Coralline algal calcification: A morphological and process-based understanding

How the Ecology of Calcified Red Macroalgae is Investigated under a Chemical Approach? A Systematic Review and Bibliometric Study

Characteristics such as calcareous morphology and life cycle are used to understand the ecology of calcified rhodophytes. However, there is limited information regarding their chemical profiles and biological activities. Therefore, a systematic review (PRISMA) was conducted to assess the influence of the chemistry of calcareous rhodophytes on ecological interactions in the marine environment. The keywords used were: ["Chemical AND [Ecology OR Interaction OR Response OR Defense OR Effect OR Cue OR Mediated OR Induce]"] AND ["Red Seaweed" OR "Red Macroalgae" OR Rhodophy?] AND [Calcified OR Calcareous] in Science Direct, Scielo, PUBMED, Springer, Web of Science, and Scopus. Only English articles within the proposed theme were considered. Due to the low number of articles, another search was conducted with three classes and 16 genera. Finally, 67 articles were considered valid. Their titles, abstracts, and keywords were analyzed using IRaMuTeQ through factorial, hierarchical and similarity classification. Most of the studies used macroalgae thallus to evaluate chemical mediation while few tested crude extracts. Some substances were noted as sesquiterpene (6-hydroxy-isololiolide), fatty acid (heptadeca5,8,11-triene) and dibromomethane. The articles were divided into four classes: Herbivory, Competition, Settlement/Metamorphosis, and Epiphytism. Crustose calcareous algae were associated with studies of Settlement/Metamorphosis, while calcified algae were linked to herbivory. Thus, the importance of chemistry in the ecology of these algae is evident,and additional studies are needed to identify the substances responsible for ecological interactions. This study collected essential information on calcified red algae, whose diversity appears to be highly vulnerable to the harmful impacts of ongoing climate change.

Genome of Halimeda opuntia reveals differentiation of subgenomes and molecular bases of multinucleation and calcification in algae

Algae mostly occur either as unicellular (microalgae) or multicellular (macroalgae) species, both being uninucleate. There are important exceptions, however, as some unicellular algae are multinucleate and macroscopic, some of which inhabit tropical seas and contribute to biocalcification and coral reef robustness. The evolutionary mechanisms and ecological significance of multinucleation and associated traits (e.g., rapid wound healing) are poorly understood. Here, we report the genome of Halimeda opuntia, a giant multinucleate unicellular chlorophyte characterized by interutricular calcification. We achieve a high-quality genome assembly that shows segregation into four subgenomes, with evidence for polyploidization concomitant with historical sea level and climate changes. We further find myosin VIII missing in H. opuntia and three other unicellular multinucleate chlorophytes, suggesting a potential mechanism that may underpin multinucleation. Genome analysis provides clues about how the unicellular alga could survive fragmentation and regenerate, as well as potential signatures for extracellular calcification and the coupling of calcification with photosynthesis. In addition, proteomic alkalinity shifts were found to potentially confer plasticity of H. opuntia to ocean acidification (OA). Our study provides crucial genetic information necessary for understanding multinucleation, cell regeneration, plasticity to OA, and different modes of calcification in algae and other organisms, which has important implications in reef conservation and bioengineering.

Evaluation and Development of Analytical Procedures to Assess Buffering Capacity of Carbonate Ruminant Feed Buffers

The inclusion of rumen buffers in ruminant feeds has gained widespread adoption for the prevention of rumen acidosis, thereby avoiding the negative production and health consequences of low rumen pH and resulting in improved feed efficiency. Benchmarking and quality controlling the performance of rumen buffer materials is of significant interest to feed mills and end-user producers. The aim of this study was to evaluate, develop and optimise a laboratory protocol to consistently and robustly evaluate rumen buffering materials in order to predict their in vivo efficacy. Three different methods were evaluated for determining the buffering potential of carbonate buffer materials: (a) 2 and 8 h static pH, (b) 8 h fixed HCl acid load addition and (c) 3 h acidotic diet simulation using acetic acid. Buffer material, threshold pH, test duration and interactions between all three variables were significant (p < 0.001) in evaluating the performance of the buffer materials. The acidotic diet simulation was found to provide a different ranking of materials to the 8 h fixed HCl acid load methodology. The results highlight the importance of method selection and test parameters for accurately evaluating the potential efficacy of rumen buffer materials.

