Siliceous structures and silicification in flagellated protists

Summary Flagellated protists produce a diverse range of siliceous structures, such as internal and external skeletons, scales, spines, bristles, cell walls, cyst walls, and loricae. The different groups of silica-depositing flagellates, i.e., chrysophytes/synurophytes, choanoflagellates, dinoflagellates, ebriids, silicoflagellates, thaumatomastigids, and the genusPetasaria are reviewed. Brief mention is also given to those algal groups in which silicification is uncommon and rare (i.e., chlorophytes, euglenophytes, haptophytes/prymnesiophytes, xanthophytes/tribophytes), but in which silicified structures nevertheless occur in few flagellate genera. Special attention is given to aspects of morphology and development of the different siliceous structures as well as on aspects of systematics and taxonomy.     

Combining geometric morphometrics,molecular phylogeny,and micropaleontology to assess evolutionary patterns in Mallomonas (Synurophyceae: Heterokontophyta)

Synurophytes, also known as scaled chrysophytes, are ecologically important algae that produce an array of siliceous structures upon which their taxonomy is based. Despite occupying a key position within the photosynthetic heterokonts, the evolutionary history of synurophytes remains poorly constrained. Here, modern and Middle Eocene siliceous scales of the morphotaxon Mallomonas insignis are used as a model to investigate synurophyte evolutionary patterns. Structural details of scale morphology were examined comparatively with scanning electron microscopy and scored for geometric morphometric analyses to assess the stability of shape characters. Although consistent size differences exist (modern scales are larger than Eocene counterparts), the populations cannot be differentiated on the basis of shape or microstructural detail, implying considerable evolutionary stasis in scale morphology. A time‐calibrated relaxed molecular clock analysis using a three‐gene concatenated data set (27 strains) suggests that the M. insignis lineage predates the available fossil record, having diverged from closest congeneric taxa in the Cretaceous (≥94 Ma). However, the molecular analysis also implies that considerable genetic variability is present within several morphotaxa of Mallomonas, implying that substantial genetic variability has arisen despite the retention of uniform scale morphologies, and resulting in the widespread occurrence of cryptic taxa. Results from the synurophyte lineage are consistent with the notion of protracted ghost ranges (>10 Ma) implied by the molecular phylogenies of other algal groups, together pointing to the paucity of the fossil record of these organisms on these timescales.     

SEPERATION OF MALLOMONAS DUERRSCHMIDTIAE SP. NOV. FROM M. CRASSISQUAMA AND M. PSEUDOCORONATA: IMPLICATIONS FOR PALEOLIMNOLOGICAL RESEARCH1

A new species, Mallomonas duerrschmidtiae, with characteristics common to both Mallomonas crassisquama (Asmund) Fott and Mallomonas pseudocoronata Prescott, is described from acidic lakes low in specific conductance and total phosphorus concentration. Characteristics of scales, bristles and spines serve to separate the three taxa. The length and area of scales of M. duerrschmidtiae are significantly larger than those of M. crassisquama but smaller than those of M. pseudocoronata. Although the anterior submarginal ribs of scales of M. duerrschmidtiae may become extended to form short wings, the scales lack the large forward projecting anterior wings characteristic of scales of M. pseudocoronata. Features of the dome and junction between the arms of the V-rib and anterior submarginal ribs also serve to distinguish between the three species. Cells of M. duerrschmidtiae also possess long, smooth and thick spines on their posterior scales and lack helmet bristles. M. duerrschmidtiae has different maxima along pH, temperature, specific conductance, total phosphorus and seasonal gradients than either M. crassisquama or M. pseudocoronata. Discriminant analysis, based on nine morphological characters, was used to successfully classify body scales of the three species. The importance of M. duerrschmidtiae as a bioindicator in future lake monitoring and paleolimnological inference studies is discussed.     

The taxonomy of problematic Mallomonas species (Synurophyceae). I. Mallomonas hindonii and M. canina

Whole cells with complete cell armour (siliceous scales and bristles) of Mallomonas hindonii and Mallomonas canina (Synurophyceae) have been found in Ontario. It is concluded that these two species are not synonymous, as has been suggested previously, and attention is given to structural features of scales and bristles which can be used to distinguish between them. Both species are restricted to softwater/acidic environments.     

