Site-specific variation in gene expression from Symbiodinium spp. associated with offshore and inshore Porites astreoides in the lower Florida Keys is lost with bleaching and disease stress

Heat stress in symbiotic dinoflagellates: Implications on oxidative stress and cellular changes

Global warming has been shown to harmfully affect symbiosis between Symbiodiniaceae and other marine invertebrates. When symbiotic dinoflagellates (the genus Breviolum) were in vitro exposed to acute heat stress of +7 °C for a period of 5 days, the results revealed the negative impact on all physiological and other cellular parameters measured. Elevated temperatures resulted in a severe reduction in algal density of up to 9.5-fold, as well as pigment concentrations, indicating the status of the physiological stress and early signs of photo-bleaching. Reactive oxygen species (ROS) were increased in all heated dinoflagellate cells, while the antioxidant-reduced glutathione levels initially dropped on day one but increased under prolonged temperature stress. The cell viability parameters were reduced by 97 % over the heating period, with an increased proportion of apoptotic and necrotic cells. Autofluorescence (AF) for Cy5-PE 660-20 was reduced from 1.7-fold at day 1 to up to 50-fold drop at the end of heating time, indicating that the AF changes were highly sensitive to heat stress and that it could be an extremely sensitive tool for assessing the functionality of algal photosynthetic machinery. The addition of the drug 5-AZA-2'-deoxycytidine (5-AZA), which inhibits DNA methylation processes, was assessed in parallel and contributed to some alterations in algal cellular stress response. The presence of drug 5-AZA combined with the temperature stress had an additional impact on Symbiodiniaceae density and cell complexity, including the AF levels. These variations in cellular stress response under heat stress and compromised DNA methylation conditions may indicate the importance of this epigenetic mechanism for symbiotic dinoflagellate thermal tolerance adaptability over a longer period, which needs further exploration. Consequently, the increased ROS levels and changes in AF signals reported during ongoing heat stress in dinoflagellate cells could be used as early stress biomarkers in these microalgae and potentially other photosynthetic species.

Indications of symbiotic state influencing melanin-synthesis immune response in the facultative coral Astrangia poculata

Increased ocean warming is causing detrimental impacts to tropical corals worldwide. Compounding the effects of heat stress, incidences of tropical coral disease have risen concurrently. While tropical coral responses to these impacts are well studied, temperate coral responses remain largely unknown. The present study focused on the immune response of the temperate coral Astrangia poculata to increased temperature and disease. Symbiotic and aposymbiotic A. poculata were collected from Narragansett Bay, Rhode Island (USA) in summer and winter seasons and exposed to control (18°C) versus elevated temperatures (26°C) in the presence of an immune stimulant (i.e. lipopolysaccharide) for a 12 h period. Prophenoloxidase (PPO) and melanin concentrations from the melanin-synthesis pathway were assessed via spectrophotometry to examine immune responses. While PPO measurements were higher on average in symbiotic corals compared with aposymbiotic corals, temperature and season did not significantly affect this metric. Melanin was significantly higher in symbiotic compared to aposymbiotic corals, implying that symbiotic state may be important for melanin-synthesis response. Conversely, melanin as an immune response may be of less importance in aposymbiotic A. poculata due to the potential capacity of other immune responses in this species. In addition, differences in resource allocation to immune investment as a result of symbiosis is plausible given melanin production observed within the present study. However, thermal stressors may reduce the overall influence of symbiosis on melanin production. Future studies should build upon these results to further understand the entirety of innate immunity responses in temperate coral species.

The genome of the mustard hill coral, Porites astreoides

Anthropogenic effects have contributed to substantial declines in coral reefs worldwide. However, some corals are more resilient to environmental changes and have increased in relative abundance, thus these species may shape future reef communities. Here, we provide the first draft reference genome for the mustard hill coral, Porites astreoides, collected in Bermuda. DNA was sequenced via Pacific Biosciences (PacBio) HiFi long-read technology. PacBio read assembly with FALCON UnZip resulted in a 678-Mbp assembly with 3051 contigs with an N50 of 412,256 and the BUSCO completeness analysis resulted in 90.9% of the metazoan gene set. An ab initio transcriptome was also produced with 64,636 gene models with a transcriptome BUSCO completeness analysis of 77.5% versus the metazoan gene set. Functional annotation was completed for 86.6% of proteins. These data are valuable resources for improving biological knowledge of P. astreoides, facilitating comparative genomics for corals, and supporting evidence-based restoration and human-assisted evolution of corals.

