Exploration of resting cysts (stages) and their relevance for possibly HABs-causing species in China

First insights into the distribution and diversity of toxic dinoflagellate cysts in the surface sediments of Dakhla Bay (African Atlantic coast): relationships with environmental factors and mollusk intoxication events

Dakhla Bay, situated on the African Atlantic coast, has witnessed sporadic harmful algal blooms (HABs) caused by toxic dinoflagellate species over the past two decades. In this study, we investigated the distribution, abundance, and diversity of dinoflagellate cysts, with a focus on potentially toxic species that develop in this ecosystem where such data are lacking. Sediment samples were collected in April 2018 through coring at 49 stations distributed across the bay. The highest abundance of dinoflagellate cysts was recorded at 304 cysts/g dry sediment, observed at the inner part of the bay, indicating that this area is the preferential zone for cyst accumulation. Pearson's tests revealed significant positive correlations (P < 0.05) between cyst abundance and the water content, organic matter, and fine fraction (< 63 μm) of the sediment. Cyst morphotypes of potentially toxic dinoflagellate species known to produce saxitoxins, such as Alexandrium minutum, Alexandrium tamarense species complex, Gymnodinium catenatum, and yessotoxins, such as Lingulodinium polyedrum and Gonyaulax cf. spinifera, were identified in the sediment of Dakhla Bay. These findings were further supported by our long-term monitoring period (2005-2018), underscoring the presence of these HAB species in Dakhla Bay. During our survey, sporadic mollusk intoxication events were recorded at station PK25 for the grooved razor shell Solen marginatus and at station Boutelha for the oyster Crassostrea gigas. Paralytic shellfish toxin concentrations exceeded the sanitary threshold (80 μg STX di-HCl eq/100 g of shellfish meat) only twice, in December 2006 and January 2007 at station PK25. Contamination by amnesic shellfish toxins occurred sporadically but never reached the sanitary threshold of 20 µg/g of shellfish meat. Lipophilic shellfish intoxication occurred multiple times in the two investigated areas. These observations suggest that the cysts of the identified HAB species germinated and inoculated the water column, resulting in the observed intoxication events. Relatively low levels of intoxication could be attributed to the moderate abundances of cysts, which may reduce the seeding capacity. This could be explained by the significant interaction of Dakhla Bay with the Atlantic Ocean, characterized by hydrological dynamics that impede the deposition and accumulation of cysts in the bay's sediments. This study reaffirms the importance of investigating dinoflagellate cysts in assessing the diversity of HAB species and evaluating associated sanitary risks.

Evidence for the production of asexual resting cysts in a free-living species of Symbiodiniaceae (Dinophyceae)

