Vegetative Filaments Research Articles

Tip extension and simultaneous multiple fission in a filamentous bacterium

Organisms display an immense variety of shapes, sizes, and reproductive strategies. At microscopic scales, bacterial cell morphology and growth dynamics are adaptive traits that influence the spatial organization of microbial communities. In one such community-the human dental plaque biofilm-a network of filamentous Corynebacterium matruchotii cells forms the core of bacterial consortia known as hedgehogs, but the processes that generate these structures are unclear. Here, using live-cell time-lapse microscopy and fluorescent D-amino acids to track peptidoglycan biosynthesis, we report an extraordinary example of simultaneous multiple division within the domain Bacteria. We show that C. matruchotii cells elongate at one pole through tip extension, similar to the growth strategy of soil-dwelling Streptomyces bacteria. Filaments elongate rapidly, at rates more than five times greater than other closely related bacterial species. Following elongation, many septa form simultaneously, and each cell divides into 3 to 14 daughter cells, depending on the length of the mother filament. The daughter cells then nucleate outgrowth of new thinner vegetative filaments, generating the classic "whip handle" morphology of this taxon. Our results expand the known diversity of bacterial cell cycles and help explain how this filamentous bacterium can compete for space, access nutrients, and form important interspecies interactions within dental plaque.

3D-reconstructions of zygospores in Zygnema vaginatum (Charophyta) reveal details of cell wall formation, suggesting adaptations to extreme habitats.

The streptophyte green algal class Zygnematophyceae is the immediate sister lineage to land plants. Their special form of sexual reproduction via conjugation might have played a key role during terrestrialization. Thus, studying Zygnematophyceae and conjugation is crucial for understanding the conquest of land. Moreover, sexual reproduction features are important for species determination. We present a phylogenetic analysis of a field-sampled Zygnema strain and analyze its conjugation process and zygospore morphology, both at the micro- and nanoscale, including 3D-reconstructions of the zygospore architecture. Vegetative filament size (26.18 ± 1.07 μm) and reproductive features allowed morphological determination of Zygnema vaginatum, which was combined with molecular analyses based on rbcL sequencing. Transmission electron microscopy (TEM) depicted a thin cell wall in young zygospores, while mature cells exhibited a tripartite wall, including a massive and sculptured mesospore. During development, cytological reorganizations were visualized by focused ion beam scanning electron microscopy (FIB-SEM). Pyrenoids were reorganized, and the gyroid cubic central thylakoid membranes, as well as the surrounding starch granules, degraded (starch granule volume: 3.58 ± 2.35 μm3 in young cells; 0.68 ± 0.74 μm3 at an intermediate stage of zygospore maturation). Additionally, lipid droplets (LDs) changed drastically in shape and abundance during zygospore maturation (LD/cell volume: 11.77% in young cells; 8.79% in intermediate cells, 19.45% in old cells). In summary, we provide the first TEM images and 3D-reconstructions of Zygnema zygospores, giving insights into the physiological processes involved in their maturation. These observations help to understand mechanisms that facilitated the transition from water to land in Zygnematophyceae.

Morphology, taxonomy and phylogenetic positions of two Spirogyra species (Zygnematophyceae, Streptophyta) with replicate and semi-replicate transverse walls from Japan

