ULTRASTRUCTURAL CHARACTERIZATION OF THE LYTIC CYCLE OF AN INTRANUCLEAR VIRUS INFECTING THE DIATOM CHAETOCEROS CF. WIGHAMII(BACILLARIOPHYCEAE) FROM CHESAPEAKE BAY,USA1

Numerous microalgal species are infected by viruses that have the potential to control phytoplankton dynamics by reducing host populations, preventing bloom formation, or causing the collapse of blooms. Here we describe a virus infecting the diatom Chaetoceros cf. wighamii Brightw. from the Chesapeake Bay. To characterize the morphology and lytic cycle of this virus, we conducted a time‐course experiment, sampling every 4 h over 72 h following viral inoculation. In vivo fluorescence began to decline 16 h after inoculation and was reduced to <19% of control cultures by the end of experiment. TEM confirmed infection within the first 8 h of inoculation, as indicated by the presence of virus‐like particles (VLP) in the nuclei. VLP were present in two different arrangements: rod‐like structures that appeared in cross‐section as paracrystalline arrays of hexagonal‐shaped profiles measuring 12 ± 2 nm in diameter and uniformly electron‐dense hexagonal‐shaped particles measuring ～ 22–28 nm in diameter. Nuclei containing paracrystalline arrays were most prevalent early in the infection cycle, while cells containing VLP increased and then declined toward the end of the cycle. The proportion of nuclei containing both paracrystalline arrays and VLP remained relatively constant. This pattern suggests that rod‐like paracrystalline arrays fragmented to produce icosahedral VLP. C. cf. wighamii nuclear inclusion virus (CwNIV) is characterized by a high burst size (averaged 26,400 viruses per infected cell) and fast generation time that could have ecological implications on C. cf. wighamii population control.     

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.     

STRUCTURE,LIFE HISTORY AND SYSTEMATICS OF RHOICOSPHENIA (BACILLARIOPHYTA). I. THE VEGETATIVE CELL OF RH. CURVATA1

Rhoicosphenia Grun. has been placed by some authors in the monoraphid group with Achnanthes Bory and Cocconeis Ehrenb., and by others near Gomphonema Ehrenb. In order to clarify the systematic position of the genus, the morphology and anatomy of the vegetative cells of Rh. curvata (Kütz.) Grun. were investigated using light and electron microscopy. The structure and formation of the two types of valve are described, and the heterovalvy shown to be of a different type from that of the monoraphids; on the basis of raphe, valve and girdle structure a close relationship between these and Rhoicosphenia is unlikely. Rhoicosphenia shows many resemblances to Gomphonema but the types of pore occlusion present, coupled with apparently slight differences in the mucilage-secreting structures and the girdle, suggest that classification in the same family is unwise. The cryptic asymmetry of the valves, and in particular of the raphe system, is noted and explained with reference to their formation; with respect to this asymmetry two configurations of the valves can occur (named cis and trans types) and the distribution of these in raphid genera is discussed briefly. In view of the lack of evidence in raphid diatoms supporting a classification of bands into copulae and pleurae, it is recommended that this practice be suspended.     

COMPARATIVE ULTRASTRUCTURE OF THE VALVES OF SOME CYCLOTELLA SPECIES (BACILLARIOPHYCEAE)1

Cyclotella atomus Hust., C. meneghiniana Kütz., C. comta (Ehr.) Kütz., C. antiqua W. Sm., C. michiganiana Skv., C. ocellata Pant., C. glomerata Bachmann, C. pseudostelligera Hust., and C. stelligera (Cleve et Grun.) V. H., were selected for ultrastructure investigations on the basis of their ready availability. Specimens were examined employing both transmission and scanning electron microscopy. Strutted processes, labiate processes, spines and other special features were surveyed in all species. On the basis of these observations the species of Cyclotella observed seem to be in one of 3 morphological groups, the meneghiniana group, the comta group, and the stelligera group.     

TAXONOMY AND FRUSTULAR STRUCTURE OF THE MARINE CENTRIC DIATOM PARALIA SULCATA1

Light and electron microscopy were used to investigate the complex structure of the frustule of Paralia sulcata (Ehrenb.) Cleve. Rimoportulae are reported for the first time in this diatom and two types of linking processes are described. The ease with which the cingulum is lost is explained with regard to its attachment to the valve. Two kinds of heterovalvy were observed and the taxonomic significance of one of these is discussed. The validity of Heiberg's genus Paralia is confirmed and a type slide of the species is designated.     

