Rich diversity and active spatial-temporal dynamics of Thalassiosira species revealed by time-series metabarcoding analysis.

Thalassiosira is a species-rich genus in Bacillariophyta that not only contributes positively as primary producer, but also poses negative impacts on ecosystems by causing harmful algal blooms. Although taxonomical studies have identified a large number of Thalassiosira species, however, the composition of Thalassiosira species and their geographical distribution in marine ecosystems were not well understood due primarily to the lack of resolution of morphology-based approaches used previously in ecological expeditions. In this study, we systematically analyzed the composition and spatial-temporal dynamic distributions of Thalassiosira in the model marine ecosystem Jiaozhou Bay by applying metabarcoding analysis. Through analyzing samples collected monthly from 12 sampling sites, 14 Thalassiosira species were identified, including five species that were not previously reported in Jiaozhou Bay, demonstrating the resolution and effectiveness of metabarcoding analysis in ecological research. Many Thalassiosira species showed prominent temporal preferences in Jiaozhou Bay, with some displaying spring-winter preference represented by Thalassiosira tenera, while others displaying summer-autumn preference represented by Thalassiosira lundiana and Thalassiosira minuscula, indicating that the temperature is an important driving factor in the temporal dynamics. The application of metabarcoding analysis, equipped with appropriate molecular markers with high resolution and high specificity and databases of reference molecular marker sequences for potential all Thalassiosira species, will revolutionize ecological research of Thalassiosira species in Jiaozhou Bay and other marine ecosystems.

Composition and spatial-temporal dynamics of phytoplankton community shaped by environmental selection and interactions in the Jiaozhou Bay.

The Jiaozhou Bay as a model marine ecosystem in China has been intensively investigated over the last 90 years. However, detailed phytoplankton community composition, spatial-temporal dynamics, and its assembly mechanism were still unclear. To address these, we systematically examined the composition and spatial-temporal dynamics of phytoplankton in the Jiaozhou Bay through high-throughput sequencing of 18S rDNA V4. Analysis of 468 samples from 12 sampling sites over one full year revealed much higher phytoplankton diversity than previous reports, and strong seasonal succession patterns. Some phytoplankton also showed spatial variations, although the phytoplankton community didn't show significant distance-decay pattern. Environmental factors (especially temperature), species-species interrelationships and unique resting stages were uncovered to be the main contributors instead of stochastic process in shaping the phytoplankton community assembly. The overwhelming positive correlations between phytoplankton and other protists suggested that coevolution might be critical in this marine ecosystem. Complementary distributions of different amplicon sequence variants (ASVs) of same genera, such as Skeletonema marinoi (ASV_2) and Skeletonema tropicum (ASV_263) of the genus Skeletonema, suggested that phytoplankton have evolved differentially to exploit a wide range of ecological niches. This study laid a solid foundation for asertaining phytoplankton composition and spatial-temporal dynamics in temperate seawaters and mechanisms underlying phytoplankton community assembly, allowing in-depth studies of marine ecology.

Comparative Analysis of Chloroplast Genomes of Seven Chaetoceros Species Revealed Variation Hotspots and Speciation Time.

Chaetoceros is a species-rich diatom genus with broad distribution and plays an important role in global carbon cycle and aquatic ecosystems. However, genomic information of Chaetoceros species is limited, hindering advanced researches on Chaetoceros biodiversity and their differential impact on ecology. In this study, we constructed full-length chloroplast genomes (cpDNAs) for seven Chaetoceros species, including C. costatus, C. curvisetus, C. laevisporus, C. muelleri, C. pseudo-curvisetus, C. socialis, and C. tenuissimus. All of these cpDNAs displayed a typical quadripartite structure with conserved genome arrangement and specific divergence. The sizes of these cpDNAs were similar, ranging from 116,421 to 119,034 bp in size, and these cpDNAs also displayed similar GC content, ranging from 30.26 to 32.10%. Despite extensive synteny conservation, discrete regions showed high variations. Divergence time estimation revealed that the common ancestor of Chaetoceros species, which formed a monophyletic clade at approximately 58 million years ago (Mya), split from Acanthoceras zachariasii at about 70 Mya. The availability of cpDNAs of multiple Chaetoceros species provided valuable reference sequences for studying evolutionary relationship among Chaetoceros species, as well as between Chaetoceros species and other diatom species.

