Individual and interactive effects of warming and CO 2 on Pseudo-nitzschia subcurvata and 1 Phaeocystis antarctica, two dominant phytoplankton from the Ross Sea, Antarctica 2 Zhi Zhu 1 , Pingping Qu 1 , Jasmine Gale 1 , Feixue Fu 1 , David A. Hutchins 1 3 1. Department of Biological Science, University of Southern California, Los Angeles, CA 90089, 4 USA. 5 Correspondence to: David A. Hutchins (dahutch@usc.edu) 6 7 Abstract: We investigated the effects of temperature and CO 2 variation on the growth and 8 elemental composition of cultures of the diatom Pseudo-nitzschia subcurvata and the 9 prymnesiophyte Phaeocystis antarctica, two ecologically dominant phytoplankton species 10 isolated from the Ross Sea, Antarctica. To obtain thermal functional response curves, cultures 11 were grown across a range of temperatures from 0 o C to 14 o C. In addition, a competition 12 experiment examined the relative abundance of both species at 0 o C and 6 o C. CO 2 functional 13 response curves were conducted from 100 to 1730 ppm at 2 o C and 8 o C to test for interactive 14 effects between the two variables. The growth of both phytoplankton was significantly affected 15 by temperature increase, but with different trends. Growth rates of P. subcurvata increased with 16 temperature from 0°C to maximum levels at 8°C, while the growth rates of P. antarctica only 17 increased from 0°C to 2°C. The maximum thermal limits of P. subcurvata and P. antarctica 18 where growth stopped completely were 14°C and 10°C, respectively. Although P. subcurvata 19 outcompeted P. antarctica at both temperatures in the competition experiment, this happened 20 much faster at 6°C than at 0°C. For P. subcurvata, there was a significant interactive effect in 21 which the warmer temperature decreased the CO 2 half saturation constant for growth, but this 22 was not the case for P. antarctica. The growth rates of both species increased with CO 2 increases 23 up 425 ppm, and in contrast to significant effects of temperature, the effects of CO 2 increase on 24 their elemental composition were minimal. Our results suggest that future warming may be more 25 favorable to the diatom than to the prymnesiophyte, while CO 2 increases may not be a major 26 factor in future competitive interactions between Pseudo-nitzschia subcurvata and Phaeocystis 27 antarctica in the Ross Sea. 28 1 Biogeosciences Discuss., doi:10.5194/bg-2017-18, 2017 Manuscript under review for journal Biogeosciences Published: 3 February 2017 c  Author(s) 2017. CC-BY 3.0 License.