of Zygogonium ericetorum. The purple morph had significantly higher amounts of phenolic compounds per dry weight. All peaks had a spectral absorption maximum in the UV-B, and some of them also had a smaller maximum in the UV-A. Two compounds (peaks with RT 13.9 and 25.2 min) showed broad absorption, indicating a more complex phenolic constitution.

FIG. 5

Quantification of peaks from the HPLC analysis of the secondary phenolic compounds; peak area · mg dry · weight⁻¹ vs. retention time of major peaks are shown; values represent means ± SD of two HPLC runs.

FIG. 6

Folin-Ciocalteu (FC) assay of total phenols, phenols and tannins; comparisons of the purple and green morphs in mg · g⁻¹ dry weight gallic-acid equivalents (n = 5, P < 0.001).

FIG. 7

The effect of increasing photon fluence densities up to 406 μmol photons · m⁻² · s⁻¹ on the relative electron transport rate (rETR) in the green and purple morphs of Zygogonium ericetorum (n = 5, mean value ± SD).

FIG. 8

Changes in photosystem II efficiency (Fv/Fm: optimum quantum yield) in the green and purple morphs of Zygogonium ericetorum (n = 5, mean value ± SD) during 150 min desiccation, followed by 8 d recovery after rehydration with stock culture medium. Significances of differences among the treatments were calculated by one-way ANOVA (P < 0.001). Different letters represent significant differences among the time points as revealed by Tukey's post hoc test.