Endosymbiont Density Research Articles

Coral species-specific loss and physiological legacy effects are elicited by an extended marine heatwave.

Marine heatwaves are increasing in frequency and intensity, with potentially catastrophic consequences for marine ecosystems such as coral reefs. An extended heatwave and recovery time-series that incorporates multiple stressors and is environmentally realistic can provide enhanced predictive capacity for performance under climate change conditions. We exposed common reef-building corals in Hawai'i, Montipora capitata and Pocillopora acuta, to a 2-month period of high temperature and high PCO2conditions or ambient conditions in a factorial design, followed by 2months of ambient conditions. High temperature, rather than high PCO2, drove multivariate physiology shifts through time in both species, including decreases in respiration rates and endosymbiont densities. Pocillopora acuta exhibited more significantly negatively altered physiology, and substantially higher bleaching and mortality than M. capitata. The sensitivity of P. acuta appears to be driven by higher baseline rates of photosynthesis paired with lower host antioxidant capacity, creating an increased sensitivity to oxidative stress. Thermal tolerance of M. capitata may be partly due to harboring a mixture of Cladocopium and Durusdinium spp., whereas P. acuta was dominated by other distinct Cladocopium spp. Only M. capitata survived the experiment, but physiological state in heatwave-exposed M. capitata remained significantly diverged at the end of recovery relative to individuals that experienced ambient conditions. In future climate scenarios, particularly marine heatwaves, our results indicate a species-specific loss of corals that is driven by baseline host and symbiont physiological differences as well as Symbiodiniaceae community compositions, with the surviving species experiencing physiological legacies that are likely to influence future stress responses.

Physiological and microbiome adaptation of coral Turbinaria peltata in response to marine heatwaves.

Against the backdrop of global warming, marine heatwaves are projected to become increasingly intense and frequent. This trend poses a potential threat to the survival of corals and the maintenance of entire coral reef ecosystems. Despite extensive evidence for the resilience of corals to heat stress, their ability to withstand repeated heatwave events has not been determined. In this study, we examined the responses and resilience of Turbinaria peltata to repeated exposure to marine heatwaves, with a focus on physiological parameters and symbiotic microorganisms. In the first heatwave, from a physiological perspective, T. peltata showed decreases in the Chl a content and endosymbiont density and significant increases in GST, caspase-3, CAT, and SOD levels (p < .05), while the effects of repeated exposure on heatwaves were weaker than those of the initial exposure. In terms of bacteria, the abundance of Leptospira, with the potential for pathogenicity and intracellular parasitism, increased significantly during the initial exposure. Beneficial bacteria, such as Achromobacter arsenitoxydans and Halomonas desiderata increased significantly during re-exposure to the heatwave. Overall, these results indicate that T. peltata might adapt to marine heatwaves through physiological regulation and microbial community alterations.

Competitive effects of the macroalga Caulerpa taxifolia on key physiological processes in the scleractinian coral Turbinaria peltata under thermal stress.

