Alternative photosynthetic electron pathways in different clades of Symbiodinium: the Mehler reaction

peer reviewedThe high productivity of coral reef ecosystems is largely attributed to the mutualistic symbiosis between reef-building corals and their intracellular dinoflagellate in the genus Symbiodinium commonly referred to as zooxanthellae. These photosynthetic algae translocate a majority of their photosynthetically fixed carbon to the host and contribute greatly to their metabolic needs (Muscatine, 1990) and the calcification process (Gattuso, 1999). In the natural environment the holobiont have to cope with significant daily variations in light intensities that sometimes exceed Symbiodinium photosynthetic capacity. Fortunately, photosynthetic organisms possess regulatory features that help to ensure that high light intensities can be endured without the accumulation of photodamage. Thus, the regulation of photosynthesis can be viewed as a dynamic balance between photosynthetic efficiency (photochemical quenching) and photoprotection processes (i.e. non-photochemical quenching). Among them, the role of O2 as an alternative electron acceptor within the chloroplast could play a critical role (Ort & Baker, 2002). Under particular environmental conditions when sinks for photosynthetic electrons are scarce, the direct reduction of oxygen by the PSI could sustain significant levels of photosynthetic electron flux by initiating the ΔpH formation and of NPQ, regulating the ratio of ATP/NADPH to match the requirements of carbon reduction. However, this process leads to the formation of reactive oxygen species that are rapidly detoxified by superoxide dismutase and ascorbate peroxidase. An additional electron flux associated with this oxygen pathway is directed to the reduction of monodehydroascorbate (MDA), which is generated as a result of peroxide reduction by ascorbate (Asada, 2000). The present study aimed to highlight the existence of alternative photosynthetic electron pathways and more especially the Mehler ascorbate peroxidase pathway in different clades of Symbiodinium, cultivated at low and high light intensities. -Muscatine L (1990) The role of symbiotic algae in carbon and energy flux in reef corals. In: Dubinsky Z(ed) Ecosystems of the world: coral reefs. Elsevier, Amsterdam, pp 1-9. -Gattuso JP, Allemand D and M Frankignoulle (1999) Photosynthesis and calcification at cellular, organismal and community levels in coral reefs: A review on interactions and control by carbonate chemistry. American Zoologist 39(1): 160-183. -Ort, D. R. and N. R. Baker (2002). A photoprotective role for O2 as an alternative electron sink in photosynthesis? Current Opinion in Plant Biology 5(3): 193-198. -Asada, K. (2000) The water-water cycle as alternative photon and electron sinks. Philosophical Transactions of the Royal Society B-Biological Sciences 355(1402): 1419–1431