Paradoxical effects of temperature and solar irradiance on the photodegradation state of killed phytoplankton

The aim of this paper was to study the effects of temperature and irradiance on the photodegradation state of killed phytoplankton cells. For this purpose, killed cells of the diatom Chaetoceros neogracilis RCC2022 were irradiated (photosynthetically active radiation) at 36 and 446 J · s^?1 · m^?2 (for the same cumulative dose of irradiation energy) and at two temperatures (7°C and 17°C). Analyses of specific lipid tracers (fatty acids and sterols) revealed that low temperatures and irradiances increased photooxidative damages of monounsaturated lipids (i.e., palmitoleic acid, cholesterol and campesterol). The high efficiency of type II photosensitized degradation processes was attributed to: (i) the relative preservation of the sensitizer (chlorophyll) at low irradiances allowing a longer production of singlet oxygen and (ii) the slow diffusion rate of singlet oxygen through membranes at low temperatures inducing more damages. Conversely, high temperatures and irradiances induced (i) a rapid degradation of the photosensitizer and a loss of singlet oxygen by diffusion outside the membranes (limiting type II photosensitized oxidation), and (ii) intense autoxidation processes degrading unsaturated cell lipids and oxidation products used as photodegradation tracers. Our results may explain the paradoxical relationship observed in situ between latitude and photodegradation state of phytoplankton cells.