New branched Porolithon species (Corallinales, Rhodophyta) from the Great Barrier Reef, Coral Sea, and Lord Howe Island

Porolithon is one of the most ecologically important genera of tropical and subtropical crustose (non-geniculate) coralline algae growing abundantly along the shallow margins of coral reefs and functioning to cement reef frameworks. Thalli of branched, fruticose Porolithon specimens from the Indo-Pacific Ocean traditionally have been called P. gardineri, while massive, columnar forms have been called P. craspedium. Sequence comparisons of the rbcL gene both from type specimens of P. gardineri and P. craspedium and from field-collected specimens demonstrate that neither species is present in east Australia and instead resolve into four unique genetic lineages. Porolithon howensis sp. nov. forms columnar protuberances and loosely attached margins and occurs predominantly at Lord Howe Island; P. lobulatum sp. nov. has fruticose to clavate forms and free margins that are lobed and occurs in the Coral Sea and on the Great Barrier Reef (GBR); P. parvulum sp. nov. has short (<2 cm), unbranched protuberances and attached margins and is restricted to the central and southern GBR; and P. pinnaculum sp. nov. has a mountain-like, columnar morphology and occurs on oceanic Coral Sea reefs. A rbcL gene sequence of the isotype of P. castellum demonstrates it is a different species from other columnar species. In addition to the diagnostic rbcL and psbA marker sequences, the four new species may be distinguished by a combination of features including thallus growth form, margin shape (attached or unattached), and medullary system (coaxial or plumose). Porolithon species, because of their ecological importance and sensitivity to ocean acidification, need urgent documentation of their taxonomic diversity.

Environmental impacts on the structural integrity of British rhodoliths

Coralline algae form complex habitats which are biodiversity hotspots. Experimental studies suggest that climate change will decrease coralline algal structural integrity. These experiments, however, lack information on local morphological variability and how much structural change would be needed to threaten habitat formation. Here, using finite element modelling, we assess variability in cellular structure and chemical composition of the carbonate skeleton of four coralline algal species from Britain in contemporary and historical specimens collected over the last 130 years. Cellular and mineral properties are highly variable within species, between sites and through time, with structurally weaker cells in the southern species and contemporary material compared to northern taxa and historical material. Yet, temporal differences in strength were smaller than spatial differences. Our work supports long term experiments which show the adaptation potential of this group. Our results suggest that future anthropogenic climate change may lead to loss of habitat complexity in the south and expansion of structurally weaker southern species into northern sites.

Cell wall organic matrix composition and biomineralization across reef-building coralline algae under global change

Crustose coralline algae (CCA) are one of the most important benthic substrate consolidators on coral reefs through their ability to deposit calcium carbonate on an organic matrix in their cell walls. Discrete polysaccharides have been recognized for their role in biomineralization, yet little is known about the carbohydrate composition of organic matrices across CCA taxa and whether they have the capacity to modulate their organic matrix constituents amidst environmental change, particularly the threats of ocean acidification (OA) and warming. We simulated elevated pCO₂ and temperature (IPCC RCP 8.5) and subjected four mid-shelf Great Barrier Reef species of CCA to 2 months of experimentation. To assess the variability in surficial monosaccharide composition and biomineralization across species and treatments, we determined the monosaccharide composition of the polysaccharides present in the cell walls of surficial algal tissue and quantified calcification. Our results revealed dissimilarity among species' monosaccharide constituents, which suggests that organic matrices are composed of different polysaccharides across CCA taxa. We also observed that species differentially modulate composition in response to ocean acidification and warming. Our findings suggest that both variability in composition and ability to modulate monosaccharide abundance may play a crucial role in surficial biomineralization dynamics under the stress of OA and global warming.

Transcriptomic stability or lability explains sensitivity to climate stressors in coralline algae

BACKGROUND

Crustose coralline algae (CCA) are calcifying red macroalgae that play important ecological roles including stabilisation of reef frameworks and provision of settlement cues for a range of marine invertebrates. Previous research into the responses of CCA to ocean warming (OW) and ocean acidification (OA) have found magnitude of effect to be species-specific. Response to OW and OA could be linked to divergent underlying molecular processes across species.

RESULTS

Here we show Sporolithon durum, a species that exhibits low sensitivity to climate stressors, had little change in metabolic performance and did not significantly alter the expression of any genes when exposed to temperature and pH perturbations. In contrast, Porolithon onkodes, a major coral reef builder, reduced photosynthetic rates and had a labile transcriptomic response with over 400 significantly differentially expressed genes, with differential regulation of genes relating to physiological processes such as carbon acquisition and metabolism. The differential gene expression detected in P. onkodes implicates possible key metabolic pathways, including the pentose phosphate pathway, in the stress response of this species.

