Algal Clades Research Articles

Ordovician origin and subsequent diversification of the brown algae

Brown algae are the only group of heterokont protists exhibiting complex multicellularity. Since their origin, brown algae have adapted to various marine habitats, evolving diverse thallus morphologies and gamete types. However, the evolutionary processes behind these transitions remain unclear due to a lack of a robust phylogenetic framework and problems with time estimation. To address these issues, we employed plastid genome data from 138 species, including heterokont algae, red algae, and other red-derived algae. Based on a robust phylogeny and new interpretations of algal fossils, we estimated the geological times for brown algal origin and diversification. The results reveal that brown algae first evolved true multicellularity, with plasmodesmata and reproductive cell differentiation, during the late Ordovician Period (ca. 450 Ma), coinciding with a major diversification of marine fauna (the Great Ordovician Biodiversification Event) and a proliferation of multicellular green algae. Despite its early Paleozoic origin, the diversification of major orders within this brown algal clade accelerated only during the Mesozoic Era, coincident with both Pangea rifting and the diversification of other heterokont algae (e.g., diatoms), coccolithophores, and dinoflagellates, with their red algal-derived plastids. The transition from ancestral isogamy to oogamy was followed by three simultaneous reappearances of isogamy during the Cretaceous Period. These are concordant with a positive character correlation between parthenogenesis and isogamy. Our new brown algal timeline, combined with a knowledge of past environmental conditions, shed new light on brown algal diversification and the intertwined evolution of multicellularity and sexual reproduction.

Selection drives convergent gene expression changes during transitions to co-sexuality in haploid sexual systems

Co-sexuality has evolved repeatedly from unisexual (dioicous) ancestors across a wide range of taxa. However, the molecular changes underpinning this important transition remain unknown, particularly in organisms with haploid sexual systems such as bryophytes, red algae and brown algae. Here we explore four independent events of emergence of co-sexuality from unisexual ancestors in brown algal clades to examine the nature, evolution and degree of convergence of gene expression changes that accompany the breakdown of dioicy. The amounts of male versus female phenotypic differences in dioicous species were not correlated with the extent of sex-biased gene expression, in stark contrast to what is observed in animals. Although sex-biased genes exhibited a high turnover rate during brown alga diversification, some of their predicted functions were conserved across species. Transitions to co-sexuality consistently involved adaptive gene expression shifts and rapid sequence evolution, particularly for male-biased genes. Gene expression in co-sexual species was more similar to that in females rather than males of related dioicous species, suggesting that co-sexuality may have arisen from ancestral females. Finally, extensive convergent gene expression changes, driven by selection, were associated with the transition to co-sexuality. Together, our observations provide insights on how co-sexual systems arise from ancestral, haploid UV sexual systems.

Organismal and cellular interactions in vertebrate-alga symbioses.

Photosymbioses, intimate interactions between photosynthetic algal symbionts and heterotrophic hosts, are well known in invertebrate and protist systems. Vertebrate animals are an exception where photosynthetic microorganisms are not often considered part of the normal vertebrate microbiome, with a few exceptions in amphibian eggs. Here, we review the breadth of vertebrate diversity and explore where algae have taken hold in vertebrate fur, on vertebrate surfaces, in vertebrate tissues, and within vertebrate cells. We find that algae have myriad partnerships with vertebrate animals, from fishes to mammals, and that those symbioses range from apparent mutualisms to commensalisms to parasitisms. The exception in vertebrates, compared with other groups of eukaryotes, is that intracellular mutualisms and commensalisms with algae or other microbes are notably rare. We currently have no clear cell-in-cell (endosymbiotic) examples of a trophic mutualism in any vertebrate, while there is a broad diversity of such interactions in invertebrate animals and protists. This functional divergence in vertebrate symbioses may be related to vertebrate physiology or a byproduct of our adaptive immune system. Overall, we see that diverse algae are part of the vertebrate microbiome, broadly, with numerous symbiotic interactions occurring across all vertebrate and many algal clades. These interactions are being studied for their ecological, organismal, and cellular implications. This synthesis of vertebrate-algal associations may prove useful for the development of novel therapeutics: pairing algae with medical devices, tissue cultures, and artificial ecto- and endosymbioses.