Complete mitochondrial genome of the harmful algal bloom species Pseudo-nitzschia delicatissima (Bacillariophyceae, Bacillariophyta).

Pseudo-nitzschia is an important genus of diatoms with many species capable of inducing harmful algae blooms (HABs) in coastal and oceanic waters, some of which produce the toxin domoic acid (DA), a neurotoxin that causes amnesic shellfish poisoning (ASP). Pseudo-nitzschia delicatissima is a cosmopolitan species that can induce HABs and produce DA. Nevertheless, mitochondrial genome of P. delicatissima has not been revealed. In this study, we determined the complete mitochondrial genome of P. delicatissima for the first time. The circular mitochondrial genome was 42,182 bp in length with GC content of 30.37%. It consisted of 65 genes including 39 protein-coding genes (PCGs), 24 tRNA genes, and two rRNA genes. This mitogenome has a group II intron, located in the cytochrome c oxidase subunit genes (cox1), with orf790 identified inside the intron region. Phylogenetic analysis revealed that P. delicatissima was clustered well with P. multiseries. This analysis is valuable for studying the evolutionary relationships among Pseudo-nitzschia species, and for comparative analysis of P. delicatissima strains.

Mitochondrial genome and phylogenomic analysis of Pseudo-nitzschia micropora (Bacillariophyceae, Bacillariophyta).

The number of species in the genus Pseudo-nitzschia has increased to 56, including 26 species known to produce domoic acid (DA), which is harmful to marine animals and human health. The lack of genomic sequences of Pseudo-nitzschia species has been a limiting factor in the studies of genetic and evolutionary relationships of Pseudo-nitzschia species. Here, the complete mitochondrial genome sequence of Pseudo-nitzschia micropora was determined for the first time, which was 38,792 bp in length with the overall AT content being 69.98%. The mitochondrial genome encoded 62 genes, including 36 protein-coding genes (PCGs, including orf157), 24 transfer RNA (tRNA) genes and two ribosomal RNA (rRNA) genes. Phylogenetic tree analysis suggests that the P. micropora had a closer relationship with P. cuspidate than that with P. multiseries. The availability of the complete mitochondrial genome of P. micropora would be useful for researching the evolutionary relationships of Pseudo-nitzschia species.

Complete chloroplast genome of Eucampia zodiacus (Mediophyceae, Bacillariophyta).

The cosmopolitan phytoplankton species Eucampia zodiacus Ehrenberg 1839 is a common harmful algal bloom (HAB) species with significant negative ecological impact. However, molecular information for this HAB species is limited. In this study, the complete chloroplast genome (cpDNA) of E. zodiacus was constructed for the first time. The circular genome was 118,107 bp in length, containing a pair of inverted repeats (IR) (6991 bp each). The overall GC content of E. zodiacus cpDNA was 31.64%. It encoded 169 genes, including 131 protein-coding genes (PCGs), 30 tRNA genes, one tmRNA gene, one ncRNA gene, and six rRNA genes in IR regions. Phylogenetic analysis using concatenated PCGs of 56 diatom cpDNAs strongly supported that E. zodiacus was closely related to Cerataulina daemon, which belongs to the same family Hemiaulaceae according to AlgaeBase. Syntenic analysis revealed nearly identical gene order between the two cpDNAs, except for an inversion in the small single-copy (SSC) region.

Reconstruction of the evolutionary biogeography reveal the origins and diversification of oysters (Bivalvia: Ostreidae).

