| Summary: | The Southern Ocean (SO) is one of the most productive oceanic regions in the world. Despite a high abundance of macronutrients such as nitrate and phosphorus, some areas have a surprisingly low amount of phytoplankton biomass. These areas are known as high nutrient low chlorophyll regions (HNLC) and are often limited by the micronutrient iron (Fe). The influx of Fe to the surface Southern Ocean has varied over geological timescales and is expected to change further in a future climate. Additionally, climate change will alter surface ocean mixing depth which can constrain phytoplankton productivity. To identify and understand the effects of Fe and light availability on phytoplankton, this study assessed ecophysiological and photophysiological responses of two SO phytoplankton species: the haptophyte Emiliania huxleyi (E. hux.) and the diatom Fragilariopsis cylindrus (F. cyl.). The species were grown under saturating and sub-saturating Fe conditions and different light conditions. A dynamic light treatment mimicked natural light variability in different mixed layer depths, inducing either light limitation, saturation and/or short-term high light stress while a continuous light environment for F. cyl. also simulated conditions in the Antarctic higher latitude summer. In all treatments, ecophysiological parameters such as growth, elemental ratios, and cellular chlorophyll a were measured. Photophysiology was examined using Fast Repetition Rate fluorometry (FRRf) over a diurnal cycle. For both species, the results indicated a clear reduction in chlorophyll a, growth rate and cellular POC under Fe limitation with weaker effects attributed to light intensity. Fe limitation reduced photochemical efficiency (photosynthetic quantum yield (Fv/Fm)) and resulted in a reduced reoxidation rate of the photosystem (τ). Light affected photophysiological parameters mainly during peak light in the dynamic lighting environment. Irrespective of Fe availability, light adaptation strategies differed between species. E.hux primarily ...
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