Moonrise timing is key for synchronized spawning in coral Dipsastraea speciosa.

Synchronized mass coral spawning typically occurs several days after a full moon once a year. It is expected that spawning day is determined by corals sensing environmental change regulated by the lunar cycle (i.e., tide or moonlight); however, the exact regulatory mechanism remains unknown. Here, we demonstrate how moonlight influences the spawning process of coral, Dipsastraea speciosa When corals in the field were shaded 1 and 3 d before the full moon or 1 d after the full moon, spawning always occurred 5 d after shading commenced. These results suggest moonlight suppresses spawning: a hypothesis supported by laboratory experiments in which we monitored the effects of experimental moonlight (night-light) on spawning day. Different night-light treatments in the laboratory showed that the presence of a dark period between day-light and night-light conditions eliminates the suppressive effect of night-light on spawning. In nature, moonrise gets progressively later during the course of the lunar cycle, shifting to after sunset following the day of the full moon. Our results indicate that this period of darkness between sunset and moonrise triggers synchronized mass spawning of D. speciosa in nature.

Green fluorescence from cnidarian hosts attracts symbiotic algae.

Reef-building corals thrive in nutrient-poor marine environments because of an obligate symbiosis with photosynthetic dinoflagellates of the genus Symbiodinium Symbiosis is established in most corals through the uptake of Symbiodinium from the environment. Corals are sessile for most of their life history, whereas free-living Symbiodinium are motile; hence, a mechanism to attract Symbiodinium would greatly increase the probability of encounter between host and symbiont. Here, we examined whether corals can attract free-living motile Symbiodinium by their green fluorescence, emitted by the excitation of endogenous GFP by purple-blue light. We found that Symbiodinium have positive and negative phototaxis toward weak green and strong purple-blue light, respectively. Under light conditions that cause corals to emit green fluorescence, (e.g., strong blue light), Symbiodinium were attracted toward live coral fragments. Symbiodinium were also attracted toward an artificial green fluorescence dye with similar excitation and emission spectra to coral-GFP. In the field, more Symbiodinium were found in traps painted with a green fluorescence dye than in controls. Our results revealed a biological signaling mechanism between the coral host and its potential symbionts.

Overexpression of BUNDLE SHEATH DEFECTIVE 2 improves the efficiency of photosynthesis and growth in Arabidopsis.

Bundle Sheath Defective 2, BSD2, is a stroma-targeted protein initially identified as a factor required for the biogenesis of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO) in maize. Plants and algae universally have a homologous gene for BSD2 and its deficiency causes a RuBisCO-less phenotype. As RuBisCO can be the rate-limiting step in CO2 assimilation, the overexpression of BSD2 might improve photosynthesis and productivity through the accumulation of RuBisCO. To examine this hypothesis, we produced BSD2 overexpression lines in Arabidopsis. Compared with wild type, the BSD2 overexpression lines BSD2ox-2 and BSD2ox-3 expressed 4.8-fold and 8.8-fold higher BSD2 mRNA, respectively, whereas the empty-vector (EV) harbouring plants had a comparable expression level. The overexpression lines showed a significantly higher CO2 assimilation rate per available CO2 and productivity than EV plants. The maximum carboxylation rate per total catalytic site was accelerated in the overexpression lines, while the number of total catalytic sites and RuBisCO content were unaffected. We then isolated recombinant BSD2 (rBSD2) from E. coli and found that rBSD2 reduces disulfide bonds using reductants present in vivo, for example glutathione, and that rBSD2 has the ability to reactivate RuBisCO that has been inactivated by oxidants. Furthermore, 15% of RuBisCO freshly isolated from leaves of EV was oxidatively inactivated, as compared with 0% in BSD2-overexpression lines, suggesting that the overexpression of BSD2 maintains RuBisCO to be in the reduced active form in vivo. Our results demonstrated that the overexpression of BSD2 improves photosynthetic efficiency in Arabidopsis and we conclude that it is involved in mediating RuBisCO activation.

Principles of chemical geometry underlying chiral selectivity in RNA minihelix aminoacylation.

