Novel methods to establish whole-body primary cell cultures for the cnidarians Nematostella vectensis and Pocillopora damicornis

In Vitro Phagocytosis of Different Dinoflagellate Species by Coral Cells

Coral-dinoflagellate symbiosis is a unique biological phenomenon, in which animal cells engulf single-celled photosynthetic algae and maintain them in their cytoplasm mutualistically. Studies are needed to reveal the complex mechanisms involved in symbiotic processes, but it is difficult to answer these questions using intact corals. To tackle these issues, our previous studies established an in vitro system of symbiosis between cells of the scleractinian coral Acropora tenuis and the dinoflagellate Breviolum minutum, and showed that corals direct phagocytosis, while algae are likely engulfed by coral cells passively. Several genera of the family Symbiodiniaceae can establish symbioses with corals, but the symbiotic ratio differs depending on the dinoflagellate clades involved. To understand possible causes of these differences, this study examined whether cultured coral cells show phagocytotic activity with various dinoflagellate strains similar to those shown by intact A. tenuis. We found that (a) A. tenuis larvae incorporate Symbiodinium and Breviolum, but not Cladocopium, and very few Effrenium, (b) cultured coral cells engulfed all four species but the ratio of engulfment was significantly higher with Symbiodinium and Breviolum than Cladocopium and Effrenium, (c) cultured coral cells also phagocytosed inorganic latex beads differently than they do dinoflagellates . It is likely that cultured coral cells preferentially phagocytose Symbiodinium and Breviolum, suggesting that specific molecular mechanisms involved in initiation of symbiosis should be investigated in the future.

Help Me, Symbionts, You're My Only Hope: Approaches to Accelerate our Understanding of Coral Holobiont Interactions

Tropical corals construct the three-dimensional framework for one of the most diverse ecosystems on the planet, providing habitat to a plethora of species across taxa. However, these ecosystem engineers are facing unprecedented challenges, such as increasing disease prevalence and marine heatwaves associated with anthropogenic global change. As a result, major declines in coral cover and health are being observed across the world's oceans, often due to the breakdown of coral-associated symbioses. Here, we review the interactions between the major symbiotic partners of the coral holobiont-the cnidarian host, algae in the family Symbiodiniaceae, and the microbiome-that influence trait variation, including the molecular mechanisms that underlie symbiosis and the resulting physiological benefits of different microbial partnerships. In doing so, we highlight the current framework for the formation and maintenance of cnidarian-Symbiodiniaceae symbiosis, and the role that immunity pathways play in this relationship. We emphasize that understanding these complex interactions is challenging when you consider the vast genetic variation of the cnidarian host and algal symbiont, as well as their highly diverse microbiome, which is also an important player in coral holobiont health. Given the complex interactions between and among symbiotic partners, we propose several research directions and approaches focused on symbiosis model systems and emerging technologies that will broaden our understanding of how these partner interactions may facilitate the prediction of coral holobiont phenotype, especially under rapid environmental change.

Harnessing the Power of Model Organisms To Unravel Microbial Functions in the Coral Holobiont

Stony corals build the framework of coral reefs, ecosystems of immense ecological and economic importance. The existence of these ecosystems is threatened by climate change and other anthropogenic stressors that manifest in microbial dysbiosis such as coral bleaching and disease, often leading to coral mortality. Despite a significant amount of research, the mechanisms ultimately underlying these destructive phenomena, and what could prevent or mitigate them, remain to be resolved. This is mostly due to practical challenges in experimentation on corals and the highly complex nature of the coral holobiont that also includes bacteria, archaea, protists, and viruses. While the overall importance of these partners is well recognized, their specific contributions to holobiont functioning and their interspecific dynamics remain largely unexplored. Here, we review the potential of adopting model organisms as more tractable systems to address these knowledge gaps. We draw on parallels from the broader biological and biomedical fields to guide the establishment, implementation, and integration of new and emerging model organisms with the aim of addressing the specific needs of coral research. We evaluate the cnidarian models Hydra, Aiptasia, Cassiopea, and Astrangia poculata; review the fast-evolving field of coral tissue and cell cultures; and propose a framework for the establishment of "true" tropical reef-building coral models. Based on this assessment, we also suggest future research to address key aspects limiting our ability to understand and hence improve the response of reef-building corals to future ocean conditions.

