Chloroplast photooxidation inhibits the expression of a set of nuclear genes

Summary Mutations or herbicides which inhibit the accumulation of carotenoid pigments in higher plants also result in the arrest of chloroplast development at a very early stage. The cause is extensive photooxidative damage within the chloroplast in the absence of protective carotenoids. Because the extent of photooxidation is dependent upon light intensity, normal chloroplast development can occur when carotenoid-deficient seedlings are grown in very dim light. Normal accumulation of chloroplastic and cytosolic mRNAs encoding chloroplast proteins proceeds only under permissive dim light conditions. Illumination with higher intensity light causes rapid chlorophyll photooxidation and the loss of two cytosolic mRNAs coding for proteins destined for the chloroplast, but does not affect another light-regulated cytosolic mRNA encoding a cytosolic protein. This experimental system may have uncovered a mechanism which coordinates the expression of genes in different cellular compartments.Abbreviations LHCP light-harvesting chlorophyll a/b protein - SSu small subunit - RuBP fibulose 1,5-bisphoshate - PEP phosphoenolpyruvate     

Distribution of carotenoids in sub-cellular and sub-plastidic fractions of radish seedlings (Raphanus sativus) grown in the presence of bleaching herbicides

The intracellular and intraplastidic distribution of carotenoids has been investigated in radish seedlings grown in the presence of the herbicides amitrole and SAN 6706. Both herbicides caused bleaching and the plants became deficient in chlorophylls and the usual chloroplast cyclic carotenoids, but accumulated the acyclic carotenoid biosynthetic intermediates 15-cis-phytoene and all-trans-lycopene. In both the untreated and herbicide-treated plants all carotenoids, including phytoene and lycopene, were contained in the plastid. In all cases the normal cyclic carotenoids were located virtually exclusively in the thylakoid or prothylakoid fraction. In amitrole-treated plants, lycopene also was contained only in the thylakoid fraction, whereas phytoene, in these and in SAN 6706-treated plants, was detected in both the thylakoid fraction and an envelope preparation. Possible implications for the biosynthesis of the carotenoids are discussed.     

Effect of Organic Acids on Reactions Catalyzed by Manganese Peroxidase from Phanerochaete chrysosporium

The effect of several organic acids on the oxidation of Mn(II) catalyzed by manganese peroxidase was studied. Reactivities of manganese peroxidase and chemically prepared Mn(III) organic acid complexes towards phenolic compounds were compared. If lactate appears to be the best complexant for manganese peroxidase activity, chemically prepared Mn(III)—lactate complex is a less effective oxidant towards phenolic compounds than other Mn(III)—complexes. Our results agree with the hypothesis that certain organic acids are involved in the catalytic cycle of manganese peroxidase. Malonate and lactate seem to be the most attractive complexants for practical applications of manganese peroxidase and were used in enzymatic treatment of hardwood kraft pulp. Bleaching of kraft pulp was studied and after alkaline extraction, a significant decrease of kappa number was measured. The bleaching was enhanced in lactate buffer.     

Forecasting intensifying disturbance effects on coral reefs

Anticipating future changes of an ecosystem's dynamics requires knowledge of how its key communities respond to current environmental regimes. The Great Barrier Reef (GBR) is under threat, with rapid changes of its reef‐building hard coral (HC) community structure already evident across broad spatial scales. While several underlying relationships between HC and multiple disturbances have been documented, responses of other benthic communities to disturbances are not well understood. Here we used statistical modelling to explore the effects of broad‐scale climate‐related disturbances on benthic communities to predict their structure under scenarios of increasing disturbance frequency. We parameterized a multivariate model using the composition of benthic communities estimated by 145,000 observations from the northern GBR between 2012 and 2017. During this time, surveyed reefs were variously impacted by two tropical cyclones and two heat stress events that resulted in extensive HC mortality. This unprecedented sequence of disturbances was used to estimate the effects of discrete versus interacting disturbances on the compositional structure of HC, soft corals (SC) and algae. Discrete disturbances increased the prevalence of algae relative to HC while the interaction between cyclones and heat stress was the main driver of the increase in SC relative to algae and HC. Predictions from disturbance scenarios included relative increases in algae versus SC that varied by the frequency and types of disturbance interactions. However, high uncertainty of compositional changes in the presence of several disturbances shows that responses of algae and SC to the decline in HC needs further research. Better understanding of the effects of multiple disturbances on benthic communities as a whole is essential for predicting the future status of coral reefs and managing them in the light of new environmental regimes. The approach we develop here opens new opportunities for reaching this goal.     