CONCLUSIONS

We suggest S. durum is more resistant to OW and OA than P. onkodes, which demonstrated a high sensitivity to climate stressors and may have limited ability for acclimatisation. Understanding changes in gene expression in relation to physiological processes of CCA could help us understand and predict how different species will respond to, and persist in, future ocean conditions predicted for 2100.

Calcification traits for cryptic species identification: Insights into coralline biomineralization

Calcareous red algae are foundation species and ecosystem engineers with a global distribution. The principles governing their calcification pathways are still debated and the morphological characters are frequently unreliable for species segregation, as shown by molecular genetics. The recent description of the new species Lithophyllum pseudoracemus, previously undetected and morphologically confused with Lithophyllum racemus, offered a challenging opportunity to test the effectiveness of microanatomy and ultrastructural calcification traits as tools for the identification of these two species, for integrative taxonomy. High resolution SEM images of molecularly identified samples showed that the different size of the perithallial cells and the features of the asexual conceptacle chambers may contribute to the separation of the two species. The two species share the same crystallite morphology in the primary and secondary cell-wall calcification, as previously described in other species belonging to the same clade. However, the perithallial secondary calcification was significantly thicker in L. racemus than in L. pseudoracemus. We described a granular calcified layer in the innermost part of the cell wall, as a putative precursor phase in the biomineralization and formation of the secondary calcification. The hypothesis of different pathways for the formation of the primary and secondary calcification is supported by the observed cell elongation associated with thicker and higher Mg/Ca primary calcification, the inverse correlation of primary and secondary calcification thickness, and the absence of primary calcification in the newly formed wall cutting off an epithallial cell from the meristem.

Structural and chemical variations in Mg-calcite skeletal segments of coralline red algae lead to improved crack resistance

The calcareous alga Jania sp. is an articulated coralline red seaweed that is abundant in the shallow waters of oceans worldwide. We have previously demonstrated that its structure is highly intricate and exhibits hierarchical organization across multiple length scales from the macro to the nano scale. Moreover, we have proven that the inner pores of its structure are helical, conveying the alga greater compliance as compared to a cylindrical configuration. Herein, we reveal new insights into the structure of Jania sp., particularly, its crystallographic variations and the internal elemental distribution of Mg and Ca. We show that the high-Mg calcite cell wall nanocrystals of Jania sp. are arranged in layers with alternating Mg contents. Moreover, we show that this non-homogenous elemental distribution assists the alga in preventing fracture caused by crack propagation. We further reveal that each one of the cell wall nanocrystals in Jania sp. is not a single crystal as was previously thought, but rather comprises Mg-rich calcite nanoparticles demonstrating various crystallographic orientations, arranged periodically within the layered structure. We also show that these Mg-rich nanoparticles are present in yet another species of the coralline red algae, Corallina sp., pointing to the generality of this phenomenon. To the best of our knowledge this is a first report on the existence of Mg-rich nanoparticles in algal mineralized tissue. We envisage that our findings on the bio-strategy found in the algae to enhance their fracture toughness will have an impact on the design of structures with superior mechanical properties. STATEMENT OF SIGNIFICANCE: Understanding the structure-property relation in biomineralized tissues is of great importance in unveiling Nature's material design strategies, which form the basis for the development of novel structural materials. Crystallographic and elemental variations in the skeletal parts of the coralline red algae and their cumulative contribution to prevention of mechanical failure are yet poorly studied. Herein, we reveal that the high-Mg calcite cell wall nanocrystals of Jania sp. are arranged in layers with alternating Mg concentrations and that this organization facilitates crack deflection, thereby preventing catastrophic fracture. We further discovered that the nanocrystals contain incoherent Mg-rich nanoparticles and suggest that they form via spinodal decomposition of the Mg-ACC precursor and self-arrange periodically throughout the alga's mineralized cell wall, a phenomenon most likely to be widespread in high-Mg calcite biomineralization.

Corallinapetrales and Corallinapetraceae: A new order and family of coralline red algae including Corallinapetra gabrielii comb. nov

