Effects of long-term starvation on a host bivalve (Codakia orbicularis, Lucinidae) and its symbiont population

Environmental Transmission of Symbionts in the Mangrove Crabs Aratus pisonii and Minuca rapax: Acquisition of the Bacterial Community through Larval Development to Juvenile Stage

Aratus pisonii and Minuca rapax are two brachyuran crabs living with bacterial ectosymbionts located on gill lamellae. One previous study has shown that several rod-shaped bacterial morphotypes are present and the community is dominated by Alphaproteobacteria and Bacteroidota. This study aims to identify the mode of transmission of the symbionts to the new host generations and to identify the bacterial community colonizing the gills of juveniles. We tested for the presence of bacteria using PCR with universal primers targeting the 16S rRNA encoding gene from gonads, eggs, and different larval stages either obtained in laboratory conditions or from the field. The presence of bacteria on juvenile gills was also characterized by scanning electron microscopy, and subsequently identified by metabarcoding analysis. Gonads, eggs, and larvae were negative to PCR tests, suggesting that bacteria are not present at these stages in significant densities. On the other hand, juveniles of both species display three rod-shaped bacterial morphotypes on gill lamellae, and sequencing revealed that the community is dominated by Bacteroidota and Alphaproteobacteria on A. pisonii juveniles, and by Alphaprotobacteria, Bacteroidota, and Acidimicrobia on M. rapax juveniles. Despite the fact that juveniles of both species co-occur in the same biotope, no shared bacterial phylotype was identified. However, some of the most abundant bacteria present in adults are also present in juveniles of the same species, suggesting that juvenile-associated communities resemble those of adults. Because some of these bacteria were also found in crab burrow water, we hypothesize that the bacterial community is established gradually during the life of the crab starting from the megalopa stage and involves epibiosis-competent bacteria that occur in the environment.

Host-symbiont stress response to lack-of-sulfide in the giant ciliate mutualism

The mutualism between the thioautotrophic bacterial ectosymbiont Candidatus Thiobius zoothamnicola and the giant ciliate Zoothamnium niveum thrives in a variety of shallow-water marine environments with highly fluctuating sulfide emissions. To persist over time, both partners must reproduce and ensure the transmission of symbionts before the sulfide stops, which enables carbon fixation of the symbiont and nourishment of the host. We experimentally investigated the response of this mutualism to depletion of sulfide. We found that colonies released some initially present but also newly produced macrozooids until death, but in fewer numbers than when exposed to sulfide. The symbionts on the colonies proliferated less without sulfide, and became larger and more rod-shaped than symbionts from freshly collected colonies that were exposed to sulfide and oxygen. The symbiotic monolayer was severely disturbed by growth of other microbes and loss of symbionts. We conclude that the response of both partners to the termination of sulfide emission was remarkably quick. The development and the release of swarmers continued until host died and thus this behavior contributed to the continuation of the association.

Host-Microbe Interactions in the Chemosynthetic Riftia pachyptila Symbiosis

All animals are associated with microorganisms; hence, host-microbe interactions are of fundamental importance for life on earth. However, we know little about the molecular basis of these interactions. Therefore, we studied the deep-sea Riftia pachyptila symbiosis, a model association in which the tubeworm host is associated with only one phylotype of endosymbiotic bacteria and completely depends on this sulfur-oxidizing symbiont for nutrition. Using a metaproteomics approach, we identified both metabolic interaction processes, such as substrate transfer between the two partners, and interactions that serve to maintain the symbiotic balance, e.g., host efforts to control the symbiont population or symbiont strategies to modulate these host efforts. We suggest that these interactions are essential principles of mutualistic animal-microbe associations.

The deep-sea tubeworm Riftia pachyptila lacks a digestive system but completely relies on bacterial endosymbionts for nutrition. Although the symbiont has been studied in detail on the molecular level, such analyses were unavailable for the animal host, because sequence information was lacking. To identify host-symbiont interaction mechanisms, we therefore sequenced the Riftia transcriptome, which served as a basis for comparative metaproteomic analyses of symbiont-containing versus symbiont-free tissues, both under energy-rich and energy-limited conditions. Our results suggest that metabolic interactions include nutrient allocation from symbiont to host by symbiont digestion and substrate transfer to the symbiont by abundant host proteins. We furthermore propose that Riftia maintains its symbiont by protecting the bacteria from oxidative damage while also exerting symbiont population control. Eukaryote-like symbiont proteins might facilitate intracellular symbiont persistence. Energy limitation apparently leads to reduced symbiont biomass and increased symbiont digestion. Our study provides unprecedented insights into host-microbe interactions that shape this highly efficient symbiosis.

