Ocean warming alters photosynthetic responses of diatom Phaeodactylum tricornutum to fluctuating irradiance

:Little is known about the combined impacts of temperature increases and fluctuating radiation regimes on the photosynthetic capability of marine diatoms. We incubated Phaeodactylum tricornutum cells under normal (18°C) vs elevated temperature (+6°C) in semicontinuous cultures and exposed them to fixed and fluctuating radiation conditions. Photosynthetic performance was determined based on the effective quantum yield of open photosystem II (PSII; F′V/F′M), the maximum relative electron transport rate (rETRmax), the light saturation parameter (Ek) and the photosynthetic efficiency parameter (α). These photophysiological parameters were affected more by irradiance than by the increase of temperature when P. tricornutum cells were exposed to fixed light regimes, and there was no significant difference between temperature treatments. In contrast, under fluctuating radiation regimes, the impact of temperature was clearly evident. Cells incubated at high temperature showed higher F′V/F′M, rETRmax and α but lower Ek when exposed to fast cycling in irradiance (20 min per cycle). In addition, cells grown at high temperature possessed a smaller value of effective target size for photoinactivation of PSII (σi), particularly for cells exposed to fast cycling in irradiance. This indicated that they were more resistant to PSII photoinactivation. These results showed that the photosynthetic capacity of P. tricornutum cells could be favoured by the increase of temperature, and fast cycling in irradiance increased the ability of cells to use the light and provoked much less photoinactivation.