Historical biogeography of the widespread macroalga Sargassum (Fucales,Phaeophyceae)

Sargassum is a cosmopolitan brown algal genus spanning the three ocean basins of the Atlantic, Pacific and Indian Oceans, inhabiting temperate, subtropical and tropical habitats. Sargassum has been postulated to have originated in the Oligocene epoch approximately 30 mya according to a broad phylogenetic analysis of brown macroalgae, but its diversification to become one of the most widespread and speciose macroalgal genera remains unclear. Here, we present a Bayesian molecular clock study, which analyzed data from the order Fucales of the brown algal crown radiation (BACR) group to reconstruct a time-calibrated phylogeny of the Sargassum clade. Our phylogeny included a total of 120 taxa with 99 Sargassum species sampled for three molecular markers – ITS-2, cox3 and rbcLS – calibrated with an unambiguous Sargassaceae fossil from between the lower and middle Miocene. The analysis revealed a much later origin of Sargassum than expected at about 6.7 mya, with the genus diversifying since approximately 4.3 mya. Current geographic distributions of Sargassum species were then analyzed in conjunction with the time-calibrated phylogeny using the dispersal-extinction-cladogenesis (DEC) model to estimate ancestral ranges of clades in the genus. Results strongly support origination of Sargassum in the Central Indo-Pacific (CIP) region with subsequent independent dispersal events into other marine realms. The longer history of diversification in the ancestral CIP range could explain the much greater diversity there relative to other marine areas today. Analyses of these dynamic processes, when fine-tuned to a higher spatial resolution, enable the identification of evolutionary hotspots and provide insights into long-term dispersal patterns.     

Egg release and germling development in Sargassum horneri (Fucales, Phaeophyceae)

The mature female conceptacle of Sargassum horneri (Turner) C. Agardh has an ostiole filled with a gelatinous plug. The oogonium in the conceptacle has cell walls that can be differentiated into a dense outer and a less dense inner microfibrillar layer. Just prior to egg release, stalk material is produced inside the outer layer and the inner layer disappears. At this stage the gelatinous plug is extruded and mucilage is released through the ostiole. The released eggs are retained on the receptacle by the stalk and are surrounded by a large amount of the mucilage. Three-celled germlings form a primary wall with a polylamellated structure of microfibril layers. In multicellular germlings that have differentiated into thallus and rhizoids, the peripheral thallus cells have an outer cell wall consisting of a microfibril layer under the primary wall, while the cell wall of the rhizoid tip has an amorphous structure. The germlings are released from the stalk and become attached to the substratum by an adhesive substance secreted from rhizoidal cells.     

Ecosystem functioning impacts of the invasive seaweed Sargassum muticum (Fucales,Phaeophyceae)

Ongoing changes in natural diversity due to anthropogenic activities can alter ecosystem functioning. Particular attention has been given to research on biodiversity loss and how those changes can affect the functioning of ecosystems, and, by extension, human welfare. Few studies, however, have addressed how increased diversity due to establishment of nonindigenous species (NIS) may affect ecosystem function in the recipient communities. Marine algae have a highly important role in sustaining nearshore marine ecosystems and are considered a significant component of marine bioinvasions. Here, we examined the patterns of respiration and light‐use efficiency across macroalgal assemblages with different levels of species richness and evenness. Additionally, we compared our results between native and invaded macroalgal assemblages, using the invasive brown macroalga Sargassum muticum (Yendo) Fensholt as a model species. Results showed that the presence of the invader increased the rates of respiration and production, most likely as a result of the high biomass of the invader. This effect disappeared when S. muticum lost most of its biomass after senescence. Moreover, predictability–diversity relationships of macroalgal assemblages varied between native and invaded assemblages. Hence, the introduction of high‐impact invasive species may trigger major changes in ecosystem functioning. The impact of S. muticum may be related to its greater biomass in the invaded assemblages, although species interactions and seasonality influenced the magnitude of the impact.     

Comparison of antifouling properties of native and invasive Sargassum (Fucales,Phaeophyceae) species

The invasiveness of algal species can be facilitated by chemo-ecological traits that allow the establishment of invasive species in a highly competitive environment. Anti-bacterial, anti-quorum sensing, anti-diatom and anti-larval properties of the invasive brown macroalga Sargassum muticum and three native Sargassum species from Oman waters were compared in laboratory and field experiments to assess whether these traits have the potential to facilitate the invasion process. Only the extract of S. muticum inhibited bacterial growth of four marine bacterial strains and quorum sensing in the reporter strain Chromobacterium violaceum CV017. Settlement, growth and survival of the diatom Cylindrotheca closterium and larvae of the bryozoan Bugula neritina were significantly inhibited by all Sargassum extracts in laboratory experiments. However, crude extracts of S. muticum had the strongest antifouling effect. Natural tissue-level concentrations of S. muticum extract reduced diatom density to about 20% compared with the controls. Larval mortality increased by 80–90% compared with controls with S. muticum extract diluted to one-third natural levels. Significant anti-diatom activity of S. muticum was confirmed in the field experiments with Sargassum extracts embedded in a phytagel matrix. Comparison of non-polar compounds by gas chromatography–mass spectrometry demonstrated that S. muticum extracts had overall fewer secondary metabolites but more species-unique compounds than extracts of native Sargassum spp. The greater antifouling defence of invasive vs. native Sargassum species indicates a selective trait that may contribute to the invasion success of S. muticum.     

