Paradoxical Effects of Seawater Temperature and Solar Irradiance on the Photo-degradation State of Senescent Phytoplankton.

In this study, we intented to deconvolve the effects of temperature and irradiance on the photo-oxidation state of senescent phytoplanctonic cells. For this purpose, killed cells of the diatom Chaetoceros neogracile have been irradiated (PAR radiations) at 36 and 446 J s^-1 m^-2 (for a same cumulative energetic dose) and at two well distinct temperatures (7 and 17°C). Analyses of specific tracers revealed that low temperatures and irradiances amplify photo-oxidative damages of mono-unsaturated lipid components of this diatom (i.e. palmitoleic acid, cholesterol and campesterol). The high efficiency of Type II photosensitized degradation processes under these conditions 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 the membranes at low temperatures inducing more damages. Conversely, high temperatures and irradiances induce not only a rapid degradation of the photosensitizer and a loss of singlet oxygen by diffusion outside the membranes (limiting thus type II photosensitized oxidation), but also intense autoxidation processes degrading not only unsaturated lipid components of the cells but also the oxidation products employed as tracers of photodegradation. These results allow to explain the paradoxical correlation observed in situ between the latitude and the photo-degradation state of phytoplankton.