Long-term changes in Noctiluca scintillans blooms along the Chinese coast from 1933 to 2020

Long-term changes in spatiotemporal distribution of Noctiluca scintillans in the southern North Sea

To assess the spatiotemporal evolution of the heterotrophic dinoflagellate Noctiluca scintillans in the North Sea, the Helgoland Roads time series and Continuous Plankton Recorder survey were analysed using generalized additive models. Over the last decades, blooms of N. scintillans have occurred more frequently and intensively in many regions. This harmful algal bloom forming species can alter food webs, reduce ecosystem productivity, and lead to economic losses while causing lower aquacultural yields. After the 1990s, N. scintillans abundances have significantly increased by 1.65-fold and a significant prolongation of the bloom window was found (from 27.5 to 98 days in recent decades) off the island of Helgoland, Germany. Significant correlations were found between bloom initiation and nutrients, as well as light availability since these factors lead to increased prey availability. Highest abundances of N. scintillans were associated with water temperatures around 17 °C and wind speed below 6 ms-1 causing dense surface accumulations. Solar radiation of more than 200 Wm-2 was identified as a main driver for post-bloom conditions as it can deteriorate the cells and lead to the decline of N. scintillans abundances. In the southern North Sea, N. scintillans occurrences have intensified and spread since the 1980s with hotspots identified as the coastal waters adjacent to the estuaries of the Elbe and Rhine rivers.

Physical drivers of Noctiluca scintillans (Dinophyceae) blooms outbreak in the northern Taiwan Strait: A numerical study

The red Noctiluca scintillans (RNS) blooms often break out near Pingtan Island, in the northern Taiwan Strait from April to June. It is essential to gain insights into their formation mechanism to predict and provide early warnings for these blooms. Previous studies and observations showed that RNS blooms are the most likely to occur when winds are weak and shifting in direction. To explore this phenomenon further, we employed a high-resolution coastal model to investigate the hydrodynamics influencing RNS blooms around Pingtan Island from April to June 2022. The model results revealed that seawater exhibited weak circulation but strong stratification during RNS blooms. Residence time were examined through numerical experiments by releasing passive neutrally buoyant particles in three bays of Pingtan Island. The results showed a significantly longer residence time during RNS blooms, indicating reduced flushing capabilities within the bays, which could give RNS a stable environment to multiply and aggregate. This hydrodynamic condition provided a favorable basis for RNS blooms breakout near Pingtan Island. The shifts and weakening of the prevailing northeast wind contributed substantially to weakening the flow field around Pingtan Island and played a crucial role in creating the hydrodynamics conducive to RNS blooms. Our study offers fresh insights into the mechanisms underpinning RNS blooms formation near Pingtan Island, providing a vital framework for forecasting RNS blooms in this region.

Noctiluca scintillans bloom alters the composition and carbohydrate utilization of associated bacterial community and enriches potential pathogenic bacterium Vibrio anguillarum

Noctiluca scintillans (red) is a widely distributed heterotrophic dinoflagellate and a prominent red tide forming species. This study investigated the effects of Noctiluca blooms on marine microbial diversity and functionality using multi-omics approaches. Our findings revealed significant differences in the community composition of Noctiluca-associated bacteria compared to those associated with autotrophic plankton and free-living bacteria in the surrounding seawater. The dominant bacterial groups within the Noctiluca-associated community shifted at various bloom stages, which could be attributed to changes in prey composition of Noctiluca. During the non-bloom stage, Burkholderiaceae, Carnobacteriaceae, and Pseudomonadaceae dominated the community, while Vibrionaceae became dominant during the bloom stage, and Saprospiraceae, Crocinitomicaceae, and Pirellulaceae thrived during the post-bloom stage. Compared to the non-bloom stage, Noctiluca-associated bacterial community at the bloom stage exhibited significant down-regulation of genes related to complex carbohydrate metabolism, while up-regulation of genes related to glucose transportation and utilization. Furthermore, we identified Vibrio anguillarum, a potential pathogenic bacterium to marine fish, as a major component of the Vibrionaceae family during the bloom stage. The occurrence of V. anguillarum associated with Noctiluca blooms may be attributed to the increased availability of its preferred carbon sources and its high capabilities in glucose transportation, motility and chemotaxis. Moreover, the presence of Vibrio infection genes (hap, hlyA, rtxA) encoding vibriolysin, hemolysin, and RTX (Repeats-in-toxin) toxin in the V. anguillarum genome, with the hap gene showing high expression levels during Noctiluca blooms, indicates an elevated risk of infection. This study underscores the unique composition of the bacterial community associated with red tide forming heterotrophic dinoflagellates and suggests that Noctiluca cells may serve as reservoirs and vectors for pathogenic bacteria, potentially posing a threat to fish-farming and the health of other marine organisms.

