Ultrastructure of the Oral Apparatus of Mesodinium rubrum from Korea

Mesodinium rubrum Lohmann is a photosynthetic marine ciliate that has functional chloroplasts of cryptophyte origin. Little is known about the oral ultrastructure of M. rubrum compared with several reports on the sequestration of nuclei and plastids from prey organisms, such as Geminigera cryophila and Teleaulax species. Here, we describe the fine structure of the oral apparatus of a M. rubrum strain from Gomso Bay, Korea. The cytopharynx was cone‐shaped and supported by 20–22 ribbons of triplet microtubules. At the anterior end of the cytopharynx, an annulus anchored small cylinders composed of 11 microtubules. The small cylinders were spaced at regular intervals, each reinforced by one set of the triplet microtubules. At the opening of the cytostome, larger 14‐membered microtubular cylinders were set adjacent to the small, 11‐membered microtubular cylinders, each pair surrounded by separate membranes, however, only the large cylinders extended into the oral tentacles. There were 20–22 oral tentacles each having one to five extrusomes at its tip. At the anterior end of the oral apparatus, microtubular bands supporting the cytostome curved posteriad, extending beneath the cell cortex to the kinetosomes of the somatic cirri. The microtubular bands were connected by striated fibers and originated from kinetosomes anchored by fibers. Each cirrus consisted of eight cilia associated with 16 kinetosomes. The ultrastructure of M. rubrum from Korea provides information useful for taxonomic characterization of the genus Mesodinium and relevant to developing a better understanding of the acquisition of foreign organelles through phagocytosis by M. rubrum.     

Ultrastructure and Molecular Phylogeny of Mesodinium coatsi sp. nov., a Benthic Marine Ciliate

Mesodinium is a globally distributed ciliate genus forming frequent and recurrent blooms in diverse marine habitats. Here, we describe a new marine species, Mesodinium coatsi n. sp., originally isolated from interstitial water of surface sand samples collected at Mohang Beach, Korea. The species was maintained under a mixotrophic growth condition for longer than 1 yr by providing a cryptomonad, Chroomonas sp., as the sole prey. Cell morphology and subcellular structure were examined by light microscopy, scanning, and transmission electron microscopy, and molecular phylogeny was inferred from nuclear‐encoded 18S rDNA sequence data. Like other Mesodinium species, M. coatsi consisted of two hemispheres separated by two types of kinetids, and had tentacles located at the oral end of the cell. Several food vacuoles were observed in the cytoplasm, and partially digested prey cells sometimes existed in food vacuoles. Kinetids and the associated accessory structures were quite similar to those previously reported, but M. coatsi was differentiated from other marine Mesodinium species by ultrastructural characters of the dikinetids, polykinetids, and tentacles. We also provided a detailed illustration of infraciliature. Molecular phylogeny revealed that M. coatsi and Mesodinium chamaeleon were closely related to each other.     

纤毛虫念珠异列虫射出胞器的超微结构观察

应用扫描电镜术和透射电镜术显示,纤毛虫念珠异列虫(Anteholosticha monilata)的射出胞器早期发生在细胞质深处,附近有不同类型的囊泡结构。成熟后射出胞器向表膜迁移,结构由不同电子密度片层的体部、结晶状的中心轴杆部和多层膜的帽部组成。受外界刺激时胞器冲破皮层射出,形态呈"蘑菇"状。据上述观察结果推测:该射出胞器具有防御作用,它可能起源于高尔基体活动产生的小泡;在亲缘关系较近的纤毛虫中,其射出胞器可能具有相似的分化特征。     

The ultrastructure of the extrusomes in <Emphasis Type="Italic">Pseudourostyla cristata</Emphasis>, a hypotrichous ciliated protozoan

By using scanning and transmission electron microscopy, the present study demonstrates a great number of trichocyst-like extrusomes distributed in the cortical cytoplasm of the protozoan Pseudourostyla cristata, a hypotrichous ciliate. Of these, the mature organelles are rod-shaped with a cap consisting of tubular structures, a tip located at the apex of the cap, a body consisting of strateform structures of uneven electron density and an elongated shaft located along the longitudinal central axis of the body. The electron microscopic observations suggest that the extrusive organelles in P. cristata might undergo a morphogenetic process including the following sequential events: the occurrence of the vesicles in the cytoplasm, the condensation of the fibrous substances within the vesicles, the appearance of the electron-dense shaft, and the formation of the cap. In contrast with a large quantity of extrusomes in trophozoit P. cristata, there are no such extrusive organelles in the encysted cells of the ciliate. The phenomena that P. cristata ciliates can readily enter physiological reorganization or encysting phases and discharge a great number of their extrusomes when prepared for SEM and TEM observation suggest that the extrusive process of the extrusomes in P. cristata might have an important influence on the life activity of the ciliate and could be one of the causes leading to the physiological reorganization and the encysting of the ciliate. These reactions of P. cristata might be a protective or defensive response to the environmental changes.     