ABSTRACT Characteristics of transverse walls in filaments, and modes of conjugation are essential for species identification in the genus Spirogyra. Although most Spirogyra species have plane transverse walls, species with other types of transverse walls (replicate, semi-replicate and colligate types) have also been described. Recent molecular phylogenetic studies grouped species with replicate transverse walls in a single clade. However, phylogenetic positions of Spirogyra species with semi-replicate and colligate transverse walls have not yet been clarified. We examined vegetative and reproductive traits of two Spirogyra species with replicate and semi-replicate transverse walls collected in Japan, under culture conditions. One of them resembled most S. crassispina in having replicate transverse walls and both scalariform and lateral conjugation, with conjugation tubes separated from the filaments in lateral conjugation. However, these two species differed in their vegetative filaments, zygospores and mesospores; and the molecular phylogenetic analyses placed the Japanese strain with replicate transverse walls in a lineage separate from other previously known species with replicate transverse walls. Vegetative filaments of the other Japanese strain had semi-replicate transverse walls resembling those of previously known species S. unduliseptum and S. narcissiana. This strain differs from S. unduliseptum by the variation in zygospore ornamentation and from S. narcissiana by the presence of three types of conjugation; and it formed an independent lineage that was phylogenetically separated from two Spirogyra lineages characterized by having replicate transverse walls. The two Japanese strains reported on are described herein as S. minuticrassispina sp. nov. and S. tertia sp. nov.

Photophysiological investigations of the temperature stress responses of Zygnema spp (Zygnematophyceae) from subpolar and polar habitats (Iceland, Svalbard)

ABSTRACT Zygnematophyceae are main primary producers in polar hydro-terrestrial habitats, characterized by extreme abiotic conditions. They are expected to be strongly impacted by climate change, leading to threats to subpolar and polar ecosystems. Two isolates of Zygnema from a subpolar (Iceland, Zygnema sp. I) and a polar Island (Svalbard, Zygnema sp. B) were compared by their photophysiological performance and phenolic content. A phylogenetic analysis was performed in a newly isolated Zygnema I where also morphology and ultrastructure were characterized. The rbcL sequence of the Icelandic Zygnema I was identical to that of Zygnema V, previously isolated from Svalbard, and phylogenetic analysis placed this strain into Zygnema clade 2. Average width of vegetative filaments (28 ± 0.7 µm) and ultrastructure was similar to closely related Zygnema strains. Zygnema I and Zygnema B were exposed to three different treatment temperatures (15, 20 and 25°C) for two weeks, then photophysiological parameters and cellular phenol contents were acquired. The maximum electron transport rate increased significantly with elevated temperatures, but non-photochemical quenching did not change. Net photosynthetic oxygen production was higher in Zygnema B, but decreased in both strains from 10 to 15°C measuring temperature. Zygnema I showed a significant decrease between 15/25°C-treated cultures above 20°C measuring temperature. The phenolic content did not change significantly with experimental treatments, the spectral absorption at 350 nm was significantly lower in Zygnema B when compared with Zygnema I. Taken together our results indicate that Zygnema B and Zygnema I cannot adapt to elevated temperatures.

Changes in Envelope Structure and Cell-Cell Communication during Akinete Differentiation and Germination in Filamentous Cyanobacterium Trichormus variabilis ATCC 29413.

Planktonic freshwater filamentous cyanobacterium Trichormus variabilis ATCC 29413 (previously known as Anabaena variabilis) can differentiate heterocysts and akinetes to survive under different stress conditions. Whilst heterocysts enable diazotrophic growth, akinetes are spore-like resting cells that make the survival of the species possible under adverse growth conditions. Under suitable environmental conditions, they germinate to produce new vegetative filaments. Several morphological and physiological changes occur during akinete formation and germination. Here, using scanning electron microscopy (SEM), we found that the mature akinetes had a wrinkled envelope, and the surface of the envelope smoothened as the cell size increased during germination. Thereupon, the akinete envelope ruptured to release the short emerging filament. Focused ion beam–scanning electron microscopy (FIB/SEM) tomography of immature akinetes revealed the presence of cytoplasmic granules, presumably consisting of cyanophycin or glycogen. In addition, the akinete envelope architecture of different layers, the exopolysaccharide and glycolipid layers, could be visualized. We found that this multilayered envelope helped to withstand osmotic stress and to maintain the structural integrity. Furthermore, by fluorescence recovery after photobleaching (FRAP) measurements, using the fluorescent tracer calcein, we found that intercellular communication decreased during akinete formation as compared with the vegetative cells. In contrast, freshly germinating filaments restored cell communication.

