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Photophysiology in Two Major Southern Ocean Phytoplankton Taxa: Photosynthesis and Growth of "Phaeocystis antarctica" and "Fragilariopsis cylindrus" under Different Irradiance Levels

Kevin R. Arrigo, Matthew M. Mills, Lindsey R. Kropuenske, Gert L. van Dijken, Anne-Carlijn Alderkamp and Dale H. Robinson
Integrative and Comparative Biology
Vol. 50, No. 6 (December 2010), pp. 950-966
Published by: Oxford University Press
Stable URL: http://www.jstor.org/stable/40931217
Page Count: 17
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Abstract

The Ross Sea, Antarctica, supports two distinct populations of phytoplankton, one that grows well in sea ice and blooms in the shallow mixed layers of the Western marginal ice zone and the other that can be found in sea ice but thrives in the deeply mixed layers of the Ross Sea. Dominated by diatoms (e.g. Fragilariopsis cylindrus) and the prymnesiophyte Phaeocystis antarctica, respectively, the processes leading to the development of these different phytoplankton assemblages are not well known. The goal of this article was to gain a better understanding of the photophysiological characteristics that allow each taxon to dominate its specific habitat. Cultures of F. cylindrus and P. antarctica were each grown semi-continuously at four different constant irradiances (5, 25, 65, and 125µmol quanta/m²/s). Fragilariopsis cylindrus produced far less photosynthetic pigment per cell than did P. antárctica but much more photoprotective pigment. Fragilariopsis cylindrus also exhibited substantially lower rates of photosynthesis and growth but also was far less susceptible to photoinhibition of cell growth. Excess photosynthetic capacity, a measure of the ability of phytoplankton to exploit variable light environments, was significantly higher in both strains of P. antarctica than in F. cylindrus. The combination of these characteristics suggests that F. cylindrus has a competitive advantage under conditions where mixed layers are shallow and light levels are relatively constant and high. In contrast, P. antarctica should dominate waters where mixed layers are deep and light levels are variable. These results are consistent with distributions of phytoplankton in the Ross Sea and suggest that light is the primary factor determining composition of phytoplankton communities.

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