The Xanthophyll Cycle in Aquatic Phototrophs and Its Role in the Mitigation of Photoinhibition and Photodynamic Damage

Solar energy is the initial power of photosynthesis. Plants and algae cannot proceed in the absence of light, and limited light conditions will limit photosynthesis. However, the conversion of solar energy into chemical energy is a potentially hazardous business that photosynthetic organisms expertly master. Whenever sunlight can actually be converted to chemical energy, there is minimal potential for problems. However, no leaf or algal cell can utilize all the light absorbed by the antenna system during exposure to full sunlight. Excessive light may be potentially dangerous to phototrophic organisms because it has the potential to be transferred to the formation of reactive oxygen species (ROS), which can result in cell damage (Ledford &Niyogi, 2005). It can also inhibit photosynthesis and lead to photooxidative destruction of the photosynthetic apparatus photoinhibition (DemmigAdams &Adams, 2006; Lu &Vonshak, 1999). It is known that photosynthesis is the basis of crop yield in plants and primary production in algae, and photoinhibition has an obvious adverse effect on photosynthesis and the accumulation of dry weight, which could lead to a decrease of carbon assimilation by about 10%. Thus, the ability of plants and algae to dissipate excessive light energy in order to resist photoinhibition, would significantly affect plant and alga yield and primary production. In the first step of the photosynthetic process, light is intercepted by a variety of lightabsorbing substances, the photosynthetic pigments. These pigments are associated with proteins forming light-harvesting 'antennae' that have a large optical cross-section for absorbing photons whose energy is efficiently transmitted to reaction centers (Dubinsky, 1992; Emerson &Arnold, 1932; Kirk, 1994). The light energy absorbed by the chlorophyll of photosynthetic organisms drives photosynthesis and is also dissipated as heat and fluorescence. To avoid massive ROS accumulation, phytoplankton and plants employ a host of protective mechanisms (Kanervo et al., 2005; Lavaud et al., 2002) – including various alternative energy-dissipation pathways (Adams et al., 2006) and multiple antioxidant systems