The origin of homochirality in L-amino acid in proteins is one of the mysteries of the evolution of life. Experimental studies show that a non-enzymatic aminoacylation reaction of an RNA minihelix has a preference for L-amino acid over D-amino acid. The reaction initiates by approaching of a 3'-oxygen of the RNA minihelix to the carbonyl carbon of an aminoacyl phosphate oligonucleotide. Here, employing molecular dynamics simulations, we examined the possible mechanisms that determine this chiral selectivity. The simulation system adopted a geometry required for the chemical reaction to occur more frequently with L-alanine than that with D-alanine. For L-alanine, the structure with this geometry was formed by a combination of stable dihedral angles along alanyl phosphate backbone with a canonical RNA structure, where the methyl group of alanine was placed on the opposite side of the approaching 3'-hydroxyl group with respect to the carbonyl plane. For D-alanine, the methyl group and the 3'-hydroxyl group were placed on the same side with respect to the carbonyl plane, which significantly decreased its ability to approach 3'-oxygen close to the carbonyl carbon compared to L-alanine. The mechanism suggested herein can explain experimentally observed chiral preferences.

Heat Induction of Cyclic Electron Flow around Photosystem I in the Symbiotic Dinoflagellate Symbiodinium.

Increases in seawater temperature impair photosynthesis (photoinhibition) in the symbiotic dinoflagellate Symbiodinium within cnidarian hosts, such as corals and sea anemones, and may destroy their symbiotic relationship. Although the degree of photoinhibition in Symbiodinium under heat stress differs among strains, the differences in their responses to increased temperatures, including cyclic electron flow (CEF), which sustains photoprotective thermal energy dissipation, have not been investigated. Here, we examined CEF in cultured Symbiodinium cells or those in an endosymbiotic relationship within a cnidarian host. The light-dependent reduction of the primary electron donor photosystem I, i.e. P700(+), was enhanced in any Symbiodinium cell by increasing temperatures, indicating CEF was induced by heat, which was accompanied by thermal energy dissipation activation. The critical temperatures for inducing CEF were different among Symbiodinium strains. The clade A strains with greater susceptibility to photoinhibition, OTcH-1 and Y106, exhibited higher CEF activities under moderate heat stress than a more phototolerant clade B strain Mf1.05b, suggesting that the observed CEF induction was not a preventive measure but a stress response in Symbiodinium.

Mitochondrial Alternative Pathway-Associated Photoprotection of Photosystem II is Related to the Photorespiratory Pathway.

Respiratory electron transport has two ubiquinol-oxidizing pathways, the cytochrome pathway (CP) and the alternative pathway (AP). The AP, which is catalyzed by the alternative oxidase (AOX), is energetically wasteful but may alleviate PSII photoinhibition under light conditions excessive for photosynthesis. However, its mechanism remains unknown. We used Arabidopsis aox1a mutants lacking AOX activity and studied the mutation's effects on photoinhibition by measuring the decrease in the maximum quantum yield of PSII (Fv/Fm) after high light exposure. Since the CP compensates for the lack of AOX, we monitored the extent of photoinhibition under conditions where CP activity is partially inhibited by antimycin A. When leaves were exposed to high light at 350 µmol m-2 s-1, the decline in Fv/Fm was significantly faster in the aox1a mutants than in the wild type. However, under conditions where photorespiration was suppressed by high CO2 or low O2 levels, the decline in Fv/Fm was suppressed in the aox1a mutants, but not in the wild type, making the difference between the wild type and mutants small. Our results demonstrate that the lack of the AP causes an acceleration of PSII photoinhibition in relation to the photorespiratory pathway, suggesting that the AP can support the activity of the photorespiratory pathway under high light conditions.

Novel Characteristics of Photodamage to PSII in a High-Light-Sensitive Symbiodinium Phylotype.

