3. Hypometabolism and antioxidative defense systems in marine invertebrates

During normal cell respiration, approximately 0.1–0.2% of the oxygen consumed by aerobic cells is converted into reactive oxygen species (ROS), due to the uncompleted reduction of molecular oxygen in the mitochondrial electron transport chain [1]. ROS includes the superoxide anion radical (O2·), hydrogen peroxide (H2O2), alkyl peroxides, singlet oxygen (O2) and the hydroxyl radical (·OH). They are also generated as by-products of several oxygenases, or can be the result of cycle-redox reactions promoted by some pollutants. ROS can be involved in several cellular signaling pathways, including the cell cycle, stress responses, and energy metabolism, and are also important components of defense mechanisms of phagocytic cells [2,3]. However, when in excess ROS can be very deleterious to living organisms because of their ability to oxidize key cell components, such as lipids, proteins and nucleic acids, which sometimes lead to cell death.