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.     

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.     

Delimitation of cryptic species of the Scytosiphon lomentaria complex (Scytosiphonaceae,Phaeophyceae) in Japan,based on mitochondrial and nuclear molecular markers

Scytosiphon lomentaria (Scytosiphonaceae, Ectocarpales) is believed to include some cryptic species, particularly in the Pacific. We attempted to delimit these species in Japan using mitochondrial cox1 and cox3 and nuclear ITS2 and the second intron of the centrin gene (cetn‐int2). Fifty‐three cox1+cox3 mitotypes, 26 ITS2 ribotypes and 45 cetn‐int2 haplotypes were found in 107 samples collected from 33 localities in Japan. Based on phylogenetic analyses, similar sequence types were grouped into ten mitogroups, eight ribogroups and six cetn‐int2 haplogroups (sequence‐type groups). From the molecular trees and combinations of the mito‐, ribo‐ and haplogroups, three cryptic species were apparent (Groups I–III). Group I, widely distributed on Pacific coasts, was highly supported by all molecular trees, whereas Groups II (North Pacific) and III (Northwestern Pacific and Australasia) were more closely related to each other. However, sequence‐type‐group combinations that would be characteristic of hybrids between Groups II and III were not detected, suggesting no gene flow between the two Groups. Further investigations of an additional 127 sympatrically growing plants supported the absence of gene flow between Groups II and III. Four samples did not belong to any of the Groups I–III and possibly represent additional species.     

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.     

Morphological changes of sperm nuclei during spermatogenesis in the brown alga Cystoseira hakodatensis (Fucales, Phaeophyceae)

Morphological changes and chromatin condensation of sperm nuclei were observed during spermatogenesis in the fucalean brown alga Cystoseira hakodatensis (Yendo) Fensholt. Ultrastructural studies have shown that the mature spermatozoid has an elongated and concave nucleus with condensed chromatin. The morphological changes and the chromatin condensation process during spermatogenesis was observed. Nuclear size decreased in two stages during spermatogenesis. During the first stage, spherical nuclei decreased in size as they were undergoing meiotic divisions and the subsequent mitoses within the antheridium. During the second stage, the morphological transformation from a spherical into an elongated nucleus occurred. Afterwards, chromatin condensed at the periphery in each nucleus, and chromatin‐free regions were observed in the center of the nucleus. These chromatin‐free regions in the center of nucleus were compressed by the peripheral chromatin‐condensed region. As the result, the elongated and concave nucleus of the mature sperm consisted of uniformly well‐condensed chromatin.     

In situ growth rates of Sargassum horneri (Fucales,Phaeophyta)1

The relative growth rate of the brown alga Sargassum horneri (Turner) C. Agardh in the sea at Maizuru Bay, Japan, was investigated from summer to winter to examine the impact of declining solar radiation and temperature. The observed change in the growth rate between summer and winter was insignificant, in spite of a major decline in solar radiation and water temperature. The mean relative growth rate was 4.6% per day, equivalent to the daily net photosynthetic production previously reported for this species.     

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.     

Turbinaria foliosasp. nov. (Fucales, Phaeophyceae) from the Sultanate of Oman, with a census of currently recognized species in the genus Turbinaria

Turbinaria foliosasp. nov. is described on the basis of several collections made from Dhofar, Sultanate of Oman. The new species is known only from southern Oman, a region of the northern Arabian Sea that is strongly impacted by the upwelling from the summertime monsoon. It is distinguished from other species of the genus by the shape of the leaves and by their loose, non‐congested arrangement. Air vesicles, embedded in the leaves, may be present, but more often are absent. A census of the currently recognized species in the genus Turbinaria is provided. Reference is made to Sargassum turbinarioides and Sargassum turbinati‐folium. These species possess discrete blades and spherical vesicles and thus conform to Sargassum.     

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.     

Photoperiodic regulation of receptacle induction in Sargassum horneri (Phaeophyceae) using clonal thalli

We developed a clonal culture of Sargassum horneri to investigate the effect of photoperiod on reproduction in this species. Regenerated vegetative thalli were obtained using lateral branches excised from a thallus grown from a single embryo under short‐day conditions (SD = 10:14 h light : dark cycle). Lateral branches excised from the SD‐regenerated thallus became vegetative thalli that remained in that phase as long as they were cultured under SD. When an excised lateral branch was cultured under long‐day conditions (LD = 14:10 h light : dark cycle), it began to enter the reproductive phase while still less than 50 mm long. Induction of the reproductive phase was accompanied by a distinctive morphological change – suppression of blade formation at the apical region of the branch; elongation of branches without blades was then followed by differentiation of receptacles bearing conceptacles on their surface. Apices of receptacles were able to interconvert between reproductive and vegetative phases, as blades resprouted upon transfer from LD to SD. The critical day length for induction of receptacle formation was between 13 and 14 h; receptacle formation was also induced under SD conditions with night breaks (NBs). These results strongly suggest that reproductive regulation of S. horneri is a photoperiodic long‐day response. NBs with blue and green light were effective for reproductive induction but not with red light. This suggests that blue‐ and/or green‐light photoreceptors are involved in the photoperiodic reproductive response of S. horneri.     

