Journal of Phycology

Unlike most other heterotrophic protist lineages which synthesize glycogen, alveolates accumulate -glucan storage polysaccharides in the form of starch. Within Alveolata, ciliates do so by using a simple pathway analogous to that used for glycogen metabolism in eukaryotes. However, Myzozoa have evolved five additional enzymatic steps which are common to Dinoflagellata, Chromerids and Apicomplexa. Moreover, dinoflagellates have acquired seven additional enzymes that distinguish them from the Chromerids-Apicompexa lineages, while chromerids possess only a single enzyme not found in dinoflagellates. Such modifications are not found in other eukaryotic lineages save for those resulting directly from primary or secondary plastid endosymbiosis. The many differences affecting the starch pathway of Myzozoa can be explained by a single secondary or tertiary plastid endosymbiosis of a starch-containing alga followed by distinct compartmentalisation outcomes of starch metabolism in Dinoflagellata relative to chromerids and Apicomplexa.

This study addresses the functional diversity of wild Limnospira platensis strains from two continents. Wild strains retain regulatory and metabolic flexibility often lost during domestication, making them essential for exploring natural adaptation within the species evolutionary history. We investigated the comparative growth performance of five Limnospira strains under controlled temperature and light regimes to identify phenotypic differences attributable to single environmental variables. We then evaluated their biochemical and biophysical responses under naturally fluctuating light conditions, linking energy use strategies to phylogenetic relationships. Our analyses reveal that while certain traits, such as fine-tuning of photosynthetic reactions, are shared among closely related strains, overall growth performance is highly variable even within phylogenetically similar groups. For example, photosensitive strains (e.g., UTEX 3086) differ markedly from high light tolerant strains (e.g., UTEX LB 2342) in their mechanisms of coping with light stress. Together, these comparative analyses highlight how environmental sensitivity and energy-use strategies vary across strains of the same species, reflecting broader patterns of diversification within Limnospira.

Hydractiniidae is a diverse family of colonial hydrozoans exhibiting substantial variation in colony organization, reproduction and life-cycle strategies, including species with and without a medusa stage. Several hydractiniids are also established model organisms, yet species boundaries and distributions remain incompletely resolved, limiting comparisons of reproductive and developmental traits across taxa. Here, we combine morphological and molecular data to reassess hydractiniid diversity, species identities, and life-cycle variation in material from the North Atlantic and Arctic Ocean. We integrate detailed observations of colonies and reproductive structures with DNA barcoding and mitochondrial genomes to evaluate species boundaries and provide molecular support for taxonomic identifications. Our results confirm 10 species in the study area, occurring from shallow waters to 2600 m depth and from coastal fjords to deep-sea hydrothermal vents. We further show that some historical species identifications and distribution records require reassessment. We establish a new genus for Arctic hydractiniid species (Bonneviactinia gen. nov.) and reassign two species to Schuchertinia. Mapping reproductive strategies onto the phylogeny reveals a broad pattern of increasing retention of medusa characters, from fixed sporosacs through reduced and fixed medusoids and non-liberated eumedusoids to fully developed, free-swimming feeding medusae, suggesting a progressive evolutionary trajectory in life-cycle complexity across Hydractiniidae. Furthermore, we clarify the identity of Podocoryna carnea and, through rearing experiments from polyp stage to medusae, characterize its morphological variation and developmental stages. By linking morphologically documented specimens and vouchers to genetic data, we establish reference resources that will facilitate future taxonomic, phylogenetic and biodiversity studies, including molecular surveys of marine communities. Together, these results demonstrate the value of integrative taxonomy for resolving hydractiniid diversity and provide a comparative foundation for investigating the evolution of reproductive strategies and life-cycle complexity in Medusozoa.