The high molecular diversity in Noctiluca scintillans is dominated by intra-genomic variations revealed by single cell high-throughput sequencing of 18S rDNA V4

The application of high-throughput sequencing (HTS) technologies has revolutionized research on phytoplankton biodiversity by generating an unprecedented amount of molecular data in marine ecosystem surveys. However, high-level of molecular diversity uncovered in HTS-based metabarcoding analyses may lead to overinterpretation of phytoplankton diversity due to excessive intra-genomic variations (IGVs). The aims in this study are to explore the nature of phytoplankton molecular diversity and to test the hypothesis. We carried out single-cell metabarcoding analysis of 18S rDNA V4 sequences obtained in single Noctiluca scintillans cells isolated from various sites in coastal waters of China. Results showed that each single N. scintillans cell harbored a high level of IGVs with about 100 amplicon sequence variants (ASVs). The large numbers of non-dominant ASVs identified in N. scintillans cells, which might correspond to the larger numbers of ASVs annotated as N. scintillans and showed similar temporal dynamics in metabarcoding analyses, could inflate the inter-species diversity or intra-species genetic diversity. In addition, there were large numbers of additional ASVs that were not annotated as N. scintillans. These non-N. scintillans ASVs might represent diverse preys for N. scintillans, consistent with previous reports that N. scintillans may act as chance predator of a broad-spectrum preys. This single-cell study has unambiguously demonstrated that the existence of high levels of IGVs in N. scintillans and most likely many other phytoplankton species, demonstrating that the majority of the molecular diversity revealed in metabarcoding analysis, which were generally interpreted as the sum of inter-species diversity and intra-species diversity, actually included high levels of IGVs and should be interpreted with caution.

In situ diets of the bloom-forming dinoflagellate Noctiluca scintillans in Daya Bay

Red Noctiluca scintillans is a common heterotrophic dinoflagellate that forms blooms in temperate, subtropical, and tropical coastal ecosystems. The diet of this species plays an important role in its cell growth, development, and reproduction. Because limited gene diversity data are available regarding prey of this species, its diet in Daya Bay during a boreal winter bloom is reported using an integrated approach involving light microscopy, single cell isolation and plastid 16S rDNA cloning, and 18S rDNA V4 and V9 region amplification using isolated cells and environmental DNA as templates with high-throughput sequencing. While conventional light microscopy reveals the diet of this species to comprise Coscinodiscus sp. and Stephanopyxis turris (diatoms), copepod eggs, and detritus, plastid gene diversity identifies a diet comprising diatoms, cyanobacteria, and bacteria, and 18S rDNA high-throughput sequencing reveals a diet comprising 36 eukaryote families (primarily copepods, as well as diatoms, dinoflagellates, Ochrophyta, Haptophytes, Chordata, Cercozoans, Chlorophyta, Polychaeta, and ciliates). Dietary staples include copepods, diatoms, dinoflagellates, Ochrophyta, and Synechococcus. High copepod abundance in prey may reflect their relatively high abundance in environmental seawater. Thus, N. scintillans is generally omnivorous but prefers dominant phytoplankton taxa, including Rhizosoleniaceae, Leptocylindraceae, and Cymatosiraceae (diatoms), as well as Gonyaulacaceae (dinoflagellates). An integrated multi-disciplinary approach provides a more comprehensive picture of N. scintillans diet in Daya Bay, and an improved understanding of this species' ecological niche and trophic role in marine ecosystems.