Acquired phototrophy in ciliates: a review of cellular interactions and structural adaptations

Many ciliates acquire the capacity for photosynthesis through stealing plastids or harboring intact endosymbiotic algae. Both phenomena are a form of mixotrophy and are widespread among ciliates. Mixotrophic ciliates may be abundant in freshwater and marine ecosystems, sometimes making substantial contributions toward community primary productivity. While mixotrophic ciliates utilize phagotrophy to capture algal cells, their endomembrane system has evolved to partially bypass typical heterotrophic digestion pathways, enabling metabolic interaction with foreign cells or organelles. Unique adaptations may also be found in certain algal endosymbionts, facilitating establishment of symbiosis and nutritional interactions, while reducing their fitness for survival as free-living cells. Plastid retaining oligotrich ciliates possess little selectivity from which algae they sequester plastids, resulting in unstable kleptoplastids that require frequent ingestion of algal cells to replace them. Mesodinium rubrum (=Myrionecta rubra) possesses cryptophyte organelles that resemble a reduced endosymbont, and is the only ciliate capable of functional phototrophy and plastid division. Certain strains of M. rubrum may have a stable association with their cryptophyte organelles, while others need to acquire a cryptophyte nucleus through feeding. This process of stealing a nucleus, termed karyoklepty, was first described in M. rubrum and may be an evolutionary precursor to a stable, reduced endosymbiont, and perhaps eventually a tertiary plastid. The newly described Mesodinium"chamaeleon," however, is less selective of which cryptophyte species it will retain organelles, and appears less capable of sustained phototrophy. Ciliates likely stem from a phototrophic ancestry, which may explain their propensity to practice acquired phototrophy.     

Symbionticism revisited: a discussion of the evolutionary impact of intracellular symbioses.

Wallin (1927) first published the notion that the fusion of bacteria with host cells was the principal source of genetic novelty for speciation. He suggested that mitochondria are transitional elements in this process. While the significance that he attributed to symbiosis now seem excessive, he was one of the first authors to be aware of the evolutionary potential of symbiotic events and his view of mitochondria may not seem strange to many cell biologist today. The most significant evolutionary development which has been attributed to intracellular symbiosis is the origin of eukaryotic cellular organization. The current status of the 'serial endosymbiosis hypothesis' is briefly review. The case for the symbiotic origin of the chloroplast, based principally on 16 S RNA oligonucleotide cataloguing, is very strong. Mitochondrial origins are more obscure but also appear to be symbiotic due to recent 18 S cataloguing from wheat embryos. The probablility of the multiple origin of some eukaryotic organelles is also examined, the processes in question being the acquisition of distinct stocks of chloroplasts from disparate photosynthetic prokaryotes and the secondary donation of organelles from degenerate eukaryotic endosymbionts to their hosts, with specific reference to the dinoflagellates Peridinium balticum, Kryptoperidinium foliaceum and the ciliate Mesodinium rubrum. It is concluded that the evolutionary potential of intracellular symbiosis ('cytobiosis': a term introduced in this paper) is great, with the best established influence being on the origin of eukaryotic chloroplasts. Together with the potential effects of viral vectors, symbiosis serves as a supplementary speciation mechanism capable of producing directed evolutionary changes. It is likely that these processes will explain some of the apparent anomalies in evolutionary rates and direction which are not readily explicable by the conventional synthetic theory of evolution.     

Vertical migration of autotrophic micro-organisms during a vernal bloom at the coastal Baltic Sea – coexistence through niche separation

Vertical migration of two dinoflagellate species (Peridiniellacatenata and Scrippsiella hangoei) and a phototrophic ciliate(Mesodinium rubrum) were studied during the peak and decline of avernal bloom at the SW coast of Finland. During the diel cycle, part of thepopulations of P. catenata and M. rubrum wereobserved in the deeper layers with elevated nutrient concentrations, whileS. hangoei remained in the upper nutrient depleted mixed layer.Using a correspondence analysis the vertical distribution patterns of thespecies and chlorophyll a were examined over a temporal scale of hoursand weeks. The vertical migration was reflected in much higher variabilityin the depth distribution of P. catenata and M. rubrum over a diel scale, compared to S. hangoei. The analysisrevealed also significant differences in species specific depth distributionpatterns over both time scales. It is discussed that the co-existence of thetwo dominant dinoflagellate species during the vernal bloom is due to nicheseparation through behavioural adaptations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Vibrios associated with red tides caused by Mesodinium rubrum