A diverse array of photosynthetic phytoplankton drives primary production in equatorial surface waters. Among these, the cyanobacterium Prochlorococcus is an important contributor to net primary production in these typically iron-limited, high-nutrient and low-chlorophyll (HNLC) regions. Here, we explore the diversity of these organisms, in part, through targeted enrichment of Prochlorococcus cells using field-based high-speed cell sorting techniques. We demonstrate that the genomes of Prochlorococcus belonging to the low-light adapted LLI clade, and isolated from the surface of the Equatorial Pacific Ocean, are depleted in functions related to the assimilation of urea, nitrite, and amino acids. These are the first examples of LLI Prochlorococcus that have lost the ability to use nitrite, a trait considered to be a core feature of this clade. All new equatorial cultures of LLI Prochlorococcus appear to use a distinct isoform of protoporphyrinogen IX oxidase (HemG), for the biosynthesis of a chlorophyll precursor, that does not require the use of iron-containing heme. In contrast, the heme-dependent HemJ isoform is typically used by Prochlorococcus found outside equatorial HNLC waters. Together, these findings suggest that low-light adapted Prochlorococcus in the equatorial ocean possess accessory gene content that reflects adaptation to the generally iron-limited but nitrogen-replete conditions of surface waters.

Hydrodynamics is a strong regulator of phytoplankton cell densities and population connectivity. However, the extent to which hydrodynamics, specifically dilution and advection, dictates the internal physiological state and overall bloom dynamics remains poorly understood. We conducted an integrated study of the harmful diatom Pseudo-nitzschia australis in two neighboring coastal bays with similar abiotic conditions but contrasting bloom trajectories. Using a combination of field observations, transcriptomics, domoic acid (DA) measurements, and hydrodynamic simulations, we show that a three-fold difference in dilution rates alone explains a 25-day shift in bloom timing and an order-of-magnitude difference in cell densities. Transcriptomic profiling revealed distinct metabolic and stress-associated gene modules that synchronized with hydrodynamically-mediated nutrient depletion rate, linking residence times to cellular mortality. Furthermore, directional connectivity between the bays may explain population-level toxin heterogeneity, as advection of older cells, releasing DA outside the cells, may have decreased the proportion of intracellular DA in the destination bay. Our results illustrate that hydrodynamics do not merely modulate population transport; they structure the full physiological life cycle of a bloom, from initiation to nutrient-driven demise. This highlights the necessity of considering hydrodynamics as a primary explanation for bloom dynamics with cascading effects on bloom onset, cell density, nutrient consumption, cell physiology and mortality.

Background: Long non protein coding (lnc)RNAs are ubiquitous transcripts known to be major regulators of cellular processes in eukaryotes. In diatoms, specific lncRNA expression is tightly correlated to nutrient fluctuations, which suggests function, but their biological relevance remains to be established. In order to fill this knowledge gap, we sought to design a protocol to generate diatom lncRNA knockout mutants by CRISPR-Cas9 enabling subsequent functional studies to shed light into their mechanistic roles. Since lncRNAs do not contain open reading frames that can be targeted by point mutations for the generation of functional knockouts, we designed a multiplexed CRISPR-Cas9 plasmid harboring two independent gRNA for the specific ablation of lncRNA loci in P. tricornutum. As proof of concept, we used our plasmid-based system to target three phosphate depletion responsive lncRNAs. Results: Using the golden gate based uLoop system, we assembled a single plasmid containing all the necessary components to generate CRISPR-Cas9 large genomic ablations of lncRNAs in P. tricornutum using bacterial conjugation. Three intergenic lncRNAs (Linc91, Linc77 and Linc24) responsive to phosphate fluctuations were selected and independently targeted. Around 32% of the exconjugants presented a monoallelic ablation of the targeted loci and 11% presented a biallelic ablation on the first PCR-screening round. Mutated lines, harboring biallelic lncRNAs ablations for each lncRNA targeted, were successfully generated for each of the targeted lncRNAs. Additionally, a first line of phenotypic analysis revealed significant growth rate differences for one of the mutants ({Delta}Lnc24) under both full and phosphate depleted conditions, suggesting its involvement in cell division regulation under nutrient fluctuations. Conclusions: This work provides a simple and efficient method for the generation of lncRNA knock-out mutants in P. tricornutum. Furthermore, this approach can be easily adapted to other microalgae amenable to transformation, enabling the enhancement of our understanding of these important cellular regulatory transcripts in the marine environment.