Paradise lost: End‐of‐century warming and acidification under business‐as‐usual emissions have severe consequences for symbiotic corals

Despite recent efforts to curtail greenhouse gas emissions, current global emission trajectories are still following the business‐as‐usual representative concentration pathway (RCP) 8.5 emission pathway. The resulting ocean warming and acidification have transformative impacts on coral reef ecosystems, detrimentally affecting coral physiology and health, and these impacts are predicted to worsen in the near future. In this study, we kept fragments of the symbiotic corals Acropora intermedia (thermally sensitive) and Porites lobata (thermally tolerant) for 7 weeks under an orthogonal design of predicted end‐of‐century RCP8.5 conditions for temperature and pCO₂ (3.5°C and 570 ppm above present‐day, respectively) to unravel how temperature and acidification, individually or interactively, influence metabolic and physiological performance. Our results pinpoint thermal stress as the dominant driver of deteriorating health in both species because of its propensity to destabilize coral–dinoflagellate symbiosis (bleaching). Acidification had no influence on metabolism but had a significant negative effect on skeleton growth, particularly when photosynthesis was absent such as in bleached corals or under dark conditions. Total loss of photosynthesis after bleaching caused an exhaustion of protein and lipid stores and collapse of calcification that ultimately led to A. intermedia mortality. Despite complete loss of symbionts from its tissue, P. lobata maintained small amounts of photosynthesis and experienced a weaker decline in lipid and protein reserves that presumably contributed to higher survival of this species. Our results indicate that ocean warming and acidification under business‐as‐usual CO₂ emission scenarios will likely extirpate thermally sensitive coral species before the end of the century, while slowing the recovery of more thermally tolerant species from increasingly severe mass coral bleaching and mortality. This could ultimately lead to the gradual disappearance of tropical coral reefs globally, and a shift on surviving reefs to only the most resilient coral species.     

Turbid reefs moderate coral bleaching under climate‐related temperature stress

Thermal‐stress events that cause coral bleaching and mortality have recently increased in frequency and severity. Yet few studies have explored conditions that moderate coral bleaching. Given that high light and high ocean temperature together cause coral bleaching, we explore whether corals at turbid localities, with reduced light, are less likely to bleach during thermal‐stress events than corals at other localities. We analyzed coral bleaching, temperature, and turbidity data from 3,694 sites worldwide with a Bayesian model and found that K_d490, a measurement positively related to turbidity, between 0.080 and 0.127 reduced coral bleaching during thermal‐stress events. Approximately 12% of the world's reefs exist within this “moderating turbidity” range, and 30% of reefs that have moderating turbidity are in the Coral Triangle. We suggest that these turbid nearshore environments may provide some refuge through climate change, but these reefs will need high conservation status to sustain them close to dense human populations.     

Host autophagic degradation and associated symbiont loss in response to heat stress in the symbiotic anemone,Aiptasia pallida

