Responses of Subantarctic Marine Phytoplankton to Ozone Decrease and Increased Temperature

Abstract

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.