An increased abundance of macroalgae has been observed in coral reefs damaged by climate change and local environmental stressors. Macroalgae have a sublethal effect on corals that includes the inhibition of their growth, development, and reproduction. Thus, this study explored the effects of the macroalga, Caulerpa taxifolia, on the massive coral, Turbinaria peltata, under thermal stress. We compared the responses of the corals' water-meditated interaction with algae (the co-occurrence group) and those in direct contact with algae at two temperatures. The results show that after co-culturing with C. taxifolia for 28 days, the density content of the dinoflagellate endosymbionts was significantly influenced by the presence of C. taxifolia at ambient temperature (27 °C), from 1.3 × 106 cells cm-2 in control group to 0.95 × 106 cells cm-2 in the co-occurrence group and to 0.89 × 106 cells cm-2 in the direct contact group. The chlorophyll a concentration only differed significantly between the control and the direct contact group at 27 °C. The protein content of T. peltata decreased by 37.2% in the co-occurrence group and 49.0% in the direct contact group compared to the control group. Meanwhile, the growth rate of T. peltata decreased by 57.7% in the co-occurrence group and 65.5% in the direct contact group compared to the control group. The activity of the antioxidant enzymes significantly increased, and there was a stronger effect of direct coral contact with C. taxifolia than the co-occurrence group. At 30 °C, the endosymbiont density, chlorophyll a content, and growth rate of T. peltata significantly decreased compared to the control temperature; the same pattern was seen in the increase in antioxidant enzyme activity. Additionally, when the coral was co-cultured with macroalgae at 30 °C, there was no significant decrease in the density or chlorophyll a content of the endosymbiont compared to the control. However, the interaction of macroalgae and elevated temperature was evident in the feeding rate, protein content, superoxide dismutase (SOD), and catalase (CAT) activity compared to the control group. The direct contact of the coral with macroalga had a greater impact than water-meditated interactions. Hence, the competition between coral and macroalga may be more intense under thermal stress.

Cellular plasticity facilitates phenotypic change in a dominant coral’s Symbiodiniaceae assemblage

Coral-associated dinoflagellates (Symbiodiniaceae) are photosynthetic endosymbionts that influence coral acclimation, as indicated by photo-endosymbiotic phenotypic variance across different environmental conditions. Symbiont shuffling (shifts in endosymbiont community composition), changes in endosymbiont cell density, and cellular plasticity have all been proposed as acclimation mechanisms. However, few studies have been able to partition which of the three strategies were responsible for observed phenotypic variance. Using a combination of metabarcoding and flow cytometry, we simultaneously characterized Acropora pulchra-associated Symbiodiniaceae assemblages at the community, population, and individual level under natural environmental conditions to deduce whether seasonal phenotypic change and site-related phenotypic variation of Symbiodiniaceae assemblages is a product of symbiont shuffling or cellular plasticity. Symbiodiniaceae assemblages displayed season-specific phenotypic variance, while Symbiodiniaceae community composition was geographically structured and cell density showed limited data structure. Based on these patterns, we reveal that cellular plasticity of Symbiodiniaceae was the source of a phenotypic variation, thus indicating that cellular plasticity is a mechanism for acclimation to mild environmental change.

A theoretical model for host-controlled regulation of symbiont density.

There is growing empirical evidence that animal hosts actively control the density of their mutualistic symbionts according to their requirements. Such active regulation can be facilitated by compartmentalization of symbionts within host tissues, which confers a high degree of control of the symbiosis to the host. Here, we build a general theoretical framework to predict the underlying ecological drivers and evolutionary consequences of host-controlled endosymbiont density regulation for a mutually obligate association between a host and a compartmentalized, vertically transmitted symbiont. Building on the assumption that the costs and benefits of hosting a symbiont population increase with symbiont density, we use state-dependent dynamic programming to determine an optimal strategy for the host, i.e., that which maximizes host fitness, when regulating the density of symbionts. Simulations of active host-controlled regulation governed by the optimal strategy predict that the density of the symbiont should converge to a constant level during host development, and following perturbation. However, a similar trend also emerges from alternative strategies of symbiont regulation. The strategy which maximizes host fitness also promotes symbiont fitness compared to alternative strategies, suggesting that active host-controlled regulation of symbiont density could be adaptive for the symbiont as well as the host. Adaptation of the framework allowed the dynamics of symbiont density to be predicted for other host-symbiont ecologies, such as for non-essential symbionts, demonstrating the versatility of this modelling approach.

