Patricia Elena Thomé

La capacidad inmune de los cnidarios les permite mantener la integridad de sus tejidos y prevenir infecciones ante amenazas de daño. Los cnidarios cuentan con un sistema inmune innato que incluye la melanina y enzimas, como la fenoloxidasa, que se activa en presencia de patógenos, generando compuestos reactivos y tóxicos para su eliminación. La anémona Exaiptasia diaphana es un organismo modelo en estudios sobre simbiosis, nutrición e inmunidad, ya que mantiene una simbiosis facultativa con las microalgas de la familia Symbiodiniaceae, como los corales; E. diaphana también puede existir en un estado aposimbiótico. Por tanto, E. diaphana es un modelo adecuado para estudiar el sistema inmune de los cnidarios. Para conocer la actividad inmune básica de esta anémona, nos preguntamos si existe variabilidad en la actividad de la enzima fenoloxidasa a lo largo del día, considerando 2 fases lunares (luna llena y luna nueva). Se tomaron muestras de mucus de 6 anémonas simbióticas y 6 aposimbióticas, previamente aclimatadas y mantenidas en cajas de 6 pozos con agua de mar filtrada bajo condiciones de luz natural. La actividad de fenoloxidasa se estimó cada 6 h durante 48 h, con 2 repeticiones por fase lunar. La actividad de fenoloxidasa presentó mayor actividad a las 14:00 h, mientras que la actividad nocturna fue baja. La actividad de la enzima se incrementó bajo luna llena en las anémonas aposimbióticas (H = 17.47, P = 0.0005). Además, encontramos una relación negativa entre la densidad de simbiontes y la actividad de fenoloxidasa (R2 = 0.50, P = 0.04). Estos resultados nos permiten sugerir que E. diaphana puede mantener una inmunidad constitutiva y constante en condiciones de laboratorio, la cual responde a ciclos biológicos, como los ciclos circadianos y lunares.

Cassiopea xamachana is a tropical medusa that lives in symbiosis with dinoflagellate algae, serving as a model organism for symbiotic studies. The symbiosis is necessary for this medusa to complete its life cycle. The symbiotic partners maintain a metabolic exchange of organic molecules that constitute an important source of energy for the animal host, with free organic molecules, like glucose and glycerol, being the primary source. This molecular exchange can be facilitated by cellular internal membrane transport proteins, such as Glucose membrane transporters (GLUTs) and Glycerol transport-like aquaglyceroporins (GLP-like), probably located at the symbiosomal interface. The present study was conducted in October 2021, evaluating the expression of transporter coding genes GLUT3, GLUT8, and GLP9 (two genes) by qPCR under conditions of symbiosis and after the loss of symbionts. Symbiotic medusae donated from Xcaret Park, Mexico (20° 34′ 24.59″ N; -87° 07′ 5.40″ W) were sampled and compared to medusae with an experimental decrease of algal symbionts. In agreement with glucose being an important mobile molecule, our results showed higher transcription levels for glucose transporters GLUT3 and GLUT8 in control compared to bleached medusae. By contrast, bleached medusae showed a higher expression of aquaglyceroporin transporters GLP9-1 and GLP9-2, probably associated with glycerol production after lipid catabolism, to compensate for lower organic carbon levels due to the loss of symbionts. Our results highlight the importance of free carbon molecules transported from symbiont to host and agree with glucose being an energy fuel for this symbiotic association.

Cassiopea xamachana is a model system for studies in animal symbiosis with algal symbionts. This medusa is also associated with a microbial community that can impact its health, but this community has not been thoroughly studied. Shifts in the bacterial community following the loss of symbionts involving stress, environmental changes, or seasonal fluctuations can be complex, as the role of symbionts in structuring this community is not well established. To understand the interplay among microbial associates with this host, we explored the experimental diminishing of algal symbionts, and the influence of seasonal fluctuations over the structure of the bacterial community, through 16S rRNA gene high-throughput sequencing. Results showed that Gammaproteobacteria, Bacteroidia, and Alphaproteobacteria were dominant in all the mucus samples at the beginning of the experiments. However, after 28 days, bleached medusas showed a marked increase in Gammaproteobacteria, specifically in the genus Vibrio, as evidenced by Linear Discriminant Analysis of Effect Size (LEfSe). Seasons also resulted in shifts of the bacterial community, although bacterial genera were distinct from those found in bleached medusas, suggesting temporal associations with the host. According to PERMANOVA analysis, seasonal fluctuations affected the dominant bacterial members (p = 0.07), but symbiont presence was a more significant driver (p=0.001). We found the bacterial community of C. xamachana is like that of other jellyfish and corals, which furthers the interest in this animal as a study model. Defining relevant bacterial genera can help us understand the functional role of the holobiont members that assemble and maintain a healthy microbial community. Also, studies in other regions where C. xamachana distributes can help us define a core bacterial community for this medusa.