STUDIES ON ULTRASTRUCTURE AND THREE‐GENE PHYLOGENY OF THE GENUS MALLOMONAS (SYNUROPHYCEAE)1

The genus Mallomonas, a common and often abundant member of the planktic community in many freshwater habitats worldwide, consists of 180 species divided into 19 sections and 23 series. Classification of species is based largely on ultrastructural characteristics of the siliceous scales and bristles that collectively form a highly organized covering over the cell. However, the relative importance of the different siliceous features of the scales, such as the dome, V rib, and secondary structures, as well as the different types of scales, in understanding the evolution and phylogeny of the genus is little known. In this study, we investigated the scale and bristle ultrastructure, along with sequences of three genes, for 19 isolates (18 species) of Mallomonas (18 isolates were from Korean habitats). The isolates represented nine of the 19 sections. Sequences for both the nuclear SSU and LSU rDNA and plastid LSU of RUBISCO (rbcL) genes for each of the 19 Mallomonas isolates and four outgroups were determined. Bayesian and maximum‐likelihood (ML) analyses of the data revealed that Mallomonas consists of two strongly supported clades. Mallomonas bangladeshica (E. Takah. et T. Hayak.) Siver et A. P. Wolfe was at the base of the first clade that included taxa from the sections Planae and Heterospinae, both of which lack a V rib on the shield of the scales. Our results indicated that the sections Planae and Heterospinae should be combined. The second clade, with Mallomonas insignis Penard and Mallomonas punctifera Korshikov at the base, contained taxa from the sections Mallomonas, Striatae, Akrokomae, Annulatae, Torquatae, Punctiferae, and Insignes, all of which have V ribs or well‐developed marginal ribs on the scales. Sister relationships between Mallomonas and Striatae were strongly supported, but interrelations among the remaining sections were not resolved, probably due to inclusion of too few species. Our results suggest that the current classification of the genus Mallomonas at the section level will require some revision. Additional species will need to be added in future analyses.     

SECRETION AND DEPLOYMENT OF BRISTLES IN MALLOMONAS SPLENDENS (SYNUROPHYCEAE)1

Mallomonas splendens (G. S. West) Playfair has a cell covering of siliceous scales and bristles. Interphase cells bear four anterior and four posterior bristles that each articulate, at their flexed basal ends via a complex of labile fibers (the fibrillar complex), on a specialized body scale (a base-plate scale). Body scales, base-plate scales and bristles are formed independently of each other and at different times in silica deposition vesicles (SDVs) that are associated with one of the two chloroplasts. The fine structure of scale and bristle morphogenesis in M. splendens agrees with that previously described for Synura and Mallomonas. Four new posterior bristles are formed at late interphase with their basal ends towards the cell posterior. The fibrillar complex is formed in situ on the bristle in the SDV. Mature bristles are secreted one by one onto the surface of the protoplast, beneath the layer of body scales, where the basal ends of the bristles adhere to the plasma membrane via the fibrillar complex. The extrusion of posterior bristles and their deployment onto the cell surface was monitored with video. A fine cellular protuberance accompanies the bristles as they are extruded from beneath the scale layer with their basal ends leading. When distant from the cell, the basal ends of the bristles appear attached to the protuberance, possibly by way of their fibrillar complexes. Once bristles are fully extruded, and their tips free in the surrounding environment, the bristle bases are drawn back to the posterior apex of the cell, apparently by the now shortening protuberance. Thus a 180° reorientation of the posterior bristles has been effected outside the cell. Thin-sections of cells that are extruding bristles show a threadlike, cytoplasmic extension of the cell posterior which may be analogous to the protuberance seen in live cells. Four new posterior base-plate scales are secreted after the bristles have reoriented. Scanning electron microscopy indicates that the fibrillar complex is involved in positioning the bristles onto their respective base-plate scales. Anterior bristles are formed in new daughter cells in the same orientation as the posterior bristles; thus they are extruded tip first and no reorientation is required.     

High molecular mass glycoproteins associated with the siliceous scales and bristles of Mallomonas splendens (Synurophyceae) may be involved in cell surface development and maintenance

The elaborate scale case of Mallomonas splendens (Synurophyceae) consists of an overlapping arrangement of siliceous scales. In addition, siliceous bristles are attached to specialized base plate scales located at both the anterior and posterior ends of the cells. We have generated monoclonal antibodies against molecules associated with the scale case of M. splendens. One of these antibodies, designated MsS.H9, labelled a proteinaceous epitope of high-molecular-mass cell surface glycoproteins. Immunofluorescence and immunoelectron microscopy demonstrated that only two regions of M. splendens scale cases were labelled by MsS.H9, namely, the upper surface of the scales that contact neighboring scales and the bases of the bristles. Immunoelectron microscopy using thin sections of M. splendens cells showed these labelling sites corresponded to the amorphous material at the sites of scale-to-scale overlap and to a fibrillar complex located at scale-to-bristle attachment sites. Scales and bristles of M. splendens are formed within the cell, in silica deposition vesicles. Immunolabelling of cell sections containing developing scales and bristles showed that MsS.H9 labelling sites were present very early in the formation of these cell surface components. MsS.H9 labelling was also found associated with developing flagellar hairs whereas no labelling was detected on these structures after their deployment onto the flagellum. The location of MsS.H9 labelling sites strongly suggests that the molecule(s) recognized by the antibody plays a role in the adhesion of the individual components making up the scale case of M. splendens.Abbreviations CER chloroplast endoplasmic reticulum - ER endoplasmic reticulum - SDV silica deposition vesicle This work was supported by a grant from the Australian Research Council to R.W. We thank Dr. P. L. Beech for Fig. 13, Dr. L. Perasso for technical assistance and the Plant Cell Biology Group for the use of their monoclonal facilities.     