Dinoflagellates from the genus Symbiodinium form symbiotic relationships with many marine invertebrates, including reef-building corals. Symbiodinium is genetically diverse, and acquiring suitable Symbiodinium phylotypes is crucial for the host to survive in habitat environments, such as high-light conditions. The sensitivity of Symbiodinium to high light differs among Symbiodinium phylotypes, but the mechanism that controls light sensitivity has not yet been fully resolved. In the present study using high-light-tolerant and -sensitive Symbiodinium phylotypes, we examined what determines sensitivity to high light. In growth experiments under different light intensities, Symbiodinium CS-164 (clade B1) and CCMP2459 (clade B2) were identified as high-light-tolerant and -sensitive phylotypes, respectively. Measurements of the maximum quantum yield of photosystem II (PSII) and the maximum photosynthetic oxygen production rate after high-light exposure demonstrated that CCMP2459 is more sensitive to photoinhibition of PSII than CS-164, and tends to lose maximum photosynthetic activity faster. Measurement of photodamage to PSII under light of different wavelength ranges demonstrated that PSII in both Symbiodinium phylotypes was significantly more sensitive to photodamage under shorter wavelength regions of light spectra (<470 nm). Importantly, PSII in CCMP2459, but not CS-164, was also sensitive to photodamage under the regions of light spectra around 470-550 and 630-710 nm, where photosynthetic antenna proteins of Symbiodinium have light absorption peaks. This finding indicates that the high-light-sensitive CCMP2459 has an extra component of photodamage to PSII, resulting in higher sensitivity to high light. Our results demonstrate that sensitivity of PSII to photodamage differs among Symbiodinium phylotypes and this determines their sensitivity to high light.

Thermal acclimation of the symbiotic alga Symbiodinium spp. alleviates photobleaching under heat stress.

A moderate increase in seawater temperature causes coral bleaching, at least partially through photobleaching of the symbiotic algae Symbiodinium spp. Photobleaching of Symbiodinium spp. is primarily associated with the loss of light-harvesting proteins of photosystem II (PSII) and follows the inactivation of PSII under heat stress. Here, we examined the effect of increased growth temperature on the change in sensitivity of Symbiodinium spp. PSII inactivation and photobleaching under heat stress. When Symbiodinium spp. cells were grown at 25°C and 30°C, the thermal tolerance of PSII, measured by the thermal stability of the maximum quantum yield of PSII in darkness, was commonly enhanced in all six Symbiodinium spp. tested. In Symbiodinium sp. CCMP827, it took 6 h to acquire the maximum PSII thermal tolerance after transfer from 25°C to 30°C. The effect of increased growth temperature on the thermal tolerance of PSII was completely abolished by chloramphenicol, indicating that the acclimation mechanism of PSII is associated with the de novo synthesis of proteins. When CCMP827 cells were exposed to light at temperature ranging from 25°C to 35°C, the sensitivity of cells to both high temperature-induced photoinhibition and photobleaching was ameliorated by increased growth temperatures. These results demonstrate that thermal acclimation of Symbiodinium spp. helps to improve the thermal tolerance of PSII, resulting in reduced inactivation of PSII and algal photobleaching. These results suggest that whole-organism coral bleaching associated with algal photobleaching can be at least partially suppressed by the thermal acclimation of Symbiodinium spp. at higher growth temperatures.

Efficacy and safety of lanthanum carbonate in pre-dialysis CKD patients with hyperphosphatemia: a randomized trial.

BACKGROUND: Lanthanum carbonate (LC), an effective non-calcium phosphate binder is widely used to manage hyperphosphatemia in patients with chronic kidney disease (CKD) on dialysis. Recently, the additional indication for control of hyperphosphatemia in CKD patients not on dialysis has been approved. METHODS: A multicenter, randomized, double-blind, placebo-controlled trial to confirm the efficacy and safety of LC in Japanese hyperphosphatemic stage 4 - 5 CKD patients not on dialysis. After a 4-week run-in period, 143 eligible subjects with serum phosphate levels of 5.6 - 11.0 mg/dL were randomized (2 : 1) to receive LC or placebo (88 vs. 55) for 8 weeks; 119 subjects completed the study (76 vs. 43). The starting LC dose was 750 mg/day, which was then up-titrated to 2,250 mg/day as needed while tolerated. Primary efficacy analysis was performed on the intent-to-treat (ITT) population of 141 patients (86 vs. 55). RESULTS: LC produced a significantly greater reduction in serum phosphate level compared with placebo after 8 weeks of treatment (difference, 0.97 (95% CI: 0.58, 1.37) mg/ dL; p < 0.0001). The cumulative proportion of subjects with controlled phosphate levels ≤ 4.6 mg/dL was higher in the LC group than the placebo group (59.56% vs. 10.46%). LC caused significantly greater reductions in serum Ca × P product and urinary phosphate excretion compared with placebo. The safety profile of LC was similar to that of placebo. CONCLUSIONS: This study demonstrated the effectiveness of LC to control hyperphosphatemia in pre-dialysis CKD patients.