Organ transcriptomes of the lucinid clam Loripes orbiculatus (Poli, 1791) provide insights into their specialised roles in the biology of a chemosymbiotic bivalve

BACKGROUND

The lucinid clam Loripes orbiculatus lives in a nutritional symbiosis with sulphur-oxidizing bacteria housed in its gills. Although our understanding of the lucinid endosymbiont physiology and metabolism has made significant progress, relatively little is known about how the host regulates the symbiosis at the genetic and molecular levels. We generated transcriptomes from four L. orbiculatus organs (gills, foot, visceral mass, and mantle) for differential expression analyses, to better understand this clam's physiological adaptations to a chemosymbiotic lifestyle, and how it regulates nutritional and immune interactions with its symbionts.

RESULTS

The transcriptome profile of the symbiont-housing gill suggests the regulation of apoptosis and innate immunity are important processes in this organ. We also identified many transcripts encoding ion transporters from the solute carrier family that possibly allow metabolite exchange between host and symbiont. Despite the clam holobiont's clear reliance on chemosynthesis, the clam's visceral mass, which contains the digestive tract, is characterised by enzymes involved in digestion, carbohydrate recognition and metabolism, suggesting that L. orbiculatus has a mixotrophic diet. The foot transcriptome is dominated by the biosynthesis of glycoproteins for the construction of mucus tubes, and receptors that mediate the detection of chemical cues in the environment.

CONCLUSIONS

The transcriptome profiles of gills, mantle, foot and visceral mass provide insights into the molecular basis underlying the functional specialisation of bivalve organs adapted to a chemosymbiotic lifestyle.

Mitochondrial Adaptations to Variable Environments and Their Role in Animals' Stress Tolerance

Mitochondria are the key organelles involved in energy and redox homeostasis, cellular signaling, and survival. Animal mitochondria are exquisitely sensitive to environmental stress, and stress-induced changes in the mitochondrial integrity and function have major consequences for the organismal performance and fitness. Studies in the model organisms such as terrestrial mammals and insects showed that mitochondrial dysfunction is a major cause of injury during pathological conditions and environmental insults such as hypoxia, ischemia-reperfusion, and exposure to toxins. However, animals from highly stressful environments (such as the intertidal zone of the ocean) can maintain mitochondrial integrity and function despite intense and rapid fluctuations in abiotic conditions and associated changes in the intracellular milieu. Recent studies demonstrate that mitochondria of intertidal organisms (including mollusks, crustaceans, and fish) are capable of maintaining activity of mitochondrial electron transport system (ETS), ATP synthesis, and mitochondrial coupling in a broad range of temperature, osmolarity, and ion content. Mitochondria of intertidal organisms such as mollusks are also resistant to hypoxia-reoxygenation injury and show stability or even upregulation of the mitochondrial ETS activity and ATP synthesis capacity during intermittent hypoxia. In contrast, pH optima for mitochondrial ATP synthesis and respiration are relatively narrow in intertidal mollusks and may reflect adaptation to suppress metabolic rate during pH shifts caused by extreme stress. Sensitivity to anthropogenic pollutants (such as trace metals) in intertidal mollusks appears similar to that of other organisms (including mammals) and may reflect the lack of adaptation to these evolutionarily novel stressors. The mechanisms of the exceptional mitochondrial resilience to temperature, salinity, and hypoxic stress are not yet fully understood in intertidal organisms, yet recent studies demonstrate that they may involve rapid modulation of the ETS capacity (possibly due to post-translation modification of mitochondrial proteins), upregulation of antioxidant defenses in anticipation of oxidative stress, and high activity of mitochondrial proteases involved in degradation of damaged mitochondrial proteins. With rapidly developing molecular tools for non-model organisms, future studies of mitochondrial adaptations should pinpoint the molecular sites associated with the passive tolerance and/or active regulation of mitochondrial activity during stress exposures in intertidal organisms, investigate the roles of mitochondria in transduction of stress signals, and explore the interplay between bioenergetics and mitochondrial signaling in facilitating survival in these highly stressful environments.

