From the ice to the open ocean : threats to phytoplankton productivity in the Antarctic marine ecosystem from a changing climate
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The Antarctic marine ecosystem is unique and dynamic, changing seasonally and forming specialised niche habitats including open ocean, sea ice and meltwater environments. Phytoplankton are key species in the structure and function of the Antarctic ecosystem, instrumental in the regions biogeochemistry, fundamental to the food web and strong contributors to global primary production and carbon sequestration. Understanding the photosynthetic plasticity of Antarctic phytoplankton is essential to understanding the effects global change is likely to have on primary production in the region. Through a series of experiments, this thesis explores the processes of light acclimation, photoprotection and photoinhibition in Antarctic microalgae under different environmental stressors, comparing photophysiological responses of species known to inhabit the sea-ice, meltwater and pelagic regions of Antarctic waters. The photosynthetic properties of three Antarctic diatoms (Fragilariopsis cylindrus, Pseudo-nitzschia subcurvata and Chaetoceros sp.) to changes in salinity, temperature and light were compared. Large heterogeneities in the photoprotective capacity of the three species and several distinct physiological strategies in response to the rapid changes in the ambient environment were observed (Publication I). Similarly, photosynthesis and net primary productivity was species-specific with large differences between environmental conditions (Publication II). Fast induction kinetics and pulse amplitude modulated fluorometry were used to demonstrate high levels of flexibility in light acclimation capabilities of sea ice algae from the east Antarctic. Inhibitors and pigment analyses identified xanthophyll cycling as the critical mechanism for photoprotection and preferred means by which sea ice diatoms regulated energy flow to PS1 (Publication III). While immunoblot analyses of natural communities measured minimal D1 protein breakdown in algae exposed to irradiances up to 200 µmol photons m⁻² s⁻¹. These data showed that sea ice diatoms had low intrinsic susceptibility to PSII photoinactivation and strong irradiance-dependent induction of non-photochemical quenching that was independent of protein resynthesis (Publication IV). The remaining chapters investigated photoprotective strategies and photosynthetic plasticity of phytoplankton under nutrient limitation. Nitrogen depletion in F. cylindrus had a strong influence on non-photochemical quenching capacity and resulted in the impairment of photosynthetic electron transport resulting in the formation of Qʙ non-reducing PSII centres within the photosystem (Publication V). The influence of iron-limitation and high light stress on the growth and physiology of Southern Ocean phytoplankton revealed a community-based response of measurable changes in pigment ratios, photosynthetic capacity and community composition (Publication VI). Iron-limited phytoplankton altered the allocation of photosynthetically derived energy, increasing photoprotective pigment pools and down-regulating photochemistry, at the expense of photosynthetic plasticity.
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