Sargassum boreale sp. nov. (Fucales, Phaeophyceae) from Hokkaido, Japan

A new species, Sargassum boreale Yoshida et Horiguchi is described. It belongs to the subgenus Bactrophycus section Teretia, with cylindrical receptacles and is distinct from Sargassum confusum C. Agardh, S. pallidum (Turner) C. Agardh and Sargassum microceratium (Turner) C. Agardh in having a rather elongated stem with smooth surface and distantly issuing main branches, with narrow leaves. The distinction between S. boreale and these species is also revealed by a difference in internal transcribed spacer 2 (ITS‐2) sequences. In addition to the base substitutions, the existence of a large gap in S. boreale distinguishes this species from others. Sargassum boreale is distributed around Hokkaido and Saghalien to 50°N latitude. A key to the species of section Teretia is provided.     

Haplotypic differentiation between seasonal populations of Sargassum horneri (Fucales,Phaeophyceae) in Japan

Conspecific populations with different life‐cycle seasonality, particularly of their reproductive season, have been reported for many seaweed species. However, the number of genetic analyses of such seasonal populations is limited. Herein, based on the mitochondrial cytochrome oxidase subunit 3 gene (cox3) haplotype, we evaluated the genetic diversity of winter (January–March) and spring (April–June) fertile populations of the brown alga Sargassum horneri in Sado Island, Japan. Obviously, dominant cox3 haplotypes were different between seasons (even at a single site), and the N _ST values indicated the presence of a strong genetic differentiation between the two seasonal populations. AMOVA analyses confirmed that most of the genetic diversity was between seasons rather than between regions. Herein, the possibility of allochronic isolation is discussed. Considering the phylogeny of the mitochondrial cox3 DNA sequence, autumn or winter seasonal populations such as those in this investigation have evolved independently at least twice in the species. The present study shows that a shift in breeding season could be expected to conserve the genetic diversity of S. horneri in a certain region.     

Testing systematic concepts of Sargassum (Fucales, Phaeophyceae) using portions of the rbcLS operon

The taxonomic and phylogenetic concepts within the Sargassum C. Agardh (Sargassaceae) species complex were evaluated through molecular phylogenetic analyses using portions of the chloroplast encoded rbcLS Operon. According to more conservative sequences (rbcL), Turbinaria (Turner) J. Agardh is a close and well‐supported sister lineage to the Sargassum species complex and an appropriate external outgroup for analyses of subgenera and subsections within Sargassum. Both rbcL and more rapidly evolving rbcLS spacer sequences indicated that the East Asiatic genus Myagropsis (Mertens et Turner) Fensholt, along with Sargassum sinicola Setchell et Gardner, represent the closest lineage to Sargassum and form appropriate internal outgroups. The rbcLS spacer region supported three of four subgeneric designations by J. Agardh and sectional levels within the subgenus Sargassum. However, some aspects of Agardh's system were not supported: many of the subsectional ranks or the phyletic concepts; Phyllotrichia was not monophyletic as a subgenus, and its species were also not the most ancestral of Sargassum; and subgenus Sargassum was not the most derived subgenus within the genus. This modern phylogeny suggests a deep evolutionary history for subgenus Sargassum with rapid speciation in closely related subsections and series, and a sister relationship between subgenera Arthrophycus and Bactrophycus.     

Finding of regeneration in a large number of wounded receptacles in Sargassum ringgoldianum (Phaeophyceae,Fucales)

Sargassum ringgoldianm is a canopy‐forming perennial macroalga dominant in a shallow waters along the central Pacific coast of Honshu, Japan, where macroalgal forests have been drastically declining by browsing of herbivorous fish since the 1990s. Accordingly, we tried to reveal seasonal fluctuation of browsing damage on S. ringgoldianum. In the course of the investigation, we found signs of receptacle regeneration in S. rinngoldianum, which has been reported in a few species of the order Fucales. Regenerations occurred most frequently from 3.0 mm to 3.9 mm of the basal part of original receptacles and were also observed at the tips of remaining stalks as well as branchlets that seemed to lose original receptacles. The regenerated receptacles formed antheridia and oogonia, and released eggs from them, suggesting that regenerated receptacles are functional. In addition, no regeneration was observed in only three individuals among 51 plants observed. We show herein a high regenerative ability of wounded receptacles in S. ringgoldianum. This frequent regeneration of functional receptacles is important for the persistence of the population.     