Gymnosperms have increased capacity for electron leakage to oxygen (Mehler and PTOX reactions) in photosynthesis compared with angiosperms.

Oxygen plays an important role in photosynthesis by participating in a number of O2-consuming reactions. O2 inhibits CO2 fixation by stimulating photorespiration, thus reducing plant production. O2 interacts with photosynthetic electron transport in the chloroplasts' thylakoids in two main ways: by accepting electrons from PSI (Mehler reaction); and by accepting electrons from reduced plastoquinone (PQ) mediated by the plastid terminal oxidase (PTOX). In this study, we show, using 101 plant species, that there is a difference in the potential for photosynthetic electron flow to O2 between angiosperms and gymnosperms. We found, from measurements of Chl fluorescence and leaf absorbance at 830 nm, (i) that electron outflow from PSII, as determined by decay kinetics of Chl fluorescence after application of a saturating light pulse, is more rapid in gymnosperms than in angiosperms; (ii) that the reaction center Chl of PSI (P700) is rapidly and highly oxidized in gymnosperms during induction of photosynthesis; and (iii) that these differences are dependent on oxygen. Finally, rates of O2 uptake measured by mass spectrometry in the absence of photorespiration were significantly promoted by illumination in dark-adapted leaves of gymnosperms, but not in those of angiosperms. The light-stimulated O2 uptake was around 10% of the maximum O2 evolution in gymnosperms and 1% in angiosperms. These results suggest that gymnosperms have increased capacity for electron leakage to oxygen in photosynthesis compared with angiosperms. The involvement of the Mehler reaction and PTOX in the electron flow to O2 is discussed.

Environmental pH signals the release of monosaccharides from cell wall in coral symbiotic alga.

Reef-building corals thrive in oligotrophic environments due to their possession of endosymbiotic algae. Confined to the low pH interior of the symbiosome within the cell, the algal symbiont provides the coral host with photosynthetically fixed carbon. However, it remains unknown how carbon is released from the algal symbiont for uptake by the host. Here we show, using cultured symbiotic dinoflagellate, Breviolum sp., that decreases in pH directly accelerates the release of monosaccharides, that is, glucose and galactose, into the ambient environment. Under low pH conditions, the cell surface structures were deformed and genes related to cellulase were significantly upregulated in Breviolum. Importantly, the release of monosaccharides was suppressed by the cellulase inhibitor, glucopyranoside, linking the release of carbon to degradation of the agal cell wall. Our results suggest that the low pH signals the cellulase-mediated release of monosaccharides from the algal cell wall as an environmental response in coral reef ecosystems.