Vibrios were isolated from red tides caused by Mesodinium rubrum and also throughout the year in the Ria de Pontevedra, Spain. The isolates were grouped into 14 phena by numerical toxonomy. Strains associated with red tides were restricted to four phena: phena I and II were Vibrio alginolyticus, and phena III and IV were Vibrio tubiashii and Vibrio anguillarum, respectively. V. anguillarum-like strains (phena V through XI) predominated throughout the year outside the red tide areas. Cytotoxicity assays conducted in different poikilothermic and homoiothermic cell lines showed that cytotoxin production was not necessarily associated with the species selected during the red tides.     

Vibrios associated with red tides caused by Mesodinium rubrum.

Vibrios were isolated from red tides caused by Mesodinium rubrum and also throughout the year in the Ria de Pontevedra, Spain. The isolates were grouped into 14 phena by numerical toxonomy. Strains associated with red tides were restricted to four phena: phena I and II were Vibrio alginolyticus, and phena III and IV were Vibrio tubiashii and Vibrio anguillarum, respectively. V. anguillarum-like strains (phena V through XI) predominated throughout the year outside the red tide areas. Cytotoxicity assays conducted in different poikilothermic and homoiothermic cell lines showed that cytotoxin production was not necessarily associated with the species selected during the red tides.     

Changes in bacterial populations during red tides caused by Mesodinium rubrum and Gymnodinium catenatum in North West Coast of Spain

Heterotrophic bacterial communities associated with four red tides caused by Mesodinium rubrum and Gymnodinium catenatum in two Galician Rias (North West Spain) were examined. Three of these were produced by the Mesodinium rubrum and the causative organism of a toxic bloom was Gymnodinium catenatum . In early stages of all the blooms, the diversity decreased but the total marine bacterial counts increased by one or two logs. Vibrio numbers were also incremented by two logs in two blooms of M. rubrum , while in the other bloom of this organism and in the red tide caused by G. catenatum a decrease in number of these bacteria was observed. A total of 116 bacterial strains were identified at the genus level and grouped into 12 phena. During the decomposition processes of two blooms of M. rubrum a zooplanktonic-type bacterial succession was observed ( Vibrio , pseudomonads and Moraxella-Acinetobacter ). On the other hand, during decomposition of the other red tide of M. rubrum and the bloom of G. catenatum , a typical phytoplanktonic-type succession occurred, as Pseudomonas and Moraxella groups became dominant for all the process. These results support the conflicting taxonomical position of M. rubrum . After the blooms, the changes in the community point towards the restablishment of the normal bacterial flora of the estuary (increase in diversity and decreases of bacterial numbers). Only the Vibrio strains, isolated from the non-toxic first and second red tides, displayed cytotoxic activities. A relationship among bacterial cytotoxicity and toxic effects of blooms cannot therefore be established.     

Changes in bacterial populations during red tides caused by Mesodinium rubrum and Gymnodinium catenatum in North West Coast of Spain

Heterotrophic bacterial communities associated with four red tides caused by Mesodinium rubrum and Gymnodinium catenatum in two Galician Rias (North West Spain) were examined. Three of these were produced by the Mesodinium rubrum and the causative organism of a toxic bloom was Gymnodinium catenatum. In early stages of all the blooms, the diversity decreased but the total marine bacterial counts increased by one or two logs. Vibrio numbers were also incremented by two logs in two blooms of M. rubrum, while in the other bloom of this organism and in the red tide caused by G. catenatum a decrease in number of these bacteria was observed. A total of 116 bacterial strains were identified at the genus level and grouped into 12 phena. During the decomposition processes of two blooms of M. rubrum a zooplanktonic-type bacterial succession was observed (Vibrio, pseudomonads and Moraxella-Acinetobacter). On the other hand, during decomposition of the other red tide of M. rubrum and the bloom of G. catenatum, a typical phytoplanktonic-type succession occurred, as Pseudomonas and Moraxella groups became dominant for all the process. These results support the conflicting taxonomical position of M. rubrum. After the blooms, the changes in the community point towards the restablishment of the normal bacterial flora of the estuary (increase in diversity and decreases of bacterial numbers). Only the Vibrio strains, isolated from the non-toxic first and second red tides, displayed cytotoxic activities. A relationship among bacterial cytotoxicity and toxic effects of blooms cannot therefore be established.     

