Temperature Dependence and the Effects of Ultraviolet Radiation on the Ultrastructure and Photosynthetic Activity of Carpospores in Sub-Antarctic Red Alga Iridaea cordata (Turner) Bory 1826

Abstract

The short-term effects of UV radiation and low temperature on ultrastructure, photosynthetic activity (measured as the maximal photochemical quantum yield of photosystem II: F_v/F_m), chlorophyll-a (Chl-a) contents, and UV-absorbing compounds on the carpospores of Iridaea cordata from a sub-Antarctic population were investigated. Exposure to both photosynthetically active radiation (PAR) and PAR + UV for 4 h caused ultrastructural modifications in all treatments. Under PAR + UV at 2 °C, a disruption of the chloroplast's internal organization was observed. Plastoglobuli were often found in carpospores exposed to 2 °C. 'Electron dense particles', resembling physodes of brown algae, were detected for the first time in cells exposed to PAR and PAR + UV at 8 °C. F_v/F_m decreased following 4 h exposure at 2 °C under PAR + UV (64%) and PAR (25%). At 8 °C, F_v/F_m declined by 21% only under PAR + UV. The photosynthesis of carpospores previously treated with UV partially recovered after a 4 h exposure under dim light. UV-absorbing compounds were degraded in all radiation and temperature treatments without recovery after a 4 h dim light period. Chl-a did not change, whereas total carotenoids increased under PAR at 8 °C The study indicates that although carpospores of I. cordata exhibit photoprotective mechanisms, UV radiation strongly damages their ultrastructure and physiology, which were exacerbated under low temperatures.

Keywords: photochemistry; propagules; red algae; stress tolerance; sub-Antarctic region; ultrastructure.

Figure 1

Structure of Iridaea cordata carpospores. (A) Carpospores under light microscopy and their respective ultrastructural models. (B–E) Transmission electron microscopy (TEM) of carpospores cultivated under control conditions. (B) Carpospores exhibit homogeneously distributed vacuolar spaces, starch grains in the cytoplasm, and a condensed nucleolus. (C,D) Thick cell walls (black arrowheads) and cored vesicles releasing their contents out of the plasmalemma; additionally, tubular invaginations (white arrowheads in (D) and ER are shown close to the plasmalemma. (E) Typical internal organization of red algae chloroplasts showing a single peripheral thylakoid. CV, cored vesicles; ER, endoplasmic reticulum; EP, electron-dense particles; M, mitochondria; N, nucleus; Nu, nucleolus.

Figure 2

Changes in ultrastructural organization in the chloroplast of Iridaea cordata carpospores after exposure for 4 h to PAR (A,B) and PAR + UV (C,D) treatments at two temperatures. White arrowheads in (B–D) indicate plastoglobuli.

Figure 3

Summary of major ultrastructural changes in carpospores of Iridaea cordata after 4 h of exposure to PAR and PAR + UV at two temperatures. (A,E) carpospores exposed to PAR at 2 °C; (B) carpospores exposed to PAR + UV at 2 °C; (C,D) carpospores exposed to PAR at 8 °C; (F) carpospores exposed to PAR + UV at 8 °C. CV, cored vesicles; ER, endoplasmic reticulum; EP, electron-dense particles; G, Golgi complex; M, mitochondria; N, nucleus; Nu, nucleolus. White arrowheads in C indicate nuclear membrane pores, while in E plastoglobuli.

Figure 4

Maximum photochemical efficiency of photosystem II (F_v/F_m) of Iridaea cordata carpospores after exposure (A,B) for 4 h to PAR and PAR + UV at two temperatures and recovery (C,D) under low white light (4 μmol photon m⁻² s⁻¹). Control was continuously maintained at 4 μmol photon m⁻²s⁻¹ at 8 °C (mean ± SD, n = 6). The percentage decrease in F_v/F_m (A,B) and recovery (C,D) with respect to the control is presented within the bars. Different letters indicate significant differences (p < 0.05, HSD post hoc test).

Figure 5

Chlorophyll a content in µg Chl-a g⁻¹ DW, the ratio of carotenoids (A_480nm) to Chl-a (A_665nm) in Iridaea cordata carpospores exposed for 4 h to PAR and PAR + UV treatments at 2 and 8 °C, and subsequent 4 h recovery in dim light. Control was continuously maintained at 4 μmol photon m⁻² s⁻¹ and at 8 °C. Values are means ± S.E. (n = 4). F-values and ANOVA significance are indicated. Different letters indicate significant differences (p < 0.05, HSD post hoc test).

Figure 6

Spectra of methanol extract of the Iridaea cordata carpospores. (A) Spectra of initial samples. (B,C) Spectra of methanol extracts (against control) of samples following 4 h exposure to UV radiation under two temperature treatments and subsequent 4 h recovery in dim light. Control was kept constant at 4 μmol photon m⁻² s⁻¹ and 8 °C. Each spectrum represents the average of four measurements.