COMPARATIVE ULTRASTRUCTURE OF CULTURED SPECIES OF TREBOUXIA1,2

Five species of cultured Trebouxia—T. anticipata, T. decolorans, T. erici, T. gelatinosa, and T. impressa—were examined with the electron microscope. A comparative examination of their pyrenoids revealed pyrenoglobuli associated with single pyrenoid thylakoids. The pyrenoids of T. decolorans, T. erici, and T. gelatinosa possess single thylakoids that cross or deeply penetrate the pyrenoid matrix and are often disposed in parallel arrays. T. anticipata possesses both single and double pyrenoid thylakoids within the matrix. T. impressa possesses vesiculate invaginations of thylakoid membranes into the pyrenoid matrix. The phycobiont. T. erici was examined in detail at the light and electron microscopic levels for pyrenoid alterations associated, with varied environmental regimes and with cell division. A greater amount of starch is present in cells grown in organic culture at 215 lux light intensity than in cells of similar size grown at 1075 or 3600 lux. Pyrenoglobuli are present throughout the life cycle and occur both in aplanospores and in zoospores.     

SPECIES-SPECIFIC DIFFERENCES OF PYRENOIDS IN CHLORELLA (CHLOROPHYTA)1

The structure of the pyrenoid supports the separation of Chlorella species into two groups based on cell wall chemistry and suggests evolutionary relationships. Chlorella species with a glucan-type wall exhibit quite diverse pyrenoid structures, which may indicate that these species are not closely related. Those species with glucosamine cell walls (C. kessleri, C. sorokiniana, C. vulgaris) are virtually identical in pyrenoid morphology, indicating a closer evolutionary relationship. In the species with glucosamine walls, the thylakoid that penetrates into the pyrenoid matrix, is unijormly double-layered. Pyrenoids in the species with glucan walls show various features: 1) a pyrenoid matrix only, 2) a pyrenoid traversed by a few discs of double thylakoids with many adhering pyrenoglobuli, 3) a pyrenoid penetrated with tubelike structures or 4) a pyrenoid penetrated with many single undulating thylakoids. The pyrenoid structure of the symbiotic Chlorella in Paramecium bursaria resembles those of free-living Chlorella with glucosamine walls.     

Comparative ultrastructure of pyrenoids inTrebouxia (Microthamniales,Chlorophyta)

Pyrenoid ultrastructure has been investigated from cultures of all 26 species ofTrebouxia with the aim of establishing pyrenoids as a taxonomic character. Different arrangements and forms of thylakoid lamellae within the pyrenoid matrix allow eight pyrenoid types to be distinguished. Each type is characteristic of a group of species. Thegigantea- andimpressa-type are similar, differing only in the form of the tubules: short, branched tubules mark thegigantea-type; ± long and straight invaginations theimpressa-type. Thearboricola-type is characterized by meandering pyrenoid membranes developing from lamellae parallel with each other in young autospores. Pyrenoids of thegelatinosa-type are traversed by thin parallel-arranged tubules. Few thylakoids with a curved profile are typical of theirregularis-type. Thecorticola-type is different from all others in having a distinct starch sheath closely connected with the pyrenoid matrix and no pyrenoglobuli being associated with the pyrenoid membranes. No true pyrenoids have been found inT. magna andT. erici. Within the chloroplast, they have indistinct areas with pyrenoglobuli, but without differentiated thylakoids. Pyrenoid morphology is stable in culture on different media as well as in phycobionts within lichen thalli. Comparing the pyrenoid of a lichenizedTrebouxia with that from cultured species, the identification of the phycobiont within the lichen thallus is possible, without the need of culturing the algae. This has been shown in species ofParmelia andHypogymnia. New aspects for the taxonomy and systematics ofTrebouxia are discussed.Dedicated to Prof. DrLothar Geitler on the occasion of the 90th anniversary of his birthday.     