Coral reef ecosystems are the most productive and biodiverse marine ecosystems, with their productivity levels highly dependent on the symbiotic dinoflagellates belonging to the family Symbiodiniaceae. As a unique life history strategy, resting cyst production is of great significance in the ecology of many dinoflagellate species, those HABs-causing species in particular, however, there has been no confirmative evidence for the resting cyst production in any species of the family Symbiodiniaceae. Based on morphological and life history observations of cultures in the laboratory and morpho-molecular detections of cysts from the marine sediments via fluorescence in situ hybridization (FISH), cyst photography, and subsequent singe-cyst PCR sequencing, here we provide evidences for the asexual production of resting cysts by Effrenium voratum, the free-living, red tide-forming, and the type species of the genus Effrenium in Symbiodiniaceae. The evidences from the marine sediments were obtained through a sequential detections: Firstly, E. voratum amplicon sequence variants (ASVs) were detected in the cyst assemblages that were concentrated with the sodium polytungstate (SPT) method from the sediments collected from different regions of China Seas by high-throughput next generation sequencing (NGS); Secondly, the presence of E. voratum in the sediments was detected by PCR using the species-specific primers for the DNA directly extracted from sediment; Thirdly, E. voratum cysts were confirmed by a combined approach of FISH using the species-specific probes, light microscopic (LM) photography of the FISH-positive cysts, and a subsequent single-cyst PCR sequencing for the FISH-positive and photographed cysts. The evidences from the laboratory-reared clonal cultures of E. voratum include that: 1) numerous cysts formed in the two clonal cultures and exhibited a spherical shape, a smooth surface, absence of ornaments, and a large red accumulation body; 2) cysts could maintain morphologically intact for a storage of two weeks to six months at 4 °C in darkness and of which 76-92 % successfully germinated through an internal development processes within a time period of 3-21 days after being transferred back to the normal culturing conditions; 3) two or four germlings were released from each cyst through the cryptopylic archeopyle in all cysts with continuous observations of germination processes; and 4) while neither sexual mating of gametes nor planozygote (cells with two longitudinal flagella) were observed, the haploidy of cysts was proven with flow cytometric measurements and direct LM measurements of fluorescence from cells stained with either propidium iodide (PI) or DAPI, which together suggest that the cysts were formed asexually. All evidences led to a conclusion that E. voratum is capable of producing asexual resting cysts, although its sexuality cannot be completely excluded, which guarantees a more intensive investigation. This work fills a gap in the knowledge about the life cycle, particularly the potential of resting cyst formation, of the species in Symbiodiniaceae, a group of dinoflagellates having unique life forms and vital significance in the ecology of coral reefs, and may provide novel insights into understanding the recovery mechanisms of coral reefs destructed by the global climate change and suggest various forms of resting cysts in the cyst assemblages of dinoflagellates observed in the field sediments, including HABs-causing species.

The resting cyst of dinoflagellate Scrippsiella acuminata host bacterial microbiomes with more diverse trophic strategies under conditions typically observed in marine sediments

Variation in the condition of marine sediments provides selective preservation milieus, which act as a key determinant for the abundance and distribution of dinoflagellate resting cysts in natural sediments. Microbial degradation is an understudied biological factor of potential importance in the processes. However, gaps remain in our knowledge about the fundamental information of the bacterial consortia associated with dinoflagellate resting cysts both in laboratory cultures and in the field. Here we used Scrippsiella acuminata as a representative of cyst-producing dinoflagellates to delineate the diversity and composition of bacterial microbiomes co-existing with the laboratory-cultured resting cysts, and to explore possible impacts of low temperature, darkness, and anoxia (the mock conditions commonly observed in marine sediments) on the associated bacterial consortia. Bacterial microbiome with high diversity were revealed associated with S. acuminata at resting stage. The mock conditions could significantly shift bacterial community structure and exert notably inhibitory effects on growth-promoting bacteria. Resting cysts under conditions typically observed in marine sediments fostered bacterial microbiomes with more diverse trophic strategies, characteristic of prominently enriched anaerobic chemotrophic bacteria generating energy via respiration with several different terminal electron acceptors, which yielded more acidic milieu unfavorable for the preservation of calcareous resting cysts. Our findings suggest that there is complex and dynamic interaction between dinoflagellates resting cysts and the associated bacterial consortia in natural sediments. This intrinsic interaction may influence the maintenance and/or accumulation of dinoflagellate resting cysts with potential of germination and initiation blooms in the field.

Transoceanic ships as a source of alien dinoflagellate invasions of inland freshwater ecosystems

