Phylogenetic Analysis Reveals that the 'Radial Centric' Diatom Orthoseira Thwaites (Orthoseiraceae, Bacillariophyta) is a Member of a 'Multipolar' Diatom Lineage

Unraveling centric diatoms from the Caatinga: Coscinodiscophyceae and Mediophyceae in northwestern Ceará, Brazil

Abstract The Caatinga is the largest tropical dry forest region in South America and harbors an impressive biological diversity. However, efforts in the studies of many groups of organisms, especially aquatic ones, are very small compared to other Brazilian regions. The state of Ceará stands out due to the major concentration of surface water in the semi-arid region, and the diatom flora is virtually unknown. We performed a pioneering and extensive taxonomic study on the centric diatoms from the Caatinga, documenting the morphological variation of the species found, expanding their geographic distribution, and reviewing taxonomic and nomenclatural aspects when relevant. The study was based on planktonic and periphytic samples collected in four hydrographic basins located in the northwestern region of Ceará, northeastern Brazil. Fifteen infrageneric diatom taxa were identified, described, and illustrated using light and scanning electron microscopy. Discostella stelligera was first recorded for northeastern Brazil and D. woltereckii is a new record to the country. Diminutive centric diatoms are well represented in the samples and their taxonomy is discussed. Terpsinoë musica occurred in a population with high phenotypic plasticity and we discussed the related taxonomic implications.

Extensive chloroplast genome rearrangement amongst three closely related Halamphora spp. (Bacillariophyceae), and evidence for rapid evolution as compared to land plants

Diatoms are the most diverse lineage of algae, but the diversity of their chloroplast genomes, particularly within a genus, has not been well documented. Herein, we present three chloroplast genomes from the genus Halamphora (H. americana, H. calidilacuna, and H. coffeaeformis), the first pennate diatom genus to be represented by more than one species. Halamphora chloroplast genomes ranged in size from ~120 to 150 kb, representing a 24% size difference within the genus. Differences in genome size were due to changes in the length of the inverted repeat region, length of intergenic regions, and the variable presence of ORFs that appear to encode as-yet-undescribed proteins. All three species shared a set of 161 core features but differed in the presence of two genes, serC and tyrC of foreign and unknown origin, respectively. A comparison of these data to three previously published chloroplast genomes in the non-pennate genus Cyclotella (Thalassiosirales) revealed that Halamphora has undergone extensive chloroplast genome rearrangement compared to other genera, as well as containing variation within the genus. Finally, a comparison of Halamphora chloroplast genomes to those of land plants indicates diatom chloroplast genomes within this genus may be evolving at least ~4–7 times faster than those of land plants. Studies such as these provide deeper insights into diatom chloroplast evolution and important genetic resources for future analyses.