Increased cell proliferation and mucocyte density in the sea anemone Aiptasia pallida recovering from bleaching

Step-by-Step Regeneration of Tentacles after Injury in Anemonia viridis-Morphological and Structural Cell Analyses

Benthic marine invertebrates, such as corals, are often subjected to injury caused by several sources. Here, the differences and characteristics in injured and health tissues in terms of cellular components are shown through a histological investigation of the soft coral Anemonia viridis at 0 h, 6 h, 24 h, and 7 days after injury caused by tentacle amputation. In addition, a new tool was used for the first time in invertebrates, positron emission tomography, in order to investigate the events that occur during regeneration within a longer time period (0 h, 24 h, and 14 days after the tentacles were cut). Higher integrated density values were measured through a densitometric analysis in sections stained with Fontana-Masson at 24 h after the tentacles were cut. This suggests an increase in melanin-like containing cells and a subsequent increase in fibroblast-like cells differentiated by amoebocytes that converge to the lesion site in the early stages of inflammation and regeneration. This work provides, for the first time, an elucidation of the events that occur during wound-healing and regeneration in basal metazoan, focusing on the characterisation of immune cells and their role. Our results indicate that Mediterranean anthozoan proves to be a valuable model for studying regeneration. Many events highlighted in this research occur in different phyla, suggesting that they are highly conserved.

Symbiosis with Dinoflagellates Alters Cnidarian Cell-Cycle Gene Expression

In the cnidarian-dinoflagellate symbiosis, hosts show altered expression of genes involved in growth and proliferation when in the symbiotic state, but little is known about the molecular mechanisms that underlie the host’s altered growth rate. Using tissue-specific transcriptomics, we determined how symbiosis affects expression of cell cycle-associated genes, in the model symbiotic cnidarian Exaiptasia diaphana (Aiptasia). The presence of symbionts within the gastrodermis elicited cell-cycle arrest in the G1 phase in a larger proportion of host cells compared with the aposymbiotic gastrodermis. The symbiotic gastrodermis also showed a reduction in the amount of cells synthesizing their DNA and progressing through mitosis when compared with the aposymbiotic gastrodermis. Host apoptotic inhibitors (Mdm2) were elevated, while host apoptotic sensitizers (c-Myc) were depressed, in the symbiotic gastrodermis when compared with the aposymbiotic gastrodermis and epidermis of symbiotic anemones, respectively. This indicates that the presence of symbionts negatively regulates host apoptosis, possibly contributing to their persistence within the host. Transcripts (ATM/ATR) associated with DNA damage were also downregulated in symbiotic gastrodermal tissues. In epidermal cells, a single gene (Mob1) required for mitotic completion was upregulated in symbiotic compared with aposymbiotic anemones, suggesting that the presence of symbionts in the gastrodermis stimulates host cell division in the epidermis. To further corroborate this hypothesis, we performed microscopic analysis using an S-phase indicator (EdU), allowing us to evaluate cell cycling in host cells. Our results confirmed that there were significantly more proliferating host cells in both the gastrodermis and epidermis in the symbiotic state compared with the aposymbiotic state. Furthermore, when comparing between tissue layers in the presence of symbionts, the epidermis had significantly more proliferating host cells than the symbiont-containing gastrodermis. These results contribute to our understanding of the influence of symbionts on the mechanisms of cnidarian cell proliferation and mechanisms associated with symbiont maintenance.

Thermal stress responses of the antipatharian Stichopathes sp. from the mesophotic reef of Mo'orea, French Polynesia

Antipatharians, also called black corals, are present in almost all oceans of the world, until extreme depths. In several regions, they aggregate in higher densities to form black coral beds that support diverse animal communities and create biodiversity hotspots. These recently discovered ecosystems are currently threatened by fishing activities and illegal harvesting for commercial purposes. Despite this, studies dedicated to the physiology of antipatharians are scarce and their responses to global change stressors have remained hardly explored since recently. Here, we present the first study on the physiological responses of a mesophotic antipatharian Stichopathes sp. (70-90 m) to thermal stress through a 16-d laboratory exposure (from 26 to 30.5 °C). Oxygen consumption measurements allowed identifying the physiological tipping point of Stichopathes sp. (T_opt = 28.3 °C; 2.7 °C above mean ambient condition). Our results follow theoretical predictions as performances start to decrease beyond T_opt, with lowered oxygen consumption rates, impairment of the healing capacities, increased probability of tissue necrosis and stress responses activated as a function of temperature (i.e. increase in mucocyte density and total antioxidant capacity). Altogether, our work indicates that Stichopathes sp. lives at suboptimal performances during the coldest months of the year, but also that it is likely to have low acclimatization capacity and a narrow thermal breadth.