Seasonal wild dance of dual endosymbionts in the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea)

Most sap-feeding insects maintain obligate relationships with endosymbiotic bacteria that provide their hosts with essential nutrients. However, knowledge about the dynamics of endosymbiont titers across seasons in natural host populations is scarce. Here, we used quantitative PCR to investigate the seasonal dynamics of the dual endosymbionts “Candidatus Carsonella ruddii” and “Ca. Psyllophila symbiotica” in a natural population of the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea: Psyllidae). Psyllid individuals were collected across an entire year, covering both summer and overwintering generations. Immatures harboured the highest titers of both endosymbionts, while the lowest endosymbiont density was observed in males. The density of Carsonella remained high and relatively stable across the vegetative period of the pear trees, but significantly dropped during the non-vegetative period, overlapping with C. pyricola’s reproductive diapause. In contrast, the titer of Psyllophila was consistently higher than Carsonella’s and exhibited fluctuations throughout the sampling year, which might be related to host age. Despite a tightly integrated metabolic complementarity between Carsonella and Psyllophila, our findings highlight differences in their density dynamics throughout the year, that might be linked to their metabolic roles at different life stages of the host.

The roles of heating rate, intensity, and duration on the response of corals and their endosymbiotic algae to thermal stress

Anthropogenic ocean warming is one of the biggest threats to marine organisms worldwide. However, it remains unclear how the duration and intensity of thermal anomalies affect organismal stress responses and thermal thresholds. We used detailed tracking of coral endosymbiont and host physiology and dose-response analyses to compare the effects of multiple heating rates, intensities, and exposure durations on two reef-building corals, Acropora hemprichii and Porites lobata, from adjacent sides of a reef (protected vs. exposed) in the Central Red Sea known to differ in high-frequency (< 24 h) temperature variability. Corals were exposed to acute heat exposures (18 h) with four target temperatures (32 °C, 35 °C, 36.5 °C, and 38 °C), versus prolonged heat exposures lasting 7–15 days where temperatures were raised 0.5 and 1.5 °C day−1 to four target temperatures (32 °C, 33.5 °C, 35 °C, and 36.5 °C). In the prolonged experiment, dose-response curves assessing algal endosymbiont Fv/Fm revealed little initial effect of temperature, before an exponential decline above 34 °C for both species. Temperature at time of measurement and degree heating hours above 34 °C (DHH34) were the variables most strongly associated with declines in Fv/Fm. The Fv/Fm thermal thresholds for P. lobata from the high-variability protected site were higher than the exposed site in the faster heating, prolonged heat stress experiment despite minimal differences in endosymbiont density, chlorophyll-a, and host protein between sites. Together, our dose-response analysis revealed complex effects of DHH34, heating rate, and species-specific differences in the influence of local thermal histories shaping thermotolerance limits for these corals.

Acclimatization of a coral-dinoflagellate mutualism at a CO2 vent

Ocean acidification caused by shifts in ocean carbonate chemistry resulting from increased atmospheric CO2 concentrations is threatening many calcifying organisms, including corals. Here we assessed autotrophy vs heterotrophy shifts in the Mediterranean zooxanthellate scleractinian coral Balanophyllia europaea acclimatized to low pH/high pCO2 conditions at a CO2 vent off Panarea Island (Italy). Dinoflagellate endosymbiont densities were higher at lowest pH Sites where changes in the distribution of distinct haplotypes of a host-specific symbiont species, Philozoon balanophyllum, were observed. An increase in symbiont C/N ratios was observed at low pH, likely as a result of increased C fixation by higher symbiont cell densities. δ13C values of the symbionts and host tissue reached similar values at the lowest pH Site, suggesting an increased influence of autotrophy with increasing acidification. Host tissue δ15N values of 0‰ strongly suggest that diazotroph N2 fixation is occurring within the coral tissue/mucus at the low pH Sites, likely explaining the decrease in host tissue C/N ratios with acidification. Overall, our findings show an acclimatization of this coral-dinoflagellate mutualism through trophic adjustment and symbiont haplotype differences with increasing acidification, highlighting that some corals are capable of acclimatizing to ocean acidification predicted under end-of-century scenarios.

Effects of microplastic combined with Cr(III) on apoptosis and energy pathway of coral endosymbiont.