The coralline algal genus Corallinapetra is currently monospecific and was established on the species Corallinapetra novaezelandiae, known from a single collection from north-eastern New Zealand. On the basis of multi-gene phylogenetic analyses, Corallinapetra has been resolved apart from all currently recognized families and orders within the Corallinophycidae. We analyzed DNA sequence data from the holotype of Lithothamnion gabrielii, which has been considered a heterotypic synonym of L. muelleri, and an unidentified sample collected from Stewart Island in New Zealand, using psbA, rbcL, and COI-5P genes. We also observed detailed morpho-anatomical characters with light and scanning electron microscopy. Our phylogenetic analyses showed that L. gabrielii and the sample from New Zealand belonged to the same clade as Corallinapetra, distinct from other families and orders in the Corallinophycidae. Members of this clade are distinguishable from other families and orders in the Corallinophycidae by possessing sporangia that are surrounded by remnant sterile filaments that are weakly calcified in mature multiporate sporangial conceptacles that produce zonately divided tetrasporangia. Therefore, we propose that Corallinapetra be placed in its own family, Corallinapetraceae and order, Corallinapetrales, and that L. gabrielii should be assigned to Corallinapetra, as C. gabrielii, to reflect their phylogenetic relationships. We also obtained a partial rbcL sequence data from the lectotype of L. muelleri, the generitype of Lithothamnion. Comparison of the L. muelleri type sequence with L. gabrielii unambiguously demonstrated that these two species are not conspecific, and confirm the placement of L. muelleri within the Hapalidiales.

Calcification in free-living coralline algae is strongly influenced by morphology: Implications for susceptibility to ocean acidification

Rhodolith beds built by free-living coralline algae are important ecosystems for marine biodiversity and carbonate production. Yet, our mechanistic understanding regarding rhodolith physiology and its drivers is still limited. Using three rhodolith species with different branching morphologies, we investigated the role of morphology in species’ physiology and the implications for their susceptibility to ocean acidification (OA). For this, we determined the effects of thallus topography on diffusive boundary layer (DBL) thickness, the associated microscale oxygen and pH dynamics and their relationship with species’ metabolic and light and dark calcification rates, as well as species’ responses to short-term OA exposure. Our results show that rhodolith branching creates low-flow microenvironments that exhibit increasing DBL thickness with increasing branch length. This, together with species’ metabolic rates, determined the light-dependent pH dynamics at the algal surface, which in turn dictated species’ calcification rates. While these differences did not translate in species-specific responses to short-term OA exposure, the differences in the magnitude of diurnal pH fluctuations (~ 0.1–1.2 pH units) between species suggest potential differences in phenotypic plasticity to OA that may result in different susceptibilities to long-term OA exposure, supporting the general view that species’ ecomechanical characteristics must be considered for predicting OA responses.

High Magnesium Calcite and Dolomite composition carbonate in Amphiroa (Lithophyllaceae, Corallinales, Rhodophyta): further documentation of elevated Mg in Corallinales with climate change implications

Species of the calcified, articulate coralline Amphiroa are key components of many shallow marine ecosystems. Understanding their mineral composition is important as their susceptibility to dissolution, due to ocean acidification, may vary with mineral composition. We studied the distribution of Mg-calcite, very high magnesium calcite (VHMC), and dolomite within Amphiroa species to elucidate their mineral properties and susceptibility to dissolution. Results revealed that the asymmetrical X-ray diffraction (XRD) pattern typical of Amphiroa globally represents high levels of VHMC and dolomite composition carbonate. The dolomite seems most likely to be disordered, but higher resolution XRD is required for confirmation. The calcified long sides of medullary cells have predominantly VHMC/dolomite and the corners have bands of VHMC/dolomite. Epithallial cell walls are low Mg-calcite, and cortical cells are low Mg-calcite with bands of VHMC. VHMC/dolomite is more stable than Mg-calcite, and this may provide a competitive advantage for Amphiroa species as seawater pH declines. Further work is required to determine the metabolic controls on VHMC/dolomite mineral formation.

Interplay of microbial communities with mineral environments in coralline algae

Coralline algae are worldwide carbonate builders, considered to be foundational species and biodiversity hotspots. Coralline habitats face increasing pressure from human activities and effects related to Global Change, yet their ecological properties and adaptive responses remain poorly understood. The relationships of the algal microbiota with the mineral bioconstructions, as well as plasticity and resilience of coralline holobionts in a changing environment, are of particular interest. In the Gulf of California, Neogoniolithon trichotomum (Rhodophyta) is the main carbonate builder in tidal pools. We performed a multi-disciplinary assessment of the N. trichotomum microstructure using XRD, SEM microscopy and SR-FTIR spectromicroscopy. In the algal perithallus, magnesium-calcite and aragonite were spatially segregated and embedded in a polysaccharide matrix (rich in sulfated polysaccharides). Mg-calcites (18-19 mol% Mg) were the main mineral components of the thallus overall, followed by iron carbonates related to dolomite (ankerite) and siderite. Minerals of late evaporitic sequences (sylvite and bischofite) were also present, suggesting potential halophilic microenvironments within the algal thalli. The diverse set of abundant halophilic, halotolerant and oligotrophic taxa, whose abundance increase in the summer, further suggests this condition. We created an integrated model, based on environmental parameters and the microbiota distribution, that identified temperature and nutrient availability (particularly nitrate and silicate) as the main parameters related to specific taxa patterns. Among these, Hahella, Granulossicoccus, Ferrimonas, Spongiibacteraceae and cyanobacterial Xenococcaceae and Nostocaceae change significantly between seasons. These bacterial components might play relevant roles in algal plasticity and adaptive responses to a changing environment. This study contributes to the understanding of the interplay of the prokaryotic microbiota with the mineral microenvironments of coralline algae. Because of their carbonates with potential resistance to dissolution in a higher pCO₂ world and their seasonally dynamic bacteria, coralline algae are relevant targets to study coastal resilience and carbonated systems responses to changing environments.