Chlorella sorokiniana is an industrially important microalga, but its genetic manipulation remains limited by poor transformation efficiency. Although several barriers have been proposed, the role of intracellular nuclease activity has received little attention. Here, we compared the degradation of exogenous double-stranded DNA in whole-cell lysates from transformation-recalcitrant (Chlorella sorokiniana and Tetradesmus obliquus) and transformation-proficient (Phaeodactylum tricornutum, Microchloropsis gaditana, and Nannochloropsis oceanica) microalgae. C. sorokiniana rapidly degraded both linear and plasmid DNA, while T. obliquus exhibited intermediate activity. In contrast, no detectable degradation was observed in the transformation-proficient species. DNA degradation in C. sorokiniana occurred within minutes, and nuclease activity in both recalcitrant species was inhibited by EDTA and Zn2+, consistent with a metal ion-dependent endonuclease. These findings suggest that intracellular nuclease activity may represent an underexplored barrier to exogenous DNA stability during transformation and provide a potential target for improving genetic transformation of recalcitrant microalgae.

The transformative impacts of global bleaching events are now a ubiquitous feature of coral reefs, often resulting in taxonomically simplified coral assemblages that can persist long-term due to ongoing ocean warming, heatwaves, and localized anthropogenic stressors. Understanding to what extent these processes shape symbiont biodiversity continues to be a major knowledge gap. Here, we examined Acropora host and symbiont communities on Kiritimati (Republic of Kiribati), seven and a half years after the 20152016 El Nino-associated marine heatwave, which caused a near total loss of the genus in the atoll's shallow reefs. We quantified Acropora cover and growth-form composition at 37 sites spanning a gradient of local human disturbance and characterised Symbiodiniaceae communities using ITS2 DNA metabarcoding from 109 coral tissue samples at 13 of the sites. Corymbose and tabular Acropora growth forms remained evenly represented across the atoll and cover declined with increasing human disturbance. Symbiodiniaceae communities were dominated by Cladocopium with 93 % of colonies associated with a single ITS2 profile. Communities were homogenous, with low diversity and insignificant differences amongst growth forms, but slight differences across disturbance levels. DNA barcoding of the PaxC intron identified the majority of host colonies as most similar to Acropora granulosa (62 %) and Acropora hyacinthus (24 %). Although dominance by a single symbiont lineage is common among Acropora species across the Pacific, the low Symbiodiniaceae diversity observed at Kiritimati contrasts with the substantial within- and among-lineage diversity documented elsewhere. The depauperate Symbiodiniaceae communities observed in this study may thus represent a lasting legacy of the 2015-2016 disturbance, with the recovering Acropora assemblage representing an early stage of community reassembly drawing from a potentially depleted environmental symbiont pool.

The most studied symbiosis between land vertebrates and algae, is the one between Ambystoma maculatum and Oophila amblystomatis, where the latter colonizes the amphibian''s eggs. Although clear benefit given by algal growth for the salamander embryo has been observed, mutualism is not widely accepted. Mutualistic hypotheses posit benefits from the host to the algae which include protection, N- and C-sources, but no experimental evidence has been obtained. In fact, recent studies suggest that the algae are stressed during this relationship and propose fermentation rather than photosynthesis as main driver for algal growth. In this work, we analyzed embryonic host developmental rate, algal and embryo growth and photosynthetic function of eggs housed at different light intensities. We provide evidence that photosynthetic growth is the main driver for Oophila and that higher algal growth yields faster developmental rate and embryo size. Also, we provide strong evidence that rates of photosynthetic electron transport (and the concomitant O2 evolution) must be higher than those of consumption by the embryo for this symbiosis to be mutualistic. Since photosynthetic rate is dependent on light, this work illustrates the importance of light intensity in this symbiotic relationship.

