Effects of Light and Iron on Growth and Physiology of a Polar Diatom, <italic>Fragilariopsis cylindrus</italic>
Bennett, Jennifer Melville
DiTullio, Giacomo R
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The Ross Sea, one of the most productive Southern Ocean regions, accounts for a substantial proportion of global primary production and is responsible for up to one fourth of the C0<sub>2</sub> export in this ocean. Both primary and export production in this region are thought to be mediated by the interaction of light and iron (Fe) bio-availability. Future climate change may produce significant changes in the mixing-irradiance regime, and in the supply of macro- and micro-nutrients, in the highly productive waters of the Antarctic continental shelf. In this context, there is a pressing need to understand the responses of the major groups of Antarctic phytoplankton to such environmental changes. The diatom <italic>Fragilariopsis cylindrus</italic> is a prolific species on the Antarctic shelf, inhabiting both sea-ice (low irradiance) and open-water (high irradiance) regimes. Laboratory culture experiments were performed to examine the growth and physiology of this diatom under nutrient-replete conditions at irradiances of 5-500 μE m<super>-2</super>s<super>-1</super>, on both acute and long term timescales. These allowed the sub-optimal, optimal, and supra-optimal irradiance for growth (5, 100, and 500 μE m<super>-2</super>s<super>-1</super>, respectively) to be assessed for this species, under which growth at varying Fe concentrations (0 - 1000nM Fe-EDTA) could then be examined. Cell number, biovolume, photosynthetic efficiency (F<sub>v</sub>/F<sub>m</sub>) and effective absorption cross section of PSII (σ<sub>PSII</sub>), photosynthetic and photoprotective pigments, reactive oxygen species (ROS), and intracellular particulate dimethylsulfoniopropionate (DMSP<sub>P</sub>) were measured in these experiments. The results indicated that <italic>F. cylindrus</italic> maintained relatively high growth rates (μ= 0.1-0.4) over a wide range of irradiance levels under nutrient replete conditions, probably using various physiological mechanisms including xanthophyll cycling and decreasing effective absorption cross section at higher irradiance. These mechanisms were also employed under light and iron stress, accompanied by an approximate 25% decrease in growth rate (µ) values. DMSP<sub>P</sub> levels (up to 60 mM) may also be serving as an antioxidant free-radical scavenging pool under both iron and light stress, thereby preventing oxidative damage, within the photosynthetic apparatus.