Cell wall characteristics during sexual reproduction of Mougeotia sp. (Zygnematophyceae) revealed by electron microscopy, glycan microarrays and RAMAN spectroscopy

Mougeotia spp. collected from field samples were investigated for their conjugation morphology by light-, fluorescence-, scanning- and transmission electron microscopy. During a scalarifom conjugation, the extragametangial zygospores were initially surrounded by a thin cell wall that developed into a multi-layered zygospore wall. Maturing zygospores turned dark brown and were filled with storage compounds such as lipids and starch. While M. parvula had a smooth surface, M. disjuncta had a punctated surface structure and a prominent suture. The zygospore wall consisted of a polysaccharide rich endospore, followed by a thin layer with a lipid-like appaerance, a massive electron dense mesospore and a very thin exospore composed of polysaccharides. Glycan microarray analysis of zygospores of different developmental stages revealed the occurrence of pectins and hemicelluloses, mostly composed of homogalacturonan (HG), xyloglucans, xylans, arabino-galactan proteins and extensins. In situ localization by the probe OG7-13AF 488 labelled HG in young zygospore walls, vegetative filaments and most prominently in conjugation tubes and cross walls. Raman imaging showed the distribution of proteins, lipids, carbohydrates and aromatic components of the mature zygospore with a spatial resolution of ~ 250 nm. The carbohydrate nature of the endo- and exospore was confirmed and in-between an enrichment of lipids and aromatic components, probably algaenan or a sporopollenin-like material. Taken together, these results indicate that during zygospore formation, reorganizations of the cell walls occured, leading to a resistant and protective structure.

Genomic and transcriptomic insights into the habitat adaptation of the diazotrophic paddy-field cyanobacterium Nostoc sphaeroides.

Nitrogen-fixing cyanobacteria are common in paddy fields, one of the most productive wetland ecosystems. Here, we present the complete genome of Nostoc sphaeroides, a paddy-field diazotroph used for food and medicine for more than 1700 years and deciphered the transcriptional regulation during the developmental transition from hormogonia to vegetative filaments with heterocysts. The genome of N. sphaeroides consists of one circular chromosome (6.48 Mb), one of the largest ever reported megaplasmids (2.34 Mb), and seven plasmids. Multiple gene families involved in the adaption to high solar radiation and water fluctuation conditions were found expanded, while genes involved in anoxic adaptation and phosphonate utilization are located on the megaplasmid, suggesting its indispensable role in environmental adaptation. Distinct gene expression patterns were observed during the light-intensity-regulated transition from hormogonia to vegetative filaments, specifically, genes encoding proteins involved in photosynthetic light reaction, carbon fixation, nitrogen metabolism and heterocyst differentiation were significantly upregulated, whereas genes related to cell motility were down-regulated. Our results provide genomic and transcriptomic insights into the adaptation of a filamentous nitrogen-fixing cyanobacterium to the highly dynamic paddy-field habitat, suggesting N. sphaeroides as an excellent system to understand the transition from aquatic to terrestrial habitats and to support sustainable rice production.

Effects of calcium ion on the colony formation, growth, and photosynthesis in the edible cyanobacterium Nostoc sphaeroides