Heterotrophic flagellates play a major role in nutrient recycling and shaping aquatic microbial communities. Many motile species have evolved diverse predatory strategies to exploit microbial prey, from bacteria to small eukaryotes. Excavates, deep-branching eukaryotic lineages with distinctive biflagellate morphology and conserved cytoskeletal features, exhibit varied feeding behaviours, including grazing, raptorial feeding, and filter-feeding. However, the temporal dynamics of their motility, feeding, and spatial distribution in relation to bacterial population density remain under-explored. Here, we quantify the behavioural ecology of excavates using high-resolution, single-cell tracking and quantitative analysis. Focussing on two Discoba species - the marine Jakoba libera and freshwater Reclinomonas americana - we identify different behavioural patterns: J. libera swims, forages and feeds, while R. americana swims and feeds. Prey availability differentially affects motility: J. libera increases the number of swimming cells when food is scarce, while R. americana shows the opposite trend. Spatial statistics reveal fundamental differences in their feeding and redistribution strategies. J. libera acts as an active forager, whereas R. americana behaves as a strict filter feeder. A third species, the anaerobic Kipferlia bialata (from a different clade), adopts a foraging strategy similar to J. libera. Together, these results elucidate the adaptive relationship between prey availability, feeding modes, and motility in bacteriovorous excavate flagellates, offering new insights into the ecological and evolutionary dynamics of microbial predators.

The endosymbiosis between tropical reef-building corals and dinoflagellate algae (family Symbiodiniaceae) underpins the healthy functioning of coral reef ecosystems. Although dominant host-symbiont pairings differ across spatial scales (e.g., bioregion, reef zone, and depth), corals often associate with multiple symbionts. Where diverse symbionts occur together, their local-scale ecology is poorly understood. Moreover, whilst coral morphological plasticity generates a structurally complex spatial habitat for endosymbiosis, the role of host geometry in facilitating symbiont co-occurrence is completely unexplored. Here, we integrated high-throughput sequencing of the ITS2 gene region with digital 3D photogrammetry to evaluate symbiont distribution within coral structural habitat and determine the influence of host morphology on symbiont community composition. Symbiont composition was characterised along a vertical gradient (coral apex to base) across 50 Hydnophora microconos colonies from Rarotonga, Cook Islands. We identified dominance by the genera Durusdinium and Cladocopium, with inter-genus co-occurrence recognised in 60% of coral colonies. The co-occurrence of two Cladocopium lineages (C3f and C3k) was validated based on congruence with the plastid minicircle psbA non-coding region (psbAncr). Geometric quantification from digital 3D reconstructions of coral colonies revealed that single symbiont taxon colonies were morphologically distinct, whilst multi-symbiont colonies exhibited a broader morphological range. Where Cladocopium-Durusdinium co-occurrence was identified, surface orientation indicated increased Durusdinium prevalence toward the base of coral colonies, corresponding to regions with the highest occlusion of modelled downwelling light. Collectively, these findings suggest that coral structural heterogeneity supports Symbiodiniaceae diversity and highlights the value of resolving microhabitat-level variation to understand symbiont co-occurrence.

