Photosynthesis and photoinhibition at low temperatures: physiological responses of Antarctic rhodophytes

The environment of Antarctica represents one of the most challenging and harshest ecosystems, characterized by very low temperatures and a strong seasonality in light availability. Due to the abiotic conditions, inhabiting organisms are highly adapted to their habitat and possess the ability to cope flexibly with changing environmental factors. In the context of global climate change, the Antarctic and especially the Antarctic Peninsula undergo the most rapid and significantly changing regions worldwide. The combination of low temperatures and high light intensities are challenging conditions for photosynthetic organisms, as low temperatures reduce for instance enzymatic processes and the turn-over of the D1 centre protein of photosystem II. This protein plays a crucial role in photosynthetic function.Low water temperatures also decrease membrane fluidity, resulting in an impairment of transfer processes for instance through the thylakoid membrane of chloroplasts. In addition, photosynthetic activity is particularly sensitive to low temperatures, as enzymatic secondary reactions are strictly temperature-dependent, while primary reactions are not. The present thesis investigated the physiological performance, the acclimation potential and tolerance limits of the endemic Antarctic rhodophyte Palmaria decipiens. In various experiments conducted under laboratory and field conditions, the alga was exposed to changing light and temperature levels. It was hypothesized that in particular the combination of photosynthetically active radiation (PAR, 400-700 nm) and low temperatures lead to stress responses in the organism. To estimate the responses of P. decipiens, the alga was exposed over different periods of time to natural and artificial radiation. The macroalgae investigated in the present study revealed a relatively high acclimation potential within their species-specific tolerance ranges. Obviously, this tolerance is limited and particularly rapid and pronounced changes can impact seaweeds negatively. To obtain realistic predictions in a more ecological context, investigations should involve interactive effects of the abiotic and biotic conditions. In summary, this study demonstrates that Antarctic rhodophytes are very well adapted to their environment and are, in spite of this, still able to acclimate and cope with certain changes in the abiotic conditions. The results obtained during the present study also indicate that the combined effects of environmental conditions is an important issue that has to be considered when adressing ecological questions