Developments in the taxonomy of silica‐scaled chrysophytes – from morphological and ultrastructural to molecular approaches

Taxonomy in silica‐scaled chrysophytes has gone through three morphological phases. From primary studies of the cell morphology in the 18th century, the focus was in the 20th century replaced by studies of the silica structures of the cell envelope. Now, in the latest decades the importance of DNA sequencing has been recognized, not only to support the taxonomic framework but also to obtain new understanding of taxonomic relations among particular taxa. In the first part of this review, we provide a historical overview of the developments in the taxonomy of scale‐bearing chrysophytes. In the second part, we present a phylogenetic reconstruction of chrysophyte algae, updated by newly obtained SSU rDNA and rbcL sequences of several isolated Synura, Mallomonas and Chrysosphaerella species. We detected significant incongruence between the phylogenies obtained from the different datasets, with the SSU rDNA phylogram being the most congruent with the morphological data. Significant saturation of the first rbcL codon position could indicate the presence of positive selection in the rbcL dataset. Within the Synurales, the relationships revealed by the phylogenetic analyses highlight the artificial infragenetic classification of Mallomonas and Synura, and the occurrence of cryptic diversity within a number of traditionally defined species. Finally, three new combinations are proposed based on the phylogenetic analyses: Tessellaria lapponica, Synura asmundiae and S. bjoerkii.     

Size variations of planktonic Aulacoseira Thwaites (Diatomae) in water and in sediment from Finnish lakes of varying trophic state

We measured siliceous frustules of diatoms belonging to the genus Aulacoseira Thwaites in phytoplankton samples of 33 lakes in Finland, and found some significant relationships between the frustule dimensions and the concentrations of total phosphorous, phosphate phosphorus, total nitrogen and nitrate nitrogen in lake water. The size alterations appeared to be species specific and the studied taxa showed different patterns of morphometic change along the nutrient gradients. Some taxa were clearly smaller in eutrophic and mesotrophic lakes. We also observed size reductions of diatoms in the upper parts of sediment cores from two lakes which have apparently become more nutrient rich during recent decades.      

Marked recent increases of colonial scaled chrysophytes in boreal lakes: implications for the management of taste and odour events

1. Lake managers suspect that taste and odour‐causing algal blooms are increasing in frequency and intensity, although long‐term monitoring records are scarce, and a number of critical scientific and management questions remain unanswered. 2. In nutrient‐poor lakes and reservoirs, these events are caused primarily by sporadic outbreaks of some chrysophyte algae, which leave identifiable markers in lake sediments. We examine the siliceous remains of these organisms in more than fifty boreal lakes at broad temporal and spatial scales. 3. Colonial scaled chrysophytes, including the taste and odour‐causing Synura petersenii, have increased markedly in more that 90% of the lakes examined since pre‐industrial times. 4. Detailed stratigraphic analyses of two lakes show a rise in the abundance of colonial taxa in the 1930s to 1950s, with a sharp increase over the past two decades. 5. An examination of biogenic silica and biological ratios in Crosson Lake, Ontario, Canada, indicate that these changes represent true increases in the absolute abundance of colonial chrysophytes. 6. Rapid increases over the past two decades indicate that these trends are the result of one or more anthropogenic stressors that are operating at a broad, regional scale.     

Further observations on the scaled Chrysophycean and Synurophycean flora of the Ocala National Forest, Florida, U.S.A.