Ecology and Fisheries: Dark Carbon on Your Dinner Plate

Chemosynthetic primary production by symbiotic microbes powers entire ecosystems in the remote deep sea. New research shows that in shallow waters chemosynthetic symbioses can contribute substantially to a vital economic resource - lobster fisheries in the Caribbean Sea.

Caribbean Spiny Lobster Fishery Is Underpinned by Trophic Subsidies from Chemosynthetic Primary Production

The Caribbean spiny lobster, Panulirus argus, is one of the most valuable fisheries commodities in the Central American region, directly employing 50,000 people and generating >US$450 million per year [1]. This industry is particularly important to small island states such as The Bahamas, which exports more lobster than any other country in the region [1]. Several factors contribute to this disproportionally high productivity, principally the extensive shallow-water banks covered in seagrass meadows [2], where fishermen deploy artificial shelters for the lobsters to supplement scarce reef habitat [3]. The surrounding seabed communities are dominated by lucinid bivalve mollusks that live among the seagrass root system [4, 5]. These clams host chemoautotrophic bacterial symbionts in their gills that synthesize organic matter using reduced sulfur compounds, providing nutrition to their hosts [6]. Recent studies have highlighted the important role of the lucinid clam symbiosis in maintaining the health and productivity of seagrass ecosystems [7, 8], but their biomass also represents a potentially abundant, but as yet unquantified, food source to benthic predators [9]. Here we undertake the first analysis of Caribbean spiny lobster diet using a stable isotope approach (carbon, nitrogen, and sulfur) and show that a significant portion of their food (∼20% on average) is obtained from chemosynthetic primary production in the form of lucinid clams. This nutritional pathway was previously unrecognized in the spiny lobster's diet, and these results are the first empirical evidence that chemosynthetic primary production contributes to the productivity of commercial fisheries stocks.

Nitrogen fixation in a chemoautotrophic lucinid symbiosis

The shallow water bivalve Codakia orbicularis lives in symbiotic association with a sulfur-oxidizing bacterium in its gills. The endosymbiont fixes CO₂ and thus generates organic carbon compounds, which support the host's growth. To investigate the uncultured symbiont's metabolism and symbiont-host interactions in detail we conducted a proteogenomic analysis of purified bacteria. Unexpectedly, our results reveal a hitherto completely unrecognized feature of the C. orbicularis symbiont's physiology: the symbiont's genome encodes all proteins necessary for biological nitrogen fixation (diazotrophy). Expression of the respective genes under standard ambient conditions was confirmed by proteomics. Nitrogenase activity in the symbiont was also verified by enzyme activity assays. Phylogenetic analysis of the bacterial nitrogenase reductase NifH revealed the symbiont's close relationship to free-living nitrogen-fixing Proteobacteria from the seagrass sediment. The C. orbicularis symbiont, here tentatively named 'Candidatus Thiodiazotropha endolucinida', may thus not only sustain the bivalve's carbon demands. C. orbicularis may also benefit from a steady supply of fixed nitrogen from its symbiont-a scenario that is unprecedented in comparable chemoautotrophic symbioses.

Characterization and Expression of the Lucina pectinata Oxygen and Sulfide Binding Hemoglobin Genes