The combined effect of polyethylene (PE) microplastics and chromium (Cr(III)) on the scleractinian coral Acropora pruinosa (A. pruinosa) was investigated. The endpoints analysed in this study included the endosymbiont density, the chlorophyll a + c content, and the activity of enzymes involved in apoptosis (caspase-1, caspase-3), glycolysis (lactate dehydrogenase, LDH), the pentose phosphate pathway (glucose-6-phosphate dehydrogenase, G6PDH) and electron transfer coenzyme (nicotinamide adenine dinucleotide, NAD+/NADH). During the 7-day exposure to PE and Cr(III) stress, the endosymbiont density and chlorophyll content decreased gradually. The caspase-1 and caspase-3 activities increased in the high-concentration Cr(III) exposure group. Furthermore, the LDH and G6PDH activities decreased significantly, and the NAD+/NADH was decreased significantly. In summary, the results showed that PE and Cr(III) stress inhibited the endosymbiont energy metabolism enzymes and further led to endosymbiont apoptosis in coral. In addition, under exposure to the combination of stressors, when the concentration of Cr(III) remained at 1 × 10-2mg/L, the toxic effects of heavy metals on the endosymbiont were temporarily relieved with elevated PE concentrations. In contrast, when coral polyps were exposed to 5mg/L PE and increasing Cr(III) concentrations, their metabolic activities were seriously disturbed, which increased the burden of energy consumption. In the short term, the toxic effect of Cr(III) was more obvious than that of PE because Cr(III) exposure leads to endosymbiont apoptosis and irreversible damage. This is the first study to provide insights into the combined effect of microplastic and Cr(III) stress on the apoptosis and energy pathways of coral endosymbionts. This study suggested that microplastics combined with Cr(III) are an important factor affecting the apoptosis and energy metabolism of endosymbionts, accelerating the collapse of the balance between the coral host and symbiotic endosymbiont.

Positive interactions between corals and damselfish increase coral resistance to temperature stress.

By the century's end, many tropical seas will reach temperatures exceeding most coral species' thermal tolerance on an annual basis. The persistence of corals in these regions will, therefore, depend on their abilities to tolerate recurrent thermal stress. Although ecologists have long recognized that positive interspecific interactions can ameliorate environmental stress to expand the realized niche of plants and animals, coral bleaching studies have largely overlooked how interactions with community members outside of the coral holobiont shape the bleaching response. Here, we subjected a common coral, Pocillopora grandis, to 10 days of thermal stress in aquaria with and without the damselfish Dascyllus flavicaudus (yellowtail dascyllus), which commonly shelter within these corals, to examine how interactions with damselfish impacted coral thermal tolerance. Corals often benefit from nutrients excreted by animals they interact with and prior to thermal stress, corals grown with damselfish showed improved photophysiology (Fv /Fm ) and developed larger endosymbiont populations. When exposed to thermal stress, corals with fish performed as well as control corals maintained at ambient temperatures without fish. In contrast, corals exposed to thermal stress without fish experienced photophysiological impairment, a more than 50% decline in endosymbiont density, and a 36% decrease in tissue protein content. At the end of the experiment, thermal stress caused average calcification rates to decrease by over 80% when damselfish were absent but increase nearly 25% when damselfish were present. Our study indicates that damselfish-derived nutrients can increase coral thermal tolerance and are consistent with the Stress Gradient Hypothesis, which predicts that positive interactions become increasingly important for structuring communities as environmental stress increases. Because warming of just a few degrees can exceed corals' temperature tolerance to trigger bleaching and mortality, positive interactions could play a critical role in maintaining some coral species in warming regions until climate change is aggressively addressed.

Environmental memory gained from exposure to extreme pCO2 variability promotes coral cellular acid-base homeostasis.