Pigment composition and regulation are critical for efficient photosynthesis in the ocean, where light intensity decreases and the spectrum narrows with depth. Fucoxanthin (Fx), the main carotenoid of several microalgal lineages, harvests the blue-green light prevailing in the deep euphotic zone. In pelagophytes, the acylated derivative 19'-butanoyloxyfucoxanthin (19'-BFx) is abundant, yet its function and regulation remain unclear. Here, we investigated how irradiance and spectral quality shape photoacclimation in Pelagomonas calceolata, an abundant and cosmopolitan low-light pelagophyte. Cultures were grown under blue or white light across 2-60 umol photons m^-2 s^-1. We analysed growth, PSII photophysiology, pigment composition, gene expression levels, and the impact of spectral shifts on the 19'-BFx/Fx ratio. Pelagomonas calceolata grew optimally under dim blue light and displayed low non-photochemical quenching (NPQ) under assay conditions. Transcriptomes revealed broad remodelling driven mainly by irradiance and modulated by light colour, including carotenoid-related processes. Dim blue light progressively increased the 19'-BFx/Fx ratio, whereas white light kept it low and reversed the response after a blue-to-white shift. These results identify reversible, spectrum-dependent 19'-BFx accumulation as a key component of photoacclimation in P. calceolata and separate irradiance-driven from spectrum-dependent cellular responses. This plasticity may explain the ecological success of P. calceolata in low-light oceans.

The identification and use of stress-tolerant coral in outplanting efforts has been proposed as a method of restoring and bolstering reef health under increasing environmental change. Prior research has focused on understanding survival in cross-reef transplantation, but evidence of long term resiliency of outplanted corals remains limited. Here, we report data at a five-year timepoint of a reciprocal transplant study of the stress-tolerant Caribbean corals Pseudodiploria strigosa and Siderastrea siderea between a nearshore and offshore site in Belize. Extreme marine heat wave conditions in August of 2024 revealed species and site-specific bleaching patterns. Through assessments of mortality, energy reserves, and chlorophyll concentrations, P. strigosa appear to be more limited in long-term acclimatization potential to the nearshore environment while S. siderea maintain acclimatization capacity displayed 17 months post transplant to nearshore. This work conveys information relevant to managers in selecting species for restoration efforts based on short-term growth and long-term resilience in native versus non-native sites, offers insight into potential mechanisms driving these trends, and takeaways on site specific management decisions for successful coral outplanting success.

Mesophotic coral ecosystems are commonly interpreted as thin biological veneers over relict shallow-water reef structures, but their capacity to generate late Holocene carbonate deposits remains poorly quantified. Here we combine diver excavations, rubble-bed thickness surveys, radiocarbon age-depth modelling, sediment grain-size analyses, coral-rubble assemblages, trace-element geochemistry, and modern Alveopora allingi growth measurements from two 40-65 m deep coral-rubble fields in the northern Gulf of Eilat/Aqaba, Red Sea. Rubble deposits are at least 1.0-1.5 m thick and accumulated at mean rates of 2.21 {+/-} 0.52 mm yr-1 at the Dekel site and 1.33 {+/-} 0.13 mm yr-1 at the IUI site, implying persistent carbonate accumulation over centuries to >1 kyr. Age-depth models indicate quasi-continuous deposition despite pronounced faunal changes, including a shift from A. allingi-dominated rubble to upper layers enriched in Euphyllia paradivisa at Dekel. The dominant modern rubble-forming coral, A. allingi, extends at 3.04 {+/-} 0.87 mm yr-1 and has skeletal density of 1.17 {+/-} 0.27 g cm-3. First-order cover-based accretion estimates at present coral cover fall well below the radiocarbon-derived rates, indicating that long-term mesophotic rubble accumulation integrates historical cover, secondary calcification and sediment retention rather than present live coral cover alone. These data demonstrate that mesophotic rubble fields can act as active carbonate factories and palaeoecological archives, challenging the assumption that modern mesophotic habitats are geomorphologically passive.