The clam Lucina pectinata lives in sulfide-rich muds and houses intracellular symbiotic bacteria that need to be supplied with hydrogen sulfide and oxygen. This clam possesses three hemoglobins: hemoglobin I (HbI), a sulfide-reactive protein, and hemoglobin II (HbII) and III (HbIII), which are oxygen-reactive. We characterized the complete gene sequence and promoter regions for the oxygen reactive hemoglobins and the partial structure and promoters of the HbI gene from Lucina pectinata. We show that HbI has two mRNA variants, where the 5'end had either a sequence of 96 bp (long variant) or 37 bp (short variant). The gene structure of the oxygen reactive Hbs is defined by having 4-exons/3-introns with conservation of intron location at B12.2 and G7.0 and the presence of pre-coding introns, while the partial gene structure of HbI has the same intron conservation but appears to have a 5-exon/ 4-intron structure. A search for putative transcription factor binding sites (TFBSs) was done with the promoters for HbII, HbIII, HbI short and HbI long. The HbII, HbIII and HbI long promoters showed similar predicted TFBSs. We also characterized MITE-like elements in the HbI and HbII gene promoters and intronic regions that are similar to sequences found in other mollusk genomes. The gene expression levels of the clam Hbs, from sulfide-rich and sulfide-poor environments showed a significant decrease of expression in the symbiont-containing tissue for those clams in a sulfide-poor environment, suggesting that the sulfide concentration may be involved in the regulation of these proteins. Gene expression evaluation of the two HbI mRNA variants indicated that the longer variant is expressed at higher levels than the shorter variant in both environments.

Contrasted responses of Ruditapes decussatus (filter and deposit feeding) and Loripes lacteus (symbiotic) exposed to polymetallic contamination (Port-Camargue, France)

The use of symbiotic bivalve species to assess the effect of anthropogenic metal pollution was rarely investigated whereas data on filter feeding bivalves are common. The aim of this study was the exposure of two bivalve species, Ruditapes decussatus and Loripes lacteus to polymetallic pollution gradient, originating from harbor activities (Port-Camargue, south of France). Both bivalves differ by their trophic status, filter and deposit feeder for Ruditapes and symbiotic for Loripes that underlies potential differences in metal sensibility. The bivalves were immerged in July (for Ruditapes during 2 and 8 days) and in August 2012 (for Loripes during 2, 6 and 8 days) in the water column of the harbor, at 3 stations according to pollution gradient. Metal concentrations (Cu, Mn, Zn) in the water column were quantified as dissolved metals (measured by ICP-MS) and as labile metals (measured by ICP-MS using DGT technique). For each exposure time, accumulation of metals in the soft tissue of bivalves ("bioaccumulation") was measured for both species. In addition, specific parameters, according to the trophic status of each bivalve, were investigated: filtering activity (specific clearance rate, SCR) for Ruditapes, and relative cell size (SSC) and genomic content (FL1) of bacterial symbionts hosted in the gills of Loripes. The SCR of Ruditapes drops from 100% (control) to 34.7% after 2 days of exposure in the less contaminated site (station 8). On the other hand, the relative cell size (SSC) and genomic content (FL1), measured by flow cytometry were not impacted by the pollution gradient. Bioaccumulation was compared for both species, showing a greater capability of Cu accumulation for Loripes without lethal effect. Mn, Fe and Zn were generally not accumulated by any of the species according to the pollution gradient. The trophic status of each species may greatly influence their respective responses to polymetallic pollution.

Comparative modifications in bacterial gill-endosymbiotic populations of the two bivalves Codakia orbiculata and Lucina pensylvanica during bacterial loss and reacquisition

Until now, the culture of sulphur-oxidizing bacterial symbionts associated with marine invertebrates remains impossible. Therefore, few studies focused on symbiont's physiology under stress conditions. In this study, we carried out a comparative experiment based on two different species of lucinid bivalves (Codakia orbiculata and Lucina pensylvanica) under comparable stress factors. The bivalves were starved for 6 months in sulphide-free filtered seawater. For C. orbiculata only, starved individuals were then put back to the field, in natural sediment. We used in situ hybridization, flow cytometry and X-ray fluorescence to characterize the symbiont population hosted in the gills of both species. In L. pensylvanica, no decrease in symbiont abundance was observed throughout the starvation experiment, whereas elemental sulphur slowly decreased to zero after 3 months of starvation. Conversely, in C. orbiculata, symbiont abundance within bacteriocytes decreased rapidly and sulphur from symbionts disappeared during the first weeks of the experiment. The modifications of the cellular characteristics (SSC--relative cell size and FL1--genomic content) of the symbiotic populations along starvation were not comparable between species. Return to the sediment of starved C. orbiculata individuals led to a rapid (2-4 weeks) recovery of symbiotic cellular characteristics, comparable with unstressed symbionts. These results suggest that endosymbiotic population regulation is host-species-dependent in lucinids.