Domoic acid biosynthesis in the red alga Chondria armata suggests a complex evolutionary history for toxin production

Domoic acid (DA), the causative agent of amnesic shellfish poisoning, is produced by select organisms within two distantly related algal clades: planktonic diatoms and red macroalgae. The biosynthetic pathway to isodomoic acid A was recently solved in the harmful algal bloom-forming diatom Pseudonitzschia multiseries, establishing the genetic basis for the global production of this potent neurotoxin. Herein, we sequenced the 507-Mb genome of Chondria armata, the red macroalgal seaweed from which DA was first isolated in the 1950s, identifying several copies of the red algal DA (rad) biosynthetic gene cluster. The rad genes are organized similarly to the diatom DA biosynthesis cluster in terms of gene synteny, including a cytochrome P450 (CYP450) enzyme critical to DA production that is notably absent in red algae that produce the simpler kainoid neurochemical, kainic acid. The biochemical characterization of the N-prenyltransferase (RadA) and kainoid synthase (RadC) enzymes support a slightly altered DA biosynthetic model in C. armata via the congener isodomoic acid B, with RadC behaving more like the homologous diatom enzyme despite higher amino acid similarity to red algal kainic acid synthesis enzymes. A phylogenetic analysis of the rad genes suggests unique origins for the red macroalgal and diatom genes in their respective hosts, with native eukaryotic CYP450 neofunctionalization combining with the horizontal gene transfer of N-prenyltransferases and kainoid synthases to establish DA production within the algal lineages.

Rappemonads are haptophyte phytoplankton

Rapidly accumulating genetic data from environmental sequencing approaches have revealed an extraordinary level of unsuspected diversity within marine phytoplankton,1-11 which is responsible for around 50% of global net primary production.12,13 However, the phenotypic identity of many of the organisms distinguished by environmental DNA sequences remains unclear. The rappemonads represent a plastid-bearing protistan lineage that to date has only been identified by environmental plastid 16S rRNA sequences.14-17 The phenotypic identity of this group, which does not confidently cluster in any known algal clades in 16S rRNA phylogenetic reconstructions,15 has remained unknown since the first report of environmental sequences over two decades ago. We show that rappemonads are closely related to a haptophyte microalga, Pavlomulina ranunculiformis gen. nov. et sp. nov., and belong to a new haptophyte class, the Rappephyceae. Organellar phylogenomic analyses provide strong evidence for the inclusion of this lineage within the Haptophyta as a sister group to the Prymnesiophyceae. Members of this new class have a cosmopolitan distribution in coastal and oceanic regions. The relative read abundance of Rappephyceae in a large environmental barcoding dataset was comparable to, or greater than, those of major haptophyte species, such as the bloom-forming Gephyrocapsa huxleyi and Prymnesium parvum, and this result indicates that they likely have a significant impact as primary producers. Detailed characterization of Pavlomulina allowed for reconstruction of the ancient evolutionary history of the Haptophyta, a group that is one of the most important components of extant marine phytoplankton communities.

Ediacaran algal cysts from the Doushantuo Formation, South China

AbstractEarly-middle Ediacaran organic-walled microfossils from the Doushantuo Formation studied in several sections in the Yangtze Gorges area, South China, show ornamented cyst-like vesicles of very high diversity. These microfossils are diagenetically permineralized and observed in petrographic thin-sections of chert nodules. Exquisitely preserved specimens belonging to seven species of Appendisphaera, Mengeosphaera, Tanarium, Urasphaera and Tianzhushania contain either single or multiple spheroidal internal bodies inside the vesicles. These structures indicate reproductive stages, endocyst and dividing cells, respectively, and are preserved at early to late ontogenetic stages in the same taxa. This new evidence supports the algal affiliations for the studied taxa and refutes previous suggestions of Tianzhushania being animal embryo or holozoan. The first record of a late developmental stage of a completely preserved specimen of T. spinosa observed in thin-section demonstrates the interior of vesicles with clusters of identical cells but without any cavity that is diagnostic for recognizing algal cysts vs animal diapause cysts. Various lines of evidence to infer biological affinities of these microfossils – morphology, reproductive characters, spatial arrangement of cells, and biochemical properties of the vesicle wall – are collectively characteristic of algal clades. Recognizing the biological affinities of these microfossils is key to understanding whether animals capable of producing such morphologically complex diapause cysts had an early Ediacaran fossil record (633–610 Ma), or the microfossils were non-animal holozoans or algae as argued herein for Tianzhushania spinosa and other studied microfossils.

