title: Paradoxical effects of temperature and solar irradiance on the photodegradation state of killed phytoplankton.
abstract: 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.
authors.affiliation: Aix Marseille Université, CNRS/INSU, Université de Toulon, IRD, Mediterranean Institute of Oceanography (MIO) UM 110, Marseille, 13288, France.
authors.forename: Rémi
authors.initials: R
authors.lastname: Amiraux
authors.affiliation: CNRS, UMR 7144, Marine Phototrophic Prokaryotes Team, Station Biologique de Roscoff, Roscoff, 29680, France.|Sorbonne Universités, UPMC Univ Paris 06, UMR 7144, Oceanic Plankton Group, Station Biologique de Roscoff, Roscoff, 29680, France.
authors.forename: Christian
authors.initials: C
authors.lastname: Jeanthon
authors.affiliation: Aix-Marseille University, Mediterranean Institute of Oceanography (MIO), Marseille Cedex 9, 13288, France.|CNRS-INSU/IRD UM 110, Université du Sud Toulon-Var, La Garde, 83957, France.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Vaultier
authors.affiliation: Aix-Marseille University, Mediterranean Institute of Oceanography (MIO), Marseille Cedex 9, 13288, France.|CNRS-INSU/IRD UM 110, Université du Sud Toulon-Var, La Garde, 83957, France.
authors.forename: Jean-François
authors.initials: JF
authors.lastname: Rontani
mesh_terms: D017377:Diatoms / Q000502:physiology*; D010782:Photolysis; D010839:Phytoplankton / Q000502:physiology*; D013472:Sunlight; D013696:Temperature
keywords: Chaetoceros neogracilis; PAR; POM; abiotic degradation; autoxidation; irradiance; lipid tracers; photooxidation; temperature