Ocean acidification is a growing threat to coral growth and the accretion of coral reef ecosystems. Corals inhabiting environments that already endure extreme diel pCO2 fluctuations, however, may represent acidification-resilient populations capable of persisting on future reefs. Here, we examined the impact of pCO2 variability on the reef-building coral Pocillopora damicornis originating from reefs with contrasting environmental histories (variable reef flat versus stable reef slope) following reciprocal exposure to stable (218 ± 9) or variable (911 ± 31) diel pCO2 amplitude (μtam) in aquaria over eight weeks. Endosymbiont density, photosynthesis and net calcification rates differed between origins but not treatment, whereas primary calcification (extension) was affected by both origin and acclimatization to novel pCO2 conditions. At the cellular level, corals from the variable reef flat exhibited less intracellular pH (pHi) acidosis and faster pHi recovery rates in response to experimental acidification stress (pH 7.40) than corals originating from the stable reef slope, suggesting environmental memory gained from lifelong exposure to pCO2 variability led to an improved ability to regulate acid–base homeostasis. These results highlight the role of cellular processes in maintaining acidification resilience and suggest that prior exposure to pCO2 variability may promote more acidification-resilient coral populations in a changing climate.

Recovery of a symbiotic octocoral Sinularia heterospiculata after heat stress exposure

ABSTRACT Extending our knowledge about the bleaching and recovery processes in octocorals can offer valuable insights to predict future reef responses to continuing climate change. The octocoral Sinularia heterospiculata was exposed to a heat stress (32°C) for 2 days and then recovered within 205 days. The coral’s response was assessed by measuring total lipids, chlorophylls , carotenoids, and relative endosymbiont density. During the bleaching period, the coral lost half of its dinoflagellate symbionts; during the recovery period, it could completely restore their population. Photosynthetic activity of dinoflagellate symbionts was reduced after 1 week of recovery. However, the level of lipids was stable. The contents of chlorophyll b after the 2-day bleaching period increased relative to their proportion in the endosymbionts, which may be associated with the chlorophyll repackaging and absorption of more light. The content of carotenoids increases, which is probably associated with the complementary function of carotenoids as antioxidants.

Physiological costs of facultative endosymbionts in aphids assessed from energy metabolism

Abstract Many insect species harbour heritable bacterial endosymbionts. Some facultative endosymbionts provide benefits to their hosts under certain environmental conditions. Facultative endosymbionts are expected to impose additional energetic expenditures to their host, reducing host fitness. While there is accumulating evidence in plant sucking insects that facultative endosymbionts reduce the fitness of their host under permissive conditions, no direct energy costs associated with facultative endosymbionts have been identified. Using the standard metabolic rate (SMR) as a measure of the energy cost of self‐maintenance, we investigated whether two common facultative endosymbionts Hamiltonella defensa or Regiella insecticola increase the maintenance cost of the pea aphid Acyrthosiphon pisum which could translate into host fitness reduction (‘compensation hypothesis’). In addition, we tested if there was a link between SMR and the aphid fitness and whether it depended on endosymbiont density and aphid energetic reserves. Finally, we measured SMR at different temperatures to assess the impact of suboptimal thermal conditions on physiological cost of endosymbionts. In the presence of facultative endosymbionts, aphids expressed generally a lower fitness and a higher SMR compared to uninfected ones, in accordance with the ‘compensation hypothesis’. However, the SMR difference between infected and uninfected aphids tended to decrease with increasing temperature. Complex host genotype‐by‐symbiont genotype‐by‐temperature interactions on SMR were also revealed. Energetic budget of adult aphids appeared weakly influenced by the aphid genotype and endosymbiont species, suggesting that facultative endosymbionts primarily impact the consumption of energy resources rather than their acquisition. Density of facultative endosymbionts varied largely among aphid lines but was not associated with the fitness nor metabolic rate of aphids. This work supports the energy basis of facultative endosymbiont associated fitness costs and raises new questions about the effect of facultative endosymbionts on the energy metabolism of their host. Read the free Plain Language Summary for this article on the Journal blog.

