Field study of growth and calcification rates of three species of articulated coralline algae in British Columbia, Canada

Kelp and sea urchin settlement mediated by biotic interactions with benthic coralline algal species

Species interactions can influence key ecological processes that support community assembly and composition. For example, coralline algae encompass extensive diversity and may play a major role in regime shifts from kelp forests to urchin-dominated barrens through their role in inducing invertebrate larval metamorphosis and influencing kelp spore settlement. In a series of laboratory experiments, we tested the hypothesis that different coralline communities facilitate the maintenance of either ecosystem state by either promoting or inhibiting early recruitment of kelps or urchins. Coralline algae significantly increased red urchin metamorphosis compared with a control, while they had varying effects on kelp settlement. Urchin metamorphosis and density of juvenile canopy kelps did not differ significantly across coralline species abundant in both kelp forests and urchin barrens, suggesting that recruitment of urchin and canopy kelps does not depend on specific corallines. Non-calcified fleshy red algal crusts promoted the highest mean urchin metamorphosis percentage and showed some of the lowest canopy kelp settlement. In contrast, settlement of one subcanopy kelp species was reduced on crustose corallines, but elevated on articulated corallines, suggesting that articulated corallines, typically absent in urchin barrens, may need to recover before this subcanopy kelp could return. Coralline species differed in surface bacterial microbiome composition; however, urchin metamorphosis was not significantly different when microbiomes were removed with antibiotics. Our results clarify the role played by coralline algal species in kelp forest community assembly and could have important implications for kelp forest recovery.

Transcriptome of the coralline alga Calliarthron tuberculosum (Corallinales, Rhodophyta) reveals convergent evolution of a partial lignin biosynthesis pathway

The discovery of lignins in the coralline red alga Calliarthron tuberculosum raised new questions about the deep evolution of lignin biosynthesis. Here we present the transcriptome of C. tuberculosum supported with newly generated genomic data to identify gene candidates from the monolignol biosynthetic pathway using a combination of sequence similarity-based methods. We identified candidates in the monolignol biosynthesis pathway for the genes 4CL, CCR, CAD, CCoAOMT, and CSE but did not identify candidates for PAL, CYP450 (F5H, C3H, C4H), HCT, and COMT. In gene tree analysis, we present evidence that these gene candidates evolved independently from their land plant counterparts, suggesting convergent evolution of a complex multistep lignin biosynthetic pathway in this red algal lineage. Additionally, we provide tools to extract metabolic pathways and genes from the newly generated transcriptomic and genomic datasets. Using these methods, we extracted genes related to sucrose metabolism and calcification. Ultimately, this transcriptome will provide a foundation for further genetic and experimental studies of calcifying red algae.

Functional characterization of a cellulose synthase, CtCESA1, from the marine red alga Calliarthron tuberculosum (Corallinales)

In land plants and algae, cellulose is important for strengthening cell walls and preventing breakage due to physical forces. Though our understanding of cellulose production by cellulose synthases (CESAs) has seen significant advances for several land plant and bacterial species, functional characterization of this fundamental protein is absent in red algae. Here we identify CESA gene candidates in the calcifying red alga Calliarthron tuberculosum using sequence similarity-based approaches, and elucidate their phylogenetic relationship with other CESAs from diverse taxa. One gene candidate, CtCESA1, was closely related to other putative red algal CESA genes. To test if CtCESA1 encoded a true cellulose synthase, CtCESA1 protein was expressed and purified from insect and yeast expression systems. CtCESA1 showed glucan synthase activity in glucose tracer assays. CtCESA1 activity was relatively low when compared with plant and bacterial CESA activity. In an in vitro assay, a predicted N-terminal starch-binding domain from CtCESA1 bound red algal floridean starch extracts, representing a unique domain in red algal CESAs not present in CESAs from other lineages. When the CtCESA1 gene was introduced into Arabidopsis thaliana cesa mutants, the red algal CtCESA1 partially rescued the growth defects of the primary cell wall cesa6 mutant, but not cesa3 or secondary cell wall cesa7 mutants. A fluorescently tagged CtCESA1 localized to the plasma membrane in the Arabidopsis cesa6 mutant background. This study presents functional evidence validating the sequence annotation of red algal CESAs. The relatively low activity of CtCESA1, partial complementation in Arabidopsis, and presence of unique protein domains suggest that there are probably functional differences between the algal and land plant CESAs.

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.

Transcriptomic and Physiological Responses of the Tropical Reef Calcified Macroalga Amphiroa fragilissima to Elevated Temperature1