NEW ULTRASTRUCTURAL ASPECTS OF PYRENOIDS OF THE LICHEN PHOTOBIONT TREBOUXZA (MICROTHAMNIALES,CHLOROPHYTA)1

The pyrenoid structure of Trebouxia, a photobiont of two lichen species, Umbilicaria cinereorufescens (Schaer.) Frey and Parmelia sulcata Taylor, was investigated. In both lichen species, the pyrenoid of the photobiont exhibited straight, unbranched, long or short tubules. In the first lichen species, multiple pyrenoids were observed occasionally, while in the second one, homogeneous masses, called protein bodies, appeared between the thylakoids. These protein bodies were previously observed in some other species of the family Umbilicariaceae. Serial sections from single pyrenoids showed that tubules of the Impressa-type pyrenoid were closely associated with pyrenoglobuli. The three-dimensional reconstruction of a complete chloroplast of a P. sulcata algal cell showed that the protein bodies were spatially separate structures. Immunolocalization techniques to detect the presence of ribulose-bisphosphate carboxylase (Rubisco) in the chloroplast showed that this enzyme was present primarily in the pyrenoid matrix. When protein bodies were present in the chloroplast, Rubisco appeared to be localized in these structures. The presence of pyrenoid satellites and protein bodies with reactivity to anti-Rubisco may be related to the nutritional conditions of the thalli.     

COMPARATIVE STUDIES OF PYRENOID ULTRASTRUCTURE IN ALGAE OF THE MONOSTROMA COMPLEX123

The pyrenoid structure in 15 species of the Monostroma complex is very diverse us revealed by a study of the morphology of the pyrenoid matrix, associated starch shell, and pattern of intrapyrenoidalthylakoid bands. From these characteristics 8 types of pyrenoid structure were classified. The variation of pyrenoid structure was shown not only among the species studied, but also between the alternation of generations (M. angicava and M. nitidum). In M. fuscum var. splendens, M. groenlandicum, M. undulatum, and M. zostericola pyrenoid structure is the same throughout the life cycle. The pyrenoid matrix of M. zostericola is surrounded by a double membrane that prevents the direct connection of the pyrenoid matrix with chloroplast thylakoids. The pyrenoid also lacks a starch shell. These findings support the establishment of a new genus Kornmannia by Bliding to include M. zostericola. In addition, similarities in pyrenoid ultrastructure suggest an affinity of Capsosiphon fulvescens with M. groenlandicum.     

CHLOROPLAST INCLUSIONS IN ZOOSPORES OF OEDOCLADIUM1

Chloroplast inclusions have been studied in zoospores of Oedocladium carolinianum and their ultrastructure compared with the same inclusions previously described in the related genera Oedogonium and Bulbochaete. Structure of the mature pyrenoids is consistent in all 3 genera; the pyrenoid matrix is penetrated by branched cytoplasmic channels delimited by a double membrane system continuous with the chloroplast envelope. Pyrenoids typically arise de novo in zoospores of O. carolinianum. No evidence for the bipartition of a parent pyrenoid has been observed. The incipient pyrenoids of Oedocladium are similar to those found in zoospores of Oedogonium and Bulbochaete, but they frequently demonstrated a crystalline matrix. However, a crystalline matrix was never observed in any mature pyrenoid, even those immediately adjacent to incipient pyrenoids with crystalline structure. Other chloroplast inclusions typical of Oedogonium and Bulbochaete zoospores are the eyespot and striated microtubules. Although the zoospores of O. carolinianum possess striated microtubules, the presence of an eyespot has not been observed.     

THE ULTRASTRUCTURE OF LICHENS. I. A GENERAL SURVEY

The fine structure of 10 lichens was examined. A comparison was made of the storage products of the algal symbiont (Trebouxia) in situ in the desiccated and hydrated states of the lichens. All the Trebouxia phycobionts, with the exception of that in Usnea strigosa, had lipid-containing globules in the pyrenoid. The globules were present in both the hydrated and desiccated conditions. Trebouxia in the hydrated condition contained starch granules in the chloroplast as well as the lipid-containing globules in the pyrenoid. The cell wall of Trebouxia consists of an outer electron-dense layer and an inner electron-light layer. Fungal haustoria (in Lecanora rubina) rupture the outer layer of the algal cell wall and invaginate the inner layer. A thick polysaccharide fibrillar material surrounds the fungal cells. Many bacteria were observed within this material. Septa and lomasomes are described. Ellipsoidal bodies, which appear to be an integral and unique part of the lichen fungal ultrastructure, were observed associated with membrane profiles.     