Coral bleaching involves the loss of essential photosynthetic dinoflagellates (Symbiodinium sp.) from host gastrodermal cells in response to temperature or light stress. Although numerous potential cellular bleaching mechanisms have been proposed, there remains much uncertainty regarding which cellular events occur during early breakdown of the host–dinoflagellate symbiosis. In this study, transmission electron microscopy was used to conduct a detailed examination of symbiotic tissues of the tropical anemone Aiptasia pallida during early stages of host stress. Bleaching was induced by exposing specimens to a stress treatment of 32.5±0.5°C at 140±7 μ mol photons m^?2 s^?1 light intensity for 12 h, followed by 12 h at 24±1°C in darkness, repeated over a 48 h period. Ultrastructural examinations revealed numerous dense autophagic structures and associated cellular degradation in tentacle tissues after ~12 h of the stress treatment. Anemones treated with rapamycin, a known autophagy inducer, exhibited the same ultrastructural characteristics as heat‐stressed tissues, confirming that the structures observed during heat stress treatment were autophagic. In addition, symbionts appeared to be expelled from host cells via an apocrine‐like detachment mechanism from the apical ends of autophagic gastrodermal cells. This study provides the first ultrastructural evidence of host autophagic degradation during thermal stress in a cnidarian system and also supports earlier suggestions that autophagy is an active cellular mechanism during early stages of bleaching.     

Differential antioxidant response between two Symbiodinium species from contrasting environments

High sea surface temperature accompanied by high levels of solar irradiance is responsible for the disruption of the symbiosis between cnidarians and their symbiotic dinoflagellates from the genus Symbiodinium. This phenomenon, known as coral bleaching, is one of the major threats affecting coral reefs around the world. Because an important molecular trigger to bleaching appears related to the production of reactive oxygen species (ROS), it is critical to understand the function of the antioxidant network of Symbiodinium species. In this study we investigated the response of two Symbiodinium species, from contrasting environments, to a chemically induced oxidative stress. ROS produced during this oxidative burst reduced photosynthesis by 30 to 50% and significantly decreased the activity of superoxide dismutase. Lipid peroxidation levels and carotenoid concentrations, especially diatoxanthin, confirm that these molecules act as antioxidants and contribute to the stabilization of membrane lipids. The comparative analysis between the two Symbiodinium species allowed us to highlight that Symbiodinium sp. clade A temperate was more tolerant to oxidative stress than the tropical S. kawagutii clade F. These differences are very likely a consequence of adaptation to their natural environment, with the temperate species experiencing conditions of temperature and irradiance much more variable and extreme.     

Coral microbiome diversity reflects mass coral bleaching susceptibility during the 2016 El Niño heat wave

Repeat marine heat wave‐induced mass coral bleaching has decimated reefs in Seychelles for 35 years, but how coral‐associated microbial diversity (microalgal endosymbionts of the family Symbiodiniaceae and bacterial communities) potentially underpins broad‐scale bleaching dynamics remains unknown. We assessed microbiome composition during the 2016 heat wave peak at two contrasting reef sites (clear vs. turbid) in Seychelles, for key coral species considered bleaching sensitive (Acropora muricata, Acropora gemmifera) or tolerant (Porites lutea, Coelastrea aspera). For all species and sites, we sampled bleached versus unbleached colonies to examine how microbiomes align with heat stress susceptibility. Over 30% of all corals bleached in 2016, half of which were from Acropora sp. and Pocillopora sp. mass bleaching that largely transitioned to mortality by 2017. Symbiodiniaceae ITS2‐sequencing revealed that the two Acropora sp. and P. lutea generally associated with C3z/C3 and C15 types, respectively, whereas C. aspera exhibited a plastic association with multiple D types and two C3z types. 16S rRNA gene sequencing revealed that bacterial communities were coral host‐specific, largely through differences in the most abundant families, Hahellaceae (comprising Endozoicomonas), Rhodospirillaceae, and Rhodobacteraceae. Both Acropora sp. exhibited lower bacterial diversity, species richness, and community evenness compared to more bleaching‐resistant P. lutea and C. aspera. Different bleaching susceptibility among coral species was thus consistent with distinct microbiome community profiles. These profiles were conserved across bleached and unbleached colonies of all coral species. As this pattern could also reflect a parallel response of the microbiome to environmental changes, the detailed functional associations will need to be determined in future studies. Further understanding such microbiome‐environmental interactions is likely critical to target more effective management within oceanically isolated reefs of Seychelles.