| Summary: | With global warming, the intensity and frequency of extreme episodic weather events such as typhoons are rising in tropical and subtropical regions, disturbing the water column and shifting phytoplankton therein from deep to surface layers, and exposing them to high light as well as nutrients. To explore how phytoplankton respond to such environmental changes, we tracked the growth, cell compositions and physiology of small <i>Thalassiosira pseudonana</i> and large <i>Thalassiosira punctigera</i> from simulated ambient to upward-shifted light and nitrogen (N) conditions. Shifting to high levels of light caused a limited effect on the growth of small <i>T. pseudonana</i>, but reduced that of large <i>T. punctigera</i> by 36%, with supplemental N alleviating the light-caused growth reduction. The upward-shifted light reduced the cellular pigments contents in small <i>T. pseudonana</i>, but not in large <i>T. punctigera</i>. The upward-shifted light reduced the photosynthetic capability (F<sub>V</sub>/F<sub>M</sub>) of both species, as well as the light utilization efficiency (α) and maximal relative electron transport rate (rETRmax), but it enhanced their dark reparations. Moreover, the upward-shifted light did not affect the superoxide dismutase (SOD) activity of small <i>T. pseudonana</i>, but it did enhance that of large <i>T. punctigera.</i> In addition, the supplemental N showed a limited effect on cellular pigments and the dark respiration of <i>T. pseudonana</i>, but it reduced that of <i>T. punctigera</i>. Our results showed that the growth responses of <i>Thalassiosira</i> to upward-shifted light and nitrogen vary with species and possibly with cell size, indicating that the field species composition might change after the occurrence of extreme weather events.
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