Dunaliella tertiolecta (Chlorophyta) Avoids Cell Death Under Ultraviolet Radiation By Triggering Alternative Photoprotective Mechanisms

Abstract

The effect of different ultraviolet radiation (UVR) treatments combining PAR (P), UVA (A) and UVB (B) on the molecular physiology of Dunaliella tertiolecta was studied during 6 days to assess the response to chronic UVR exposure. UVR reduced cell growth but did not cause cell death, as shown by the absence of SYTOX Green labeling and cellular morphology. However, caspase-like enzymatic activities (CLs), (regarded as cell death proteases), were active even though the cells were not dying. Maximal quantum yield of fluorescence (Fv /Fm ) and photosynthetic electron transport rate (ETR) dropped. Decreased nonphotochemical quenching (NPQ) paralleled a drop in xanthophyll cycle de-epoxidation under UVB. Reactive oxygen species (ROS) and D1 protein accumulation were inversely correlated. PAB exhibited elevated ROS production at earlier times. Once ROS decayed, D1 protein recovered two-fold compared with P and PA at later stages. Therefore, PsbA gene was still transcribed, suggesting ROS involvement in D1 recovery by its direct effect on mRNA-translation. We add evidence of an UVB-induced positive effect on the cells when P is present, providing photoprotection and resilience, by means of D1 repair. This allowed cells to survive. The photoprotective mechanisms described here (which are counterintuitive in principle) conform to an important ecophysiological response regarding light stress acclimation.

Non-photochemical quenching, a non-invasive probe for monitoring microalgal grazing: an early indicator of predation by Oxyrrhis marina and Euplotes sp.

“…Further, this NPQ elevation relaxes from day 1 onwards. Similar NPQ relaxation after stress exposure has been reported in a study that describes the multiple photoprotective mechanisms of D. tertiolecta (Ihnken, Kromkamp, & Beardall, 2011;Segovia et al, 2015). The reduction in NPQ levels in the grazing culture under abiotic stress conditions is consistent with the observations from the grazing experiments alone.…”

“…An integrated approach using NPQ assessment along with traditional microscopy and molecular methods would enable the deployment of preventive strategies to avoid biomass loss in mass cultures. Different algal species have different acclimation potential and can respond to stress conditions differently (Segovia et al, 2015). Therefore, the development of NPQ as an indicator of grazing requires systematic testing across different prey and predator species.…”

Type II-Metacaspases are involved in cell stress but not in cell death in the unicellular green alga Dunaliella tertiolecta

“…Survival of the cells under UVR (i.e. same experiment as this work) occurred at the expense of growth [42] and preservation of cell viability was parallel to Type-II-MCs accumulation over the experimental time (Fig. 2A) .…”

“…All light measurements were carried out inside the experimental cylinders once they were wrapped with the UVC cut-off filter. Experimental irradiances (unweighted) were 9.5 Wm -2 UVA and 0.5 Wm -2 UVB (UVR:PAR ratio =0.38) according to [42]. The experimental PAR:UVR ratio was specifically chosen to be able to analyse the contribution of total UVR on the physiological response, without being masked by both the effects of high PAR and UVA and UVB wavelengths separation.…”

“…Strikingly, CD was not detected in the unicellular green alga D. tertiolecta stressed with UVR in repeated experiments of this study. Cells survived chronic UVR exposure by induction of DNA repair mechanisms [41] in which the activation of antioxidant enzymes had a priority role scavenging reactive oxygen species (ROS) [18], alternative photoprotective mechanisms were triggered [42] and repair-genes were actively transcribed [43]. CLs were measured in these studies despite of cells not being dead, therefore suggesting an underlying UVR-managing stress role for CLs.…”