Larger diatoms are more sensitive to temperature changes and prone to succumb to warming stress

Abstract The effects of ocean warming on phytoplankton have been extensively studied, but little has been documented on responses of differently sized diatoms to changes of temperature over wide ranges. We grew three diatom species, Thalassiosira pseudonana , Thalassiosira weissflogii , and Thalassiosira punctigera , with cell volumes of 199, 1805, and 13,599 μ m 3 , respectively, at seven temperatures (5, 10, 15, 20, 25, 30, and 35°C) over 15 generations, and investigated their physiological responses. The optimum growth temperature ( T opt ), maximum growth rates ( μ max ), and thermal niche width ( w ) decreased by 16%, 48%, and 29% per order of magnitude in cell size increment, respectively. The cell volume of each species diminished significantly and linearly with increased temperatures, by 22% in T. punctigera , 17% in T. weissflogii , and 11% in T. pseudonana , respectively. However, the cellular biogenic silica (BSi) contents increased with rising temperature till the optimum growth temperature for growth, with the sensitivity of BSi to temperature changes ( Q 10BSi ) ranging from 1.4 in the largest to 3.4 in the smallest diatom. The largest diatom T. punctigera showed the highest deactivation energy and succumbed to warmer conditions over its T opt , while the smallest was the most tolerant of warming. Although the moderate increase in growth temperature increased the diatoms' BSi levels, warming to temperatures higher than the optimum temperature for growth significantly reduced it. Our results suggest that diatoms acclimated to warmer conditions diminish their cell size but increase their mineralization of silica, which has profound significance in ocean carbon and silica biogeochemical cyclings.