Alternative photosynthetic electron pathways in symbiotic dinoflagellates of reef-building corals

Conference (2012, July 11)

The 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. In the ... [more ▼]

The 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. 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). In addition to the linear electron flow (LEF) operating during oxygenic photosynthesis, alternative electron flows (AEF) have been widely described in higher plants and microalgae but not in Symbiodinium. The present study aimed to highlight the existence of the Mehler ascorbate peroxidase pathway (reduction of oxygen by PSI), chlororespiration (oxidation by molecular O2 of the PQ pool) and cyclic electron flow around PSI. We report that the presence of particular AEF and/or their amplitude vary from one clade to another. These processes could play a key role under particular environmental conditions when sinks for photosynthetic electrons are scarce. Indeed, they 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 and reducing the excitation pressure over the photosynthetic apparatus. [less ▲]

Increased number of mucocytes in Aiptasia pallida following heat-induced bleaching

Poster (2012)

Establishment of endosymbiosis: The case of cnidarians and Symbiodinium

in Journal of Experimental Marine Biology and Ecology (2012), 420–421

The symbiosis between cnidarians and Symbiodinium algae (dinoflagellates) is the keystone responsible for the formation of the huge and important structures that are coral reefs. Today many environmental ... [more ▼]

The symbiosis between cnidarians and Symbiodinium algae (dinoflagellates) is the keystone responsible for the formation of the huge and important structures that are coral reefs. Today many environmental and/or anthropogenic threats compromise this tight relationship and lead to more frequent events of drastic loss of Symbiodinium pigments and eventually of algae themselves from cnidarians, better known as cnidarian bleaching. While the mechanisms underlying the collapse of the algae–coral symbiosis are progressively getting unraveled, the understanding of the mechanisms involved in the de novo infection of bleached cnidarians by Symbiodinium remains elusive. In this review, we describe the various steps needed to establish a stable symbiotic relationship between Symbiodinium and cnidarians. We review the mechanisms implicated in host–symbiont recognition and in symbiosome formation and persistence, with a special emphasis on the role played by lectins and Rab proteins. A better understanding of these molecular mechanisms may contribute to the development of strategies to promote post-bleaching recovery of corals. [less ▲]

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

Poster (2010, December)

The 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 ... [more ▼]

The 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. [less ▲]

SYMBIODINIUM SP. CAN STAY ALIVE THROUGH THE GUT AND IN THE FAECES OF CNIDARIA. PREDATORS. THE CASE OF CORALLIOPHILLA MEYENDORFFI AND ANEMONIA VIRIDIS.

Poster (2010)

The gastropod Coralliophilla meyendorffi is a common predator of the zooxanthellate anemone Anemonia viridis. Zooxanthella from the anemones are an important constitutent of the mollusc faeces. Cell ... [more ▼]

The gastropod Coralliophilla meyendorffi is a common predator of the zooxanthellate anemone Anemonia viridis. Zooxanthella from the anemones are an important constitutent of the mollusc faeces. Cell integrity, occurence of flagellated forms, live/dead proportions and mitotic index of Symbiodinium collected from the faeces and cultivated in vitro were examined and compared to that of algae in hospite. The results show that most algae withstand digestive processes of the predator, staying alive and dividing actively in the faeces. Motile (lagellated) algae arise from dividing cells and escape the fecal pellets. Impact on coral bleaching recovery is discussed. [less ▲]

Increased mitosis in the bleached gastrodermis of the sea anemone A. pallida

Poster (2010)

Today, coral bleaching represents a major concern for marine biologists, especially considering the upsurge of this phenomenon possibly linked to climate change. Bleached corals, deprived of most of their ... [more ▼]

Today, coral bleaching represents a major concern for marine biologists, especially considering the upsurge of this phenomenon possibly linked to climate change. Bleached corals, deprived of most of their energy incomes, may show a partial or total mortality, which ultimately lead to shifts in reef communities. Studies focusing on cellular bleaching mechanisms have shown different ways by which algae may be expelled from gastrodermal host cells. Among those mechanisms, major emphasis has been put on host cell death, most probably due to both apoptosis and necrosis. Recovering gastrodermis is then expected to undergo regeneration process in order to be reinfected by new algae. We describe here this regeneration process in the bleached sea anemone model A. pallida. [less ▲]

L'anémone hermatypique Aiptasia pallida Verrill (1864): photophysiologie en rapport avec le phénomène de blanchiment

Master's dissertation (2007)