Distributional range shifts of Western Atlantic benthic Sargassum species (Fucales,Phaeophyceae) under future climate change scenarios

Climate change is altering the world’s marine biota, in particular, their geographic distribution. Sargassum species are foundation species that play critical ecological roles in tropical benthic communities, providing food, habitat heterogeneity and shelter for a wide range of marine organisms. To understand how future changes in abiotic variables could affect the distribution of Sargassum species along the Western Atlantic Ocean, we performed Ecological Niche Models (ENM) for 12 benthic Sargassum species. We projected present and future habitat suitability distributions under the RCP 4.5 and RCP 8.5 IPCC scenarios. We fit ENM and created ensembles from different algorithms. Our results predict changes in species latitudinal range (niche suitability) in the order of 0.5˚ to 8.1˚ northward, and 0˚ to 5.5˚ southward. Six species are likely to reduce their suitability area from 10% to 80%, while other six species are likely to expand their suitability area from 4% to 168%. Overall, changes in suitability area and latitudinal ranges will increase at larger latitudes for most species while suitability areas will decrease at lower latitudes for half of the species. This pattern is consistent with the expected tropicalization of temperate latitudes following global warming. Such changes can produce considerable losses in ecosystem services maintained by healthy Sargassum beds, particularly at lower latitudes. Our findings highlight the need to improve Sargassum conservation policies and management strategies to avoid the negative effects caused by losses in Sargassum forests.     

Seasonal studies on the alginate and its biochemical composition I: Sargassum polycystum (Fucales), Phaeophyceae

Investigations were made on the brown seaweed Sargassum polycystum C. Agardh collected from Rameswaram Coast, Tamil Nadu. The alginates extracted from ‘leaf’, ‘stem’ and entire thallus of S. polycystum were investigated for their viscosity and chemical constituents, namely β‐D‐mannuronic acid (M‐block), α‐L‐guluronic acid (G‐block) and alternating sequences of β‐D‐mannuronic acid and α‐L‐guluronic acid (MG‐block) for six different seasons between August 1998 and November 1999. Significant seasonal variation (P< 0.05) was observed with high yield of alginate in February. The alginate extracted from the ‘leaf’ region showed a maximum yield whereas the ‘stem’ region exhibited maximum viscosity. The amount of G‐block was found to be more than M‐ and MG‐blocks in all the samples tested. The amount of G‐block was high in ‘stem’ followed by leaf and entire thallus. A positive correlation was recorded between viscosity and G‐block. Among the three alginates, the ratio of M/G was low in the ‘stem’ followed by ‘leaf’ and entire thallus.     

Genetic and physiological insights into the metabolic syndrome

The metabolic syndrome (MetS) is a common phenotype that is clinically defined by threshold values applied to measures of central obesity, dysglycemia, dyslipidemia, and/or elevated blood pressure, which must be present concurrently in any one of a variety of combinations. Insulin resistance, although not a defining component of the MetS, is nonetheless considered to be a core feature. MetS is important because it is rapidly growing in prevalence and is strongly related to the development of cardiovascular disease. To define etiology, pathogenesis and expression of MetS, we have studied patients, specifically Canadian families and communities. One example is familial partial lipodystrophy (FPLD), a rare monogenic form of insulin resistance caused by mutations in either LMNA, encoding nuclear lamin A/C (subtype FPLD2), or in PPARG, encoding peroxisomal proliferator-activated receptor-gamma (subtype FPLD3). Because it evolves slowly and recapitulates key clinical and biochemical attributes, FPLD seems to be a useful monogenic model of MetS. A second example is the disparate MetS prevalence between two Canadian aboriginal groups that is mirrored by disparate prevalence of diabetes and cardiovascular disease. Careful phenotypic evaluation of such special cases of human MetS by using a wide range of diagnostic methods, an approach called "phenomics," may help uncover early presymptomatic disease biomarkers, which in turn might reveal new pathways and targets for interventions for MetS, diabetes, and atherosclerosis.     

Electron and immunofluorescence microscopy on the fertilization ofFucus distichus (Fucales,Phaeophyceae)

Summary Processes of fertilization and zygote development inFucus distichus were studied by indirect immunofluorescence microscopy using anti- tubulin antibody and electron microscopy. Just after plasmogamy, sperm aster formation occurs during migration of a sperm nucleus toward an egg nucleus at the center of cytoplasm. Only sparse microtubules (MTs) exist around the egg nucleus. The sperm aster can be observed till karyogamy, but afterwards vanishes. Accompanying sperm aster formation, cortical MTs which are reticulately arranged develop further in the zygotes. In 4 h-old zygotes, characteristic structures which are composed of fine granular masses and consist of intermixed dense and lighter staining areas appear around the nucleus. These structures cannot be detected with anti- tubulin immunofluorescence microscopy. The two centrioles derived from the sperm separate and migrate to both poles. In 4 h-and 8 h-old zygotes, there are no defined MT foci around the zygote nucleus and MTs radiate from the circumference of it. In 12 h-old zygotes, each centriole has migrated to the poles and derivative centrioles are generated. The fine granular masses also migrate to both poles and finally disappear accompanying the appearance of numerous MTs radiating from the poles. Therefore, two distinct MT foci appear from 12 h onwards. Progressive stages of nuclear division were also examined with electron and immunofluorescence microscopy in 16 h-old zygotes. The sperm chloroplast with an eyespot and the sperm mitochondria with an intercristal tubular structure, which are distinctive from those of egg, can be detected after plasmogamy and karyogamy. The sperm chloroplast is still present in 16 h-old zygotes.