Auxosporulation in Paralia guyana MacGillivary (Bacillariophyta) and Possible New Insights into the Habit of the Earliest Diatoms

A high-quality genome of the early diverging tychoplanktonic diatom Paralia guyana

The diatom Paralia guyana is a tychoplanktonic microalgal species that represents one of the early diverging diatoms. P. guyana can thrive in both planktonic and benthic habitats, making a significant contribution to the occurrence of red tide events. Although a dozen diatom genomes have been sequenced, the identity of the early diverging diatoms remains elusive. The understanding of the evolutionary clades and mechanisms of ecological adaptation in P. guyana is limited by the absence of a high-quality genome assembly. In this study, the first high-quality genome assembly for the early diverging diatom P. guyana was established using PacBio single molecular sequencing. The assembled genome has a size of 558.85 Mb, making it the largest diatom genome on record, with a contig N50 size of 26.06 Mb. A total of 27,121 protein-coding genes were predicted in the P. guyana genome, of which 22,904 predicted genes (84.45%) were functionally annotated. This data and analysis provide innovative genomic resources for tychoplanktonic microalgal species and shed light on the evolutionary origins of diatoms.

Identifying chemolithotrophic and pathogenic-related gene expression within suspended sediment flocs in freshwater environments: A metatranscriptomic assessment

The introduction and proliferation of pathogenic organisms in aquatic systems is a serious global issue that consequently leads to economic, financial, and health concerns. Health and safety related to recreational water use is typically monitored through water quality assessments that are outdated and can be misleading. These traditional methods focus on broad taxa groups, provide no insight into the active community or source of contamination, and the sediment compartments (bed and suspended) are often overlooked. To bridge this knowledge gap, our study aimed to 1) examine the metatranscriptome of the microbial community associated with suspended sediment (SS) in freshwater systems; 2) explore the influence of SS in tributaries to the littoral zone of the receiving lake; and 3) compare the SS fraction with previously reported nearshore bed sediment data. Samples were collected seasonally from Lake St. Clair and Lake Erie. Beaches in this region are influenced by both agriculture runoff and continued urban expansion. Results show that both adjacent tributary and beach SS have similar microbial functional diversity and are strongly correlated by site and season. We identified expression of transcripts encoding sequences with similarities to genes involved in nine bacterial infectious disease pathways, including legionellosis (sdhA) and Vibrio cholerae pathogenesis. According to MG-RAST gene categories, lake samples typically showed higher overall expression (p < 0.05) of transcripts with similarities to genes involved in infectious disease pathways compared to the tributaries, with summer upregulated (p < 0.05) compared to fall. Our data suggests SS acts as a strong vector for pathogen transport, making this facet an important area for further research as it pertains to human health regarding recreational water use. To our knowledge, this work is the first to investigate SS in aquatic microbial communities using metatranscriptomic analyses and has significant potential to help address growing issues of microbial contamination impacting freshwater security.

Structure and Development of the Auxospore in Ardissonea crystallina (C. Agardh) Grunow Demonstrates Another Way for a Centric to Look Like a Pennate

Reproductive development in Ardissonea crystallina revealed a unique mode of enlargement involving a combination of novel and known structures. In light microscopy, auxospores of this elongated polar centric diatom were superficially similar to the auxospores of pennates. With SEM we found three different components in the auxospore wall. In the youngest, nearly spherical cell-stage, the wall consisted only of a delicate veil containing minute siliceous spherules. Incunabular elements developed underneath this layer. Second, a previously unknown form of specifically modified incunabular scales shaped the subsequent ellipsoidal-capsule auxospore stage. Third, there was a clear contribution of scales to the development of scaly transverse perizonial bands (or scaly bands, for brevity). Such bands, although noted by previous researchers, have not been fully appreciated for the evolutionary information they may convey: possibly common among polar centrics but not pennates. Finally, we propose maintaining the term transverse perizonium to refer to these bands in polar diatoms, but to introduce the differentiation of scaly bands described here from pinnate bands (currently known as typical of pennates). Further research into band types among polar centrics may provide new insights into the relationship between the groups within polar centrics that are currently unresolved by molecular methods.