A cell's ability to maintain its volume is a prerequisite for its essential physiological functions. Changes in extracellular osmolarity induce the flux of water and solutes across the cell and nuclear membrane and, therefore, alters cell and nuclear volumes. Here, we investigated the equilibrium and dynamic volumetric responses of cells and nuclei in adherent human hepatoma (HepG2) cell monolayers using live three-dimensional imaging. Through image segmentation, 3D reconstruction, and volume rendering, precise cell and nuclear volumes were quantified. We found that during hypoosmotic stress, cells swell and lose osmolytes through active volume regulation, and nuclei undergo greater relative expansion than cells. During hyperosmotic stress, we found that both cells and nuclei shrink rapidly, develop irregular morphologies and undergo passive volume regulation. Furthermore, we explored the osmotic responses in these cells by restoring isosmotic conditions following osmotic stress and found that nuclei returned to volumes greater than their initial isosmotic volume, whereas cytoplasmic volume returned to a value below its initial isosmotic volume following swelling. By fitting coupled transport models water and solute permeability estimates for both cells and nuclei were made from their dynamic responses to different anisosmotic media with and without the presence of permeating solutes dimethyl sulfoxide and ethylene glycol. Together, these quantitative measurements of cell, cytoplasm, and nuclear volumetric responses and associated transport parameters have important implications for understanding cell physiology and the mechanism of osmoregulation.

Background Comparisons of single-cell and single-nucleus RNA sequencing (scRNA-seq and snRNA-seq) data have been described in some mammalian tissues and, subsequently, in Drosophila, but remain unexplored in most invertebrate lineages. The xenacoelomorphs occupy key phylogenetic positions, yet they differ anatomically from mammals. They have a reduced extracellular matrix, high-salt body fluid, and no circulatory system. Despite these differences, they possess a well-developed nervous system. One of the xenacoelomorphs, the photosymbiotic acoel (Praesagittifera naikaiensis) also harbours symbiotic Tetraselmis algae, whose RNA can be co-captured with host RNA. Results We compared scRNA-seq and snRNA-seq data from whole P. naikaiensis specimens. Both methods yielded high-quality data with comparable gene detection but a larger share of scRNA-seq reads derived from symbiotic algae. Gene-level analyses revealed that neural genes were enriched in snRNA-seq relative to non-neural genes. Cross-method label transfer and integration-based validation identified six snRNA-seq clusters, including some neural populations, that lacked a clear scRNA-seq counterpart. In contrast, three cell populations, including muscle and metabolically active clusters, were reciprocally validated as captured by both methods. Conclusions Our results show that key snRNA-seq advantages, particularly the enhanced recovery of neural transcripts, are recapitulated in our dataset, which is consistent with previous reports in mammals. Furthermore, snRNA-seq reduces symbiont-derived reads and recovers several cell populations underrepresented in scRNA-seq. These findings provide practical guidance for cell atlas construction in non-model, symbiotic invertebrates.

1. Protecting and restoring corals is critical to conserve marine biodiversity and ecosystem services in the Anthropocene. Knowledge of coral larval dispersal dynamics can be used to identify important sites for coral metapopulation connectivity and to spatially optimise coral conservation efforts. However, we know little regarding the larval dispersal of many coral species and our understanding of the relationships between coral larval dispersal and life-history traits (e.g., reproductive mode, colony morphology), phylogenetic relationships, and biogeography is incomplete. These knowledge gaps impede coral conservation. 2. In this research, we systematically reviewed the species, regions, and methodologies of studies of coral larval dispersal. Additionally, we conducted a meta-analysis to quantify the heterogeneity in reported coral larval duration values and identified the roles of life-history traits in driving that heterogeneity. 3. We found that various methods (experimental, biophysical modelling, and population genetics) were used to study 221 coral species across many regions. Soft and temperate corals received comparatively lower research attention. Our meta-analysis reveals that reproductive mode and colony morphology explain significant heterogeneity in larval settlement times, and that closely-related species have more similar larval settlement times. 4. Our findings suggest that reproductive mode and colony morphology could be used to predict larval duration in under-studied coral species, and that larval duration could be approximated from closely-related corals when trait data is unavailable. Our results support strategies focused on modelling differing larval durations across corals with different life-history traits to predict realised dispersal in under-studied coral species and identify conservation priority sites.