VALVE AND BAND MORPHOLOGY OF SOME FRESHWATER DIATOMS. III. PRE- AND POST-AUXOSPORE FRUSTULES AND THE INITIAL CELL OF MELOSIRA ROESEANA1

Frustules of a clonal culture of Melosira roeseana Rabenh. were examined with light and scanning electron microscopy. Vegetative valves in the post-auxospore (full size) stage exhibit a larger width/length ratio than those in the pre-auxospore (size-reduced) stage. Cells form chains by linking spines of adjacent valves which occur at the periphery of the valve face-mantle junction. Three or jour large pores occur at the center of the valve face, with the diameter of each pore tapering from the inner to the outer valve surface; these pores are often occluded by siliceous processes. Features of M. roeseana, not shown previously for Melosira, include a “stepped” mantle, on only one of the two valves resulting from the same cell division, flattened processes attached to short siliceous stalks on the valve face, disk-like processes on the mantle, and an open girdle band with up to eight antiligulae. Siliceous scales on the surface of the initial cell are remnants of the auxospore wall. The epivalve of the initial cell is larger in diameter than the hypovalve, and both valves lack linking spines and a step on the valve surface. The initial, cell epicingulum consists of only two bands; the hypocingulum has up to seven. Initial cells with four or more hypocingular bands divide to form new post-auxospore filaments. Melosira roeseana should not be included in the genus Melosira as it is presently defined by the type species, M. nurnmuloides C. Ag. Major differences include irregular linking spines, a closed pseudoloculate valve construction, and labiate processes on the valve face and mantle of M. nummuloides, compared with well-defined linking spines, a valve constructed of a basal siliceous layer perforated by poroid areolae, and labiate processes lacking on the valve of M. roeseana.     

OBSERVATIONS OF A TUBE-DWELLING DIATOM: NAVICULA HAMULIFERA (BACILLARIOPHYCEAE)1

Colonies of the tube-dwelling diatom Navicula hamulifera Grunow living on mangrove prop roots in Indian River. Florida and at La Parguera, Puerto Rico, were studied using light and electron microscopy. Observations of the tube morphology and cell structure of this diatom from fresh samples and cultures are described, as well as the ultrastructural morphology of its frustule. The formation of tubes by this diatom is reported for the first time. Comparisons are made with the closest species; Navicula delognei V.H. and Navicula pseudocomoides Hendey.     

Fine structure of two Hygropetra species,Hygropetra gelasina sp. nov. and Hygropetra balfouriana (Bacillariophyceae), and the taxonomic position of the genus with special reference to Frankophila

A small diatom, Hygropetra gelasina sp. nov., obtained from wet moss is described. This diatom is similar to Hygropetra balfouriana (Grunow ex Cleve) Krammer & Lange‐Bertalot, which was found in the same moss sample. Fine structural observations revealed that H. gelasina has a reduced raphe slit and depressions along the margin of the axial area, at the proximal ends of the striae. Both species are characterized by multiple rows of areolae in each stria and a hexagonal pattern of sub‐pores similar to that in Pinnularia, but differing in the position of the areola occlusions or hymenes, which are internal in Hygropetra. Comparison with Frankophila, which shares the characteristics of reduced raphe slits and areola structure with Hygropetra, provides a reference for future taxonomic study of these related genera.     

STRUCTURE,LIFE HISTORY AND SYSTEMATICS OF RHOICOSPHENIA (BACILLARIOPHYTA). II. AUXOSPORE FORMATION AND PERIZONIUM STRUCTURE OF RH. CURVATA1

Reproduction in Rhoicosphenia curvata (Kütz.) Grun. is isogamous. The two auxospores formed expand parallel to the apical axes of the gametangial cells. Expansion is bipolar and leads to the formation of a slightly curved, tapering cell, in which the initial valves are laid down. The perizonium consists of transverse and longitudinal bands. The transverse series, of 35 or so bands, is laid down centrifugally as the auxospore expands and can be classified into three groups on the basis of band morphology. All except the central band are open hoops, orientated so that their ends lie in the midline of the less convex, ventral side of the auxospore. The bands have fimbriate margins on one or both sides, and overlap one another from center to either pole. The longitudinal series includes 5 bands—a wide central band, with two on either side: again, the bands overlap one another from the center outwards. The initial epivalve of the new generation forms beneath the dorsal side of the auxospore, on the opposite side from the longitudinal perizonial series. Comparisons are made with other genera and the relevance of auxospore studies to an understanding of diatom morphogenesis is discussed.     