Signaling pathways are fundamental for the establishment and maintenance of diverse symbioses. The symbiosis of cnidarians and dinoflagellate algae is the foundation for the ecological success of coral reefs, involving the transfer of photosynthetic products from the symbiont to host. However, signal transduction pathways for this symbiosis remain uncharacterized. Cultured and natural cnidarian symbionts can produce glycerol, one of the main translocated photosynthates. Here, we investigate whether a signal transduction pathway may be involved in inducing glycerol synthesis in cultured symbionts under an osmotic stress model. We evaluated the effect of specific inhibitors of the main transduction pathways, p38, JNK, and ERK 1/2 in Brevolium minutum, the symbiont of the Aiptasia model system. We found that glycerol production and the specific activity of the enzyme Gpdh were selectively inhibited by a p38 Mitogen‐Activated Protein Kinase (MAPK) inhibitor. Additionally, the phosphorylation of a putative p38‐like protein was rapidly detected. Finally, we studied the presence of each of the components of the p38 MAPK pathway in silico in genomes and transcriptomes reported up to date for different symbiont types. We propose a model for the arrangement of this pathway in the family of dinoflagellate symbionts known as Symbiodiniaceae.

Reef corals in the Mexican Reef System have been severely affected by the emergence of a white syndrome that resembles both White Plague II and SCTLD descriptions. Meandroid scleractinian coral species are among the most severely affected. To gain insight into this affliction we conducted a broad study in the brain coral Pseudodiploria strigosa at a rear reef site in the NE Mexican Caribbean. We describe macro and microscopical signals of the disease, characterize the outbreak dynamics, the tissue histopathology, explore immunological responses in the individuals, and compare microbial assemblages associated with the surface mucus layer of healthy and unhealthy colonies. At the study site, the white syndrome outbreak on P. strigosa showed a high incidence rate in summer-fall and a low one in winter, as well as low survival expectation of diseased colonies at the end of the study. After 306 days of observation, out of 96 tracked colonies, eight remained apparently healthy and seven were diseased. No effective resistance to colony disease progression was observed once white syndrome signs developed. Tissue loss rate during the study varied among colonies (mean = 10.8 cm², s.d. = 7.8 cm²) suggesting a complex relation between causal agents and colony resistance. The deterioration of tissues was evidenced from the basal to the surface body wall of polyps (up to 66% hypertrophy and liquefactive necrosis in unhealthy colonies), implying that microscopic alterations begin before macroscopic signals develop, suggesting this may be a systemic disease. We measured high levels of phenoloxidase (two orders of magnitude higher PO activity than P. strigosa affected by BBD) and antibacterial activity without significant reduction in unhealthy samples from the mucus layer, indicative of an enhanced immunological response. Results showed that opportunistic bacteria dominated damaged colonies, where six genera of the Bacteroidia class were found with significant changes in unhealthy colonies after DeSeq2 analysis. Nevertheless, histological observations did not support infection of the tissues. The opportunistic overload seems to be contained within the mucus layer but may be associated with the mortality of tissues in a yet unclear way. Future research should focus on experimental infections, the tracking of natural infections, and the immunocompetence of corals in the face of environmental pressures due to local, regional, and global impacts. If environmental deterioration is the primary cause of the continuing emergence and re-emergence of lethal coral diseases, as has been proposed by many authors, the only true option to effectively help preserve the coral reef biodiversity and services, is to restore the environmental quality of reef waters at the local scale and reduce greenhouse gases at the global scale.