Coralline macroalgae contribution to ecological services of carbon storage in a disturbed seagrass meadow

Coralline macroalgae are globally distributed rhodopyhtes that remove carbon from their immediate environment and transform it into carbonate sediments through the senescence of their calcified tissues. In this study, the calcium carbonate (CaCO₃) stocks in the tissue of Jania adhaerens and sediments in Tanjung Adang Shoal, Johor were quantified for a 13-month study period. The detailed maps of the geographical distribution based on the spatial and temporal variations of biomass and CaCO₃ were also assessed. The highest amount of biomass, CaCO₃ and organic carbon (OC) stocks in the tissues showed the highest in May 2018 and May 2019. The biomass values ranged from 65 to 143 g DW m^-2, which contained 53-147 g CaCO₃ m^-2 and 3-11 g OC m^-2. These findings provided insights into the biogeochemical cycling of these inputs, which can be used to estimate the overall carbon budget of the macrophyte meadow.

Inorganic carbon uptake strategies in coralline algae: Plasticity across evolutionary lineages under ocean acidification and warming

Dissolved inorganic carbon (DIC) assimilation is essential to the reef-building capacity of crustose coralline algae (CCA). Little is known, however, about the DIC uptake strategies and their potential plasticity under ongoing ocean acidification (OA) and warming. The persistence of CCA lineages throughout historical oscillations of pCO₂ and temperature suggests that evolutionary history may play a role in selecting for adaptive traits. We evaluated the effects of pCO₂ and temperature on the plasticity of DIC uptake strategies and associated energetic consequences in reef-building CCA from different evolutionary lineages. We simulated past, present, moderate (IPCC RCP 6.0) and high pCO₂ (RCP 8.5) and present and high (RCP 8.5) temperature conditions and quantified stable carbon isotope fractionation (¹³ε), organic carbon content, growth and photochemical efficiency. All investigated CCA species possess CO₂-concentrating mechanisms (CCMs) and assimilate CO₂ via diffusion to varying degrees. Under OA and warming, CCA either increased or maintained CCM capacity, which was associated with overall neutral effects on metabolic performance. More basal taxa, Sporolithales and Hapalidiales, had greater capacity for diffusive CO₂ use than Corallinales. We suggest that CCMs are an adaptation that supports a robust carbon physiology and are likely responsible for the endurance of CCA in historically changing oceans.

Plasticity of adult coralline algae to prolonged increased temperature and pCO2 exposure but reduced survival in their first generation

Crustose coralline algae (CCA) are vital to coral reefs worldwide, providing structural integrity and inducing the settlement of important invertebrate larvae. CCA are known to be impacted by changes in their environment, both during early development and adulthood. However, long-term studies on either life history stage are lacking in the literature, therefore not allowing time to explore the acclimatory or potential adaptive responses of CCA to future global change scenarios. Here, we exposed a widely distributed, slow growing, species of CCA, Sporolithon cf. durum, to elevated temperature and pCO2 for five months and their first set of offspring (F1) for eleven weeks. Survival, reproductive output, and metabolic rate were measured in adult S. cf. durum, and survival and growth were measured in the F1 generation. Adult S. cf. durum experienced 0% mortality across treatments and reduced their O2 production after five months exposure to global stressors, indicating a possible expression of plasticity. In contrast, the combined stressors of elevated temperature and pCO2 resulted in 50% higher mortality and 61% lower growth on germlings. On the other hand, under the independent elevated pCO2 treatment, germling growth was higher than all other treatments. These results show the robustness and plasticity of S. cf. durum adults, indicating the potential for them to acclimate to increased temperature and pCO2. However, the germlings of this species are highly sensitive to global stressors and this could negatively impact this species in future oceans, and ultimately the structure and stability of coral reefs.