Nitrate enrichment & heat stress impacts the physiology of the coral A. kenti and the composition of its associated microbiome

Coral reefs are unarguably under increasing pressures arising from various environmental stressors. Coral survival in the face of environmental change relies heavily on nutrient exchanges between the host and the photosynthetic endosymbionts. While the functional contribution of the coral microbiome remains poorly understood, increasing evidence suggests that associated microorganisms are essential for coral resilience as they are intricately linked to nutrient cycling and energy flows in the ecosystem. Nitrogen underpins many aspects of coral holobiont functioning but the effect of its availability in its most abundant environmental form, nitrate, on the coral response to stress is equivocal: while nitrate sustains symbiont communities, it has also been reported to have adverse effects on the response to oxidative stress and to accentuate bleaching. In this study, using a crossed treatment experimental design in a mesocosm setup, we investigated the responses of the coral Acropora kenti to a nitrate enrichment of 5 µM in combination with a heat stress of 4 DHW over a period of 3 weeks. Corals’ health was monitored throughout the experiment and corals’ physiological response to the different treatments was assessed at the start of the stress and at the end of the experiment by measuring respiration rates, photosynthetic capacity, growth rates, symbiont densities, pigment and protein contents. In addition, corals were sampled to identify the composition of the associated symbiont and microbial communities using high-throughput sequencing of the genes ITS2 and 16S respectively. The heat stress treatment induced moderate to severe bleaching that was not alleviated by the increased nitrate supply. Nuances in the physiological data and the integration with the sequencing data give valuable inputs into the holobiont’s functioning by disentangling the effect of nitrate availability and heat stress on the resilience of the coral and the stability of its associated symbiotic and microbial communities

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)

Nitrate in the coral symbiosis: from the regulation of its assimilation to its impact on the physiology of the holobiont

In oligotrophic reef systems, coral holobionts are remarkably efficient at assimilating nitrogen through heterotrophic feeding or the uptake of dissolved inorganic nitrogen. Symbiodiniaceae are vital partners of the symbiosis for nutrient assimilation. In addition to providing translocated photosynthates, they account for most of the uptake of dissolved inorganic nitrogen. Although NO3- is the most abundant source of nitrogen in the ocean, little is known about the mechanisms regulating its assimilation by the holobiont. Coral hosts are unable to reduce nitrate as they lack the necessary enzymes, whereas Symbiodiniaceae have been shown to express the enzyme nitrate reductase (NR). However, the evidence supporting the active reduction of nitrate by the symbiotic algae during symbiosis is scarce and equivocal. This research aimed at deciphering the pathways of NO3- assimilation in both free-living Symbiodiniaceae and in hospite symbionts while also investigating the relevance of inorganic nitrogen source in physiological responses to stress. We investigated the expression and regulation of NR both in free-living Symbiodiniaceae and in in hospite symbionts using a combined western blot and qRT-PCR approach. We showed that the expression and regulation of NR in free-living Symbiodiniaceae is a dynamic and reversible process impacted by NO3- and NH4+ concentrations. Symbionts from N-depleted corals incubated with NO3- enriched seawater showed an increase in NR synthesis over time. Interestingly, NR protein synthesis did not correlate with NR gene expression, hinting towards a potential post-transcriptional regulation of the enzyme. Additionally, we investigated the impacts of inorganic N source (NO3- vs NH4+ vs N depletion) in combination with stress on the physiology of Symbiodiniaceae (photosynthetic responses, ROS and NO production). The availability of inorganic nitrogen improved photosynthetic capacity while reducing ROS production. Moreover, preliminary experiments showed that NO3- and NH4+ had differential effects on the physiological responses of Symbiodiniaceae subjected to stress

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

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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