Ultrastructural Features of Allantosoma intestinalis,a Suctorian Ciliate Isolated from the Large Intestine of the Horse1

ABSTRACT. Allantosoma intestinalis, a suctorian ciliate isolated from the intestine of the horse, was studied utilizing light and electron optical methods. These small sausage-shaped organisms have a varying number of tentacles (between one and 12) located at each extremity of the body. The microtubular axoneme of each tentacle in cross-section consists of two files of microtubules arranged in a daisy-like configuration. Haptocysts occur in the tentacle shaft, abutted to the plasma membrane of the knob of the tentacle, and in the cell body. The haptocysts are bottle-shaped, with prominent annular striations around their midportion. The cell is covered by three membranes, an outer plasma membrane, an outer alveolar, and an inner alveolar membrane. A thin epiplasmic layer is found beneath the inner alveolar membrane, and a single row of microtubules underlies the epiplasm. The subpellicular microtubules are arranged parallel to each other forming a corset around the cell along the long axis: such a system is not characteristic of suctorians. A field of diminutive kinetosomes (each 180 nm long, max. of 15 per field), lacking cilia, was found below the cortex. The function of these prokinetosomes is unknown. A ciliated swarmer has not been observed, only the nonciliated adult. The characteristics of Allantosoma are compared with those of other suctorian genera.     

Ultrastructural studies of the commensal suctorian,Choanophrya infundibulifera hartog

Summary The feeding tentacles of Choanophrya contain a central canal lined by microtubules. Only one tentacle develops during metamorphosis of the embryo into the adult, but others develop at intervals throughout adult life. Each tentacle forms adjacent to a solitary, subcortical kinetosome which lies parallel to the body surface, lacks accessory elements and never develops a cilium. Small condensations of electron-dense material and short bundles of microtubules form adjacent to the cartwheel region of the kinetosome. Initially these bundles are orientated randomly but later they become radially arranged and curved into prolamellae around a disc-shaped condensation centre, to form a paddlewheel-like tentacle primordium 0.8–1.1 m in diameter. The condensation centre consists of alternating concentric electron-dense and electron-transparent zones, and lies with its axis perpendicular to both the kinetosome and the cortex. The microtubules in each prolamella increase in number and pairs of short tip microtubules develop between adjacent prolamellae. Subsequently the developing lamellae become enclosed by a cylinder of ring microtubules. Once all the microtubule components of the tentacle primordium are established it increases in length by addition of material to the basal ends of the microtubules to form a short microtubule canal. As the canal elongates the epiplasm above it disappears and the pellicle membranes become uplifted around the protruding tentacle. An epiplasmic collar differentiates around the growing tentacle whilst spheroid vesicles and solenocysts begin to accumulate in the surrounding cytoplasm.This investigation was supported by the J.S. Dunkerley Fellowship in Protozoology, awarded by the University of Manchester.     

Trichites of Strombidium (Ciliophora, Oligotrichida) are extrusomes

The trichites of Strombidium and related genera have been considered either as a cytoskeletal armature or as extrusomes. To demonstrate their true nature, a study was undertaken on two marine Strombidium species by ultrastructural and cytochemical analysis as well as in vivo experiments. Trichites, extending from the cortex into the cell, are rod-shaped, membrane-bounded, and have a complex structure. The following elements of the trichites, are distinguishable: an electron-transparent lumen, a laminated layer, and a compact layer. In trichites of one species, thin "rings" surround the lumen. Numerous short, curved tubules with a polysaccharide wall are present in the cytoplasm surrounding the trichites. At the cortical end, each trichite is enveloped by a "cap" of electron-dense proteinaceous material. In some cases, the cortical alveoli appear interrupted, forming a "hole" for trichite ejection. Ejection of rod-shaped structures, up to 5 times longer than resting trichites, was obtained by in vivo treatments with dextran and aminoethyldextran. Negative staining indicated that these structures were transformed trichites. As no other possible extrusive structures were observed in the cytoplasm of Strombidium, trichites were considered extrusomes.     