Calcium ions are crucial for the growth and development of photosynthetic organisms, whereas its eco-physiological significance on the edible cyanobacterium Nostoc sphaeroides remains unclear. The percentage of spherical microcolonies from N. sphaeroides vegetative filaments was up to 88–93% after 18 days at higher Ca2+ levels (0.25, 1, and 4 mM), but only about 51% at lower Ca2+ levels (0.01 and 0.06 mM). When cultured at higher Ca2+ levels, N. sphaeroides specific growth rates based on the chlorophyll a content and absolute growth rates based on the dry weight significantly increased by 13–18% and 23–30%, respectively, compared to those at lower Ca2+ levels. The colonies cultured at higher Ca2+ levels exhibited 13–20% increase in light-limited photosynthetic efficiency, 28–42% increase in the maximum electron transport rate and 57–95% increase in the light-saturated photosynthetic rate relative to those at lower Ca2+ levels. The higher Ca2+-treated colonies were more tolerant to high light than lower Ca2+-treated colonies and more rapid recovery of photoinhibited colonies induced by the subsequent dim light was also observed in higher Ca2+-treated colonies. We further found the photosynthetic recovery of photoinhibited colonies required the influx of apoplastic Ca2+ and the maintenance of photosynthetic activity for desiccated colonies during rehydration was also extracellular Ca2+-dependent. These results indicated that higher levels of Ca2+ promoted colony formation, growth, and photosynthesis of N. sphaeroides, suggesting available calcium ions in natural environment may be an important factor affecting its productivity and distribution.

Detail description of Lithophyllum okamurae (Lithophylloideae, Corallinales), a widely distributed crustose coralline alga in marine ecosystems

Lithophyllum okamurae is one of the important encrusting coralline algae, which plays important roles as primary producer, carbonate sediment builder, and habitat provider in the marine ecosystems. In this study, L. okamurae was collected from tropical coast of Sanya, and firstly described based on both detailed morph-anatomical characteristics and molecular studies of typic DNA sequences. The structure of the thalli of L. okamurae was pseudoparenchymatous construction with radially organized dimerous organizations in the crustose portion. The pseudoparenchymatous construction were composed of three parts, including 1 to 3 layers of epithelia cells which had flatten to round outermost walls, one layer of square or rectangular cells of the hypothallia and multiple layers of square or elongated rectangular peripheral cells. Palisade cells were observed, and the cells of the contiguous vegetative filaments were connected by secondary pit-connections with cell fusions absent. The carposporangial conceptacles, the spermatangial conceptacles, the bisporangial conceptacles and the tetrasporangial conceptacles were observed, and all these four kinds of conceptacles were uniporate. The spermatangial conceptacles were slightly convex and buried at shallow depths in the thalli tissues, and the carposporangial conceptacles and asexual conceptacles were protruding and conical. Phylogenetic studies based on DNA barcoding markers of 18S rDNA, COI, rbcL and psbA revealed that L. okamurae clustered with the closest relation of L. atlanticum, and formed a distinct branch. Based on the comparative anatomical features and the molecular data, the detailed description of the valid species of L. okamurae was firstly given in this study to provide theoretical basis for algae resources utilization and conservation in marine ecosystems.

Desmonostoc danxiaense sp. nov. (Nostocales, Cyanobacteria) from Danxia mountain in China based on polyphasic approach

The recently established genus Desmonostoc was characterized by forming macroscopic colonies with diffluent mucilaginous envelopes embedding long vegetative filaments and possessing long chains of akinetes. The establishment of this new genus was further supported by the clustering of the 16S rRNA gene, which have a distinctive phylogenetic placement outside of Nostoc. In this study, a new cyanobacterial species was isolated from a wet rocky wall in Danxia mountain, Guangdong province, southern China, and the novel strains of this new species were evaluated by combining morphological characteristic and molecular data on the 16S rRNA gene and 16S-23S internally transcribed spacer (ITS). This new taxon was found to be closest to Desmonostoc species. The separation of the new species described here as Desmonostoc danxiaense, using morphological and molecular traits, was based on differences in phenotypic, 16S rRNA gene, ITS sequence and its secondary structure.