The distributions of silica-scaled chrysophytes in 30 freshwater ponds located in the Ocala National Forest, Florida, U.S.A., relative to chemical gradients are described. Phytoplankton, periphyton, and surface sediments from each of the 30 sites were collected in March of 2000 and later analyzed extensively with both scanning electron microscopy (SEM) and light microscopy (LM) for scaled chrysophytes. In addition, water samples were used to measure a suite of chemical characteristics, including specific conductance, pH, alkalinity, total phosphorus, total nitrogen, chlorophyll- a , chloride, sulfate and base cation concentrations. Overall, waterbodies included in this study are oligotrophic, dilute, poorly buffered and low in pH. Including six previously described species which are largely known only from the Ocala National Forest, we have identified forty-nine taxa of silica-scaled Chry-sophyceae and Synurophyceae, 23 of which were present in 5 or more waterbodies, indicating that lakes in this region are quite diverse and abundant in scaled chrysophytes. The most important species included Synura petersenii, S. echinulata and one recently reported new species from this region, Mallomonas wujekii , which was found in 73% of the lakes in this survey. The number of taxa found per lake ranged from 2 to 23 and observations include new records of several rarely reported species. Although the flora includes species commonly found in more northern regions, it also includes a group of taxa that appear to be endemic to the region and others commonly found in more tropical regions. Lastly, a new form found in ten ponds, Mallomonas transsylvanica f. curvata , is described.     

Molecular phylogeny and evolution of phenotype in silica‐scaled chrysophyte genus Mallomonas

The evolution of phenotypes is highly understudied in protists, due to the dearth of morphological characters, missing fossil record, and/or unresolved phylogeny in the majority of taxa. The chrysophyte genus Mallomonas (Stramenopiles) forms species‐specific silica scales with extraordinary diversity of their ornamentation. In this paper, we molecularly characterized three additional species to provide an updated phylogeny of 43 species, and combined this with evaluations of 24 morphological traits. Using phylogenetic comparative methods, we evaluated phylogenetic signal in traits, reconstructed the trait evolution, and compared the overall phylogenetic and morphological diversity. The majority of traits showed strong phylogenetic signal and mostly dynamic evolution. Phylogenetic relatedness was often reflected by the phenotypic similarity. Both V‐rib and dome are very conservative structures that are presumably involved in precise scale overlap and bristle attachment, respectively. Based on modern species, it seems the dome firstly appeared on apical and/or caudal scales, and only later emerged on body scales. Bristle was presumably present in the common ancestor and gradually elongated ever since. However, most other morphological traits readily changed during the evolution of Mallomonas.     

Taxonomic significance of asymmetrical helmet and lance bristles in the genus Mallomonas (Synurophyceae) and their discovery in Eocene lake sediments

Complex bristle types formed by species in the genus Mallomonas include those with helmet or lance-shaped apices. The ornamentation on each side of the helmet has been thought to be equivalent or symmetrical, whereas on a lance-shaped bristle an expanded portion folds over one side of the shaft to form an asymmetrical structure. We describe, for the first time, helmet bristles with a distinctly asymmetrical design, also formed by the folding of a siliceous membrane over one side of the helmet. We postulate that the asymmetrical helmet represents a structure that combines the formation of a symmetrical helmet and a lance-shaped design on the same bristle. Further, we report structurally similar asymmetrical helmet bristles, lance-shaped bristles and scales that are unambiguously assigned to Mallomonas asmundiae in Middle Eocene sediments from a maar lake in northern Canada, supporting the hypothesis that scale and bristle morphology in the Synurophyceae has undergone extensive prolonged evolutionary stasis. Given differences in scale morphology and the presence of asymmetrical helmet bristles, we transfer the North American endemic Mallomonas acaroides var. muskokana to the rank of species. Further, we formally describe Mallomonas dispar and M. lancea, fossil species with asymmetrical helmet bristles and lance-shaped bristles, respectively. The taxonomic and biogeographic significance of asymmetrical and lance-bearing bristles is discussed.     

Obervations on the occurrence of scales and bristles of mallomonas spp. (Chrysophyceae) in the micro-laminated sediments of a small lake in finnish North Karelia

Scales and bristles ofMallomonas crassisquama (Asmund) Fott andM. elongata Reverdin are reported from the laminated sediments of Laukunlampi, a small kettlehole lake in N. Karelia, Finland. The white summer laminae in the deeper sediment are composed almost entirely ofM. crassisquama scales and bristles. The taxonomy, ecology and distribution ofM. crassisquama andM. elongata are briefly discussed and factors influencing the preservation of scales and bristles in sediments are considered. It is suggested that analysis of sedimentary associations of scales, bristles and cysts could be used to improve cyst taxonomy inMallomonas.     

