Response of dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) cell quotas to oxidative stress in three phytoplankton species

peer reviewedSeveral phytoplankton species produce the metabolites dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) but their intracellular roles need to be better understood. To improve the understanding of the DMSP antioxidant function suggested by Sunda et al. (2002), we exposed the diatom Skeletonema costatum, the Prymnesiophyceae Phaeocystis globosa and the dinoflagellate Heterocapsa triquetra to experimental treatments known to cause potential oxidative stress (high light intensities (HL); HL with 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); menadione sodium bisulfite (MSB)). DMSP and DMSO concentrations decreased after 6h in all treatments indicating an interaction with Reactive Oxygen Species (ROS) produced. DMSP and DMSO-to-cell ratios in control conditions were higher for H. triquetra, while being unable to grow under HL. DMSP and DMSO-to-cell carbon were the highest for P. globosa while the other species had similar values. During long-term treatment, these ratios were not increased in high-light grown cells of P. globosa and S. costatum. Overall, this illustrates that (1) the DMSP- and DMSO-to-cell or carbon seems to be not indicative of the capability of the species to tolerate an oxidative stress, (2) these molecules could react with ROS and lower their cellular concentration, but no clues demonstrated that these molecules are part of the antioxidant response of the cell

Biophysical analysis of bioenergetics on coral slices

peer reviewedCoral algal symbiont (Symbiodiniaceae) has been subjected to different kind of studies to understand the role of photosynthesis and its regulatory mechanisms in the healthiness of coral reefs. Many biophysical studies have been conducted on free living algae, but few less on symbiotic corals due to technical constraints to have optically optimal biological samples. In this study we describe the use of small fragments of corals to study bioenergetic processes. Coral slices of less than one square centimeter were first prepared from apical branches of Stylophora pistillata colonies growing in aquarium conditions. The area of living tissue, number of polyps and photosynthetic pigments were quantified. In vivo biophysical measurements were carried out (yield of chlorophyll a fluorescence, P700 absorption changes, oxygen exchanges). From these measurements we concluded that photosystem II activity and respiratory rate of S. pistillata slices were very similar to those previously reported on whole nubbins measurements (Holcomb et al., 2014; Sorek and Levy, 2012). Photosystem I measurements gave also very stable signals, comparable to those obtained from green plants. We conclude that these results obtained on small fragments of corals are representatives of whole coral colony for biophysical analyses. Finally, these methodologies have been then applied on coral species (Pachyseris, Pocillopora, Porites sp.) sampled at two different sites of the Palau coral reef, which are exposed to different physico-chemical environments (pH and light intensity)

Comparison of the photosynthetic electron transport in shallow and mesophotic colonies of the reef-building coral Stylophora pistillata

peer reviewe

O2: Dr Jekyll and Mr Hyde in symbiotic dinoflagellates (Symbiodinium) from reef-building corals

Coral reefs are among the most beautiful and complex of all ecosystems on Earth. Although they cover less than 1% of the world’s oceans area, this marine ecosystem harbors a huge biodiversity and is vital to human society and industries. The foundation of coral reefs relies on the fragile mutualistic relationship between reef-building corals and their photosynthetic dinoflagellates of the genus Symbiodinium. However, this symbiosis is highly sensitive to environmental or anthropogenic disturbances and may be disrupted, thus leading to the coral bleaching phenomenon. It has been reported that the initial steps of this process are linked to photosynthesis and the antioxidant network in Symbiodinium. However the nature of the cellular mechanisms leading to the generation of reactive oxygen species and to the disruption of the symbiosis is not completely unraveled. Therefore, this study aimed to highlight the existence of photosynthetic alternative electron flows reducing molecular oxygen and the way by which they can induces an oxidative stress, in four Symbiodinium strains belonging to three different clades. Joint measurements of oxygen evolution, PSI and PSII activities by chlorophyll a fluorescence and spectrophotometric measurements allowed us to demonstrate that photoreduction of oxygen by the so-called Mehler reaction is the main electron sink at the onset of photosynthesis and during steady state photosynthesis. When Symbiodinium cells were exposed to high light conditions, the Mehler reaction and the ascorbate-glutathione cycle (water-water cycle) acted as a safety valve and drained up to 50% of the electrons from PSII, protecting it from photoinhibition and dissipating rapidly the excess photon energy by downregulation of PSII. As long as the WWC efficiency was maintained in the chloroplasts of Symbiodinium, ROS generated as a by-product of the Mehler reaction did not significantly damage target molecules and induced an acclimatory response through up-regulation of enzymes involved in the antioxidant response (superoxide dismutase, ascorbate peroxidase, glutathione reductase). Nevertheless, when cells were exposed to light stress and elevated temperature (33°C), the WWC supported 75% of the electrons coming from PSII. This increase generated twice more H2O2 than during the treatment at 26°C and resulted in the inactivation of target enzymes of the WWC. Therefore, this means that under these conditions the photoprotective functions of the WWC can no longer be maintained, thus opening the way to ROS accumulation and to the induction of coral bleaching.We found that the response to oxidative stress differed between and within Symbiodinium clades. Symbiodinium clade A was less sensitive to the chemical induced oxidative stress than the others investigated strains. These variations are most likely related to their geographic origin, their thermal history, as well as to their physiological adaptations to the local environment. They may contribute to the explanation of why coral colonies and coral species have been found to differ in their susceptibilities to bleach. However, although the antioxidant response differs to some extent, some common traits were conserved. Among them, Diatoxanthin, a xanthophyll pigment involved in the non-photochemical quenching process could also have an antioxidant function. In addition, it seems that the ubiquitin-proteasome pathway is involved in the antioxidant response by eliminating carbonylated protein