Coral reefs are known as 'treasure troves of biodiversity' because of the enormous variety of different fish, crustaceans and other marine life they support. Colonies of marine animals, known as corals, which are anchored to rocks on the sea bed, form the main structures of a coral reef. Many corals rely on partnerships with microscopic algae known as dinoflagellates for most of their energy needs. The dinoflagellates use sunlight to make sugars and other carbohydrates and they give some of these to the coral. In exchange, the coral provides a home for the dinoflagellates inside its body. The algae live inside special compartments within coral cells known as symbiosomes. These compartments have a lower pH (that is, they are more acidic) than the rest of the coral cell. Previous studies have shown that the algae release sugars into the symbiosome but it remains unclear what triggers this release and whether it only occurs when the algae are in a partnership. Ishii et al. studied a type of dinoflagellate known as Breviolum sp. that had been grown in sea water-like liquid in a laboratory. The experiments found that the alga released two sugar molecules known as glucose and galactose into its surroundings even in the absence of a host coral. Increasing the acidity of the liquid caused the alga to release more sugars and resulted in changes to some of the structures on the surface of its cells. The alga also produced an enzyme, called cellulase, to degrade the wall that normally surrounds the cell of an alga. Treating the alga with a drug that inhibits the activity of cellulase also suppressed the release of sugars from the cells. These findings suggest that when dinoflagellates enter acidic environments, like the guts of marine animals or symbiosomes inside coral cells, the decrease in pH can activate the algal cellulase enzyme, which in turn triggers the release of sugars for the coral. This research will provide a new viewpoint to those interested in how partnerships between animals and algae are sustained in marine environments. It also highlights the importance of the alga cell wall in establishing partnerships with corals. Further work will seek to clarify the precise biological mechanisms involved.

The roles of ATP synthase and the cytochrome b6/f complexes in limiting chloroplast electron transport and determining photosynthetic capacity.

In C(3) plants, CO(2) assimilation is limited by ribulose 1,5-bisphosphate (RuBP) regeneration rate at high CO(2). RuBP regeneration rate in turn is determined by either the chloroplast electron transport capacity to generate NADPH and ATP or the activity of Calvin cycle enzymes involved in regeneration of RuBP. Here, transgenic tobacco (Nicotiana tabacum 'W38') expressing an antisense gene directed at the transcript of either the Rieske iron-sulfur protein of the cytochrome (Cyt) b(6)/f complex or the δ-subunit of chloroplast ATP synthase have been used to investigate the effect of a reduction of these complexes on chloroplast electron transport rate (ETR). Reductions in δ-subunit of ATP synthase content did not alter chlorophyll, Cyt b(6)/f complex, or Rubisco content, but reduced ETR estimated either from measurements of chlorophyll fluorescence or CO(2) assimilation rates at high CO(2). Plants with low ATP synthase content exhibited higher nonphotochemical quenching and achieved higher ETR per ATP synthase than the wild type. The proportional increase in ETR per ATP synthase complex was greatest at 35°C, showing that the ATP synthase activity can vary in vivo. In comparison, there was no difference in the ETR per Cyt b(6)/f complex in plants with reduced Cyt b(6)/f content and the wild type. The ETR decreased more drastically with reductions in Cyt b(6)/f complex than ATP synthase content. This suggests that chloroplast ETR is more limited by Cyt b(6)/f than ATP synthase content and is a potential target for enhancing photosynthetic capacity in crops.

Different thermal sensitivity of the repair of photodamaged photosynthetic machinery in cultured Symbiodinium species.

Coral bleaching caused by heat stress is accompanied by photoinhibition, which occurs under conditions where the rate of photodamage to photosystem II (PSII) exceeds the rate of its repair, in the symbiotic algae (Symbiodinium spp.) within corals. However, the mechanism of heat stress-induced photoinhibition in Symbiodinium still remains poorly understood. In the present work, we have investigated the effect of elevated temperature on the processes associated with the repair of photodamaged PSII in cultured Symbiodinium (OTcH-1 and CS-73). Severe photoinhibition was observed at temperature exceeding 32 degrees C in Symbiodinium CS-73 cells grown at 25-34 degrees C but not in cultures of the more thermally tolerant Symbiodinium OTcH-1. After photoinhibition treatment by strong light, photodamaged PSII was repaired close to initial levels under low light at 25 degrees C in both OTcH-1 and CS-73. However, the repair was strongly inhibited by increased temperature exceeding 31 degrees C in CS-73 but only weakly in OTcH-1. We found that inhibition of the repair process in CS-73 is attributed to impairment of both protein synthesis-dependent and -independent repair processes and is at least partially caused by suppression of the de novo synthesis of thylakoid membrane proteins and impairment of the generation of DeltapH across the thylakoid membrane, respectively. Our results suggest that acceleration of photoinhibition by moderate heat stress is attributed primarily to inhibition of the repair of photodamaged PSII and that the photoinhibition sensitivity of Symbiodinium to heat stress is determined by the thermal sensitivity of the PSII repair processes.

