Antarctic microalgae: physiological acclimation to environmental change

Abstract

<p>Sea ice algal communities play a very significant role in primary production in the Southern Ocean, being the only source of fixed carbon for all other life in this habitat and contributing up to 22% of Antarctic primary production in ice-covered regions. Therefore it is important to understand how these organisms adapt to this highly variable and harsh environment Previous studies have described their acclimation to changes in environmental conditions but we still do not understand the physiological basis of these responses. This study examines the effects of varying levels of photosynthetically active radiation (PAR), ultraviolet-B (UV-B) radiation and temperature on bottom ice algal communities and individual algal species using pulse-amplitude modulation (PAM) fluorometry, the production of mycosporine-like amino acids (MAAs) and superoxide dismutase (SOD) activity. The experiments conducted in this thesis show that bottom ice algae are capable of acclimating to the higher levels of PAR and temperature that would likely be experienced during sea ice melt As temperature was increased past a threshold temperature of thylakoid integrity, it became the major stressor, causing decreases in photosynthetic yield at around 14°C, even at ambient PAR exposure. Similarly, a thylakoid integrity experiment independently suggested that the critical temperature for the onset of thylakoid damage was 14°C, which correlated well to the 14°C incubation observations, although this is a temperature that sea ice algae are unlikely to encounter in the polar regions. It is likely that sea ice algae produce additional MAAs, known to be cellular sunscreens, in response to increasing levels of UV-B, allowing tolerance of this stressor. This is the first study in the marine environment to demonstrate that algae can produce MAAs in response to increasing PAR and temperature, even in the absence of UV-B, indicating that MAAs may be more than just sunscreen compounds. The levels of UV-B used in this study were representative of those likely to be faced by the algae during sea ice melt. With increasing temperature, the algae maintained photosynthetic yield and decreased MAA production, implying that the rise in temperature aids the algae with another element of photoprotection such as enzymatic repair. As these results contrasted with previous studies of bottom ice algae that showed no additional MAA production in response to higher levels of PAR and UV-B, it was hypothesized that this difference was attributed to variations in species composition that could modify the productivity of the community. The short-term effects of increasing PAR and UV -B on three unialgal cultures of Thalassiosira sp., Fragilariopsis sp. (from the Ross Sea), and Chaetoceros sp. (from the Antarctic Peninsula) were therefore examined. In unialgal culture studies, these three algal species showed higher tolerance to PAR and UV-B compared to that of the mixed culture of bottom ice algae, although there remained species-specific variation. Both Ross Sea species showed increasing photosynthetic yield with increasing PAR and UV-B exposure, but there was a difference in the tolerance shown by the two species. Thalassiosira sp. tolerated higher PAR and lower UV-B and Fragilariopsis tolerated lower PAR and higher UV-B. Both species produced MAAs in response to these stressors, indicating that these compounds allowed the algae to decrease levels of photoinhibition. In comparison to the Ross Sea, the Antarctic Peninsula is an area of higher environmental variability and change, meaning that the species in both regions could have varying acclimatory capabilities. Although data from three species alone cannot conclusively demonstrate that algae from different regions have different acclimatory capabilities, they do illustrate considerable variation between species. Chaetoceros sp. from the Antarctic Peninsula region showed a higher tolerance to PAR and UV-B compared to the Ross Sea species. The former species showed an increase in photosynthetic yield in response to increasing PAR and this was accompanied by a lack of MAA production in response to the experimental levels of PAR, which indicates that the two Ross Sea species have a higher tolerance to PAR compared to the Antarctic Peninsula species. Chaetoceros sp. from the Antarctic Peninsula showed an increase in photosynthetic yield in response to high UV-B exposures, accompanied by MAA production and had no signs of photoinhibition. A further experiment was conducted to address the weaknesses in the initial methodologies, particularly related to control conditions in the short-term experiments. Common species from the Ross Sea, Antarctic Peninsula and the Arctic were exposed to a combination of increased PAR and UV-B over a period of seven days to compare acclimatory abilities using PAM and SOD activity. Thalassiosira antarctica from the Ross Sea, Chaetoceros socialis from the Antarctic Peninsula and C. socialis from the Arctic showed no significant change in quantum yield over the incubation period. This further highlights the importance of running experiments with compounding factors, as an increase in one factor could alleviate the negative effect of the other. There was an unexpected lack of change in SOD activity for all species under all treatments applied, which could indicate that the levels of PAR and UV-B used were not high enough to cause stress in these species. This work also points to the need to assay for various antioxidants, as algae are known to rely on a network of antioxidants in their defence against environmental stresses. The data from this thesis clarify the influence of PAR, UV-B and temperature on sea ice algae, and could help better evaluate the fate of these communities under various climate change scenarios. This study has made important steps towards understanding the acclimatory abilities of sea ice algae. Increasing knowledge of sea ice algal physiology, particularly of photosynthetic health in response to environmental change, will help improve predictions of productivity in the most productive ocean on this planet. Algal tolerance to increasing PAR, UV-B and temperature is remarkable, and this ability could be crucial in the context of future climate change. The productivity of these autotrophic microorganisms strongly influences secondary production that ties their fate to that of all other life in the Southern Ocean.</p>