A contaminant DNA barcode sequence reveals a new red algal order, Corynodactylales (Nemaliophycidae, Florideophyceae)

Routine DNA barcode surveys of red algae can occasionally yield contaminant sequences owing to the diverse epi/endo flora and fauna that can inhabit these species. Often discarded as nuisance data, further exploration in this study led to the discovery of an unusual red alga with a highly characteristic vegetative development in which the diminutive primary vegetative filaments are terminated by distinctive monosporangia. This entity is described here as Corynodactylus rejiciendus G.W. Saunders gen. et sp. nov. Generating genomic data from the host and associated epiphytic flora, and subsequently subtracting out genes of the former, facilitated a multigene phylogenetic analysis supporting recognition of Corynodactylaceae fam. nov. and Corynodactylales ord. nov. as a distant sister to the order Balliales in the subclass Nemaliophycidae.

Morphological and molecular characterization of Hydrolithon rupestre (Corallinaceae, Corallinales, Rhodophyta): first report from the Mediterranean Sea

The genera Hydrolithon and Porolithon are the most discussed taxa of non-geniculate Corallinaceae, as well as the most poorly known. Anatomical observations based usually on thallus construction, number of epithallial cell layers, trichocyte arrangement, occurrence of vegetative filaments between the trichocytes, and presence/absence of cell fusions and secondary pit connections led to different interpretations mainly due to the lack of available reproductive material from type specimens. Recently molecular surveys on the phylogeny of the Corallinales supported the hypothesis of considering Hydrolithon and Porolithon as two distinct genera but, up to now, several taxonomic questions remain in part unanswered. For these reasons identification of species belonging to these genera is still very difficult and can often lead to misidentification of taxa. In this study we report the discovery of a species of the Hydrolithon group found along the coast of Vis Island, Croatia, Adriatic Sea. Samples were identified through morpho-anatomical examinations and the phylogenetic position of this species was investigated through molecular analyses based on the nuclear nSSU and the plastidial psbA markers. This finding represents the first report for the Mediterranean Sea of Hydrolithon rupestre, one of the few encrusting coralline algae reported to overgrow live corals. The species can represent a possible threat to populations of Adriatic scleractinian corals.

Morphological examination and phylogenetic analyses of Phycopeltis spp. (Trentepohliales, Ulvophyceae) from Tropical China.

During an investigation of Trentepohliales (Ulvophyceae) from tropical areas in China, four species of the genus Phycopeltis were identified: Phycopeltis aurea, P. epiphyton, P. flabellata and P. prostrata. The morphological characteristics of both young and adult thalli were observed and compared. Three species (P. flabellata, P. aurea and P. epiphyton) shared a symmetrical development with dichotomously branching vegetative cells during early stages; conversely, P. prostrata had dishevelled filaments with no dichotomously branching filaments and no symmetrical development. The adult thalli of the former three species shared common morphological characteristics, such as equally dichotomous filaments, absence of erect hair and gametangia formed in prostate vegetative filaments. Phylogenetic analyses based on SSU and ITS rDNA sequences showed that the three morphologically similar species were in a clade that was sister to a clade containing T. umbrina and T. abietina, thus confirming morphological monophyly. Conversely, Phycopeltis prostrata, a species with erect filaments, sessile gametangia on the basal erect hair, larger length/width ratio of vegetative cells and very loosely coalescent prostrate filaments, branched separately from the core Phycopeltis group and the T. umbrina and T. abietina clade. Based on morphological and molecular evidence, the genus Phycopeltis was paraphyletic. Furthermore, the traditional taxonomic criteria for Phycopeltis must be reassessed based on phylogeny using more species. A new circumscription of the Phycopeltis and the erection of new genera are recommended.

Genetic Analysis Reveals the Identity of the Photoreceptor for Phototaxis in Hormogonium Filaments of Nostoc punctiforme