Scale Biogenesis in Synurophycean Protists: Phylogenetic Implications

The Synurophyceae is a class of golden-brown, freshwater, photosynthetic flagellates with a world-wide distribution. A well-developed taxonomy exists where genera and species are distinguished by colony or cell morphology or by the siliceous scales that cover the cells. However, phylogenetic relationships within the class are poorly understood, and incongruous taxonomic concepts occur. This study reviews scale morphology from field-collected samples and controlled culturing experiments as well as from studies of scale biogenesis. The information is used to identify homologous silicification surfaces among taxa and to document the diversity of the resulting scale structures. Thirty-two character states are coded into 11 characters in a cladistic analysis of 13 pivotal taxa. Colonial species are emphasized. One most-parsimonious phylogenetic tree is found (HI = 0, CI = 1). Synura lapponica is shown to be most closely related to Tessellaria volvocina. S. sphagnicola emerges at the base of the tree. Mallomonas caudata and the S. petersenii clade emerge from within Sectio Synura. Chrysodidymus synuroideus appears as an ancestral taxon in the Synura spinosa-like clade (i.e., Series Spinosae). The poorly understood developmental bases for some characters, especially secondary scale structures, are identified and may help focus future research.     

Multiple scale analysis of factors influencing the distribution of an invasive aquatic grass

With the expected increase in the spread of invasive species, examination of factors controlling distributions at multiple spatial scales and ecological modelling of their potential distributions are important analyses for informed decision-making. The scale-dependence of mechanisms influencing invasion by non-native species has been shown previously, indicating that studies of key factors affecting invasive species distributions at multiple spatial scales are critical for successful management. Freshwater systems are particularly vulnerable to invasive species, yet few studies have examined the environmental factors influencing distributions of invasive species at multiple spatial scales. We examined the effect of environmental variables on the predicted distribution of the invasive aquatic grass Glyceria maxima over continental, regional and local scales in Australia. We undertook an initial critical evaluation of which predictor variables were most appropriate to use at each scale, largely considering prior knowledge. On a continental scale, climatic, topographic and hydrological variables predicted well the potential distribution of G. maxima, identifying temperate regions as most susceptible to invasion. The regional analysis found that dense, woody, riparian vegetation has a strong negative impact on the occurrence of G. maxima, especially at intermediate elevations. The invasive grass was found less often on biotite granite and on fluvial geology. At a local scale, occurrence of G. maxima was related positively to soil phosphorus and nitrogen, and negatively related to soil organic carbon. The identification of key factors affecting invasive species distributions at multiple spatial scales will inform prevention schemes, assist targeted field sampling for the development of monitoring programs, and allow prioritization of control methods.     

Further observations on flagellates within sea ice in northern Bothnian Bay, the Baltic Sea

To improve our knowledge of flagellates inhabiting the Baltic Sea ice and water column during the winter, material was obtained from northern Bothnian Bay in March/April 1995. Light microscopical observations on live and fixed material and further transmission electron microscopy of whole mounts revealed 47 nanoflagellate taxa. In addition, detached scales of eight taxa were encountered. It is now evident that nearly all nanoflagellate classes are present within Bothnian Bay sea ice. The most common groups were cryptomonads, dinoflagellates, chrysophytes, prasinophytes, choanoflagellates and heterotrophic flagellates of unknown systematic position (Protista incertae sedis). Most flagellates in Baltic Sea ice biota apparently thrive in both the water column and the sea ice, while some, e.g. Paraphysomonas spp. (Chrysophyceae), heterotrophic euglenids, volvocalean chlorophytes, and some taxa of uncertain systematic affinity, are more frequently found within the sea ice. Received: 9 February 1997 / Accepted: 23 September 1997     

New insights into the complex architecture of siliceous copepod teeth

Copepods belong to the dominant marine zooplankton taxa and play an important role in particle and energy fluxes of the marine water column. Their mandibular gnathobases possess tooth-like structures, so-called teeth. In species feeding on large proportions of diatoms these teeth often contain silica, which is very probably the result of a coevolution with the siliceous diatom frustules. Detailed knowledge of the morphology and composition of the siliceous teeth is essential for understanding their functioning and their significance in the context of feeding interactions between copepods and diatoms. Based on analyses of the gnathobases of the Antarctic copepod Rhincalanus gigas, the present study clearly shows, for the first time, that the silica in the siliceous teeth features large proportions of crystalline silica that is consistent with the mineral α-cristobalite and is doped with aluminium. The siliceous structures have internal chitinous fibre networks, which are assumed to serve as scaffolds during the silicification process. The compact siliceous teeth of R. gigas are accompanied by structures with large proportions of the elastic protein resilin, likely reducing the mechanical damage of the teeth when the copepods feed on diatoms with very stable frustules. The results indicate that the coevolution with diatom frustules has resulted in gnathobases exhibiting highly sophisticated composite structures.