Roles of fat body trophocytes, mycetocytes and urocytes in the American cockroach, Periplaneta americana under starvation conditions: an ultrastructural study

In insects, trophocytes (adipocytes) are major cells of a storage organ, the fat body, from which stored glycogen and lipids are mobilized under starvation. However, cockroaches have 2 additional types of cell in the fat body: mycetocytes harboring an endosymbiont, Blattabacterium cuenoti, and urocytes depositing uric acid in urate vacuoles. These cells have not been investigated in terms of their roles under starvation conditions. To gain insight into the roles of trophocytes, mycetocytes and urocytes in cockroaches, structural changes were first investigated in the cells associated with starvation in the American cockroach, Periplaneta americana, by light and electron microscopy. The area of lipid droplets in trophocytes, the endosymbiont population and mitotic activity in mycetocytes, and the area of urate vacuoles in urocytes were analyzed in association with survival rates of the starved cockroaches. After 2 weeks of starvation, trophocytes lost glycogen rosettes and their area of lipid droplets decreased, but almost all cockroaches survived this period. However, further starvation did not reduce the area, but the survival rates dropped rapidly and all cockroaches died in 7 weeks. Endosymbionts were not affected in terms of population size and mitotic activity, even if the cockroaches were dying. The area of urate vacuoles rapidly decreased in a week of starvation and did not recover upon further starvation. These results indicate that starved cockroaches mobilize glycogen and lipids stored in trophocytes to survive for 2 weeks and then die after the exhaustion of nutrients in these cells. Endosymbionts are not digested for the recycling of nutrients, but uric acid is reused under starvation.

Plasticity of symbiont acquisition throughout the life cycle of the shallow-water tropical lucinid Codakia orbiculata (Mollusca: Bivalvia)

In marine invertebrates that acquire their symbionts from the environment, these are generally only taken up during early developmental stages. In the symbiosis between lucinid clams and their intracellular sulfur-oxidizing bacteria, it has been shown that the juveniles acquire their symbionts from an environmental stock of free-living symbiont forms, but it is not known if adult clams are still competent to take up symbiotic bacteria from the environment. In this study, we investigated symbiont acquisition in adult specimens of the lucinid clam Codakia orbiculata, using transmission electron microscopy, fluorescence in situ hybridization, immunohistochemistry and PCR. We show here that adults that had no detectable symbionts after starvation in aquaria for 6 months, rapidly reacquired symbionts within days after being returned to their natural environments in the field. Control specimens that were starved and then exposed to seawater aquaria with sulfide did not reacquire symbionts. This indicates that the reacquisition of symbionts in the starved clams returned to the field was not caused by high division rates of a small pool of remaining symbionts that we were not able to detect with the methods used here. Immunohistochemistry with an antibody against actin, a protein involved in the phagocytosis of intracellular bacteria, showed that actin was expressed at the apical ends of the gill cells that took up symbionts, providing further evidence that the symbionts were acquired from the environment. Interestingly, actin expression was also observed in symbiont-containing cells of untreated lucinids freshly collected from the environment, indicating that symbiont acquisition from the environment occurs continuously in these clams throughout their lifetime.

On the evolutionary ecology of symbioses between chemosynthetic bacteria and bivalves

Mutualistic associations between bacteria and eukaryotes occur ubiquitously in nature, forming the basis for key ecological and evolutionary innovations. Some of the most prominent examples of these symbioses are chemosynthetic bacteria and marine invertebrates living in the absence of sunlight at deep-sea hydrothermal vents and in sediments rich in reduced sulfur compounds. Here, chemosynthetic bacteria living in close association with their hosts convert CO₂ or CH₄ into organic compounds and provide the host with necessary nutrients. The dominant macrofauna of hydrothermal vent and cold seep ecosystems all depend on the metabolic activity of chemosynthetic bacteria, which accounts for almost all primary production in these complex ecosystems. Many of these enigmatic mutualistic associations are found within the molluscan class Bivalvia. Currently, chemosynthetic symbioses have been reported from five distinct bivalve families (Lucinidae, Mytilidae, Solemyidae, Thyasiridae, and Vesicomyidae). This brief review aims to provide an overview of the diverse physiological and genetic adaptations of symbiotic chemosynthetic bacteria and their bivalve hosts.