O2: Dr Jekyll and Mr Hyde in symbiotic dinoflagellates (Symbiodinium) from reef-building corals

Coral reefs are among the most beautiful and complex of all ecosystems on Earth. Although they cover less than 1% of the world’s oceans area, this marine ecosystem harbors a huge biodiversity and is vital to human society and industries. The foundation of coral reefs relies on the fragile mutualistic relationship between reef-building corals and their photosynthetic dinoflagellates of the genus Symbiodinium. However, this symbiosis is highly sensitive to environmental or anthropogenic disturbances and may be disrupted, thus leading to the coral bleaching phenomenon. It has been reported that the initial steps of this process are linked to photosynthesis and the antioxidant network in Symbiodinium. However the nature of the cellular mechanisms leading to the generation of reactive oxygen species and to the disruption of the symbiosis is not completely unraveled. Therefore, this study aimed to highlight the existence of photosynthetic alternative electron flows reducing molecular oxygen and the way by which they can induces an oxidative stress, in four Symbiodinium strains belonging to three different clades. Joint measurements of oxygen evolution, PSI and PSII activities by chlorophyll a fluorescence and spectrophotometric measurements allowed us to demonstrate that photoreduction of oxygen by the so-called Mehler reaction is the main electron sink at the onset of photosynthesis and during steady state photosynthesis. When Symbiodinium cells were exposed to high light conditions, the Mehler reaction and the ascorbate-glutathione cycle (water-water cycle) acted as a safety valve and drained up to 50% of the electrons from PSII, protecting it from photoinhibition and dissipating rapidly the excess photon energy by downregulation of PSII. As long as the WWC efficiency was maintained in the chloroplasts of Symbiodinium, ROS generated as a by-product of the Mehler reaction did not significantly damage target molecules and induced an acclimatory response through up-regulation of enzymes involved in the antioxidant response (superoxide dismutase, ascorbate peroxidase, glutathione reductase). Nevertheless, when cells were exposed to light stress and elevated temperature (33°C), the WWC supported 75% of the electrons coming from PSII. This increase generated twice more H2O2 than during the treatment at 26°C and resulted in the inactivation of target enzymes of the WWC. Therefore, this means that under these conditions the photoprotective functions of the WWC can no longer be maintained, thus opening the way to ROS accumulation and to the induction of coral bleaching.We found that the response to oxidative stress differed between and within Symbiodinium clades. Symbiodinium clade A was less sensitive to the chemical induced oxidative stress than the others investigated strains. These variations are most likely related to their geographic origin, their thermal history, as well as to their physiological adaptations to the local environment. They may contribute to the explanation of why coral colonies and coral species have been found to differ in their susceptibilities to bleach. However, although the antioxidant response differs to some extent, some common traits were conserved. Among them, Diatoxanthin, a xanthophyll pigment involved in the non-photochemical quenching process could also have an antioxidant function. In addition, it seems that the ubiquitin-proteasome pathway is involved in the antioxidant response by eliminating carbonylated protein

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

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