Oysters (Bivalvia: Ostreidae Rafinesque, 1815) live in the intertidal and shallow subtidal areas worldwide. Despite their long evolutionary histories, abundant fossil records, global distribution, and ecological significance, a systematic time-dependent biogeographical analysis of this family is still lacking. Using combined mitochondrial (COI and 16S rRNA) and nuclear (18S rRNA, 28S rRNA, H3 and ITS2) gene makers for 80% (70/88) of the recognized extant Ostreidae, we reconstructed the global phylogenetic and biogeographical relationships throughout the evolutionary history of oysters. The result provided a holistic view of the origin, migration and dispersal patterns of Ostreidae. The phylogenetic results and fossil evidence indicated that Ostreidae originated from the circum-Arctic region in the Early Jurassic. The widening of the Atlantic Ocean and changes in the Tethys Ocean further facilitated their subsequent diversification during the Cretaceous and the Palaeogene periods. In particular, Crassostrea and Saccostrea exhibited relatively low dispersal abilities and their major diversifications were consistent with the tectonic events. Environmental adaptations and reproductive patterns, therefore, should play key roles in the formation of oyster distribution patterners, rather than the dispersal ability of their planktonic larvae. The diversity dynamics inferred by standard phylogenetic are consistent with the fossil record, however, further systematic classification, especially for fossil genus Ostrea, would enhance our understanding on extant and fossil oysters. The present study of the historical biogeography of oysters provides new insights into the evolution and speciation of oysters. Our findings also provide a foundation for the assessment of evolutionary patterns and ecological processes in intertidal and inshore life.

Definition of a High-Resolution Molecular Marker for Tracking the Genetic Diversity of the Harmful Algal Species Eucampia zodiacus Through Comparative Analysis of Mitochondrial Genomes.

The cosmopolitan phytoplankton species Eucampia zodiacus is a common harmful algal bloom (HAB) species that have been found to cause HABs in essentially all coastal regions except the Polar regions. However, molecular information for this HAB species is limited with only a few molecular markers. In this project, we constructed the mitochondrial genome (mtDNA) of E. zodiacus, which was also the first mtDNA constructed for any species in the order Hemiaulales that includes 145 reported species (including two additional HAB species Cerataulina bicornis and Cerataulina pelagica). Comparative analysis of eight E. zodiacus strains revealed that they could not be distinguished using common molecular markers, suggesting that common molecular markers do not have adequate resolution for distinguishing E. zodiacus strains. However, these E. zodiacus strains could be distinguished using whole mtDNAs, suggesting the presence of different genotypes due to evolutionary divergence. Through comparative analysis of the mtDNAs of multiple E. zodiacus strains, we identified a new molecular marker ezmt1 that could adequately distinguish different E. zodiacus strains isolated in various coastal regions in China. This molecular marker ezmt1, which was ∼400 bp in size, could be applied to identify causative genotypes during E. zodiacus HABs through tracking the dynamic changes of genetic diversity of E. zodiacus in HABs.

Metabarcoding analysis of harmful algal species in Jiaozhou Bay.

Accurate detection of the composition and dynamics of harmful algal bloom (HAB) species is critical for studying the mechanisms of HAB formation and for developing means for predicting the occurrences of HABs. Jiaozhou Bay is an epitome of China's coastal ecosystem and an ideal site for HAB research with the accumulation of decades of historical investigation records. Nevertheless, most of these earlier studies on phytoplankton communities applied primarily morphology-based approaches with limited resolution in phytoplankton species identification, especially for those with small-sized cells and for cryptic species. Through analyzing samples collected at 12 spatially isolated locations using metabarcoding methods, 89 phytoplankton species, including 34 Bacillariophyta, 25 Dinoflagellata, 7 Cryptophyta, 11 Chlorophyta, 8 Ochrophyta and 2 Haptophyta species were detected. Of those, 70 species had never been reported in Jiaozhou Bay in the previous expedition investigations, demonstrating the strength of the metabarcoding analysis approach. The distribution of many algal species demonstrated unique patterns, which were likely influenced by interactions among phytoplankton species or by predation by groups such as Ciliophora and Cercozoa, in addition to environmental factors such as temperature and nutritional conditions. Among these algal species, 28 were annotated as HAB species, among which 13 were reported for the very first time in Jiaozhou Bay including a mixtotrophic dianoflagellate Heterocapsa rotundata and a chain-forming diatom Skeletonema marinoi, both ranked among the top 10 most abundant ASVs. The present study represents a first attempt to study HAB species and other phytoplankton species in Jiaozhou Bay using the metabarcoding approach, which revealed substantially more algal species in Jiaozhou Bay than previously identified and sets a solid foundation for further research on the mechanisms of HAB formation.