An Easily Accessible Microfluidic Chip for High-Throughput Microalgae Screening for Biofuel Production

Microalgae are important green energy resources. With high efficiency in fixing carbon dioxide, microalgae are broadly applied for biofuel production. Integrating various cultivation parameters, we applied ultraviolet (UV) mutagenesis, one of the most common approaches, to induce genomic mutation in microalgae and thus enhance the production of lipid content, but the screening process is convoluted and labor-intensive. In this study, we aimed to develop an accessible microfluidic platform to optimize the biofuel production of microalgae. Instead of traditional lithography, we designed hanging-drop microfluidic chips that were fabricated using a cheap computer numerical control (CNC) micro-milling technique. On each chip, we cultured in parallel Botryococcus braunii, one of the most common freshwater microalgae for biofuel production, in sets of ten separated hanging drops (~30 µL each); we monitored their growth in each individual drop for more than 14 days. To optimize the culturing conditions, using drops of varied diameter, we first identified the influence of cell density on algae growth and lipid production. After introducing UV-induced random mutations, we quantified the lipid content of the microalgae in situ; the optimized UV-C dosage was determined accordingly. In comparison with wild-type B. braunii, the results showed increased biomass growth (137%) and lipid content (149%) of the microalgae mutated with the desired UV process. Moreover, we showed a capacity to modulate the illumination on an addressed chip area. In summary, without using an external pump system, we developed a hanging-drop microfluidic system for long-term microalgae culturing, which can be easily operated using laboratory pipettes. This microfluidic system is expected to facilitate microalgae mutation breeding, and to be applied for algae cultivation optimization.

Sea anemones (Exaiptasia pallida) use a secreted adhesive and complex pedal disc morphology for surface attachment

Background

The mechanism by which sea anemones attach to surfaces underwater remains elusive, which is surprising given their ubiquitous distribution in the world’s oceans and tractability for experimental biology. Their adhesion is mechanically interesting, bridging the interface between very hard and soft materials. The Cnidaria are thought to have evolved adhesion to surfaces at least 505 Ma ago implying that, among the Metazoa, only Porifera developed this capability earlier. The purpose of this study was primarily to address an existing hypothesis, that spirocysts (a sticky class of cnidocyst) facilitate adhesion to surfaces, as observed during prey capture.

Results

We demonstrated conclusively that spirocysts were not involved in the pedal disc adhesion of Exaiptasia pallida. Second, we applied a variety of imaging methods to develop an understanding of the true adhesion mechanism. Morphological studies using scanning electron microscopy identified a meshwork of adhesive material, unique to the pedal disc. Serial block-face SEM highlighted four classes of cells that could secrete the adhesive from the pedal disc ectoderm. A variety of histochemical techniques identified proteins, glycans and quinones in the cell contents and secreted adhesive, with variation in contents of specific cell-types in different areas of the body.

Conclusions

Spirocysts are not used by Exaiptasia pallida for adhesion to surfaces. Instead, a structurally and compositionally complex secreted glue was observed, firmly attaching the animals underwater. The results of this study provide a basis for further investigations of adhesion in Cnidaria, and establish E. pallida as a new model organism for bioadhesion research.

Host and Symbiont Cell Cycle Coordination Is Mediated by Symbiotic State, Nutrition, and Partner Identity in a Model Cnidarian-Dinoflagellate Symbiosis

Biomass regulation is critical to the overall health of cnidarian-dinoflagellate symbioses. Despite the central role of the cell cycle in the growth and proliferation of cnidarian host cells and dinoflagellate symbionts, there are few studies that have examined the potential for host-symbiont coregulation. This study provides evidence for the acceleration of host cell proliferation when in local proximity to clusters of symbionts within cnidarian tentacles. The findings suggest that symbionts augment the cell cycle of not only their enveloping host cells but also neighboring cells in the epidermis and gastrodermis. This provides a possible mechanism for rapid colonization of cnidarian tissues. In addition, the cell cycles of symbionts differed depending on nutritional regime, symbiotic state, and species identity. The responses of cell cycle profiles to these different factors implicate a role for species-specific regulation of symbiont cell cycles within host cnidarian tissues.

The cell cycle is a critical component of cellular proliferation, differentiation, and response to stress, yet its role in the regulation of intracellular symbioses is not well understood. To explore host-symbiont cell cycle coordination in a marine symbiosis, we employed a model for coral-dinoflagellate associations: the tropical sea anemone Aiptasia (Exaiptasia pallida) and its native microalgal photosymbionts (Breviolum minutum and Breviolum psygmophilum). Using fluorescent labeling and spatial point-pattern image analyses to characterize cell population distributions in both partners, we developed protocols that are tailored to the three-dimensional cellular landscape of a symbiotic sea anemone tentacle. Introducing cultured symbiont cells to symbiont-free adult hosts increased overall host cell proliferation rates. The acceleration occurred predominantly in the symbiont-containing gastrodermis near clusters of symbionts but was also observed in symbiont-free epidermal tissue layers, indicating that the presence of symbionts contributes to elevated proliferation rates in the entire host during colonization. Symbiont cell cycle progression differed between cultured algae and those residing within hosts; the endosymbiotic state resulted in increased S-phase but decreased G₂/M-phase symbiont populations. These phenotypes and the deceleration of cell cycle progression varied with symbiont identity and host nutritional status. These results demonstrate that host and symbiont cells have substantial and species-specific effects on the proliferation rates of their mutualistic partners. This is the first empirical evidence to support species-specific regulation of the symbiont cell cycle within a single cnidarian-dinoflagellate association; similar regulatory mechanisms likely govern interpartner coordination in other coral-algal symbioses and shape their ecophysiological responses to a changing climate.

