Summary
The fine structure and the carotenoids of three species of the genus Tribonema were investigated: Tr. viride, Tr. aequale and Tr. minus. The cells of the latter two species (cultured material) contained one nucleus, whereas the cells of the former (collected near Freiburg/Black Forest) possessed numerous nuclei. This multinucleate material is supposed to represent stages in the formation of zoospores as is also indicated by the occurrence of flagella.
The ultrastructure of the H-shaped walls which are characteristic for these Heterotrichales is described in detail. The general appearance of the cell interior of Tribonema resembles closely that of the related siphoneous genus Botrydium (Falk, 1967). This is true for mitochondria, centrioles, the parabasal position of the dictyosomes, the association of the forming face of the Golgi apparatus with the nuclear envelope, cisternae of the ER-system surrounding the plastids and for the electron-transparent DNA-containing areas at the ends of the plastids. With respect to certain other structural features, however, differences between these two Xanthophycean algae are apparent:
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a)
In Tribonema, only one distinct dictyosome per nucleus can be recognized.
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b)
The plastids lack a pyrenoid area traversed by parallel bands of thylakoids. However, some of the plastids bulge somewhat into the cytoplasm and form outpocketings, free of thylakoids. If these outpocketings do represent pyrenoids, then they belong to the “bulging type” found in some brown algae, Euglenophyceae and Haptophyceae. c) In Tribonema plastids the three thylakoids forming a band are appressed and are not separated by an interthylakoidal space.
These structural differences are discussed with particular reference to their taxonomic value in assessing the position of the Xanthophyceae and other related groups of the Chromophyta.
Furthermore, evidence is presented that a limited area in the space between the plastid envelope and the surrounding ER-cisterna is occupied by a network of smooth ER-tubules. This network seems to be continuous with the periplastidal ER-cisterna. Similar tubular structures are reported for other groups of the Chromophyta. This suggests that the periplastidal network is of widespread occurrence among the Chromophyta.
Analysis of the carotenoid pigments of Tribonema has demonstrated that besides the carotenoids reported to be common in Xanthophyceae three triesters of a recently described new carotenoid, vaucheriaxanthin also are present. The esterified vaucheriaxanthin amounts up to 40% of the total carotenoid content. Since these triesters so far have been found only in Vaucheria, Botrydium and Tribonema they may provide additional taxonomic characterization for the Xanthophyceae.
Zusammenfassung
Zell-Feinbau und Carotinoidausstattung wurde bei drei Arten der Xanthophyceengattung Tribonema untersucht: Tr. viride (Freilandmaterial), Tr. aequale und Tr. minus (Kulturmaterial). Während die beiden letzteren nur einen Kern je Zelle enthielten, traten bei Tr. viride bis zu sechs Kerne je Zelle auf. Wahrscheinlich handelte es sich dabei um Fäden, die im Begriff waren, Zoosporen zu bilden, was auch durch das vereinzelte Auftreten von Geißeln nahegelegt wird.
Der Zellfeinbau von Tribonema ist dem von Botrydium sehr ähnlich. Einige bemerkenswerte Abweichungen hiervon (Dictyosomenzahl, Assoziationstyp der Thylakoide, Pyrenoid) werden unter dem Gesichtspunkt der Verwertbarkeit von Feinstrukturdaten für die Algensystematik ebenso diskutiert wie Übereinstimmungen mit anderen Chromophytengruppen (unter anderem periplastidäre ER-Cisterne, parabasale Lage des Golgi-Apparates, Lagebeziehung Kernhülle — Dictyosom). Bei den bereits für verschiedene Chromophytengruppen beschriebenen “tubulären Strukturen” im Raum zwischen Periplastidärcisterne und Plastidenhülle handelt es sich um ein flächiges Maschenwerk aus nicht mit Ribosomen besetzten, untereinander kommunizierenden Tubuli (“Periplastidär-Reticulum”), das mit der periplastidären Cisterne des ER in Verbindung steht.
Die Analyse der Carotinoide ergab, daß bei Tribonema außer den üblichen Xanthophyceen-Carotinoiden drei Triester des erst kürzlich in Vaucheria und Botrydium gefundenen, bisher unbekannten Carotinoids Vaucheriaxanthin vorkommen. Es wird angenommen, daß Vaucheriaxanthin ein Pigment von chemotaxonomischem Wert zur Charakterisierung der Xanthophyceen ist.
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Falk, H., Kleinig, H. Feinbau und Carotinoide von Tribonema (Xanthophyceae) . Archiv. Mikrobiol. 61, 347–362 (1968). https://doi.org/10.1007/BF00409671
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DOI: https://doi.org/10.1007/BF00409671