Marcelo P. Hernando

Photoacclimation to long-term ultraviolet radiation exposure of natural sub-antarctic phytoplankton communities : Fixed surface incubations versus mixed mesocosms

Abstract Solar UVB radiation (280–320 nm) is known to have detrimental effects on marine phytoplankton. Associated with the seasonal ozone hole in Antarctica, stratospheric ozone depletion occasionally influences the sub-Antarctic (Beagle Channel, Argentina) region, enhancing levels of UVB. The primary objective of this work was to study the effects of several (i.e. 6–10) days of exposure to UVB on the taxonomic composition and photosynthetic inhibition of local phytoplankton communities. For different light treatments, fixed-depth incubations placed in an outdoors water tank were compared with incubations in 1900 L mesocosms, where vertical mixing was present. Phytoplankton growth was inhibited by UV radiation (UVR) in fixed-depth experiments but not in the mixed mesocosms. Under fixed and mixed conditions alike, photosynthesis was significantly inhibited by UVB at the beginning of the experiment but no longer after several days of exposure, suggesting that cells had acclimated to radiation conditions. There was a change in species composition in response to UVR exposure in both experiments, which likely explained acclimation. In the community exposed to fixed conditions this change was from a phytoflagellate-dominated assemblage to a community with high relative abundance of diatoms after 6 days of exposure. UVA was responsible for most of the observed growth inhibition; however, the reduction in photosynthesis was produced by UVB. The reasons behind this variability in responses to UVR are associated with species-specific sensitivity and acclimation, and the previous light history of cells. In the community exposed in mesocosms, an assemblage codominated by phytoflagellates and diatoms was observed at the beginning of the experiments. After 10 days of exposure, green algae (Eutreptiella sp.) had increased, and phytoflagellates were the dominant group. The synthesis of mycosporine-like amino acids (MAAs), antioxidant enzymes and photosynthetic antenna pigments, in relation to repair and protection processes, may explain the reduced inhibition of both growth and photosynthesis that was observed in the phytoplankton community after several days of exposure. For environments such as the Beagle Channel seasonally exposed to the ozone hole, the results obtained from the fixed-depth experiments show that species can cope with UVR by means of MAA synthesis, while mixing would primarily promote a change in species composition and defense strategies.

Growth, toxin production, active oxygen species and catalase activity of Microcystis aeruginosa (Cyanophyceae) exposed to temperature stress.

Microcystis are known for their potential ability to synthesize toxins, mainly microcystins (MCs). In order to evaluate the effects of temperature on chlorophyll a (Chl a), growth, physiological responses and toxin production of a native Microcystis aeruginosa, we exposed the cells to low (23°C) and high (29°C) temperature in addition to a 26°C control treatment. Exponential growth rate was significantly higher at 29°C compared to 23°C and control, reaching 0.43, 0.32 and 0.33day(-)(1) respectively. In addition, there was a delay of the start of exponential growth at 23°C. However, the intracellular concentration of Chl a decreased significantly due to temperature change. A significant increase in intracellular ROS was observed in coincidence with the activation of enzymatic antioxidant catalase (CAT) during the first two days of exposure to 23° and 29°C in comparison to the control experiment, decreasing thereafter to nearly initial values. Five MCs were determined by LC-MS/MS analysis. In the experiments, the highest MC concentration, 205fg [Leu(1)] MC-LR.cell(-1) expressed as MC-LR equivalent was measured in the beginning of the experiment and subsequently declined to 160fg.cell(-1) on day 2 and 70fg.cell(-1) on day 4 in cells exposed to 29°C. The same trend was observed for all other MCs except for the least abundant MC-LR which showed a continuous increase during exposure time. Our results suggest a high ability of M. aeruginosa to perceive ROS and to rapidly initiate antioxidant defenses with a differential response on MC production.

Non-enzymatic antioxidant photoprotection against potential UVBR-induced damage in an Antarctic diatom (Thalassiosira sp.)

In January 1999, unialgal cultures of the diatom Thalassiosira sp., solate from natural phyto- plankton assemblages from Potter Cove, Antarctica, were exposed to solar ultraviolet radiation (UVR, 280- 400 nm) in order to study the long-term acclimation of this species. Ultraviolet radiation B (UVBR, 280-315 nm) inhibited the growth rate during the first and second days of exposure. No UVBR inhibition was observed on the third day. The initial content of α-tocopherol (13 pmol (10 4 cell) -1 ) showed a marked decrease during the exponential growth phase (4 pmol (10 4 cell -1 ) by day 3). The initial content of β-carotene (3 pmol (10 4 cell) -1 ) did not show significant differences over time in cells exposed to UVBR. Two mycosporine-like amino acids (MAAs) were identified: porphyra-334 and shinorine. Cellular concentrations of MAAs increased significantly on days 2 and 3, and exposure of the algae to UVBR significantly enhanced this value. The relative importance of MAAs concentration was significant (P < 0.05) in relation to the α-tocopherol content. A positive correlation was shown between cellular MAAs concentration and growth rate. Our results suggest that photoprotection against UV-induced damage is characterized by short-term consumption of α-tocopherol and longer-term synthesis of MAAs. The UVBR damage/repair ratio during long-term exposure involves the combined action of several endogenous factors within the cell, with MAAs synthesis being the most effective factor related to photoprotection.