In cyanobacterial Nostoc species, substratum-dependent gliding motility is confined to specialized nongrowing filaments called hormogonia, which differentiate from vegetative filaments as part of a conditional life cycle and function as dispersal units. Here we confirm that Nostoc punctiforme hormogonia are positively phototactic to white light over a wide range of intensities. N. punctiforme contains two gene clusters (clusters 2 and 2i), each of which encodes modular cyanobacteriochrome-methyl-accepting chemotaxis proteins (MCPs) and other proteins that putatively constitute a basic chemotaxis-like signal transduction complex. Transcriptional analysis established that all genes in clusters 2 and 2i, plus two additional clusters (clusters 1 and 3) with genes encoding MCPs lacking cyanobacteriochrome sensory domains, are upregulated during the differentiation of hormogonia. Mutational analysis determined that only genes in cluster 2i are essential for positive phototaxis in N. punctiforme hormogonia; here these genes are designated ptx (for phototaxis) genes. The cluster is unusual in containing complete or partial duplicates of genes encoding proteins homologous to the well-described chemotaxis elements CheY, CheW, MCP, and CheA. The cyanobacteriochrome-MCP gene (ptxD) lacks transmembrane domains and has 7 potential binding sites for bilins. The transcriptional start site of the ptx genes does not resemble a sigma 70 consensus recognition sequence; moreover, it is upstream of two genes encoding gas vesicle proteins (gvpA and gvpC), which also are expressed only in the hormogonium filaments of N. punctiforme.

Materials to the Zygnemaceae of Poland. I. Development cycle and supplementation of diagnosis of Zygnema allorgei Gauthier-Lievre 1965, a species new for the European flora

In the water sample taken on September 7, 1976 from the pond in Głowno (Łódź District) filaments of <em>Zygnema</em> were found in vegetative stage. In culture a full development cycle of this alga was observed: vegetative filaments, conjugation, zygotes and their germination. The taxon was identified as <em>Zygnema allorgei</em> Gauthier-Lievre. Its diagnosis was supplemented. It is the second site of occurance of the species in the world. <em>Z. allorgei</em> was described for the first time in 1965 in Algeria.

Materials to the Zygnemaceae of Poland. II. Observations on the development cycle of Zygnema biforme Jao. 1947, a species new for the European flora

In the water sample taken on June 27, 1983 from the pond in Młogoszyn (Płock District) vegetative filaments and initial stages of conjugation of <em>Zygnema biforme</em> Jao were detected. In culture mature zygotes and their germination were observed. It is the second site of occurrence of this species in the world.

Mougeotia ag. and Spirogyra Link (Zygnemaceae) - new species for the European flora

In samples collected in Norway the following species new for Europe, <em>Mougeotia olivacea</em> Y.J. Ling,<em> Mougeotia mysorensis</em> Iyengar, <em>Spirogyra reflexa</em> Transeau and <em>Spirogyra schmidtii</em> W. et G.S. West, are identified. The diagnosis of <em>M. olivacea</em> has been supplemented with data on the dimensions of vegetative cells and zygotes and the number of pyrenoids. The diagnosis of <em>M. mysorensis</em> has been supplemented with data on the number and distribution of pyrenoids and, often curved, vegetative filaments. The sites of <em>M. olivaceas</em> and <em>M. mysorensi</em>'s occurrence are the second one in the world. Morphological features and dimensions of <em>S. reflexa</em> and <em>S. schmidtii</em> are congruent with those given from other non-European sites.

Description of the cyanobacterial genus Desmonostoc gen. nov. including D. muscorum comb. nov. as a distinct, phylogenetically coherent taxon related to the genus Nostoc.

On the basis of data presented here and in earlier studies, Desmonostoc gen. nov. is described. The new genus includes the traditional species Nostoc muscorum Agardh ex Bornet et Flahault 1888, and several other strains previously assigned to the genus Nostoc, which present similar morphology and phylogenetic placement within the Desmonostoc lineage. The Desmonostoc clade is phylogenetically coherent according to 16S rRNA gene sequence analysis performed with four distinct approaches. In all phylogenetic trees, Desmonostoc formed a supported group separated from strains belonging to the related taxa Nostoc, Trichormus, and Mojavia. We also suggest that other clusters hosting strains which for their morphology resemble Nostoc, but are more distant from Nostoc commune cluster than Desmonostoc, should be reclassified into new genera in the future. Strains belonging to Desmonostoc form long vegetative filaments embedded in diffluent mucilaginous envelopes, except for primordial stages they never form a firm periderm, and the filaments are never densely coiled with compact trichomes as found in Nostoc. Both terminal and intercalary heterocytes occur, and mostly elliptical akinetes were differentiated apoheterocytically in long chains. Desmonostoc strains can be usually found in moist or wet meadow, field and forest soils, more rarely in periphyton, but to our knowledge, they are missing or very rare in desert areas. Some of these strains have been found to grow in association with mosses or as symbionts of cycadean plants and of Gunnera sp.