Calcareous macroalgae are of particular ecological importance as primary producers, carbonate sediment builders, and habitat providers in coral reef ecosystems. Ocean warming is a major threat to calcareous algae, but it remains unclear exactly how these algae will respond to it. In this study, the potential physiological impacts of ocean warming on the calcareous alga Amphiroa fragilissima were evaluated in laboratory experiments. Increasing temperature from 26 to 28°C had positive effects on algal growth rate and chlorophyll a content, but these parameters decreased significantly at 32°C, which is 5°C above the annual mean temperature in the study region. Algal bleaching occurred at 34°C. There were no significant differences in CaCO₃ content of thalli among different temperatures; however, calcification rate was inhibited significantly at 32 and 34°C. Transcriptome analyses using the Illumina RNA-seq platform showed that differentially expressed genes were annotated mainly in the categories of steroid biosynthesis, gap junction, ribosome, and mTOR signaling pathway. The expression levels of PsbA and PsbP were suppressed at 32°C, implying that inactivation of photosystem II could be a main reason for the decreased photosynthetic rate. Down-regulation of the genes encoding carbonic anhydrase and nitrate reductase was observed at 32°C, which could inhibit growth rate. Additionally, several genes that might be related to calcification were identified, including CAMK, CDPK, and CAM and genes encoding alpha-catenin and carbonic anhydrase. This study contributes to our understanding of the effects of temperature on algal calcification and provides a theoretical basis to protect ecological diversity of coral reef ecosystems.

Coralline algal calcification: A morphological and process-based understanding

Research purpose and findings

Coralline algae are key biological substrates of many carbonate systems globally. Their capacity to build enduring crusts that underpin the formation of tropical reefs, rhodolith beds and other benthic substrate is dependent on the formation of a calcified thallus. However, this important process of skeletal carbonate formation is not well understood. We undertook a study of cellular carbonate features to develop a model for calcification. We describe two types of cell wall calcification; 1) calcified primary cell wall (PCW) in the thin-walled elongate cells such as central medullary cells in articulated corallines and hypothallial cells in crustose coralline algae (CCA), 2) calcified secondary cell wall (SCW) with radial Mg-calcite crystals in thicker-walled rounded cortical cells of articulated corallines and perithallial cells of CCA. The distinctive banding found in many rhodoliths is the regular transition from PCW-only cells to SCW cells. Within the cell walls there can be bands of elevated Mg with Mg content of a few mol% higher than radial Mg-calcite (M-type), ranging up to dolomite composition (D-type).

Model for calcification

We propose the following three-step model for calcification. 1) A thin (< 0.5 μm) PCW forms and is filled with a mineralising fluid of organic compounds and seawater. Nanometer-scale Mg-calcite grains precipitate on the organic structures within the PCW. 2) Crystalline cellulose microfibrils (CMF) are extruded perpendicularly from the cellulose synthase complexes (CSC) in the plasmalemma to form the SCW. 3) The CMF soaks in the mineralising fluid as it extrudes and becomes calcified, retaining the perpendicular form, thus building the radial calcite. In Clathromorphum, SCW formation lags PCW creating a zone of weakness resulting in a split in the sub-surface crust. All calcification seems likely to be a bioinduced rather than controlled process. These findings are a substantial step forward in understanding how corallines calcify.

The extraordinary joint material of an articulated coralline alga. I. Mechanical characterization of a key adaptation

Flexibility is key to survival for seaweeds exposed to the extreme hydrodynamic environment of wave-washed rocky shores. This poses a problem for coralline algae, whose calcified cell walls make them rigid. Through the course of evolution, erect coralline algae have solved this problem by incorporating joints (genicula) into their morphology, allowing their fronds to be as flexible as those of uncalcified seaweeds. To provide the flexibility required by this structural innovation, the joint material of Calliarthron cheilosporioides, a representative articulated coralline alga, relies on an extraordinary tissue that is stronger, more extensible and more fatigue resistant than the tissue of other algal fronds. Here, we report on experiments that reveal the viscoelastic properties of this material. On the one hand, its compliance is independent of the rate of deformation across a wide range of deformation rates, a characteristic of elastic solids. This deformation rate independence allows joints to maintain their flexibility when loaded by the unpredictable - and often rapidly imposed - hydrodynamic force of breaking waves. On the other hand, the genicular material has viscous characteristics that similarly augment its function. The genicular material dissipates much of the energy absorbed as a joint is deformed during cyclic wave loading, which potentially reduces the chance of failure by fatigue, and the material accrues a limited amount of deformation through time. This limited creep increases the flexibility of the joints while preventing them from gradually stretching to the point of failure. These new findings provide the basis for understanding how the microscale architecture of genicular cell walls results in the adaptive mechanical properties of coralline algal joints.

Coralline algae (Rhodophyta) in a changing world: integrating ecological, physiological, and geochemical responses to global change

Coralline algae are globally distributed benthic primary producers that secrete calcium carbonate skeletons. In the context of ocean acidification, they have received much recent attention due to the potential vulnerability of their high-Mg calcite skeletons and their many important ecological roles. Herein, we summarize what is known about coralline algal ecology and physiology, providing context to understand their responses to global climate change. We review the impacts of these changes, including ocean acidification, rising temperatures, and pollution, on coralline algal growth and calcification. We also assess the ongoing use of coralline algae as marine climate proxies via calibration of skeletal morphology and geochemistry to environmental conditions. Finally, we indicate critical gaps in our understanding of coralline algal calcification and physiology and highlight key areas for future research. These include analytical areas that recently have become more accessible, such as resolving phylogenetic relationships at all taxonomic ranks, elucidating the genes regulating algal photosynthesis and calcification, and calibrating skeletal geochemical metrics, as well as research directions that are broadly applicable to global change ecology, such as the importance of community-scale and long-term experiments in stress response.