The photo-protective role of vitamin D in the microalga Emiliania huxleyi

Abstract

An essential interaction between sunlight and eukaryotes involves vitamin D production through exposure to ultraviolet (UV) radiation. While extensively studied in vertebrates, the role of vitamin D in non-animal eukaryotes like microalgae remains unclear. Here, we investigate the potential involvement of vitamin D in the UV-triggered response of Emiliania huxleyi, a microalga inhabiting shallow ocean depths that are exposed to UV. Our results show that E. huxleyi produces vitamin D₂ and D₃ in response to UV. We further demonstrate that E. huxleyi responds to external administration of vitamin D at the transcriptional level, regulating protective mechanisms that are also responsive to UV. Our data reveal that vitamin D addition enhances algal photosynthetic performance while reducing harmful reactive oxygen species buildup. This study contributes to understanding the function of vitamin D in E. huxleyi and its role in non-animal eukaryotes, as well as its potential importance in marine ecosystems.

Keywords: biological sciences; evolutionary biology; radiation biology.

Graphical abstract

Figure 1

Vitamin D treatment leads to similar cellular traits as exposure to UV (A) Cell area, (B) chlorophyll area, (C) average number of chloroplasts, and (D) cellular chlorophyll a content at day 10 of growth. Cultures were either treated with both D₂ + D₃ (0.5 μM of each) or DMSO (untreated) as control. Statistical significance of treated cultures compared to control conditions was calculated based on three biological replicates for (A–C) and six biological replicates for (D) using two-tailed t test assuming equal variances. One, two or three asterisks indicate p < 0.05, p < 0.01, and p < 0.001, respectively. Boxplots show the average and the difference between maximum and minimum value.

Figure 2

Combined treatment of vitamin D₂ and D₃ upregulates UV-responsive genes RT-qPCR analysis of genes following 1 h of UV exposure or vitamin D treatments. Top title denotes gene products. In brackets: gene identifier in E. huxleyi CCMP3266 and matching gene transcript in the E. huxleyi CCMP1516 reference genome. Upregulation of G18590 by vitamin D was observed for only two out of three replicates. Statistical significance of treated cultures compared to control conditions was calculated using two-tailed t test assuming equal variances. One, two or three asterisks indicate p < 0.05, p < 0.01, and p < 0.001, respectively.

Figure 3

Vitamin D treatment improves the algal photosynthetic performance and alleviates oxidative stress following exposure to excess light (A) Non photochemical quenching (NPQ), (B) F_v/F_m values, and (C) Φ_PSII values of vitamin D-treated and control algal cultures. Cultures were either pre-exposed to saturating light of 1,000 μmol photons m⁻² s⁻¹ followed by dark incubation and a second exposure during the PAM analysis (“pre-exposure”), or not pre-exposed (“single exposure”). (D) Fluorescence values of vitamin D-treated and control algal cultures stained with the intracellular reactive oxygen species (ROS) probe H₂DCF-DA. Cultures were exposed to regular light or excess light (130 or 1,000 μmol photons m⁻² s⁻¹, respectively) intensities for 3 h. Algae were cultivated in 50 mL and sampled for analysis at day 10 of growth. Statistically significant values (p < 0.05) of vitamin D-treated cultures compared to control conditions are marked by ∗, calculated based on three biological replicates using two-tailed paired t test for (A–C) and one-tailed paired t test for (D). Boxplots show the average and the difference between maximum and minimum value.