The pyrenoid is the site of ribulose 1,5-bisphosphate carboxylase/oxygenase accumulation in the hornwort (Bryophyta: Anthocerotae) chloroplast

Summary Chloroplasts of many species of hornworts (Anthocerotae) have a structure that resembles the pyrenoid of green algae but whether these two structures are homologous has not been determined. We utilized immunogold labelling on thin sections to determine the distribution of ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the major protein of algal pyrenoids, in sixteen hornwort species with and without pyrenoids. Several species (Phaeoceros laevis, Anthoceros punctatus, A. formosae, A. laminiferus, Folioceros fuciformis, Folioceros sp.,Dendroceros tubercularis, D. japonicus, D. validus, Notothylas orbicularis, N. temperata, andSpaerosporoceros adscendens) have uniplastidic (or primarily uniplastidic) cells with large prominent multiple pyrenoids. In all of these species, the labelling is found exclusively in the pyrenoid and, with the exception of theFolioceros, Dendroceros, andNotothylas species, the labelling is randomly distributed throughout the pyrenoid. In the exceptional species, the pyrenoids have prominent pyrenoglobuli or other inclusions that are unlabelled. InMegaceros flagellaris andM. longispirus, the cells are multiplastidic (with the exception of the apical cell and some epidermal cells) and the chloroplasts lack pyrenoids.Anthoceros fusiformis andPhaeoceros coriaceus have primarily uniplastidic cells but the chloroplasts lack pyrenoids; only an area of stroma in the center of the plastid devoid of starch, reminiscent of a pyrenoid, is found. In all of the species lacking pyrenoids, RuBisCo is found throughout the stroma, including the stromal spaces made by the so-called channel thylakoids. No preferential accumulation of RuBisCo is found in the pyrenoid-like region inA. fusiformis andP. coriaceus. These data indicate that 1) the hornwort pyrenoid is homologous to algal pyrenoids in the presence of RuBisCo; 2) that at least some of the RuBisCo in the pyrenoid must represent an active form of the enzyme; and 3) that, in the absence of pyrenoids, the RuBisCo is distributed throughout the stroma, as in higher plants.Abbreviations RuBisCo ribulose 1,5-bisphosphate carboxylase/oxygenase     

ELECTRON MICROSCOPY OF THE VOLVOCACEAE AND ASTREPHOMENACEAE

Lang, Norma J. (U. Texas, Austin.) Electron microscopy of the Volvocaceae and Astrephomenaceae. Amer. Jour. Bot. 50(3): 280-300. Illus. 1963.—Clonal cultures of Gonium sociale, G. pectorale, Pandorina morum, Eudorina elegans, Eudorina sp., Volvulina steinii, V. pringsheimii, Platydorina caudata, Pleodorina illinoisensis, P. californica, Volvox aureus, V. tertius, V. globator, V. barberi, and Astrephomene gubernaculifera representing the Volvocaceae and Astrephomenaceae in the Volvocales were examined with the electron microscope and their ultrastructure compared. The ultrastructure of the various organelles is basically similar in the species studied and no increase in cellular complexity is found to accompany the evolutionary trends evidenced in the Volvocaceae. The ultrastructure of a colonial cell is basically that of Chlamydotnonas. A cytoplasmic membrane having a unit membrane structure encompasses a cell and is continuous with the double-membraned flagellar sheaths. The flagella contain the typical 9 + 2 fibril arrangement with the 2 axial fibrils terminating in a cylinder at the flagellar base and the 9 peripheral pairs continuing into the cytoplasm as a basal body. The organelles comprising the cytoplasm are: mitochondria with plate-like cristae; dictyosomes composed of stacks of agranular cisternae; small, rough or smooth-surfaced vesicles; an endoplasmic reticulum of granule-bearing and agranular tubules, lamellae and broad cisternae; vacuoles which are either contractile, contain fine granular and fibrillar material, or have dense contents probably representing polyphosphate; lipid bodies; and dense granules 100–150 A which have been called ribosomes. The finely granular nucleoplasm is surrounded by a porous, double-membraned nuclear envelope and contains a centric nucleolus composed of dense, spherical granules. The outer membrane of the nuclear envelope bears granules and may have granular extensions into the perinuclear cytoplasm. Each extension appears to encompass one or several dictyosomes and has been termed an “amplexus.” The amplexi are agranular on the surface contiguous to a dictyosome. A double-membraned chloroplast envelope is continuous around the single, cup-shaped chloroplast. The basic chloroplast units are discs closed at each end, occurring in stacks of varying number parallel to the envelope. The presumed proteinaceous matrix of the basal pyrenoid is penetrated by elongated, tubular elements which connect with the lamellar discs. Multiple rows of granules, associated with individual discs, form the anterior stigma within the chloroplast envelope. The colonial matrix is not a structureless, mucilaginous material uniting the cells in colonies, but it has rather a highly complex structure especially around the periphery of the colony and the flagellar channels. The apparent substitution of a fibrillar layer of the colonial matrix for the discrete compact cell wall, such as is found in Chlamydomonas, implies a greater degree of complexity in the evolution of these colonial genera than is generally assumed.     