Corals are an association between animals and plants that deposit a skeleton and build reefs. Coral reefs are now in danger because the marine environment around them is degrading, causing corals to get ill more often. Scientists have used fragments of healthy coral colonies or planted baby corals to help restore reefs that are in bad shape. But for these efforts to succeed, it is vital for the scientists to select healthy corals for planting. Corals use a substance called melanin to trap bacteria that grows in their tissues. We discovered that the enzyme phenoloxidase, which is responsible for making melanin, can be found in the snotty covering of corals, called mucus. We can take samples of this mucus to measure phenoloxidase without harming the corals. By doing so, we can detect whether a coral colony is responding to the presence of harmful bacteria even before it shows any signs of illness.

Coral reefs are affected by the deterioration of the oceans due to global warming and other anthropogenic perturbations, increasing the frequency and severity of bleaching and disease. To overcome some of these conditions, reef corals and other cnidarians rely on a mucus layer housing a diverse community of beneficial microorganisms and mechanisms of innate immune response. The antimicrobial defense has been associated with the bacterial community in these organisms, but the potential antimicrobial activity of the mucus layer itself has not been explored fully. We hypothesized that the bacteria-free mucus layer of different cnidarians would show differential and specific antimicrobial and immunological responses when challenged with two potentially pathogenic bacteria. We evaluated this capability through antimicrobial properties, immune response and biochemical composition of the mucus. Results clearly showed that the mucus of healthy cnidarians has the capability of inhibiting the growth of Serratia marcescens and Aurantimonas sp. in a species-specific way that includes differences in the potency of the response. The anemone Exaiptasia pallida was particularly potent against Aurantimonas sp. while the coral Pseudodiploria strigosa and the medusa Cassiopea xamachana had similar capabilities against both bacterial strains. In coral affected by black band disease, this antibacterial capability diminished in the mucus layer, but the associated bacteria remained potent. Results showed that hydroxyproline and phenoloxidase increased in the transition zone of diseased corals, although melanin was not detected in any of the animals tested. Bleaching of anemones and medusas also diminished the antibacterial capability of the surface mucus layer, but in anemones, the associated bacteria did not show a significant reduction in their ability to inhibit the growth of the bacterial strains. The mucus of bleached medusas showed an increased inhibitory activity against Aurantimonas sp. that may be associated with a specific bacterial strain we isolated. Mucus collected from bleached anemones and medusas did not show a significant immune response. In this work, we show that the surface mucus layer itself has antibacterial properties not associated with the bacteria this layer houses; such properties diminished due to disease or bleaching, while immunological responses increased in the mucus of diseased animals.

The upside-down jellyfish Cassiopea xamachana (Scyphozoa: Rhizostomeae) has been predominantly studied to understand its interaction with the endosymbiotic dinoflagellate algae Symbiodinium. As an easily culturable and tractable cnidarian model, it is an attractive alternative to stony corals to understanding the mechanisms driving establishment and maintenance of symbiosis. Cassiopea is also unique in requiring the symbiont in order to complete its transition to the adult stage, thereby providing an excellent model to understand symbiosis-driven development and evolution. Recently, the Cassiopea research system has gained interest beyond symbiosis in fields related to embryology, climate ecology, behavior, and more. With these developments, resources including genomes, transcriptomes, and laboratory protocols are steadily increasing. This review provides an overview of the broad range of interdisciplinary research that has utilized the Cassiopea model and highlights the advantages of using the model for future research.

Our current understanding of carbon exchange between partners in the Symbiodinium-cnidarian symbioses is still limited, even though studies employing carbon isotopes have made us aware of the metabolic complexity of this exchange. We examined glycerol and glucose metabolism to better understand how photosynthates are exchanged between host and symbiont. The levels of these metabolites were compared between symbiotic and bleached Exaiptasia pallida anemones, assaying enzymes directly involved in their metabolism. We measured a significant decrease of glucose levels in bleached animals but a significant increase in glycerol and G3P pools, suggesting that bleached animals degrade lipids to compensate for the loss of symbionts and seem to rely on symbiotic glucose. The lower glycerol 3-phosphate dehydrogenase but higher glucose 6-phosphate dehydrogenase specific activities measured in bleached animals agree with a metabolic deficit mainly due to the loss of glucose from the ruptured symbiosis. These results corroborate previous observations on carbon translocation from symbiont to host in the sea anemone Exaiptasia, where glucose was proposed as a main translocated metabolite. To better understand photosynthate translocation and its regulation, additional research with other symbiotic cnidarians is needed, in particular, those with calcium carbonate skeletons.