EFFECT OF THE SINKING RATE OF TWO DIATOMS (THALASSIOSIRA SPP.) ON UPTAKE FROM LOW CONCENTRATIONS OF PHOSPHATE1

The effect of the sinking rate, or rate of medium flow (φ) on the rate of phosphate incorporation (V) by the planktonic diatoms Thalassiosira fluviatilis Hust. and T. pseudonana Hasle & Heimdal in batch and chemostat cultures was determined by passing medium at defined flow rates (0.5–25.0 mm·min^?1) over algae on membrane filters. At concentrations from 1 to 100 μg phosphorus·l^?1 V, increases with increasing velocity of flow, approaching a maximum value (V_m) as described by the empirical relationship: where K_φ is the sinking rate value when V = 1/2 V_m+ V_o and V_o is the uptake at 0 rate of flow. By comparing uptake at controlled flow with uptake in a vigorously stirred medium, the phosphate concentration in the cell boundary layer can be determined. The sinking rate that reduces the phosphate concentration in the boundary layer to half of nominal concentration in the medium is much lower for the larger T. fluviatilis than for T. pseudonana. For both diatoms, it is inversely related to the nominal concentration.     

STRUCTURE,LIFE HISTORY AND SYSTEMATICS OF RHOICOSPHENIA (BACILLARIOPHYTA). V. INITIAL CELL AND SIZE REDUCTION IN RH. CURVATA AND A DESCRIPTION OF THE RHOICOSPHENIACEAE FAM. NOV.1

The initial epivalve of Rhoicosphenia curvata (Kütz.) Grun. differs from vegetative valves in having a strongly arched section, a wide hyaline marginal strip, no pseudosepta, an unthickened margin, and a terminal raphe fissure at the head pole. The initial epivalve is of the D type, with short raphe fissures. The epicingulum consists of three bands as usual, but they are narrower and more delicate than those of vegetative cells. The initial hypovalve and hypocingulum are similar in every way to those of vegetative cells, except for the rounded section of the hypovalve. During size reduction the almost isopolar outline of the initial valves and their immediate descendants gives way to an increasingly strong heteropolarity, and this is accompanied by changes in the relative lengths of the raphe slits and the shape of the central area. Different populations have different gametangium and initial cell sizes, suggesting the presence of races within the species. The structure of the initial cell indicates that Rhoicosphenia is less closely related to the monoraphid genera than to the gomphocymbelloid genera, confirming conclusions reached from studies of the vegetative cell and auxospore formation. Rhoicosphenia should therefore be separated into a new family, the Rhoicospheniaceae, which is described.     

STUDIES ON MARINE PLANKTON DIATOMS. III. STRUCTURE AND CLASSIFICATION OF GOSSLERIELLA TROPICA1

The structure of the tropical oceanic diatom Gossleriella tropica Schütt was examined in the light and electron microscopes. The species is characterized by a primarily poroid valve, surface, it single central labiate process and a unique ring of spines attached to the cingulum. Placement of the genus in the family Rhizosoleniaceae is suggested. Although G. tropica is widely distributed in the world's oceans, it is usually confined to the lower photic zone, salinities of 34.5–35.5% and temperatures of 15–30 C.     

COMPARATIVE SEQUENCE ANALYSIS OF DIATOM SILICON TRANSPORTERS: TOWARD A MECHANISTIC MODEL OF SILICON TRANSPORT1

Silicon is an important element in biology, for organisms ranging from unicellular algae to humans. It acts as a structural material for both plants and animals, but can also function as a metabolite or regulator of gene expression, affecting a wide range of cellular processes. Molecular details of biological interaction with silicon are poorly understood. Diatoms, the largest group of silicifying organisms, are a good model system for studying this interaction. The first proteins shown to directly interact with silicon were diatom silicon transporters (SITs). Because the basis for substrate recognition lies within the primary sequence of a protein, identification of conserved amino acid residues would provide insight into the mechanism of SIT function. Lack of SIT sequences from a diversity of diatoms and high sequence conservation in known SITs has precluded identification of such residues. In this study, PCR was used to amplify partial SIT sequences from eight diverse diatom species. Multiple gene copies were prevalent in each species, and phylogenetic analysis showed that SITs generally group according to species. In addition to partial SIT sequences, full‐length SIT genes were identified from the pennate diatom, Nitzschia alba (Lewin and Lewin), and the centric diatom Skeletonema costatum (Greville) Cleve. Comparing these SITs with previously identified SITs showed structural differences between SITs of centrics and pennates, suggesting differences in transport mechanism or regulation. Comparative amino acid analysis identified conserved regions that may be important for silicon transport, including repeats of the motif GXQ. A model for silicon uptake and efflux is presented that is consistent with known aspects of transport.     