The solar action spectrum of photosystem II damage.

The production of oxygen and the supply of energy for life on earth rely on the process of photosynthesis using sunlight. Paradoxically, sunlight damages the photosynthetic machinery, primarily photosystem II (PSII), leading to photoinhibition and loss of plant performance. However, there is uncertainty about which wavelengths are most damaging to PSII under sunlight. In this work we examined this in a simple experiment where Arabidopsis (Arabidopsis thaliana) leaves were exposed to different wavelengths of sunlight by dispersing the solar radiation across the surface of the leaf via a prism. To isolate only the process of photodamage, the repair of photodamaged PSII was inhibited by infiltration of chloramphenicol into the exposed leaves. The extent of photodamage was then measured as the decrease in the maximum quantum yield of PSII using an imaging pulse amplitude modulation fluorometer. Under the experimental light conditions, photodamage to PSII occurred most strongly in regions exposed to ultraviolet (UV) or yellow light. The extent of UV photodamage under incident sunlight would be greater than we observed when one corrects for the optical efficiency of our system. Our results suggest that photodamage to PSII under sunlight is primarily associated with UV rather than photosynthetically active light wavelengths.

Heat stress causes inhibition of the de novo synthesis of antenna proteins and photobleaching in cultured Symbiodinium.

Coral bleaching, caused by heat stress, is accompanied by the light-induced loss of photosynthetic pigments in in situ symbiotic dinoflagellate algae (Symbiodinium spp.). However, the molecular mechanisms responsible for pigment loss are poorly understood. Here, we show that moderate heat stress causes photobleaching through inhibition of the de novo synthesis of intrinsic light-harvesting antennae [chlorophyll a-chlorophyll c(2)-peridinin-protein complexes (acpPC)] in cultured Symbiodinium algae and that two Clade A Symbiodinium species showing different thermal sensitivities of photobleaching also show differential sensitivity of this key protein synthesis process. Photoinhibition of photosystem II (PSII) and subsequent photobleaching were observed at temperatures of >31 degrees C in cultured Symbiodinium CS-73 cells grown at 25-34 degrees C, but not in cultures of the more thermally tolerant control Symbiodinium species OTcH-1. We found that bleaching in CS-73 is associated with loss of acpPC, which is a major antennae protein in Symbiodinium. In addition, the thermally induced loss of this protein is light-dependent, but does not coincide directly with PSII photoinhibition and is not caused by stimulated degradation of acpPC. In cells treated at 34 degrees C over 24 h, the steady-state acpPC mRNA pool was modestly reduced, by approximately 30%, whereas the corresponding synthesis rate of acpPC was diminished by >80%. Our results suggest that photobleaching in Symbiodinium is consequentially linked to the relative susceptibility of PSII to photoinhibition during thermal stress and occurs, at least partially, because of the loss of acpPC via undefined mechanism(s) that hamper the de novo synthesis of acpPC primarily at the translational processing step.

Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus.

Some sea slugs sequester chloroplasts from algal food in their intestinal cells and photosynthesize for months. This phenomenon, kleptoplasty, poses a question of how the chloroplast retains its activity without the algal nucleus. There have been debates on the horizontal transfer of algal genes to the animal nucleus. To settle the arguments, this study reported the genome of a kleptoplastic sea slug, Plakobranchus ocellatus, and found no evidence of photosynthetic genes encoded on the nucleus. Nevertheless, it was confirmed that light illumination prolongs the life of mollusk under starvation. These data presented a paradigm that a complex adaptive trait, as typified by photosynthesis, can be transferred between eukaryotic kingdoms by a unique organelle transmission without nuclear gene transfer. Our phylogenomic analysis showed that genes for proteolysis and immunity undergo gene expansion and are up-regulated in chloroplast-enriched tissue, suggesting that these molluskan genes are involved in the phenotype acquisition without horizontal gene transfer.

How do environmental stresses accelerate photoinhibition?