Cellular dynamics of conjugation in the ciliate euplotes aediculatus. II. Cellular membranes

The formation and subsequent dissolution of a common bridge of cytoplasm between conjugating ciliated protozoan cells provides an excellent opportunity to follow the dynamics of the cellular membrane systems involved in this process. In particular, separation of conjugant partners offers the chance to observe, at a fixed site on the cell surface, how the ciliate surface complex of plasma and alveolar membranes (collectively termed the “pellicle”) is constructed. Consequently, cortical and cellular membranes of Euplotes aediculatus were studied by light and electron microscopy through the conjugation sequence. A conjugant fusion zone of shared cytoplasm elaborates between the partner cells within their respective oral fields (peristomes) to include microtubules, cytosol, and a concentrated endoplasmic reticulum (heavily stained by osmium impregnation techniques) that may also be continuous with cortical ER of each cell. Cortical membranes displacd by fusion are autolyzed in acid phosphatase-positive lysosomes in the fusion zone. As conjugants separate, expansion of the plasma membrane may occur through the fusion of vesicles with the plasma membrane, presumably at bare membrane, presumably at bare membrane patches near the fusion zone. The underlying cortical alveolar membranes and their plate-like contents are reconstructed beneath the plasma membrane, apparently by multiple fusions of dense-cored alveolar precursor vesicles (APVs). These precursor vesicles themselves appear to condense directly from the smooth ER present in the fusion zone. No Golgi apparatus was visible in the fusion zone cytoplasm, and no step of APV maturation that might involve the Golgi complex was noted.     

Evidence that microtubules play a permissive role in hepatocyte very low density lipoprotein secretion

To determine whether a minimum number of assembled microtubules is required for very low density lipoprotein (VLDL) triglyceride TG) secretion in hepatocytes, antimicrotubule drugs of different concentrations were given to rats. Hepatic VLDL-TG release was subsequently measured by a liver perfusion system, and hepatocyte ultrastructural changes were analyzed by quantitative ultrastructural methods. The results demonstrate a tight coupling between the reduction in hepatocyte microtubule content and the reduction in hepatic VLDL-TG secretion which is related to the dose of colchicine or vinblastine administered. The various estimates imply that a minimum number of microtubules is necessary for hepatic VLDL secretion to proceed normally and that hepatic VLDL secretion rates reach their nadir (10-- 30% of control) when microtubules comprise less than 0.005% of the cytoplasm (or less than 10% of control values) when microtubules comprise less than 0.005% of the cytoplasm (or less than 10% of control values). At this point, hepatocyte Golgi complexes are also greatly altered; Golgi complexes with recognizable dictyosomal membranes are reduced to 15% of control values and the region is filled with large numbers of electron-dense bodies which appear to be lysosomes in the process of digesting VLDL. There is a predilection for the remaining Golgi complexes to be associated with a few segments of microtubules, even when no microtubules can be measured in random samplings of hepatocytes. Clusters of vacuoles containing VLDL are also present throughout the cytoplasm; the limiting membranes of 25% of these vacuoles are studded with ribosomes. These findings demonstrate that the administration of antimicrotubule agents results in decreases in hepatic VLDL-TG secretion which are associated with loss of microtubules and alteration of existing Golgi complexes.     

Ultrastructural studies of the commensal suctorian,Choanophyra infundibulifera hartog

Summary Tentacle structure, movement and feeding of the commensal suctorian Choanophrya infundibulifera have been examined by light, scanning and transmission electron microscopy. The tentacles possess a flattened tip and rounded shaft externally, with a neck and root region internally. There is a microtubule canal consisting of 150 ring microtubules within which are 20–35 curved lamellae each containing about 20 microtubules. Novel structural features include pairs of short oblique arranged microtubules at the tip, and a collar of epiplasm in the neck region. No haptocysts are found in Choanophrya but the tentacle cytoplasm contains two types of inclusions named solenocysts and spherical vesicles. These features are discussed in relation to the processes of tentacle movement and feeding. The rapid longitudinal movements of the tentacles are described and compared to those of other suctorians and possible mechanisms are suggested. Ingestion in Choanophrya is described and several theories involving tentacle microtubules in the feeding process are examined.This investigation was supported by the J.S. Dunkerley Fellowship in Protozoology, awarded by the University of Manchester.     

The Kinetid Structures of the Choreotrichous Ciliate Strobilidium velox and an Assessment of Its Evolutionary Lineage1

The ciliary (kinetid) structures of the ciliate Strobilidium velox have been examined with scanning and transmission electron microscopes. Somatic kineties consist of a linear row of kinetosomes (monokinetids) and short cilia lying partially beneath a thin fold of cytoplasm. The only fibrillar kinetid structure extending from the kinetosomes is a transverse ribbon of microtubules. The paroral membrane is a single-file polykinetid possessing a possible transverse ribbon of microtubules and a nematodesma. The oral polykinetids or membranelles are complex, with microtubules extending from both anterior and posterior rows of cilia. While the kinetid structures do not satisfy the criteria for the order Choreotrichida, they are similar to the tintinnids in several other relevant ways. Strobilidium velox is proposed to be an unusual ciliate that is an exception to the concept that somatic kinetids are conservative and reliable phylogenetic indicator structures.