Naked Chrysochromulina (Haptophyta) isolates from lake and river ecosystems: An electron microscopic comparison including new observations on the type species of this taxon

BackgroundChrysochromulina (Haptophyta) species are recognized as seminal contributors to marine and freshwater ecosystem function. Historically, scale vestiture is used to augment taxonomic identification of these algae, and a large literature exists concerning the morphology of many scale-covered representatives. Scale-less freshwater isolates present a new challenge. Details concerning the microanatomy of naked cells remain essentially unreported. Insight into morphological similarities/differences between scaled and naked cells provides information on the evolution of the taxon, especially as discussions defining cryptic species complexes become more germane. Using light, scanning, and transmission microscopy, we present an analysis of the cellular structure for three Chrysochromulina freshwater isolates. ResultsChrysochromulina tobinii Cattolico, Chrysochromulina parva Lackey and ChrysochromulinaAND cells are approximately 5 μm wide, saddle-shaped to globose, and devoid of scales. A lipid body is nestled close to each of two chloroplasts and numerous mitochondria. Plastoglobuli are frequently associated with thylakoidal membranes that encircle an internal pyrenoid. The Golgi apparatus has large, club-shaped cisternae. The haptonema consists of 9 microtubules within the cytoplasm, but 6 or 7 microtubules in the external portion. A rootlet system anchors the two flagella that are sub-apically inserted adjacent to the haptonema. The flagellar complex is anchored in the cytoplasm by a flat ribbon of microtubules and microtubular rootlets associated with each basal body. Fibrous rootlets, several of which are cross-banded, interconnect the two flagellar basal bodies and haptonema. ConclusionThree freshwater, scale-less Chrysochromulina isolates are almost indistinguishable in ultrastructure, even though extensive genome sequencing studies verify their distinctiveness. These freshwater representatives certainly comprise members of a cryptic species complex. Extensive morphological similarity also occurs between freshwater and scaled marine isolates. Importantly, this report addresses the misidentification of Chrysochromulina tobinii Cattolico as Chrysochromulina parva Lackey and clarifies a complex literature regarding the morphological description of the type species for this algal clade.

Chrysochromulina: Genomic assessment and taxonomic diagnosis of the type species for an oleaginous algal clade

BackgroundUntil recent studies documented their extensive contribution to primary productivity and carbon sequestration, haptophytes remained underappreciated players in global ecosystem processes. Contemporary analyses, augmented by the use of molecular probes, show the haptophyte taxon Chrysochromulina to be seminal to the ecology of both marine and freshwater ecosystems. Unfortunately, description for the type species for this clade remains enigmatic. ResultsChrysochromulina parva Lackey was re-isolated from Big Walnut Creek (Ohio), the site where the original isolate was obtained. The sequenced haploid genome of this organism is 65.7 Mb in size. Several noteworthy nuclear-encoded genes identified include a novel ftsZ (mediates organelle division) that phylogenetically clusters with the Chloroarachniophytes. Also revealed, is a complement of genes associated with meiosis and DNA repair, indicating the presence of a sexual cycle in this alga. Mitochondrial genes lost to the nucleus include all extrinsic components of the nad complex, completing a punctate pattern of transfer that is observed among algal taxa. Comparison of the newly sequenced Chrysochromulina parva Lacky isolate was made with that of Chrysochromulina tobinii (59.1 Mb) – a fresh water strain isolated from a lake in Colorado. ConclusionGenomic analysis suggests that fresh water Chrysochromulina isolates, though geographically well separated, from a related clade. The name of the type species of Chrysochromulina parva Lacky is anchored with a lectotype and epitype, and the second isolate is described as Chrysochromulina tobinii sp. nov. Chrysochromulina represents a new, extremely tractable model organism for experimental studies. This oleaginous alga has a small genome and because it represents only the second haptophyte taxon to be sequenced and assembled, presents new opportunity to examine the evolution of an algal taxon that plays an intrinsic role in ecosystem function.

Genomic Analysis of Picochlorum Species Reveals How Microalgae May Adapt to Variable Environments.

