Effects of climate-induced changes in temperature and salinity on phytoplankton physiology and stress responses in coastal Antarctica

Coastal phytoplankton assemblages from Potter Cove in Antarctica were exposed to low salinity (S-) and high temperature (T+) conditions to simulate oceanic changes resulting from global warming. The treatments were: low salinity (30) and high temperature (S-T+); low salinity and ambient temperature (1–2 °C) (S-T0); ambient salinity (34) and increased temperature (4–5 °C) (S0T+) and ambient salinity with ambient temperature (control, S0T0). Experiments were conducted in 100-L microcosms and monitored for 6 days. Compared to the control treatment, micro-size diatoms (25–50 μm) dominated the phytoplankton assemblages while prasinophyceae were less abundant at the end of the S-T+ and S0T+ treatments. Nano-size diatoms (10–20 μm) also increased significantly at the end of the experiment but only when exposed to S0T+. In S- treatments, the production of reactive oxygen/ nitrogen species (ROS/RNS) increased while phytoplankton biomass decreased. Under T+ conditions, the production of ROS/RNS was significantly lower than in T0 treatments. Throughout the experiment, α-Tocopherol (α-T) consumption may have prevented lipid damage, allowing for increases in photosynthetic rate and growth when nutrients concentrations were sufficiently high. Our results indicate that an increase in temperature can compensate for the lipid damage produced by low salinity, and stimulate carbon uptake in both conditions. This study demonstrated that the final composition of phytoplankton assemblages in all experimental treatments was strongly influenced by the original composition. Future changes in natural phytoplankton assemblages in Antarctic coastal waters will therefore depend on the planktonic species present at the time of the perturbation, which can strongly impact energy flow along food webs and the magnitude of carbon and nutrient fluxes in Antarctic waters.