Response of the Diatom Phaeodactylum tricornutum to Photooxidative Stress Resulting from High Light Exposure

Nuno Domingues,Paulo Cartaxana,Ana Rita Matos,Jorge Marques Da Silva,Senjie Lin

doi:10.1371/journal.pone.0038162

Abstract

The response of microalgae to photooxidative stress resulting from high light exposure is a well-studied phenomenon. However, direct analyses of photosystem II (PSII) D1 protein (the main target of photoinhibition) in diatoms are scarce. In this study, the response of the diatom model species Phaeodactylum tricornutum to short-term exposure to high light was examined and the levels of D1 protein determined immunochemically. Low light (LL) acclimated cells (40 µmol photons m−2 s−1) subjected to high light (HL, 1,250 µmol photons m−2 s−1) showed rapid induction of non-photochemical quenching (NPQ) and ca. 20-fold increase in diatoxanthin (DT) concentration. This resulted from the conversion of diadinoxanthin (DD) to DT through the activation of the DD-cycle. D1 protein levels under LL decreased about 30% after 1 h of the addition of lincomycin (LINC), a chloroplast protein synthesis inhibitor, showing significant D1 degradation and repair under low irradiance. Exposure to HL lead to a 3.2-fold increase in D1 degradation rate, whereas average D1 repair rate was 1.3-x higher under HL than LL, leading to decreased levels of D1 protein under HL. There were significant effects of both HL and LINC on P. tricornutum maximum quantum yield of PSII (F v/F m), showing a reduction of active PSII reaction centres. Partial recovery of F v/F m in the dark demonstrates the photosynthetic resilience of this diatom to changes in the light regime. P. tricornutum showed high allocation of total protein to D1 and an active D1-repair cycle to limit photoinhibition.

Highlights

Diatoms (Heterokontophyta, Bacillariophyceae) are a major group of microalgae, ubiquitous in marine and freshwater ecosystems, contributing to approximately 20% of the global primary photosynthetic production [1]
Exposure of P. tricornutum to high light (HL) resulted in a decrease in D1 content when compared to cells subjected to Low light (LL), if chloroplast protein synthesis was inhibited by the addition of LINC (Figs. 1 and 2)
Significant effects of HL were observed in both D1 protein degradation and re-synthesis in P. tricornutum: degradation of D1 was 3.2-x higher under HL, while average resynthesis was 1.3-x higher than in LL

Summary

Introduction

Diatoms (Heterokontophyta, Bacillariophyceae) are a major group of microalgae, ubiquitous in marine and freshwater ecosystems, contributing to approximately 20% of the global primary photosynthetic production [1]. One of the main physiological processes involved is the thermal dissipation of harmful excess energy through the xanthophyll cycle [4]. In diatoms, this cycle involves the deepoxidation of the pigment diadinoxanthin (DD) to diatoxanthin (DT) under HL, triggered by acidification of the thylakoid lumen [5,6]. Bailleul et al [8] revealed that the levels of LHCX1, an atypical member of the light-harvesting complex stress-related protein family, are directly related to the ability of P. tricornutum to quench excess energy

Methods

Results

Conclusion