Understanding how microalgae adapt to rapidly changing environments is not only important to science but can help clarify the potential impact of climate change on the biology of primary producers. We sequenced and analyzed the nuclear genome of multiple Picochlorum isolates (Chlorophyta) to elucidate strategies of environmental adaptation. It was previously found that coordinated gene regulation is involved in adaptation to salinity stress, and here we show that gene gain and loss also play key roles in adaptation. We determined the extent of horizontal gene transfer (HGT) from prokaryotes and their role in the origin of novel functions in the Picochlorum clade. HGT is an ongoing and dynamic process in this algal clade with adaptation being driven by transfer, divergence, and loss. One HGT candidate that is differentially expressed under salinity stress is indolepyruvate decarboxylase that is involved in the production of a plant auxin that mediates bacteria-diatom symbiotic interactions. Large differences in levels of heterozygosity were found in diploid haplotypes among Picochlorum isolates. Biallelic divergence was pronounced in P. oklahomensis (salt plains environment) when compared with its closely related sister taxon Picochlorum SENEW3 (brackish water environment), suggesting a role of diverged alleles in response to environmental stress. Our results elucidate how microbial eukaryotes with limited gene inventories expand habitat range from mesophilic to halophilic through allelic diversity, and with minor but important contributions made by HGT. We also explore how the nature and quality of genome data may impact inference of nuclear ploidy.

Double blind microarray-based polysaccharide profiling enables parallel identification of uncharacterized polysaccharides and carbohydrate-binding proteins with unknown specificities

Marine algae are one of the largest sources of carbon on the planet. The microbial degradation of algal polysaccharides to their constitutive sugars is a cornerstone in the global carbon cycle in oceans. Marine polysaccharides are highly complex and heterogeneous, and poorly understood. This is also true for marine microbial proteins that specifically degrade these substrates and when characterized, they are frequently ascribed to new protein families. Marine (meta)genomic datasets contain large numbers of genes with functions putatively assigned to carbohydrate processing, but for which empirical biochemical activity is lacking. There is a paucity of knowledge on both sides of this protein/carbohydrate relationship. Addressing this ‘double blind’ problem requires high throughput strategies that allow large scale screening of protein activities, and polysaccharide occurrence. Glycan microarrays, in particular the Comprehensive Microarray Polymer Profiling (CoMPP) method, are powerful in screening large collections of glycans and we described the integration of this technology to a medium throughput protein expression system focused on marine genes. This methodology (Double Blind CoMPP or DB-CoMPP) enables us to characterize novel polysaccharide-binding proteins and to relate their ligands to algal clades. This data further indicate the potential of the DB-CoMPP technique to accommodate samples of all biological sources.

JSE IBC Special Issue on Frontiers in Plant Systematics and Evolution

JSE IBC Special Issue on Frontiers in Plant Systematics and Evolution

Tidying up the genusLetharia: introducingL. lupinasp. nov. and a new circumscription forL. columbiana

AbstractWestern North America is the global centre of diversity forLetharia, a distinctive and cryptically diverse genus of lichenized fungi belonging to theParmeliaceae. The genus is characterized by a shrubby, fruticose habit and presence of vulpinic acid. Previous studies using multiple fungal nuclear loci revealed the existence of two distinct species-level lineages within the traditional concept ofL. vulpinaand four such lineages withinL. columbiana. Here we use molecular sequence data in an attempt to settle long-standing taxonomic issues in the genus. Our results confirm the widespread existence withinL. vulpinas. lat. of two distinct species-level groups, each forming a mutually exclusive partnership with a separate algal clade withinTrebouxia jamesiis. lat. Accordingly, we formally describe the segregate speciesL. lupinasp. nov. Our results also support the evolutionary independence of four candidate species previously circumscribed fromL. columbianas. lat. One of these lineages,L.‘gracilis’, has already received species recognition asL. gracilis, while a second,L.‘lucida’, is epitypified here againstL. columbianas. str. Based on results from species delimitation analyses under the multispecies coalescent model, the two remaining lineages,L.‘barbata’ andL.‘rugosa’, also warrant formal taxonomic recognition; however, we refrain from describing these species pending additional studies of diagnostic characters, ecological preference, and distributions.