HIGH LOCALIZATION OF RIBULOSE-1,5-BISPHOSPHATE CARBOXULASE/OXYGENASE IN THE PYRENOIDS OF CHLAMYDOMONAS REINHARDTII (CHLOROPHYTA), AS REVEALED BY CRYOFIXATION AND IMMUNOGOLD ELECTRON MICROSCOPY

The distribution of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the chloroplasts of the unicellular green alga Chlamydomonas reinhardtii Dangeard was examined using cryotechnique and conventional fixation for immunogold electron microscopy. Both methods provided essentially identical results, although somewhat higher densities of gold particles indicating Rubisco molecules were recognized in the pyrenoids of cryofixed cells. The gold particles were highly concentrated in the pyrenoid matrix within the chloroplasts. Even when considering the vast difference in volume between the pyrenoid and the rest of the Chloroplast, more than 99% of the total Rubisco labeling in the chloroplast was calculated to be present in the pyrenoid matrix. High localization of Rubisco in the pyrenoid matrix was also recognized regardless of cell age, based on immunofluorescence microscopy of the same en bloc samples. These results are inconsistent with a recent immunocytochemical study employing cryotechnique in which more than 90% of the total Rubisco was recognized in the thylakoid region (thylakoid membranes and stroma) of C. reinhardtii cells. Rubisco highly localized in the pyrenoid matrix may take part in active photosynthetic CO₂ fixation and/or the CO₂ concentrating mechanism .     

Observations with the electron microscope on Asteromonas gracilis Artari emend. (Stephanoptera gracilis (Artari) wisl.), with some comparative observations on Dunaliella Sp

A clone of Asteromonas gracilis has been examined in culture by means of light and electron microscopy. The morphological observations of Artari (1913) have been confirmed and extended. New observations include demonstration of an unusual type of pyrenoid somewhat resembling that of Prasinocladus, with tubular channels entering the pyrenoid centre from outside the plastid and penetrated by haustorial outgrowths from the nuclear envelope. The very different structure characteristic of Dunaliella is briefly illustrated. Other comparisons indicate that A. gracilis is the type species and sole known representative of a very distinct genus which cannot be confused with Pyramimonas and is unlikely to be synonymous with the imperfectly known genus, Stephanoptera.     

NEW PYRENOID FORMATION IN THE BROWN ALGA,SCYTOSIPHON LOMENTARIA (SCYTOSIPHONALES,PHAEOPHYCEAE)1

The Scytosiphon lomentaria (Lyngbye) Link cell characteristically has only one chloroplast with a prominent protruding pyrenoid. We observed the appearance of a new pyrenoid in each chloroplast during first mitosis in zygotes of S. lomentaria, using the freeze substitution technique. At first, a pyrenoid matrix appeared within the outermost stroma, in which thylakoid triplets and ribosomes were absent. At this time, the surface of this part remained smooth. The old pyrenoid was covered with a pyrenoid cap on the cytoplasmic side, whereas there was no pyrenoid cap on the new pyrenoid before protrusion. Irregularly shaped membranous sacs containing fine granular materials associated with the cytoplasmic side of the new pyrenoid. The sacs fused with each other and changed conformation and finally transformed into the pyrenoid cap. The new pyrenoid gradually protruded toward the cytoplasm, and the new pyrenoid cap became curved along the surface of pyrenoid. Cytokinesis occurred, and each chloroplast had two prominent protruding pyrenoids in two‐celled zygotes. We examined immunolocalization of β‐1,3‐glucans within the pyrenoid cap with a monoclonal antibody, using EM. Gold particles indicating localization of β‐1,3‐glucans were detected in vacuoles but never in the pyrenoid cap. This observation suggests that the pyrenoid cap in brown algae contains no photosynthetic products such as polysaccharide.     