Hormogonia in two nostocalean cyanophytes (cyanobacteria) from the genera Hapalosiphon and Fischerella

The formation of hormogonia in the nostocalean cyanophytes/cyanobacteria Hapalosiphon fontinalis (C. Agardh) Bornet and Fischerella sp. was studied in natural populations collected from the Klin peatbog, northern Slovakia. Hormogonia were produced terminally in lateral branches of filaments (both species), or also directly on the main branches (Fischerella sp.). In contrast to vegetative filaments, hormogonia were not ramified, lacked heterocytes, were embedded in mucilaginous envelopes, were able to move, and their cells contained aerotopes. They were released by gliding through an opening in the sheath at the end of lateral branches of filaments. Released hormogonia of H. fontinalis were solitary or agglomerated into common fascicles morphologically resembling planktic colonies of Aphanizomenon flos-aquae (L.) Ralfs ex Bornet et Flahault or Dolichospermum affine (Lemmermann) Wacklin, Hoffmann et Komarek (syn. Anabaena affinis Lemmermann). Occasionally, lateral or sessile Nostochopsis-like heterocytes and apical spherical monocytes were formed on the main filaments. Hormogonia of Fischerella sp. were formed not only in apical part of lateral trichomes, but also directly on the main trichomes. Their cells were markedly larger than the vegetative cells and possessed well-developed aerotopes. Released hormogonia remained solitary, and were not agglomerated into fascicles. Apical hormogonia were released by gliding through an opening in the sheath at the end of lateral branches of filaments, and basal hormogonia were released by breaking off the main axis. In contrast to filaments of H. fontinalis which were very common and represented the dominant species of the cyanophyte communities in the locality, filaments of Fischerella sp. were observed only in one sample and for a limited period. This is the first record of a representative of the genus Fischerella in Slovakia.

Myrionema strangulans (Chordariales, Phaeophyceae) epiphyte on Ulva spp. (Ulvophyceae) from Patagonian Atlantic coasts

Fronds of Ulva spp. from Patagonian Atlantic coasts exhibited brown spots produced by the presence of Myrionema strangulans (Chordariales, Phaeophyceae). The occurrence of M. strangulans on Ulva spp. is widely reported from several regions of the world, but there were no detailed studies about the subject. In the present study, we describe the morphology and interactions of M. strangulans with Ulva spp. as observed under light and electron microscopes, and we reconstruct all stages of its life cycle based upon in vitro experiments. The prevalence of infection by M. strangulans was 100%. In case of the strongest epiphytism, the host cuticle exhibited perforations, massive depigmentation, cellular disorganization, and cuticle rupture. It was possible to demonstrate a purely epiphytic life strategy of the organism by transmission electron microscopy. M. strangulans formed discoid thalli constituted by vegetative filaments and radiating from a central zone to a peripheral zone. Transversally, the discs were formed by two strata: a basal monostromatic and a filamentous erect stratum. From the monostromatic stratum, hyaline hairs and reproductive structures were produced. Both plurilocular and unilocular sporangia were present. Zoids from both plurilocular and unilocular sporangia were able to germinate in culture. M. strangulans exhibited a haploid–diploid, heteromorphic life cycle with thalli with three different morphologies. The haploid chromosome number was 12 ± 2 chromosomes.