Evaluation of fluorescence-based viability stains in cells dissociated from scleractinian coral Pocillopora damicornis

The application of established cell viability assays such as the commonly used trypan blue staining method to coral cells is not straightforward due to different culture parameters and different cellular features specific to mammalian cells compared to marine invertebrates. Using Pocillopora damicornis as a model, we characterized the autofluorescence and tested different fluorescent dye pair combinations to identify alternative viability indicators. The cytotoxicity of different representative molecules, namely small organic molecules, proteins and nanoparticles (NP), was measured after 24 h of exposure using the fluorescent dye pair Hoechst 33342 and SYTOX orange. Our results show that this dye pair can be distinctly measured in the presence of fluorescent proteins plus chlorophyll. P. damicornis cells exposed for 24 h to Triton-X100, insulin or titanium dioxide (TiO₂) NPs, respectively, at concentrations ranging from 0.5 to 100 µg/mL, revealed a LC50 of 0.46 µg/mL for Triton-X100, 6.21 µg/mL for TiO₂ NPs and 33.9 µg/mL for insulin. This work presents the approach used to customize dye pairs for membrane integrity-based cell viability assays considering the species- and genotype-specific autofluorescence of scleractinian corals, namely: endogenous fluorescence characterization followed by the selection of dyes that do not overlap with endogenous signals.

Cryopreservation and Cryobanking of Cells from 100 Coral Species

When coral species become extinct, their genetic resources cannot be recovered. Coral cryobanks can be employed to preserve coral samples and thereby maintain the availability of the samples and increase their potential to be restocked. In this study, we developed a procedure to determine coral species-specific requirements for cryobank freezing through determining suitable cryoprotective agents (CPAs), CPA concentrations, equilibration times, holding durations, viability rates, and cell amounts for banked coral cells, and we established the first ever coral cell cryobank. Coral cells, including supporting and gland cells, epidermal nematocysts, Symbiodiniaceae and symbiotic endoderm cells (SEC) were found from the extracted protocol. Approximately half of the corals from the experimental corals consisted of spindle and cluster cells. Gastrodermal nematocysts were the least common. The overall concentration of Symbiodiniaceae in the coral cells was 8.6%. Freezing using DMSO as a CPA was suitable for approximately half of the corals, and for the other half of species, successful cell cryopreservation was achieved using MeOH and EG. EG and DMSO had similar suitabilities for Acanthastrea, Euphyllia, Favites, Lobophyllia, Pavona, Seriatopora, and Turbinaria, as did EG and MeOH for Acropora, Echinopyllia, and Sinularia and MeOH and DMSO for Platygyra after freezing. At least 14 straws from each species of coral were cryobanked in this study, totaling more than 1884 straws (0.5 mL) with an average concentration of 6.4 × 10⁶ per mL. The results of this study may serve as a framework for cryobanks worldwide and contribute to the long-term conservation of coral reefs.

Analysis of Spatial Gene Expression at the Cellular Level in Stony Corals

Scleractinians, or stony corals, are colonial animals that possess a high regenerative capacity and a highly diverse innate immune system. As such they present the opportunity to investigate the interconnection between regeneration and immunity in a colonial animal. Understanding the relationship between regeneration and immunity in stony corals is of further interest as it has major implications for coral reef health. One method for understanding the role of innate immunity in scleractinian regeneration is in situ hybridization using RNA probes. Here we describe a protocol for in situ hybridization in adult stony corals using a digoxigenin (DIG)-labeled RNA antisense probe which can be utilized to investigate the spatial expression of immune factors during regeneration.

Development of an in vitro tissue culture system for hammer coral (Fimbriaphyllia ancora) ovaries

In vitro gonad culture systems have proven useful to investigate intrinsic mechanisms of sexual reproduction in animals. Here we describe development of an in vitro culture method for coral ovaries. Mesenterial tissues containing both ovaries and mesenterial filaments were microscopically isolated from the scleractinian coral, Fimbriaphyllia ancora, and culture conditions were optimized. M199 diluted 10× (10% M199, pH 8.1) and supplemented with 25 mM HEPES and the antibiotics, ampicillin, penicillin and streptomycin, supported oocyte survival and maintained the structural integrity of ovaries during short-term culture (~ 6 days). Addition of a commercial antibiotic-antimycotic solution (Anti-Anti) and fetal bovine serum adversely affected ovary maintenance and caused tissue disintegration. Characterization of cultured ovaries showed that there is no difference in cell proliferation of ovarian somatic cells between culture Days 1 and 6. Moreover, the presence of oogonia and expression of a major yolk protein, vitellogenin, were confirmed in ovaries cultured for 6 days. This system will be useful for studying effects of a wide range of substances on coral oogenesis.

Unlocking the single-cell mysteries of a reef-building coral

Coral reefs are one of the most important ecosystems in the world but least understood from a cellular level. In this issue of Cell, Levy et al. unravel the single-cell gene expression of the coral holobiont and open the doors to better understand the novel diversity of cell types.