ELECTRON MICROSCOPY OF THE LICHEN PHYSCZA AZPOLIA (EHRH.) NYL.1,2

The ultrastructure of the lichen Physcia aipolia was studied in the desiccated and hydrated states. No significant structural variation between these 2 states was noted for the fungus. The fungus contains unusual ellipsoidal structures heretofore unobserved. Their possible role in the formation of membrane is discussed. The plasmalemma of the fungus is convoluted while that of the alga is smooth. The convoluted projections have a “double-unit membrane” structure similar to fingerlike projections of the ellipsoidal bodies. With O_sO₄ fixation crystals are pre-served, enclosed in unit membrane sacs which extend to the convoluted surface. The pyrenoid of the associated alga does not produce starch in the desiccated condition, and electron-dense granules are present in the matrix associated with chloroplast lamellae which enter this area. In the hydrated condition, the alga contains abundant starch in the pyrenoid region, and the electron-dense granules are displaced to the preriphery of the pyrenoid starch. Mitochondria, endo-plasmic reticulum, and ribosomes are not clearly defined in the desiccated state while they are more so in the hydrated condition. Golgi bodies were not observed in the either lichen component. Finally, no fine structural basis for indicating an exchanged of materials between the alga and fungus was observed except, possibly, the convoluted plasmalemma of the fungus and the smooth plasmalemma of the alga.     

Light and electron microscopy and molecular phylogenetic analyses of Chloromonas pseudoplatyrhyncha (Volvocales,Chlorophyceae)

A strain of Chloromonas pseudoplatyrhyncha (Pascher) P. C. Silva, which has not been studied previously using cultured material, was established from a soil sample collected in Japan and examined by light microscopy, transmission electron microscopy, and molecular phylogenetic analyses. The chloroplasts of this species showed no pyrenoids under light microscopy. However, transmission electron microscopy and the staining methods with carmine after fixation in an acidified hypochlorite solution revealed that Chloromonas pseudoplatyrhyncha actually had multiple, atypical pyrenoids (pyrenoid matrices without associated starch grains) that were angular in shape and distributed in the interior regions of the lobes of the chloroplasts. Although some other species of Chloromonas have atypical pyrenoids in the chloroplast, such angular pyrenoids have not previously been reported within the Volvocales. The present molecular phylogenetic analysis, based on 18S ribosomal RNA, adenosine triphosphate synthase β‐subunit, and P700 chlorophyll a‐apoprotein A2 gene sequences, demonstrated that Chloromonas pseudoplatyrhyncha belonged to the Chloromonas lineage or Chloromonadinia, in which it occupied a basal position outside a robust, large monophyletic group consisting of 13 species of Chloromonas and Gloeomonas.     

OBSERVATIONS ON THE FINE STRUCTURE OF OEDOGONIUM. V. EVIDENCE FOR THE DE NOVO FORMATION OF PYRENOIDS IN ZOOSPORES OF OE. CARDIACUM123

Ultrastructural studies of the chloroplasts of zoospores and developing zoospores of Oedogonium carcliacum have disclosed the occurrence of numerous incipient pyrenoids. A single developing zoospore may possess several score of these structures which appear to arise de novo in the chloroplast stroma and seem to lack any direct association with mature pyrenoids which are also present in the cells. The incipient pyrenoids lack the associated starch grains and the membrane-limited channels characteristic of mature pyrenoids, but they are readily recognized in the chloroplasts since they demonstrate a greater granularity and electron density than the surrounding chloroplast stroma. The granularity and electron density of the incipient pyrenoids match the ultra-structural appearance of the matrix of mature pyrenoids. The smallest of the incipient pyrenoids examined from serial sections had a maximum diameter of less than 0.3 μ. This may be compared with the size of mature pyrenoids, many with a maximum diameter of over 5.0 μ. In all the zoospores and developing zoospores examined, only one mature pyrenoid was observed in an apparent stage of division.     

Re-examination of ultrastructures of the stellate chloroplast organization in brown algae: Structure and development of pyrenoids