The evo‐devo of multinucleate cells, tissues, and organisms, and an alternative route to multicellularity

Multinucleate cells, tissues, or organisms occur in 60 families of land plants and in five otherwise diverse algal lineages (Rhodophyceae, Xanthophyceae, Chlorophyceae, Ulvophyceae, and Charophyceae). Inspection of a morphospace constructed out of eight developmental processes reveals a large number of possible variants of multinucleate cells and organisms that, with two exceptions, are represented by one or more plant species in one or more clades. Thus, most of these permutations of developmental processes exist in nature. Inspection of the morphospace also shows how the siphonous body plan (a multinucleate cell with the capacity for indeterminate growth in size) can theoretically serve as the direct progenitor of a multicellular organism by a process similar to segregative cell division observed in siphonocladean algae. Using molecular phylogenies of algal clades, different evolutionary scenarios are compared to see how the multicellular condition may have evolved from a multinucleate unicellular progenitor. We also show that the siphonous progenitor of a multicellular organism has previously passed through the alignment-of-fitness phase (in which genetic similarity among cells/nuclei minimizes internal genomic conflict) and the export-of-fitness phase (in which genetically similar cells/nuclei collaborate to achieve a reproductively integrated multicellular organism). All that is theoretically required is the evolutionary acquisition of the capacity to compartmentalize its cytoplasm.

An extended phylogenetic analysis reveals ancient origin of "non-green" phosphoribulokinase genes from two lineages of "green" secondary photosynthetic eukaryotes: Euglenophyta and Chlorarachniophyta

BackgroundEuglenophyta and Chlorarachniophyta are groups of photosynthetic eukaryotes harboring secondary plastids of distinct green algal origins. Although previous phylogenetic analyses of genes encoding Calvin cycle enzymes demonstrated the presence of genes apparently not derived from green algal endosymbionts in the nuclear genomes of Euglena gracilis (Euglenophyta) and Bigelowiella natans (Chlorarachniophyta), the origins of these "non-green" genes in "green" secondary phototrophs were unclear due to the limited taxon sampling.ResultsHere, we sequenced five new phosphoribulokinase (PRK) genes (from one euglenophyte, two chlorarachniophytes, and two glaucophytes) and performed an extended phylogenetic analysis of the genes based on a phylum-wide taxon sampling from various photosynthetic eukaryotes. Our phylogenetic analyses demonstrated that the PRK sequences form two genera of Euglenophyta formed a robust monophyletic group within a large clade including stramenopiles, haptophytes and a cryptophyte, and three genera of Chlorarachniophyta were placed within the red algal clade. These "non-green" affiliations were supported by the taxon-specific insertion/deletion sequences in the PRK alignment, especially between euglenophytes and stramenopiles. In addition, phylogenetic analysis of another Calvin cycle enzyme, plastid-targeted sedoheptulose-bisphosphatase (SBP), showed that the SBP sequences from two genera of Chlorarachniophyta were positioned within a red algal clade.ConclusionsOur results suggest that PRK genes may have been transferred from a "stramenopile" ancestor to Euglenophyta and from a "red algal" ancestor to Chlorarachniophyta before radiation of extant taxa of these two "green" secondary phototrophs. The presence of two of key Calvin cycle enzymes, PRK and SBP, of red algal origins in Chlorarachniophyta indicate that the contribution of "non-green" algae to the plastid proteome in the "green" secondary phototrophs is more significant than ever thought. These "non-green" putative plastid-targeted enzymes from Chlorarachniophyta are likely to have originated from an ancestral red alga via horizontal gene transfer, or from a cryptic red algal endosymbiosis in the common ancestor of the extant chlorarachniophytes.

Do photobionts influence the ecology of lichens? A case study of environmental preferences in symbiotic green alga Asterochloris (Trebouxiophyceae)

