Assessment of oxidative stress in the planktonic diatom Thalassiosira pseudonana in response to UVA and UVB radiation

Induction of oxidative stress by UVA and UVB in the diatom Thalassiosira pseudonana was experimentally studied. Cells, pre-grown in a light-limited continuous culture, were incubated for 4 h at 175 µmol m-2s-1photosynthetically active radiation, with optional supplementary UVA at an unweighted dose rate of 0.70 W m-2, or 2.79 W m-2UVA plus 0.45 W m-2UVB (unweighted). A fluorescence-based measure of photosynthetic efficiency (Fv/Fm) decreased from 0.69 to 0.58 in the presence of UVB, whereas UVA caused a minor decrease of Fv/Fm. Quantitative analysis of confocal images showed a minor increase of active oxygen production associated with supplemental UVA alone, and a 100% increase with additional UVB. Cellular malondialdehyde, an indicator of lipid peroxidation by active oxygen, almost doubled under UVA and increased three-fold with additional UVB. Activities of superoxide dismutase (scavenging active oxygen) and glutathione reductase (reducing GSSG to GSH) increased in response to UVB exposure, whereas ascorbate peroxidase activities did not. UVB caused a minor decrease in the glutathione ratio GSH : (GSH + 0.5GSSG), which indicates a moderate oxidative stress.

Effects of UVB radiation and salt stress on growth, pigments and antioxidative defence of the marine diatom Cylindrotheca closterium

During emersion, diatoms are exposed to sunlight (with UVB) on tidal flats, where the salinity may increase as a result of evaporation. In order to investigate whether a combination of UV radiation and desiccation would cause oxidative stress under such conditions, an experiment has been carried out with the diatom Cylindrotheca closterium. Cell division rates, photosynthetic efficiencies, pigment contents, and activities of antioxidant enzymes (SOD, APX, MDHAR, GR) were analysed in exponential-phase batch cultures grown at 35 (normal salinity) and 70 PSU (hyperosmotic) in a 12 h:12 h light:dark cycle (268 µmol photons m-2 s-1 photosynthetically active radiation; PAR) at 15°C. Within the 12 h photoperiod, UVA (2.80 W m-2 unweighted) or UVA+UVB (4.23 and 0.22 W m-2, respectively) were supplemented during 4 h. Separately as well as in combination, UVB (3.45 kJ m-2 d-1) and salt stress (70 PSU) caused a decrease in division rates. This UVB dose caused a decrease in photosynthetic efficiency, whereas salt stress did not. Cell volumes of UVB-exposed C. closterium increased only at 70 PSU. UVB radiation and salt stress caused significant decreases in chl a, chl c and fucoxanthin contents, but the UVB effect was stronger than the salt effect. The relatively high b-carotene:chl a ratios in UVB-exposed and salt-stressed cells might indicate that b-carotene was used in scavenging singlet oxygen. In high light with UVA (at 35 PSU), SOD, APX, MDHAR and GR activities were not higher than in low light (27 µmol photons m-2 s-1). Separately and in combination, UVB and salt stress enhanced SOD activity in C. closterium, whereas APX was stimulated by UVB only. MDHAR was stimulated under UVB and salt stress, but there were no effects on GR activity. Under ambient salt and UV conditions during emersion, oxidative stress may contribute to an inhibition of growth of C. closterium [KEYWORDS: Antioxidants; Cylindrotheca closterium; Diatom; Pigments; Salt stress; Ultraviolet]

Effects of periodic, low UVA radiation on cell characteristics and oxidative stress in the marine planktonic diatom Ditylum brightwellii