Some taxa of brown algae have a so‐called ‘stellate’ chloroplast arrangement composed of multiple chloroplasts arranged in a stellate configuration, or else a single chloroplast with radiating lobes. The fine structures of chloroplasts and pyrenoids have been studied, but the details of their membrane configurations as well as pyrenoid ontogeny have not been well understood. The ultrastructure of the single stellate chloroplast in Splachnidium rugosum and Scytothamnus australis were re‐examined in the present study, as well as the stellate arrangement of chloroplasts in Asteronema ferruginea and Asterocladon interjectum, using freeze‐substitution fixation. It was confirmed that the chloroplast envelope invaginated into the pyrenoid in Splachnidium rugosum, Scytothamnus australis and Asteronema ferruginea, but chloroplast endoplasmic reticulum (CER) remained on the surface of the chloroplast. The space between the invaginated chloroplast envelope and CER was filled with electron‐dense material. In Asteronema ferruginea, CER surrounding each pyrenoid was closely appressed to the neighboring CER over the pyrenoids, so that the chloroplasts formed a stellate configuration; however, in the apical cells chloroplasts formed two or more loose groups, or were completely dispersed. The pyrenoids of Asterocladon interjectum did not have any invagination of the chloroplast envelope, but a unique membranous sac surrounded the pyrenoid complex and occasionally other organelles (e.g. mitochondria). Immunolocalization of β‐1,3‐glucans showed that the membranous sac in Asterocladon interjectum did not contain photosynthetic products such as chrysolaminaran. Observations in the dividing cells of Splachnidium rugosum and Scytothamnus australis indicated that the pyrenoid in the center of the chloroplast enlarged and divided into two before or during chloroplast division.     

THE CYTOPLASMIC FINE STRUCTURE OF THE DIATOM, NITZSCHIA PALEA

The cytoplasmic fine structure of the motile, pennate diatom, Nitzschia palea was studied in thin sections viewed in the electron microscope. The cells were fixed in OsO₄, embedded in methacrylate, and immersed in 10 per cent hydrofluoric acid (HF) for 36 to 40 hours to remove the siliceous cell wall prior to sectioning. The HF treatment did not cause any obvious cytoplasmic damage. The dictyosome complex is perinuclear, and located only in the central cytoplasm. Mitochondria are sparse in the central cytoplasm, but abundant in the peripheral cytoplasm, and fill many of the transvacuolar cytoplasmic strands. Characteristic, amorphous oil bodies fill certain cytoplasmic strands and probably are not leucosin. The pyrenoid appears to be membrane limited, and oil droplets are found adjacent to the pyrenoid. The pyrenoid of another diatom, Cymbella affinis, is also membrane-limited. The membrane limiting the pyrenoid may be a composite of the terminal portions of chloroplast discs, facilitating rapid movement of photosynthate into the pyrenoid matrix, where the characteristic oil droplets may be formed. Carinal fibrils are found singly in each carinal pore, and may be involved in the locomotion of Nitzschia palea.     

A 38-kilodalton low-CO2-inducible polypeptide is associated with the pyrenoid in Chlorella vulgaris

In the green alga Chlorella vulgaris UAM 101, a CO₂-concentrating mechanism (CCM) is induced when cells are transferred from high (5%) to low (0.03%) CO₂ concentrations. The induction of the CCM is correlated with de-novo synthesis of several polypeptides that remain to be identified. The internal carbonic anhydrase (CA; EC 4.2.1.1) activity increased 6- to 7-fold within 6 h of acclimation to air. When crude homogenates were further separated into soluble and insoluble fractions, nearly all of the CA activity was associated with the membrane fraction. Immunoblot analysis of cell homogenates probed with antibodies raised against the 37-kDa subunit of periplasmic CA of Chlamydomonas reinhardtii showed a cross-reaction with a single 38-kDa polypeptide in both high- and low-CO₂-grown cells. The up-regulation of the expression of the 38-kDa polypeptide was closely correlated with the increase in internal CA activity. Furthermore, its subcellular location was also correlated with the distribution of the activity. Immunoblot analysis of pyrenoid fractions showed that the 38-kDa polypeptide was concentrated in the pyrenoids from low-CO₂-grown cells but was not present in pyrenoids from high-CO₂-grown cells. In addition, immunogold labeling experiments showed that the protein was mainly associated with membranes crossing the pyrenoid, while it was absent from the pyrenoid matrix. These studies have identified a putative intracellular CA polypeptide associated with the pyrenoid in Chlorella vulgaris, suggesting that this structure may play an important role in the operation of the CCM and the acclimation to low CO₂ conditions. Received: 16 July 1997 / Accepted: 26 April 1998     

FINE STRUCTURE OF CALONEIS AMPHISBAENA (BACILLARIOPHYCEAE)1

The structure of the motile pennate diatom Caloneis amphisbaena Cleve is described, with emphasis on the lateral, lobed pyrenoid with neither a limiting membrane nor penetration by thylakoids, an interphase nucleus with centers of condensed chromatin, paired dictyosomes, and mitochondria cradled within the chambers of the valve. Microfilaments forming two bundles which lie beneath each raphe slit are of the same size and appearance as actin microfilaments associated with other motile systems.