Fungicides have transgenerational effects on Rhopalosiphum padi but not their endosymbionts.

While several agricultural fungicides are known to directly affect invertebrate pests, including aphids, the mechanisms involved are often unknown. One hypothesis is that fungicides with antibacterial activity suppress bacterial endosymbionts present in aphids which are important for aphid survival. Endosymbiont-related effects are expected to be transgenerational, given that these bacteria are maternally inherited. Here, we test for these associations using three fungicides (chlorothalonil, pyraclostrobin and trifloxystrobin) against the bird cherry-oat aphid, Rhopalosiphum padi, using a microinjected strain that carried both the primary endosymbiont Buchnera and the secondary endosymbiont Rickettsiella. We show that the fungicide chlorothalonil did not cause an immediate effect on aphid survival, whereas both strobilurin fungicides (pyraclostrobin and trifloxystrobin) decreased survival after 48 h exposure. However, chlorothalonil substantially reduced the lifespan and fecundity of the F1 generation. Trifloxystrobin also reduced the lifespan and fecundity of F1 offspring, however, pyraclostrobin did not affect these traits. None of the fungicides consistently altered the density of Buchnera or Rickettsiella in whole aphids. Our results suggest fungicides have sublethal impacts on R. padi that are not fully realized until the generation after exposure, and these sublethal impacts are not associated with the density of endosymbionts harbored by R. padi. However, we cannot rule out other effects of fungicides on endosymbionts that might influence fitness, like changes in their tissue distribution. We discuss these results within the context of fungicidal effects on aphid suppression across generations and point to potential field applications. © 2022 Society of Chemical Industry.

Endosymbionts moderate constrained sex allocation in a haplodiploid thrips species in a temperature-sensitive way

Maternally inherited bacterial endosymbionts that affect host fitness are common in nature. Some endosymbionts colonise host populations by reproductive manipulations (such as cytoplasmic incompatibility; CI) that increase the reproductive fitness of infected over uninfected females. Theory predicts that CI-inducing endosymbionts in haplodiploid hosts may also influence sex allocation, including in compatible crosses, however, empirical evidence for this is scarce. We examined the role of two common CI-inducing endosymbionts, Cardinium and Wolbachia, in the sex allocation of Pezothrips kellyanus, a haplodiploid thrips species with a split sex ratio. In this species, irrespective of infection status, some mated females are constrained to produce extremely male-biased broods, whereas other females produce extremely female-biased broods. We analysed brood sex ratio of females mated with males of the same infection status at two temperatures. We found that at 20 °C the frequency of constrained sex allocation in coinfected pairs was reduced by 27% when compared to uninfected pairs. However, at 25 °C the constrained sex allocation frequency increased and became similar between coinfected and uninfected pairs, resulting in more male-biased population sex ratios at the higher temperature. This temperature-dependent pattern occurred without changes in endosymbiont densities and compatibility. Our findings indicate that endosymbionts affect sex ratios of haplodiploid hosts beyond the commonly recognised reproductive manipulations by causing female-biased sex allocation in a temperature-dependent fashion. This may contribute to a higher transmission efficiency of CI-inducing endosymbionts and is consistent with previous models that predict that CI by itself is less efficient in driving endosymbiont invasions in haplodiploid hosts.