The distribution patterns of symbiotic algae are thought to be conferred mainly by their hosts, however, they may originate in algal environmental requirements as well. In lichens, predominantly terrestrial associations of fungi with algae or cyanobacteria, the ecological preferences of photobionts have not been directly studied so far. Here, we examine the putative environmental requirements in lichenized alga Asterochloris, and search for the existence of ecological guilds in Asterochloris-associating lichens. Therefore, the presence of phylogenetic signal in several environmental traits was tested. Phylogenetic analysis based on the concatenated set of internal transcribed spacer rDNA and actin type I intron sequences from photobionts associated with lichens of the genera Lepraria and Stereocaulon (Stereocaulaceae, Ascomycota) revealed 13 moderately to well-resolved clades. Photobionts from particular algal clades were found to be associated with taxonomically different, but ecologically similar lichens. The rain and sun exposure were the most significant environmental factor, clearly distinguishing the Asterochloris lineages. The photobionts from ombrophobic and ombrophilic lichens were clustered in completely distinct clades. Moreover, two photobiont taxa were obviously differentiated based on their substrate and climatic preferences. Our study, thus reveals that the photobiont, generally the subsidiary member of the symbiotic lichen association, could exhibit clear preferences for environmental factors. These algal preferences may limit the ecological niches available to lichens and lead to the existence of specific lichen guilds.

Oxidative stress induces distinct physiological responses in the two Trebouxia phycobionts of the lichen Ramalina farinacea.

Most lichens form associations with Trebouxia phycobionts and some of them simultaneously include genetically different algal lineages. In other symbiotic systems involving algae (e.g. reef corals), the relative abundances of different endosymbiotic algal clades may change over time. This process seems to provide a mechanism allowing the organism to respond to environmental stress. A similar mechanism may operate in lichens with more than one algal lineage, likewise protecting them against environmental stresses. Here, the physiological responses to oxidative stress of two distinct Trebouxia phycobionts (provisionally named TR1 and TR9) that coexist within the lichen Ramalina farinacea were analysed. Isolated phycobionts were exposed to oxidative stress through the reactive oxygen species propagator cumene hydroperoxide (CuHP). Photosynthetic pigments and proteins, photosynthesis (through modulated chlorophyll fluorescence), the antioxidant enzymes superoxide dismutase (SOD) and glutathione reductase (GR), and the stress-related protein HSP70 were analysed. Photosynthetic performance was severely impaired by CuHP in phycobionts, as indicated by decreases in the maximal PSII photochemical efficiency (F(v)/F(m)), the quantum efficiency of PSII (Φ(PSII)) and the non-photochemical dissipation of energy (NPQ). However, the CuHP-dependent decay in photosynthesis was significantly more severe in TR1, which also showed a lower NPQ and a reduced ability to preserve chlorophyll a, carotenoids and D1 protein. Additionally, differences were observed in the capacities of the two phycobionts to modulate antioxidant activities and HPS70 levels when exposed to oxidative stress. In TR1, CuHP significantly diminished HSP70 and GR but did not change SOD activities. In contrast, in TR9 the levels of both antioxidant enzymes and those of HSP70 increased in response to CuHP. The better physiological performance of TR9 under oxidative conditions may reflect its greater capacity to undertake key metabolic adjustments, including increased non-photochemical quenching, higher antioxidant protection and the induction of repair mechanisms.

Phylogeography of microbial phototrophs in the dry valleys of the high Himalayas and Antarctica

High-elevation valleys in dry areas of the Himalayas are among the most extreme, yet least explored environments on Earth. These barren, rocky valleys are subjected to year-round temperature fluctuations across the freezing point and very low availability of water and nutrients, causing previous workers to hypothesize that no photoautotrophic life (primary producers) exists in these locations. However, there has been no work using modern biogeochemical or culture-independent molecular methods to test the hypothesis that photoautotrophs are absent from high Himalayan soil systems. Here, we show that although microbial biomass levels are as low as those of the Dry Valleys of Antarctica, there are abundant microbial photoautotrophs, displaying unexpected phylogenetic diversity, in barren soils from just below the permanent ice line of the central Himalayas. Furthermore, we discovered that one of the dominant algal clades from the high Himalayas also contains the dominant algae in culture-independent surveys of both soil and ice samples from the Dry Valleys of Antarctica, revealing an unexpected link between these environmentally similar but geographically very distant systems. Phylogenetic and biogeographic analyses demonstrated that although this algal clade is globally distributed to other high-altitude and high-latitude soils, it shows significant genetic isolation by geographical distance patterns, indicating local adaptation and perhaps speciation in each region. Our results are the first to demonstrate the remarkable similarities of microbial life of arid soils of Antarctica and the high Himalayas. Our findings are a starting point for future comparative studies of the dry valleys of the Himalayas and Antarctica that will yield new insights into the cold and dry limits to life on Earth.