Environmental stress enhances the extent of photoinhibition, a process that is determined by the balance between the rate of photodamage to photosystem II (PSII) and the rate of its repair. Recent investigations suggest that exposure to environmental stresses, such as salt, cold, moderate heat and oxidative stress, do not affect photodamage but inhibit the repair of PSII through suppression of the synthesis of PSII proteins. In particular, production of D1 protein is downregulated at the translation step by the direct inactivation of the translation machinery and/or by primarily interrupting the fixation of CO2. The latter results in the creation of reactive oxygen species (ROS), which in turn block the synthesis of PSII proteins in chloroplasts.

Arabidopsis BSD2 reveals a novel redox regulation of Rubisco physiology in vivo.

Plants need light energy to drive photosynthesis, but excess energy leads to the production of harmful reactive oxygen species (ROS), resulting in oxidative inactivation of target enzymes, including the photosynthetic CO2-fixing enzyme, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). It has been demonstrated in vitro that oxidatively inactivated Rubisco can be reactivated by the addition of reducing agents. Busch et al. (in The Plant Journal, doi: 10.1111/tpj.14617, 2020) recently demonstrated that bundle-sheath defective 2 (BSD2), a stroma-targeted protein formerly known as a late-assembly chaperone for Rubisco biosynthesis, can be responsible for such reactivation in vivo. Here, we propose a working model of the novel redox regulation in Rubisco activity. Redox of Rubisco may be a new target for improving photosynthesis.

Loss of symbiont infectivity following thermal stress can be a factor limiting recovery from bleaching in cnidarians.

Increases in seawater temperature can cause coral bleaching through loss of symbiotic algae (dinoflagellates of the family Symbiodiniaceae). Corals can recover from bleaching by recruiting algae into host cells from the residual symbiont population or from the external environment. However, the high coral mortality that often follows mass-bleaching events suggests that recovery is often limited in the wild. Here, we examine the effect of pre-exposure to heat stress on the capacity of symbiotic algae to infect cnidarian hosts using the Aiptasia (sea-anemone)-Symbiodiniaceae model system. We found that the symbiont strain Breviolum sp. CS-164 (ITS2 type B1), both free-living and in symbiosis, loses the capacity to infect the host following exposure to heat stress. This loss of infectivity is reversible, however, a longer exposure to heat stress increases the time taken for reversal. Under the same experimental conditions, the loss of infectivity was not observed in another strain Breviolum psygmophilum CCMP2459 (ITS2 type B2). Our results suggest that recovery from bleaching can be limited by the loss of symbiont infectivity following exposure to heat stress.

Transcriptomic analyses highlight the likely metabolic consequences of colonization of a cnidarian host by native or non-native Symbiodinium species.

Reef-building corals and some other cnidarians form symbiotic relationships with members of the dinoflagellate family Symbiodinaceae. As Symbiodinaceae is a highly diverse taxon, the physiological interactions between its members and their hosts are assumed to differ between associations. The presence of different symbiont types is known to affect expression levels of specific host genes, but knowledge of the effects on the transcriptome more broadly remains limited. In the present study, transcriptome profiling was conducted on the tropical corallimorpharian, Ricordea yuma, following the establishment of symbiosis with either the 'homologous' symbiont Symbiodinium goreaui (also known as Cladocopium goreaui; ITS2 type C1) or 'heterologous' symbionts (predominantly S. trenchii, which is also known as Durusdinium trenchii; ITS2 type D1a) isolated from a different corallimorpharian host (Rhodactis indosinensis). Transcriptomic analyses showed that genes encoding host glycogen biosynthesis pathway components are more highly induced during colonization by the homologous symbiont than by the heterologous symbiont. Similar patterns were also observed for several other genes thought to facilitate symbiotic nutrient exchange, including those involved in lipid translocation/storage and metabolite transport. The gene expression results presented here imply that colonization by homologous or heterologous Symbiodinium types may have very different metabolic consequences for the Ricordea host, supporting the notion that even though some cnidarians may be able to form novel symbioses after bleaching, the metabolic performance of these may be compromised.This article has an associated First Person interview with the first author of the paper.