Decreased Photosynthetic Efficiency in Response to Site Translocation and Elevated Temperature Is Mitigated with LPS Exposure in Porites astreoides Symbionts

Coral reefs have been detrimentally impacted causing health issues due to elevated ocean temperatures as a result of increased greenhouse gases. Extreme temperatures have also exacerbated coral diseases in tropical reef environments. Numerous studies have outlined the impacts of thermal stress and disease on coral organisms, as well as understanding the influence of site-based characteristics on coral physiology. However, few have discussed the interaction of all three. Laboratory out-planting restoration projects have been of importance throughout impacted areas such as the Caribbean and southern Florida in order to increase coral cover in these areas. This study analyzes photosynthetic efficiency of Porites astreoides from the lower Florida Keys after a two-year reciprocal transplant study at inshore (Birthday reef) and offshore (Acer24 reef) sites to understand acclimation capacity of this species. Laboratory experiments subjected these colonies to one of three treatments: control conditions, increases in temperature, and increases in temperature plus exposure to an immune stimulant (lipopolysaccharide (LPS)) to determine their influence on photosynthetic efficiency and how stress events impact these measurements. In addition, this study is a continuation of previous studies from this group. Here, we aim to understand if these results are static or if an acclimation capacity could be found. Overall, we observed site-specific influences from the Acer24 reef site, which had significant decreases in photosynthetic efficiencies in 32 °C treatments compared to Birthday reef colonies. We suggest that high irradiance and lack of an annual recovery period from the Acer24 site exposes these colonies to significant photoinhibition. In addition, we observed significant increases in photosynthetic efficiencies from LPS exposure. We suggest host-derived antioxidants can mitigate the negative impacts of increased thermal stress. Further research is required to understand the full complexity of host immunity and symbiont photosynthetic interactions.

Variation in Immune-Related Gene Expression Provides Evidence of Local Adaptation in Porites astreoides (Lamarck, 1816) between Inshore and Offshore Meta-Populations Inhabiting the Lower Florida Reef Tract, USA

Coral communities of the Florida Reef Tract (FRT) have changed dramatically over the past 30 years. Coral cover throughout the FRT is disproportionately distributed; >70% of total coral cover is found within the inshore patch reef zone (<2 km from shore) compared to 30% found within the offshore bank reef zone (>5 km from shore). Coral mortality from disease has been differentially observed between inshore and offshore reefs along the FRT. Therefore, differences between the response of inshore and offshore coral populations to bacterial challenge may contribute to differences in coral cover. We examined immune system activation in Porites astreoides (Lamarck, 1816), a species common in both inshore and offshore reef environments in the FRT. Colonies from a representative inshore and offshore site were reciprocally transplanted and the expression of three genes monitored biannually for two years (two summer and two winter periods). Variation in the expression of eukaryotic translation initiation factor 3, subunit H (eIF3H), an indicator of cellular stress in Porites astreoides, did not follow annual patterns of seawater temperatures (SWT) indicating the contribution of other stressors (e.g., irradiance). Greater expression of tumor necrosis factor (TNF) receptor associated factor 3 (TRAF3), a signaling protein of the inflammatory response, was observed among corals transplanted to, or located within the offshore environment indicating that an increased immune response is associated with offshore coral more so than the inshore coral (p < 0.001). Corals collected from the offshore site also upregulated the expression of adenylyl cyclase associated protein 2 (ACAP2), increases which are associated with decreasing innate immune system inflammatory responses, indicating a counteractive response to increased stimulation of the innate immune system. Activation of the innate immune system is a metabolically costly survival strategy. Among the two reefs studied, the offshore population had a smaller mean colony size and decreased colony abundance compared to the inshore site. This correlation suggests that tradeoffs may exist between the activation of the innate immune system and survival and growth. Consequently, immune system activation may contribute to coral community dynamics and declines along the FRT.

Proteomic Signatures of Corals from Thermodynamic Reefs

Unlike most parts of the world, coral reefs of Taiwan’s deep south have generally been spared from climate change-induced degradation. This has been linked to the oceanographically unique nature of Nanwan Bay, where intense upwelling occurs. Specifically, large-amplitude internal waves cause shifts in temperature of 6–9 °C over the course of several hours, and the resident corals not only thrive under such conditions, but they have also been shown to withstand multi-month laboratory incubations at experimentally elevated temperatures. To gain insight into the sub-cellular basis of acclimation to upwelling, proteins isolated from reef corals (Seriatopora hystrix) featured in laboratory-based reciprocal transplant studies in which corals from upwelling and non-upwelling control reefs (<20 km away) were exposed to stable or variable temperature regimes were analyzed via label-based proteomics (iTRAQ). Corals exposed to their “native” temperature conditions for seven days (1) demonstrated highest growth rates and (2) were most distinct from one another with respect to their protein signatures. The latter observation was driven by the fact that two Symbiodiniaceae lipid trafficking proteins, sec1a and sec34, were marginally up-regulated in corals exposed to their native temperature conditions. Alongside the marked degree of proteomic “site fidelity” documented, this dataset sheds light on the molecular mechanisms underlying acclimatization to thermodynamically extreme conditions in situ.