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.

High‐sulfur, low‐oxygen environments formed by underwater sinkholes and springs create unique habitats populated by microbial mat communities. To explore the diversity and biogeography of these mats, samples were collected from three sites in Alpena, Michigan, one site in Monroe, Michigan, and one site in Palm Coast, Florida. Our study investigated previously undescribed eukaryotic diversity in these habitats and further explored their bacterial communities. Mat samples and water parameters were collected from sulfur spring sites during the spring, summer, and fall of 2022. Cyanobacteria and diatoms were cultured from mat subsamples to create a culture‐based DNA reference library. Remaining mat samples were used for metabarcoding of the 16S and rbcL regions to explore bacterial and diatom diversity, respectively. Analyses of water chemistry, alpha diversity, and beta diversity articulated a range of high‐sulfur, low‐oxygen habitats, each with distinct microbial communities. Conductivity, pH, dissolved oxygen, temperature, sulfate, and chloride had significant influences on community composition but did not describe the differences between communities well. Chloride concentration had the strongest correlation with microbial community structure. Mantel tests revealed that biogeography contributed to differences between communities as well. Our results provide novel information on microbial mat composition and present evidence that both local conditions and biogeography influence these unique communities.

Diatoms are pivotal in global oxygen, carbon dioxide, and silica cycling, contributing significantly to photosynthesis and serving as fundamental components in aquatic ecosystems. Recent advancements in genomic sequencing have shed light on their evolutionary dynamics, revealing evolutionary complex genomes influenced by symbiotic relationships and horizontal gene transfer events. By analyzing publicly available sequences for 120 plastomes and 70 mitogenomes, this paper aims to elucidate the evolutionary dynamics of diatoms across diverse lineages. Gene losses and pseudogenes were more frequently observed in plastomes compared with mitogenomes. Overall, gene losses were particularly abundant in the plastomes of Astrosyne radiata, Toxarium undulatum, and Proboscia sp. Frequently lost and pseudogenized genes were acpP, ilv, serC, tsf, tyrC, ycf42 and bas1. In mitogenomes, mttB, secY and tatA genes were lost repeatedly across several diatom taxa. Analysis of nucleotide substitution rates indicated that, in general, mitogenomes were evolving at a more rapid rate compared to plastomes. This is contrary to what was observed in synteny analyses, where plastomes exhibited more structural rearrangements than mitogenomes, with the exception of the genus Coscinodiscus and one group of species within Thalassiosira.