VALVE AND BAND MORPHOLOGY OF SOME FRESHWATER DIATOMS I. FRAGILARIA CAPUCINA VAR. MESOLEPTA1,2

Acid cleaned cells from clonal cultures of Fragilaria capucina var. mesolepta Rabh. were examined with light and scanning electron microscopy. Recently isolated cells are linear-lanceolate in shape with a median constriction. After several transfers over 25 mo, cells exhibit size diminution resulting in small elliptically shaped valves. Adjacent valves are united to one another by interlocking marginal spines. Every valve has an apical pore field at each apex. A single labiate process is present infrequtently, appearing underdeveloped most often in size-reduced cells. The girdle region consists of two cingula, each composed of a series of underlapping bands. Each pleura in the series is a discontinuous ring with a central ligula. A survey of past ultrastructure studies on the freshwater Fragilariaceae reveals that the occurrence of the apical pore field and labiate process are likely key characteristics for the family. The apical pore field of Diatoma, Asterionella and Tabellaria is positioned on the valve face, whereas the apical pore field of F. capucina var. mesolepta is located on the valve mantle, the girdle region of F. capucina var. mesolepta is basically similar to that of Gomphonema parvulum (Kütz.) Grun.     

PHYSIOLOGICAL VARIABILITY WITHIN TEN STRAINS OF CHAETOCEROS MUELLERI (BACILLARIOPHYCEAE)1

The effects of temperature, ionic composition, and conductivity on growth rates of ten strains of Chaetoceros muelleri Lemmerman (mostly var. subsalsum Johan. & Rushf.) were studied. Lipid content of stressed and unstressed cells and fatty acid composition were also determined. Considerable physiological variability was observed in the ten strains, although principal components analysis of physiological data indicated that all strains fell into one of two major groups: C. muelleri (var. muelleri and var. subsalsum) and an undescribed Chaetoceros species morphologically close to C. muelleri var. subsalsum. A high degree of agreement was found among morphological, physiological, and biochemical data sets, indicating that physiological and biochemical data may be helpful in making taxonomic decisions in diatoms, particularly in taxa with few morphological characters. We also conclude that nonmorphological characters such as those employed in the present study can be used to test phylogenetic hypotheses formulated from traditional morphological data.     

The chloroplast and mitochondrial genomes of two Gomphonema parvulum (Bacillariophyta) environmental isolates from South Carolina (United States) and Virginia (United States)

Gomphonema parvulum is a cosmopolitan freshwater diatom that is used as an indicator in water quality biomonitoring. In this study, we report the culturing of two geographically separated isolates from southeastern North America, their morphology, and the sequencing and assembly of their mitochondrial and chloroplast genomes. Morphologically, both strains fit G. parvulum sensu lato, but the frustules from a protected habitat in South Carolina were smaller than those cited in the historic data of this species from the same location as well as a second culture from Virginia. Phylogenetic analyses using the rbcL gene placed both within a clade with G. parvulum. Genetic markers, including full chloroplast and mitochondrial genomes and the nuclear small subunit rRNA gene region were assembled from each isolate. The organellar genomes of the two strains varied slightly in size due to small differences in intergenic regions with chloroplast genomes of 121,035 bp and 121,482 bp and mitochondrial genomes of 34,639 bp and 34,654 bp. The intraspecific pairwise identities of the chloroplast and mitochondrial genomes of these two isolates were 97.9% and 95.4%, respectively. Multigene phylogenetic analysis demonstrated a close relationship between G. parvulum, Gomphoneis minuta, and Didymosphenia geminata.     

THE IMPACT OF NONPHOTOCHEMICAL QUENCHING OF FLUORESCENCE ON THE PHOTON BALANCE IN DIATOMS UNDER DYNAMIC LIGHT CONDITIONS1

The nonphotochemical quenching (NPQ) of fluorescence is an important photoprotective mechanism in particular under dynamic light conditions. Its photoprotective potential was suggested to be a functional trait of algal diversity. In the present study, the influence of the photoprotective capacity on the growth balance was investigated in two diatoms, which possess different NPQ characteristics. It was hypothesized that under fluctuating light conditions Cyclotella meneghiniana Kütz. would benefit from its large and flexible NPQ potential, whereas the comparably small NPQ capacity in Skeletonema costatum (Grev.) Cleve should exert an unfavorable impact on growth. The results of the study clearly falsify this hypothesis. Although C. meneghiniana possesses a fast NPQ component, this diatom was not able to recover its full NPQ capacity under fluctuating light. On the other hand, the induction of NPQ at relatively low irradiance in S. costatum resulted in rather small differences in the fraction of energy dissipation by the NPQ mechanism in the comparison of both diatoms. Larger differences were found in the metabolic characteristics. Both diatoms differed in their biomass composition, with a higher content of lipids in C. meneghiniana but higher amounts of carbohydrates in S. costatum. Finally, the lower degree of reduction in the biomass compensated for the higher respiration rates in S. costatum and resulted in a higher quantum efficiency of biomass production. An indirect correlation between the photoprotective and the metabolic capacity is discussed.     