Responses of Subantarctic Marine Phytoplankton to Ozone Decrease and Increased Temperature

Temperature and ultraviolet B radiation (UVB, 280–315 nm) are external stressors that affect organisms in mid and high latitudes in a combined way. The combined effects of both variables on natural marine phytoplankton from the Beagle Channel (Argentina) were examined during a 7-day mesocosm experiment. We tested the hypothesis that increased temperature (HT, +3 °C) will offset negative effects on phytoplankton by UVB (natural, NUVB, and high, HUVB, simulating a 60% decrease in stratospheric ozone layer thickness). The response of the entire phytoplankton assemblage, in terms of phytoplankton biomass, community composition, reactive oxygen species (ROS), lipid damage (TBARS), nonenzymatic antioxidants (α-tocopherol (αT) and β-carotene (βC)), and mycosporine-like amino acids (MAAs), was evaluated. On the first exposure day, assemblages exposed to HUVB showed a significant increase in ROS content, regardless of the temperature, while lipid damage was significantly higher at HT and HUVB. However, on day 2, lipid damage was significantly lower possibly due to the consumption of the nonenzymatic antioxidants that protected the membranes from further damage. Under normal temperature (NT) conditions, ROS concentrations were significantly lower compared with day 1, and nonenzymatic antioxidant concentrations remained high (0.025 nmol C−1 compared with 0.05 nmol C−1 at initial time). ROS increased again in HT-HUVB and in control (NT-NUVB), in coincidence with a significant increase in UVB radiation on day 4. However, the lipid damage was significantly lower in HT-HUVB than in control conditions possibly due to a higher consumption of nonenzymatic antioxidants and probably also to a higher activity of enzymatic antioxidants by the effect of the higher temperature. The same results were observed for HT-NUVB, with low lipid damage. During all experiment no significant differences were observed in carbon-normalized MAAs. After day 4, when nutrients became limiting, high temperature significantly influenced community structure, with a negative impact on diatoms and positive on phytoflagellates, independently of the UVB doses. Our results show that subantarctic phytoplankton is able to respond to a ROS increase via antioxidant response in high irradiance conditions. In addition, increased temperature and phytoplankton community composition play a central role in this response. At lower UVB doses, diatoms were able to avoid UVB lipid damage by αT and βC synthesis. However, with maximum doses, phytoflagellates showed a best UVB adaptation to high temperature conditions.

Differential Effect of Acute Iron Overload on Oxidative Status and Antioxidant Content in Regions of Rat Brain

The hypothesis of this study is that the cerebral cortex, hippocampus, and striatum of the rat brain are differentially affected in terms of oxidative stress and antioxidant capacity by acute Fe overload because Fe is distributed in a heterogeneous fashion among different regions and cells of the brain. The effects on the lipophilic and hydrophilic cellular environment were compared between regions and with the whole brain. A single dose of Fe-dextran increased Fe deposits, reaching a maximum after 6 hr. Both in whole brain and in cortex region, the ascorbyl/ascorbate content ratio was increased after 6 hr of Fe administration, while in striatum and hippocampus, there was no significant changes after Fe overload. Total thiol content decreased in whole brain and cortex, while there were no significant changes in striatum and hippocampus after Fe overload. The content of α-tocopherol (α-T), whether measured in the whole brain or in the isolated regions, did not change following Fe treatment. Lipid radical (LR•) generation rate after Fe-dextran overload only increased in the cortex region. The LR•/α-T content ratio was increased by Fe treatment in cortex but not in the whole brain, striatum, or hippocampus, in agreement with the study tested hypothesis.

Degradación fotoquímica del carbono orgánico disuelto: Producción de peróxido de hidrógeno y efectos potenciales sobre el plancton en el Canal Beagle (Tierra del Fuego)

Se analizo la produccion de peroxido de hidrogeno (H2O2) en funcion de la dosis de radiacion solar en aguas del Canal Beagle, en la desembocadura del Rio Lapataia, Ushuaia (Tierra del Fuego, Argentina). El estudio se realizo entre Octubre y Noviembre de 2004. Durante dicho periodo se produce anualmente una disminucion en la concentracion del ozono estratosferico, lo que ocasiona un aumento en las dosis de radiacion ultravioleta B (RUVB, 280-320 nm) incidente en la superficie de la tierra, en relacion al resto de las longitudes de onda del espectro solar (radiacion ultravioleta A [RUVA, 320-400 nm] y radiacion visible [RVis, 400-700 nm]). Se observaron diferencias significativas en la foto-oxidacion del carbono organico disuelto (COD) cuando se comparo la exposicion a toda la radiacion solar contra la exposicion a longitudes de onda mayores a 295 nm y a 320 nm. Este tipo de respuesta correspondio a la exposicion a dosis mayores de 21 kJ m-2 de RUVB. La foto-produccion en aguas del Canal Beagle tiene niveles bajos (concentracion maxima de H2O2 promedio observada luego de la exposicion a la radiacion solar de 40 nM) probablemente debido a las bajas concentraciones de COD determinadas, a su baja reactividad o ambas razones. Esto explica la ausencia de potencial dano oxidativo en el plancton de la zona expuesto a las concentraciones maximas de H2O2 observadas.