Natural Variability in Caribbean Coral Physiology and Implications for Coral Bleaching Resilience

Coral reefs are among the most diverse and complex ecosystems in the world that provide important ecological and economical services. Increases in sea surface temperature linked to global climate change threatens these ecosystems by inducing coral bleaching. However, it is not fully known if natural intra- or inter-annual physiological variability is linked to bleaching resilience or recovery capacity of corals. Here, we monitored the coral physiology of three common Caribbean species (Porites divaricata, Porites astreoides, Orbicella faveolata) at six time points over 2 years by measuring the following traits: calcification, biomass, lipids, proteins, carbohydrates, chlorophyll a, algal endosymbiont density, stable carbon isotopes of the host and endosymbiotic algae, and the stable carbon and oxygen isotopes of the skeleton. The overall physiological profile of all three species varied over time and that of P. divaricata was consistently different from the two other coral species. Porites divaricata had higher energy reserves coupled with higher contributions of heterotrophically derived carbon to host tissues than both P. astreoides and O. faveolata. Consistently higher overall energy reserves and heterotrophic contributions to tissues appear to buffer against environmental stress, including bleaching events. Thus, natural physiological variability among coral species appears to be a stronger predictor of coral bleaching resilience than intra- or inter-annual physiological variability within a coral species.

Chronic sublethal pesticide exposure affects brood production, morphology and endosymbionts, but not immunity in the ant, Cardiocondyla obscurior

Abstract Insect diversity and biomass have been declining around the world. This might have severe consequences on the environment since insects provide many crucial ecosystem services. One major driver behind the decline is continuous pesticide use which also affects nontarget organisms. Even at low doses, chronic exposure to pesticides may have negative long‐term effects. Ants have not received much attention in this context, despite their importance in terrestrial ecosystems. A full factorial experimental design was used to test how ingestion of two widely used pesticides influence colonies of the ant Cardiocondyla obscurior. Colonies were exposed for 12 weeks to the herbicide glyphosate and the neonicotinoid insecticide thiacloprid, separately and in combination. Brood production and the size of new workers and queens produced in these colonies were measured. In addition, the relative densities of two endosymbionts were quantified and immunocompetence tested by measuring survival after fungal infection. The two pesticides in combination decreased brood production, which was measured as the number of produced eggs and pupae. Workers and queens produced in pesticide‐exposed colonies differed in body size from individuals produced in control colonies. Furthermore, endosymbiont densities decreased after glyphosate exposure. Neither pesticide influenced survival after fungal infection. The results show that chronic exposure to two widely used pesticides causes apparent trade‐offs in reproduction and other physiological traits of ants, potentially decreasing fitness. Even low levels of pesticides in the environment may therefore have negative effects on ant colonies, contributing to the worldwide insect decline.

Wolbachia load variation in Drosophila is more likely caused by drift than by host genetic factors

Symbiosis is a continuum of long-term interactions ranging from mutualism to parasitism, according to the balance between costs and benefits for the protagonists. The density of endosymbionts is, in both cases, a key factor that determines both the transmission of symbionts and the host extended phenotype, and is thus tightly regulated within hosts. However, the evolutionary and molecular mechanisms underlying bacterial density regulation are currently poorly understood. In this context, the symbiosis between the fruit fly and its intracellular bacteria Wolbachia (wMelPop strain) is particularly interesting to study. Although vertically transmitted, the symbiont is pathogenic, and a positive correlation between virulence and wMelPop density is observed. In addition, the number of repeats of a bacterial genomic region -Octomom- is positively correlated with Wolbachia density, underlying a potential genetic mechanism that controls bacterial density. Interestingly, the number of repeats varies between host individuals, but most likely also within them. Such genetic heterogeneity within the host could promote conflicts between bacteria themselves and with the host, notably by increasing within-host competition between symbiont genotypes through a process analogous to the tragedy of the commons. To characterize the determinisms at play in the regulation of bacterial density, we first introgressed wMelPop in different genetic backgrounds of D. melanogaster, and found different density levels and Octomom copy numbers in each host lineage. To determine whether such variations reflect a host genetic determinism on density regulation through Octomom copy number selection, we replicated the introgressions and performed reciprocal crosses on the two Drosophila populations with the most extreme density levels. In both experiments, we detected an absence of directionality in the patterns of infection, associated with a strong instability of these patterns across generations. Given that bacterial density was highly correlated with Octomom copy numbers in all experiments, these results rather suggest a strong influence of drift and a random increase in the frequency of certain bacterial variants. We then discuss how drift, both on the symbiont population during transmission and on the host population, could limit the efficiency of selection in such a symbiotic system, and the consequences of drift on the regulation of density and composition of bacterial populations.