Ardissonea crystallina has a type of sexual reproduction that is unusual for centric diatoms

Molecular phylogenetic analyses place Ardissonea crystallina (C. Agardh) Grunow and all Toxariids among the bi- and multipolar centric diatoms, almost always recovered as a derived lineage sister to Lampriscus. In all centrics where sexual reproduction has been documented, oogamy, with larger immobile eggs and smaller flagellated sperm has been observed. We were able to initiate both homothallic and heterothallic reproduction in A. crystallina. The heterothallic reproduction turned out to be non-oogamous; gametes were more or less equal in size but no flagellated cells were detected. At the same time, two mating types (“male” and “female”) were recognized by the distinct morphology and behaviour of the gametes. While no flagella were observed, periodically thin cytoplasmic projections arose on the surface of the “male” gametes. These projections similar to those found in some pennate diatoms facilitated contact with the “female” cells. In each gametangial cell, regardless of the mating type, only one gamete was formed. Thus, the Toxariids may represent a unique evolutionary group, at least in respect to their reproductive biology. The hypothesis discussed is that non-oogamous mode of reproduction could have evolved in Ardissonea (and possibly in other Toxariids) independently of the pennate lineage of diatoms.

Gametogenesis and Auxospore Development in the Bipolar Centric Diatom Brockmanniella brockmannii (Family Cymatosiraceae)

We examined gametogenesis and auxospore development in the cymatosiroid diatom Brockmanniella brockmannii. Our mating experiments demonstrated that the clones were homothallic, self-compatible, produced two eggs per oogonium and four hologenous uniflagellate male gametes per spermatogonium. Auxospores grew free in the culture medium and the most external layer of the auxospore wall was made up of spinescent scaly elements in their early globular stage of development. The anisodiametric expansion of the globular auxospore was rather limited, at best proceeding only to a sub-globular stage. Our data suggest that the initial epivalves, nearly pennate-like in outline, developed within such auxospores curled up against the ventral side of the auxospore wall, not across its equatorial plane, and apparently without the help of rings, hoops or bands characteristic of pennate-type transverse perizonia. Thus, B. brockmannii showed a mode of initial epivalve development thus far unknown among diatoms. We propose that progressive silicification of the initial epivalve facilitated the breakdown of the sub-globular auxospore dorsal wall to accommodate for its straightening. The structural features of the spinescent scales on the auxospore wall suggest that Cymatosiraceae could be related to the mediophycean family Eupodiscaceae, although most current molecular phylogenies do not support a direct relationship.

Sexual reproduction in plagiogrammacean diatoms: First insights into the early pennates

The genera Plagiogramma and Dimeregramma are members of a small, but evolutionarily important group of diatoms, the "basal" araphids. They are sister to all other pennates, both araphid and raphid taxa. Thus, their phylogenetic position carries the potential for providing insights into the earliest pennates. We documented sexual reproduction, mating system and sex cell development in the first members of the "basal" araphid clade ever investigated. The mating system in all these species involved heterothally. It was, however, more complex in P. tsawwassen, where in addition to heterothallic clones, intraclonal and polysexual clones also exist. Auxospore development and wall structure was similar in all three species and demonstrated several characters also reported from "core" araphids. Of these, vigorous, pseudopodial motility of male secondary spermatocytes and gametes was most notable because it indicates that this character was likely present in the last common ancestor of all the pennates. Pseudopodial motility of the male sex cells might have afforded sufficient compensation and/or benefits to the emerging pennates for replacing flagellated sperm, present in centrics. The characters thus far uniquely present among our plagiogrammaceans but not reported from other pennates were: the "gametic" fusion between sex-compatible secondary spermatocytes, in some cases before completion of Meiosis II in males, transverse perizonial bands produced all together or in quick succession rather than being added to the auxospore apex one at a time, and expanding auxospores with 3-4 nuclei. An initial epivalve, similar in morphology to what in some diatoms had been interpreted as a "longitudinal" perizonium, may be more widespread among pennates than thus far appreciated. In addition, we discovered two species new to science (D. acutumontgo, P. tsawwassen), and refined delineation of P. staurophorum by including metric data from the original material.

Diatom life cycles and ecology in the Cretaceous

The earliest known diatom fossils with well-preserved siliceous frustules are from Lower Cretaceous neritic marine deposits in Antarctica. In this study, we analyzed the cell wall structure to establish whether their cell and life cycles were similar to modern forms. At least two filamentous species (Basilicostephanus ornatus and Archepyrgus melosiroides) had girdle band structures that functioned during cell division in a similar way to present day Aulacoseira species. Also, size analyses of cell diameter indicated that the cyclic process of size decline and size restoration used to time modern diatom life cycles was present in five species from the Lower Cretaceous (B. ornatus, A. melosiroides, Gladius antiquus, Ancylopyrgus reticulatus, Kreagra forfex) as well as two species from Upper Cretaceous deposits (Trinacria anissimowii and Eunotogramma fueloepi) from the Southwest Pacific. The results indicate that the "Diatom Sex Clock" was present from an early evolutionary stage. Other ecological adaptations included changes in mantle height and coiling. Overall, the results suggest that at least some of the species in these early assemblages are on a direct ancestral line to modern forms.