Colpomenia borea sp. nov. (Scytosiphonaceae,Phaeophyceae) from Japan and Far East Russia

Colpomenia borea sp. nov. is described from Hokkaido, Japan and Magadan, Far East Russia based on morphological observations and molecular analyses using mitochondrial cox3 and chloroplast rbcL genes. This new species is distinguished from other Colpomenia by its small globular to ovoidal thallus up to 5 cm in diameter, and thin thallus membrane composed of a cortex of one to two-cell layers and a colorless medulla of up to three layers. This species is epiphytic on the brown alga Stephanocystis in areas protected against waves. The life history in culture of C. borea was investigated and the observed life history pattern was similar to those reported in C. peregrina and C. sinuosa. Our phylogenetic analyses supported that C. borea is a distinct species, yet it is more related to C. peregrina than C. sinuosa.     

Effect of brefeldin A and the dynamics of the actin plate on cytokinesis of zygotes in the brown alga,Silvetia babingtonii (Fucales,Phaeophyceae)

In the cytokinesis of brown algae, actin filaments appear like a plate at the intersecting region of microtubules (MTs) that emerge from the centrosomes after mitosis. The function of the actin plate itself is still unknown. To elucidate the relationship between the actin plate, MTs and membrane fusion, without inducing cytoskeleton depolymerization, the effect of brefeldin A (BFA), which prevents the production of vesicles from Golgi bodies, was examined in zygotes of Silvetia babingtonii. The beginning of mitosis was slightly delayed in zygotes under BFA compared with the controls. Almost all zygotes were inhibited for the progression of cytokinesis by BFA treatment. Ultrastructural observations showed that Golgi cisternae became fragmented or curled following continuous treatment with BFA, and the inhibitory status of cytokinesis between zygotes. The next cell cycle started before cytokinesis was completed. Although the appearance of the actin plate was not disturbed by BFA treatment, the behaviour of the actin plate during the transition between the first and second cell cycles could be classified into two patterns: it was either invisible upon the initiation of the next cell cycle, or a portion of it remained even though the next cell cycle had begun. In the latter case, a part of the actin plate seemed to associate with the new partially formed cell partition membrane, and MTs from the centrosomes were bound to it. The actin plate completely disappeared in the next mitosis, then re-emerged in the middle area of the four daughter nuclei. The results of the present study indicated that, under BFA treatment, the actin plate persisted until just before the beginning of the next mitotic phase, when the new, incomplete cell partition membrane was present, and MTs sustained the actin plate until next mitosis.     

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.     

A Virus Infection in the Marine Brown Alga Ectocarpus siliculosus (Phaeophyceae)

Laboratory cultures of Ectocarpus siliculosus originating from New Zealand showed a defect in gametangium formation. Nuclear divisions in gametangium initials are not followed by cell wall formation. In the resulting multinucleate cells nuclear DNA increases dramatically, and nuclear membranes disintegrate. Eventually, the entire structure is filled with hexagonal particles of approximately 130 nm diameter. These were isolated and shown by EM to consist of a dense core surrounded by a 3-layered shell. They are released into the culture medium when the host cells burst. Ectocarpus gametes from healthy cultures could be infected by these particles. The resulting partheno-sporophytes developed pathological symptoms, suggesting that the particles are viruses. The expression of the defect is temperature dependent. At 10°C all gametangia are abnormal, while between 15 and 20 °C defective and normal gametangia and gametes are formed on the same plant. Partheno-sporophytes developing from such gametes carry the viral particles expressed in deformed unilocular and plurilocular sporangia.     

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.     

Characterization of microsatellite loci in the marine seaweed Ascophyllum nodosum (Phaeophyceae; Fucales)

Ascophyllum nodosum L. dominates rocky intertidal shores throughout the temperate North Atlantic. Six microsatellite loci were developed for A. nodosum using enriched libraries. The number of alleles ranged from 9 to 24 and heterozygosities from 0.2213 to 0.7785. Ascophyllum is monotypic. There was no cross‐reactivity observed with Fucus serratus, F. vesiculosus or F. evanescens.     

Phylogeographic analysis of the brown alga Cutleria multifida (Tilopteridales,Phaeophyceae) suggests a complicated introduction history

In depth genetic comparisons of populations of Cutleria multifida (Tilopteridales, Phaeophyceae) collected from Europe, the northwestern Pacific Ocean, Australia and New Zealand using the DNA sequences of four gene regions (the mitochondrial cox2 and cox3 genes, the intergeneric spacer region adjacent to cox3, and the open reading frame) suggested that the northwestern European and Japanese populations were considerably greater in terms of their genetic divergence than Mediterranean, Australian or New Zealand populations. The haplotypes of the populations in northwestern European (distribution range including the type locality, seven haplotypes) and Japanese populations (seven haplotypes) were unique except for one shared haplotype. There were weak but positive correlations between the geographical distance and the genetic divergence among northwestern European and Japanese populations. Moreover, both female and male gametophytes occurred in eight of the nine Japanese localities, suggesting Japanese populations showed normal sexual heteromorphic life history of the species. In light of these results, it appears that Japanese populations were native to the area despite earlier hypothesis. In contrast, Australian and New Zealand populations were composed of only one haplotype that is very close to those found in northwestern Europe and Japan, suggesting a recent introduction history from Europe (or from northeastern Asia via Europe) by ship transport to Australia and New Zealand. The Mediterranean populations included two haplotypes identical to those found in northwestern Europe and Japan, and it is suggestive of transoceanic introductions of some populations between Mediterranean and Japanese coasts.