DISTRIBUTION OF INTERTIDAL DIATOMS ASSOCIATED WITH SEDIMENTS IN YAQUINA ESTUARY,OREGON1,2

Sediment samples and environmental data were collected from November 1973 to August 1974 to analyze the distribution of sediment-associated diatom assemblages relative to vertical, horizontal, and seasonal gradients in Yaquina Estuary, Oregon. The distribution of diatoms was regulated primarily by mean salinity and characteristics of the sediment, i.e., mean sediment size and percentage of organic carbon. Prominent taxa in the river above Yaquina Bay exhibited overlapping distributions along the salinity gradient to a location in brackish water where the mean salinity was approximately 5%_o. At this salinity, a relatively sharp discontinuity in the diatom flora existed which appeared to represent the biochemical and biophysical mechanisms involved in the osmotic regulation of fresh- and brackish-water diatoms. Relatively large disparities m the structure of diatom assemblages were found within relatively small areas of Yaquina Bay. These differences were attributed to properties of the sediment. Differences in diatom assemblages relative to variations in light intensity, water temperature and exposure to intertidal emergence were not apparent from the analysis of field data.     

PHOTOSYNTHETIC ELECTRON TRANSPORT IN CELL-FREE EXTRACTS OF DIVERSE PHYTOPLANKTON1,2

A simple and rapid procedure for preparing thylakoid membranes that are active in photosynthetic electron transport from diverse phytoplankton species is described. The method requires disruption of algal cells with glass beads, exposure to mild hypotonic stress, and subsequent enrichment of the thylakoid membranes by differential centrifugation. Isolated thylakoid membranes were assayed for photosynthetic electron transport activity by measuring rates of oxygen consumption and oxygen production, using a variety of electron donors and acceptors. In the dinoflagellate Gonyaulax polyedra Stein, a relatively broad pH optimum between 7.0 and 8.0 was determined for the whole chain electron transport from water to methyl viologen. The preparation maintained maximum activity for 45 min following the preparation. The assay for photosystem I activity in G. polyedra, determined as electron flow from ascorbate/2,6-dichlorophenolindophenol to methyl viologen, had a somewhat narrower pH optimum around 8.0. Rates of whole chain photosynthetic electron transport on a per cell and on a per chlorophyll a basis were shown to decrease dramatically with cell age in batch cultures of G. polyedra. Using the procedures optimized for G. polyedra, reproducible rates of electron transport on a per cell chlorophyll a basis were also measured in cultures of the dinoflagellate Glenodinium sp., the diatom Nitzschia closterium (Ehrenberg 1839) Wm. Smith 1853 and the chrysophyte Monochrysis lutheri Droop {= Pavlova lutheri (Droop) Green}. Other electron transport assays applied to G. polyedra, and that resulted in comparable rates to those found in other algal groups, include the photosystem II assay from water to diaminodurene/ferricyanide and the photosystem I assay from durohydroquinone to methyl viologen.     

DIVERSITY AND MORPHOLOGY OF SKELETONEMA SPECIES IN SOUTHERN BRAZIL,SOUTHWESTERN ATLANTIC OCEAN1

We examined the morphology of Skeletonema species occurring in coastal waters of southern Brazil, on the basis of LM, SEM, and TEM analyses of six isolated strains and of 13 field samples from the Patos Lagoon estuary and the adjacent Cassino Beach. We concluded that the genus Skeletonema is highly diverse in coastal waters of southern Brazil, with the occurrence of four confirmed species: S. costatum, S. pseudocostatum, S. potamos, and S. tropicum. In addition, EM details must still be provided for S. menzelii, and genetic studies should be conducted to confirm the identity of S. cf. grethae and S. subsalsum. The high diversity in this geographic area can be attributed to the proximity of the Subtropical Convergence and to the freshwater ecosystems of Patos Lagoon and La Plata River that favor the presence of warm‐, cold‐, and brackish‐water species.