Symbiont dynamics during thermal acclimation using cnidarian-dinoflagellate model holobionts

Warming oceans menace reef ecosystems by disrupting symbiosis between cnidarians and Symbiodinium zooxanthellae, thus triggering bleach episodes. Temperature fluctuations promote adjustments in physiological variables and symbiont composition, which can cause stress responses, but can also yield adaptation if fitter host-symbiont homeostasis are achieved. To understand such processes manipulative studies are required, but many reef-building cnidarians pose limitations to experimental prospects. We exposed Exaiptasia anemones to Gradual Thermal Stress (GTS) and Heat Shock (HS) exposures and monitored chlorophyll and symbiont dynamics to test the phenotypic plasticity of these photosynthetic holobionts. GTS enhanced chlorophyll concentrations and decreased Symbiodinium proliferation. A recovery period after GTS returned chlorophyll to lower concentrations and symbiont divisions to higher rates. HS triggered a stress response characterized by intense symbiont declines through degradation and expulsion, algal compensatory proliferation, and chlorophyll accumulation. Anemones pre-exposed to GTS displayed more acute signs of symbiont paucity after HS, demonstrating that recurrent stress does not always induce bleaching-resistance. Our study is the first documenting Symbiodinium C and D, along with the predominant Clade B1 in Exaiptasia anemones. C subclades found in outdoor specimens faded under laboratory exposures. Clade D emerged after HS treatments, and especially after GTS pre-exposure. This highlights the thermotolerance of D subclades found in E. pallida and shows that bleaching-recovery can involve shifts of background symbiont phylotypes. This study enlightens the capability of Exaiptasia anemones to acclimate to gradually increased temperatures, and explores into how thermal history influences in subsequent stress tolerance in symbiotic cnidarians.

Cell proliferation and migration during early development of a symbiotic scleractinian coral

In scleractinian reef-building corals, patterns of cell self-renewal, migration and death remain virtually unknown, limiting our understanding of cellular mechanisms underlying initiation of calcification, and ontogenesis of the endosymbiotic dinoflagellate relationship. In this study, we pulse-labelled the coral Stylophora pistillata for 24 h with BrdU at four life stages (planula, early metamorphosis, primary polyp and adult colony) to investigate coral and endosymbiont cell proliferation during development, while simultaneously recording TUNEL-positive (i.e. apoptotic) nuclei. In the primary polyp, the fate of BrdU-labelled cells was tracked during a 3-day chase. The pharynx and gastrodermis were identified as the most proliferative tissues in the developing polyp, and BrdU-labelled cells accumulated in the surface pseudostratified epithelium and the skeletogenic calicodermis during the chase, revealing cell migration to these epithelia. Surprisingly, the lowest cell turnover was recorded in the calicodermis at all stages, despite active, ongoing skeletal deposition. In dinoflagellate symbionts, DNA synthesis was systematically higher than coral host gastrodermis, especially in planula and early metamorphosis. The symbiont to host cell ratio remained constant, however, indicating successive post-mitotic control mechanisms by the host of its dinoflagellate density in early life stages, increasingly shifting to apoptosis in the growing primary polyp.

Body size and symbiotic status influence gonad development in Aiptasia pallida anemones

Pale anemones (Aiptasia pallida) coexist with dinoflagellates (primarily Symbiodinium minutum) in a mutualistic relationship. The purpose of this study was to investigate the role of these symbionts in gonad development of anemone hosts. Symbiotic and aposymbiotic anemones were subjected to light cycles that induced gametogenesis. These anemones were then sampled weekly for nine weeks, and gonad development was analyzed histologically. Anemone size was measured as mean body column diameter, and oocytes or sperm follicles were counted for each anemone. Generalized linear models were used to evaluate the influence of body size and symbiotic status on whether gonads were present and on the number of oocytes or sperm follicles produced. Body size predicted whether gonads were present, with larger anemones being more likely than smaller anemones to develop gonads. Both body size and symbiotic status predicted gonad size, such that larger and symbiotic anemones produced more oocytes and sperm follicles than smaller and aposymbiotic anemones. Overall, only 22 % of aposymbiotic females produced oocytes, whereas 63 % of symbiotic females produced oocytes. Similarly, 6 % of aposymbiotic males produced sperm follicles, whereas 60 % of symbiotic males produced sperm follicles. Thus, while gonads were present in 62 % of symbiotic anemones, they were present in only 11 % of aposymbiotic anemones. These results indicate that dinoflagellate symbionts influence gonad development and thus sexual maturation in both female and male Aiptasia pallida anemones. This finding substantiates and expands our current understanding of the importance of symbionts in the development and physiology of cnidarian hosts.