Changes in physiological performance and protein expression in the larvae of the coral Pocillopora damicornis and their symbionts in response to elevated temperature and acidification

Climate change causes ocean warming and acidification, which threaten coral reef ecosystems. Ocean warming and acidification cause bleaching and mortality, and decrease calcification in adult corals, leading to changes in the composition of coral communities; however, their interactive effects on coral larvae are not comprehensively understood. To examine the underlying molecular mechanisms of larval responses to elevated temperature and pCO2, we examined the physiological performance and protein expression profiles of Pocillopora damicornis at two temperatures (29 and 33 °C) and pCO2 levels (500 and 1000 μatm) for 5 d. Extensive physiological and proteomic changes were observed in coral larvae. The results indicated a significant decrease in net photosynthesis (PNET) and autotrophic capability (PNET/RD) of larvae exposed to elevated temperature but a marked increase in PNET and PNET/RD of larvae exposed to high pCO2 levels. Elevated temperature significantly reduced endosymbiont densities by 70% and photochemical efficiency, indicating that warming impaired host-symbiont symbiosis. Expression of photosynthesis-related proteins, the photosystem (PS) I reaction center subunits IV and XI as well as oxygen-evolving enhancer 1, was downregulated at higher temperatures in symbionts, whereas expression of the PS I iron‑sulfur center protein was increased under high pCO2 conditions. Furthermore, expression of phosphoribulokinase (involved in the Calvin cycle) and phosphoenolpyruvate carboxylase (related to the C4 pathway) was downregulated in symbionts under thermal stress; this finding suggests reduced carbon fixation at high temperatures. The abundance of carbonic anhydrase-associated proteins, which are predicted to exert biochemical roles in dissolved inorganic carbon transport in larvae, was reduced in coral host and symbionts at high temperatures. These results elucidate potential mechanisms underlying the responses of coral larvae exposed to elevated temperature and acidification and suggest an important role of symbionts in the response to warming and acidification.

Emersion-Associated Responses of an Intertidal Coral and Its Suitability for Transplantation to Ecologically Engineer Seawalls

There is a growing interest in transplanting corals onto the intertidal section of artificial coastal defences (e.g., seawalls) as an ecological engineering strategy to enhance biodiversity on urban shores. However, this inevitably results in exposure to the harsh environmental conditions associated with emersion (aerial exposure). Although the effects of a multitude of environmental stressors on corals have been examined, their photophysiological and gene expression responses to emersion stress remain understudied, as does the among-genotype variation in these responses. In this study, we conducted an in situ experiment to test the effects of increased daily emersion duration on a locally common intertidal coral, Dipsastraea cf. lizardensis. Coral fragments (n = 3) from five genotypically distinct colonies were subjected to two treatments: (1) increased daily emersion duration (~4.5 h d−1) and, (2) control (~3 h d−1) for three consecutive days during spring low tide. We examined the post-experimental photophysiological responses and expression level of a stress-associated gene, Hsp16. Relative to the controls, coral fragments that were exposed to longer daily emersion duration displayed significantly reduced effective quantum yield, while endosymbiont density varied significantly among genotypes across the experimental conditions. We found no significant differences in chlorophyll a concentration and Hsp16 gene expression level, suggesting that changes in these processes may be gradual and the duration of treatment that the corals were subjected to is likely within their tolerance limits. Taken together, it appears that D. cf. lizardensis displays substantial capacity to cope with sup-optimal conditions associated with emersion which makes it a promising candidate for transplantation onto intertidal seawalls. However, within-species variation in their stress response indicates that not all genotypes respond similarly to emersion, and this should be taken into account when selecting donor colonies for transplantation.