ANALYSIS OF A PLASTID MULTIGENE DATA SET AND THE PHYLOGENETIC POSITION OF THE MARINE MACROALGA CAULERPA FILIFORMIS (CHLOROPHYTA)(1).

Molecular phylogenetic relationships within the Chlorophyta have relied heavily on rRNA data. These data have revolutionized our insight in green algal evolution, yet some class relationships have never been well resolved. A commonly used class within the Chlorophyta is the Ulvophyceae, although there is not much support for its monophyly. The relationships among the Ulvophyceae, Trebouxiophyceae, and Chlorophyceae are also contentious. In recent years, chloroplast genome data have shown their utility in resolving relationships between the main green algal clades, but such studies have never included marine macroalgae. We provide partial chloroplast genome data (∼30,000 bp, 23 genes) of the ulvophycean macroalga Caulerpa filiformis (Suhr) K. Herig. We show gene order conservation for some gene combinations and rearrangements in other regions compared to closely related taxa. Our data also revealed a pseudogene (ycf62) in Caulerpa species. Our phylogenetic results, based on analyses of a 23-gene alignment, suggest that neither Ulvophyceae nor Trebouxiophyceae are monophyletic, with Caulerpa being more closely related to the trebouxiophyte Chlorella than to Oltmannsiellopsis and Pseudendoclonium.

Most corals may not change their symbionts

Many corals (stony corals and octo- corals) rely on their symbiotic algae (zooxanthellae) for survival. Under stress, zooxanthellae are ex- pelled, resulting in coral bleaching. The hypothesis that corals may survive climate change by ex- changing algal types is shown here to be potentially applicable only to a minority of corals. Data on 442 coral species from 43 studies reveal that only a few coral species may be able to change their symbionts. The ability to change symbionts seems to be linked to whether a coral species can host multiple zooxanthella clades, either at different depths on the same reef, on different reefs or at different geographic locations, or concurrently within the same colony. The combined data set shows that only 23% of coral species host multiple zoo- xanthella clades. Most coral species (77%) exhibit fidelity to a narrow subset of a single zooxanthella clade, some even to specific algal genotypes within a clade. These specific algal genotypes in coral species hosting a single algal clade do not change over time. Furthermore, no algal change occurs when a coral colony is either transplanted to differ- ent environments, or subjected to stressors such as disease or increased temperatures. For the majority of corals, therefore, algal switching does not appear to occur. These coral species will survive only if the existing host-symbiont combination withstands the changing conditions. If climate warming con- tinues, coral reefs may undergo a change in bio- diversity such that only a subset of symbiotic corals may persist.

The impact of coral bleaching on the pigment profile of the symbiotic alga, Symbiodinium

Bleaching of corals by loss of symbiotic dinoflagellate algae and/or photosynthetic pigments is commonly triggered by elevated temperatures coupled with high irradiance, and is a first-order threat to coral reef communities. In this study, a high-resolution high-performance liquid chromatography method integrated with mass spectrometry was applied to obtain the first definitive identification of chlorophyll and carotenoid pigments of three clades of symbiotic dinoflagellate algae (Symbiodinium) in corals, and their response to experimentally elevated temperature and irradiance. The carotenoids peridinin, dinoxanthin, diadinoxanthin (Dn), diatoxanthin (Dt) and beta-carotene were detected, together with chlorophylls a and c2, and phaeophytin a, in all three algal clades in unstressed corals. On exposure to elevated temperature and irradiance, three coral species (Montastrea franksi and Favia fragum with clade B algae, and Montastrea cavernosa with clade C) bleached by loss of 50-80% of their algal cells, with no significant impact to chlorophyll a or c2, or peridinin in retained algal cells. One species (Agaricia sp. with clade C) showed no significant reduction in algal cells at elevated temperature and irradiance, but lost substantial amounts of chlorophyll a and carotenoid pigments, presumably through photo-oxidative processes. Two coral species (Porites astreoides and Porites porites both bearing clade A algae) did not bleach. The impact of elevated temperature and irradiance on the levels of the photoprotective xanthophylls (Dn + Dt) and beta-carotene varied among the corals, both in pool size and xanthophyll cycling, and was not correlated to coral bleaching resistance.