A continuous culture experiment was conducted with the marine diatom Ditylum brighiwellii to assess the photo-oxidative effects of transition from dim light to high Light at three successive levels. The focus was on temporary stress by ambient WA at the water surface, via a simulation of the periodic ascent of diatoms caused by vertical mixing. Within a 14 h dim- light period, cells were exposed daily to 4 h of: (A) moderate PAR (100 mu mol photon m(-2) s(-1)), (B) high PAR (400 mu mol photon m(-2) s(-1)) and (C) high PAR (400-700 nm) plus 67 pmol photon m(-2) s(-1) (2.2 W m(-2)) UVA. Cell shape, life cycle and sinking rates were not affected by high PAR per se (mode B). The daily 4 h high-irradiance mode C, with additional UVA (WA:PAR = 0.016), caused an increase in cell diameter, a decrease in length axes, and a production of vegetative resting stages. Superoxide dismutase (SOD) activities, oxidized glutathione (GSSG) pools and malondialdehyde (MDA) contents increased as well, which indicates that this biologically effective WA dose (0.78 W m(-2)) promoted active oxygen production, oxidative stress and lipid peroxidation. Sinking rates in WA-exposed D. brightwellii were higher than in those grown without UVA. A close correlation between sinking rates and MDA contents suggests that UVA-induced lipid peroxidation has altered the membrane functions which regulate buoyancy in D. brightwellii. Although UVA-induced formation of resting cells will contribute to accelerated sinking, it is not evident that this can be considered as a stress avoidance mechanism under adverse light (WA) conditions. D. brightwellii proved to be sensitive to transitions from shade to natural surface irradiance conditions. In coastal waters, such large diatom species may experience oxidative stress after a few hours' residence near the water surface at ambient UVA levels. [KEYWORDS: cell size; Ditylum brightwellii; glutathione redox;lipid peroxidation; oxidative stress; planktonic diatom; resting stage; sinking rate; superoxide; ultraviolet-A Ultraviolet-b radiation; lipid-peroxidation; chlorella-vulgaris; photosynthetic pigment; superoxide-dismutase; thylakoid membranes; antioxidant systems; maize seedlings; sinking rates; phytoplankton]

UV- and salinity-induced oxidative effects in the marine diatom Cylindrotheca closterium during simulated emersion

The diatom Cylindrotheca closterium was exposed to transient light- and osmotic conditions as occur during its tidal emersion. The objective was to analyze how this simulated emersion contributes to the production of active oxygen species (AOS) and via this, to oxidative cell damage. Light- and salinity conditions were varied in factorial combination: low light (no UVB) or high light (unweighted UVB-dose rates of respectively 0.01; 0.07; 0.24; 1.03 W m-2) at normal (30 psu) or high salinity (60 psu). UVB (0.01–0.24 W m-2) and high salinity had a significant, negative effect on the photosynthetic efficiencies ?F/F m’ (steady-state quantum yield) and F v/F m (maximum yield). UVB at 1.03 W m-2 (15 kJ m-2 d-1) almost arrested electron transport. At ecologically relevant UVB levels, i.e. below 0.24 W m-2 (˜3.4 kJ m-2 d-1) with UVB:PAR0.07 W m-2 and at 60 psu indicated a reversal of the diatom xanthophyll cycle (diminished photoprotection) which may be caused by an enhanced AOS production. Oxidative stress and -damage to C. closterium cells were assessed applying fluorescent indicator dyes, via confocal microscopy and quantitative image analysis. AOS production rates (cellular DCF fluorescence) were stimulated by UV, and were ~50% higher at 60 psu. AOS production decreased with an increasing pre-exposure (0–4 h) to normal UVB (0.24 W m-2), which indicated a stimulation of the antioxidative defence. Non-protein thiols (indicator CMF) and glutathione pools (HPLC-analyzed) decreased with UVB-dose rates (0.01–0.24 W m-2), most likely due to AOS-mediated thiol oxidation. Hypersalinity (60 psu) and UVB (0.01–0.24 W m-2) caused membrane depolarization (dye DIBAC4(3)) and phospholipid hydrolysis (phospholipase A2 dye: bis-BODIPY FL-C11-PC). AOS production may have diminished the membrane polarity, and peroxidized the membrane lipids (HPLC-analyzed malondialdehyde) which enhanced PLA2 activity. The dyes indicated an increased oxidative (lipid) damage at a 15% inhibition of photosynthesis in this diatom, at UVB levels and salinities that can be expected in situ during its periodic tidal emersion