Marine heatwaves cause widespread coral bleaching and mortality, yet thermal responses often differ between adult and juvenile corals, with the latter frequently being more tolerant. Whether the observed enhanced tolerance of juveniles reflects intrinsic physiological differences across life stages or simply reduced environmental stress due to shaded microhabitats occupied by juveniles, or a combination of both factors, remains unclear. To test whether colony size determines thermal responses and if a threshold size exists for tolerance, we exposed 89 colonies of the reef-building coral Acropora aff. digitifera to a 6-week, 32.5 C simulated marine heatwave under controlled light conditions, with cumulative stress reaching 20 C-weeks. Contrary to expectations, ~2 C-weeks less heat stress was required to trigger bleaching and mortality in coral juveniles compared to adults. Mortality divergence across colony sizes was greatest at ~10 C-weeks, when the smallest corals exceeded 80% mortality, whereas the largest colonies showed <10% mortality. A breakpoint in survival odds was apparent at 13.4 cm diameter, corresponding to the size at onset of sexual maturity in the native wild population for this species. These results indicate an ontogenetic shift in physiological heat tolerance, with larger reproductively mature colonies showing enhanced tolerance. The apparent survival advantage of juveniles in the reef frequently reported during bleaching events could be more likely due to micro-environmental buffering than intrinsic physiological tolerance. Our findings suggest that juvenile corals may be more vulnerable to climate change than previously thought, particularly as declining habitat complexity could reduce microhabitat refugia on future reefs.

Understanding how species traits shape community properties is a central goal in ecology. But whether traits drive consistent patterns of dominance and assembly regardless of initial species abundances (evenness) remains unclear. We addressed this question using phytoplankton microcosm experiments to disentangle the effects of evenness and traits on community assembly and functioning, focusing on traits related to energy use (photosynthesis, size) alongside demographic parameters. Community assembly was highly deterministic, indicating that initial evenness had little influence on species dominance and final community structure. Instead, evenness drove community functioning. Demography was a stronger predictor of dominance than photosynthetic rates or individual size, suggesting that growth and competitive success depend on multiple metabolic processes that cannot be reduced to simpler proxies. These results show that community structure can emerge predictably across a range of initial conditions, and highlight the importance of demographic metrics over simpler metabolic descriptors for forecasting community outcomes.

Coral skeletal integrity underpins reef habitat complexity and coastal protection, yet it remains unclear whether corals persisting in chemically variable marginal habitats, such as mangrove-associated reefs, maintain sufficient mechanical resistance to withstand physical disturbance. We combined micro-computed tomography (micro-CT) morphometric analysis with compression testing to quantify skeletal microarchitecture and compressive mechanical performance of Pocillopora sp. skeletons collected across a habitat gradient at Low Isles, Great Barrier Reef, from mangrove-associated sites to exposed reef sites. Baseline skeletal properties varied among sites, with the Proximal Reef site having the densest and least porous skeletons, while mangrove-associated sites had the most porous skeletons. Similarly, Proximal Reef skeletons exhibited two-to-three-fold greater compressive stiffness and peak force than mangrove-associated skeletons. Percentage object volume was a stronger predictor of compressive stiffness than surface area-to-volume (S/V) ratio, indicating that bulk skeletal density better captured load-bearing capacity than surface architectural complexity. Structural failure mode also differed markedly among habitats: mangrove-associated skeletons underwent catastrophic fragmentation of their open, porous framework, whereas Distal and Proximal Reef skeletons failed through discrete crack-plane propagation while largely maintaining overall structural coherence. These results indicate that the physiological stress tolerance associated with mangrove habitats occurs alongside reduced mechanical resistance and more destructive failure modes, suggesting a potential trade-off between chemical stress tolerance and structural investment. Assessments of resilience in marginal coral populations should therefore consider skeletal mechanical performance alongside physiological tolerance, particularly as climate change intensifies storms and alters hydrodynamic disturbance regimes.