Davide Seveso

Coral-associated dinoflagellates undergo substantial physiological changes when transferred from the host environment to laboratory culture, yet direct compositional comparisons remain limited. We compared the proximate composition and fatty acid profiles of Symbiodinium microadriaticum (ITS2 type A1) freshly isolated from Stylophora pistillata with free-living cultures derived from the same coral-associated symbiont material and maintained under controlled conditions. Total lipid and crude protein contents did not differ significantly between states, whereas fatty acid composition differed markedly. Sixteen individual fatty acids remained significantly different after Benjamini-Hochberg false discovery rate correction. Freshly isolated samples showed higher relative abundances of several saturated, monounsaturated, and n-6 polyunsaturated fatty acids, including 16:0, 18:0, 16:1n-7, arachidonic acid, and docosatetraenoic acid, together with a higher proportion of total n-6 fatty acids. Free-living cells showed higher relative abundances of 14:0, 16:3n-4, α-linolenic acid, and 18:5n-3, together with higher proportions of total PUFA, n-3, and n-9 fatty acids and a higher docosahexaenoic acid/arachidonic acid (DHA/ARA) ratio. These contrasting signatures indicate marked state-associated biochemical differentiation despite similar bulk lipid and protein contents. The results provide a direct compositional reference for interpreting cultured Symbiodiniaceae used in algal physiology, coral-algal symbiosis research, and experimental host acquisition.

Outbreaks of crown-of-thorns sea stars (CoTS) threaten coral reef integrity and biodiversity, yet local dynamics and short-term responses to control remain insufficiently described. This study characterised an outbreaking Acanthaster population in two specific sites of the coast of Khor Fakkan (Gulf of Oman, United Arab Emirates) to resolve species identity, population composition, prey selection and the effects of targeted removals. All sequenced individuals clustered in two related haplotypes belonging to the species Acanthaster planci. Benthic surveys showed moderate live-coral cover, dominated by massive Porites sp. colonies. Moreover, the observations of 139 preyed colonies revealed pronounced genus-level selectivity, with branching and complex morphologies suffering disproportionately and massive forms largely avoided. However, the selection of massive Plesiastrea and Favites genera as preferred coral prey might suggest a shift towards less preferred coral in the CoTS diet, posing a severe threat to coral reefs’ integrity. Intensive removal reduced the local density, up to 86%, and provided substantial short-term relief, but continued monitoring is required to secure long-term reef resilience.

Current climate changes cause increasing stress to corals, which can disrupt host-symbiont relationships, leading to bleaching and mortality. While stress responses are well-documented at the coral colony and reef scales, less is known about how stress responses affect different compartments within the coral holobiont. Here, we investigated the impact of thermal stress on macro– and microscale compartments, including the endoderm-lined gastrovascular cavity (GVC) and the ectoderm-lined external tissue surface, in two tropical corals, Caulastrea curvata and Galaxea fascicularis . We assessed the coral physiological status via respirometry, variable chlorophyll fluorometry, O ₂ and H ₂ O ₂ microsensor analyses, chlorophyll measurements, and oxidative stress biomarkers during a 14-day thermal stress exposure. Under heat stress, both species showed a reduced O ₂ concentration in the GVC by up to 90% in the light, reaching hypoxic (< 50 µmol O ₂ L ⁻¹ ) to anoxic levels, while the O ₂ concentration in the external surface tissue of C. curvata decreased by 20%. Moreover, no changes in the H ₂ O ₂ dynamics of the coral’s external tissue surface were detected. Lipid peroxidation in the gastrodermis of both species increased significantly by 90–198% indicating oxidative damage, although antioxidant enzyme activity (SOD and CAT), chlorophyll content, and bulk metabolic rates (respiration and photosynthesis) remained stable. Our findings indicated that thermal stress could affect the microenvironment of corals, particularly in the GVC, without visible bleaching or major disruptions in bulk physiology. This suggested the importance of microenvironmental metrics for the early detection of coral physiological stress.

Light limitation caused by increased sedimentation and turbidity, mainly due to intense human activities, poses a particularly pronounced threat to coral reefs located near urban areas, such as those in Singapore. In this study, we investigated the photophysiological and bleaching responses of Pachyseris speciosa , a common reef building coral species found in Singapore’s turbid reefs, under low-light conditions over a four-week period. The experimental treatments included intermittent light and complete darkness, simulating possible current and potential future light scenarios for Singapore's reefs under heightened turbidity due to coastal development and climate change. Our results showed that P. speciosa experienced declines in Symbiodiniaceae density, chlorophyll-a levels, and color scores under both treatment conditions, with severe bleaching occurring in complete darkness. However, corals exposed to intermittent light exhibited only tissue paling and significant photoacclimatory adjustments. Notably, these corals maintained the stability of the maximum quantum yield of photosystem II, increased their effective quantum yield, and decreased the saturating irradiance. This suggested a potential photoadaptation to intermittent light conditions and fluctuating light levels, allowing them to sustain elevated photosynthetic performance. Our findings highlighted and confirmed the resilience and ability of P. speciosa to tolerate various limited light conditions and enhanced our understanding of coral photoacclimation under extremely low-light scenarios.

Coral reef survival is crucial for the socio-ecological interest of many countries, particularly for the Republic of Maldives, whose reef integrity influences the country’s livelihoods and economy. These ecosystems are being severely impacted by multiple stressors, leading to declines in biodiversity. In the last few decades, researchers have focused on studying coral-associated microorganisms (CAMs) and their symbiotic role in coral health and resilience. Metabarcoding analysis has been widely utilized to study CAM diversity under various conditions but provides limited information on their functional roles. Therefore, cultivation of bacterial strains remains indispensable for validating ecological and biotechnological hypotheses. In this study, we investigated the microbial community associated with two abundant corals in Maldives, Porites lobata and Acropora gemmifera, and evaluated the antimicrobial, antioxidant, and UV-protectant properties of 10 promising isolated strains. The selected CAMs, Pseudoalteromonas piscicida 39, Streptomyces parvus 79, Microbacterium sp. 92 (a potential novel species), and Micromonospora arenicola 93, exhibited antibiotic activity against a panel of pathogenic strains (MIC from 0.01 to 500 µg/mL), antioxidant (comparable effect to that of Trolox and ascorbic acid), and UV-screen activities (protection of human keratinocytes at 200 µg/mL). Genomes revealed their dual potential in contributing to coral restoration and drug discovery strategies. These findings highlight the biotechnological relevance of CAMs, representing an important step toward the identification of novel and bioactive bacterial species beneficial for coral reef ecosystems and human health.