Ships' ballast water and sediments have long been linked to the global transport and expansion of invasive species and thus have become a hot research topic and administrative challenge in the past decades. The relevant concerns, however, have been mainly about the ocean-to-ocean invasion and sampling practices have been almost exclusively conducted onboard. We examined and compared the dinoflagellate cysts assemblages in 49 sediment samples collected from ballast tanks of international and domestic routes ships, washing basins associated with a ship-repair yard, Jiangyin Port (PS), and the nearby area of Yangtze River (YR) during 2017-2018. A total of 43 dinoflagellates were fully identified to species level by metabarcoding, single-cyst PCR-based sequencing, cyst germination and phylogenetic analyses, including 12 species never reported from waters of China, 14 HABs-causing, 9 toxic, and 10 not strictly marine species. Our metabarcoding and single-cyst sequencing also detected many OTUs and cysts of dinoflagellates that could not be fully identified, indicating ballast tank sediments being a risky repository of currently unrecognizable invasive species. Particularly important, 10 brackish and fresh water species of dinoflagellate cysts (such as Tyrannodinium edax) were detected from the transoceanic ships, indicating these species may function as alien species potentially invading the inland rivers and adjacent lakes if these ships conduct deballast and other practices in fresh waterbodies. Significantly higher numbers of reads and OTUs of dinoflagellates in the ballast tanks and washing basins than that in PS and YR indicate a risk of releasing cysts by ships and the associated ship-repair yards to the surrounding waters. Phylogenetic analyses revealed high intra-species genetic diversity for multiple cyst species from different ballast tanks. Our work provides novel insights into the risk of bio-invasion to fresh waters conveyed in ship's ballast tank sediments and washing basins of shipyards.

DNA and RNA Stability of Marine Microalgae in Cold-Stored Sediments and Its Implications in Metabarcoding Analyses

The ever-increasing applications of metabarcoding analyses for environmental samples demand a well-designed assessment of the stability of DNA and RNA contained in cells that are deposited or buried in marine sediments. We thus conducted a qPCR quantification of the DNA and RNA in the vegetative cells of three microalgae entrapped in facsimile marine sediments and found that >90% of DNA and up to 99% of RNA for all microalgal species were degraded within 60 days at 4 °C. A further examination of the potential interference of the relic DNA of the vegetative cells with resting cyst detection in sediments was performed via a metabarcoding analysis in artificial marine sediments spiked with the vegetative cells of two Kareniaceae dinoflagellates and the resting cysts of another three dinoflagellates. The results demonstrated a dramatic decrease in the relative abundances of the two Kareniaceae dinoflagellates in 120 days, while those of the three resting cysts increased dramatically. Together, our results suggest that a positive detection of microalgae via metabarcoding analysis in DNA or RNA extracted from marine sediments strongly indicates the presence of intact or viable cysts or spores due to the rapid decay of relic DNA/RNA. This study provides a solid basis for the data interpretation of metabarcoding surveys, particularly in resting cyst detection.

Harmful algal blooms in China: History, recent expansion, current status, and future prospects

The impacts of harmful algal blooms (HABs) on economies, public health, ecosystems, and aquaculture across the globe have all increased in recent decades, and this has been acutely the case in China. Here, we review the history of HABs and HABs research in China, as well as recent trends in HABs and future prospects of HAB science in China. The most updated analyses demonstrated that the number of HAB events, the number of HAB species, the aerial coverage of HABs, and the impacts of HABs in Chinese waters during the 21st century were all higher than that during the last two decades of the 20th century. The increase in the number of HABs in China has been significantly correlated with the increased discharge of ammonium and total phosphorus into coastal waters (p < 0.01 for both). Notable newly recognized events this century have included chronic HABs caused by Prorocentrum donghaiense and Karenia mikimotoi, a paralytic shellfish poisoning event caused by Gymnodinium catenatum that sickened 80 people, brown tides caused by Aureococcus anophagefferens, green tides caused by Ulva prolifera, golden tides caused by Sargassum horneri, and the disruption of a nuclear power plant caused by a bloom of Phaeocystis globosa. A series of key discoveries regarding HABs has been made this century including documentation of nearly all known HAB toxins in Chinese waters, discovery of novel cyst-formation and/or life stages of multiple HABs-causing species, identification of the chemical and physical oceanographic drivers of multiple HABs including those formed by P. donghaiense, K. mikimotoi, and U. prolifera, and the successful mitigation of HABs via the use of modified clay approaches. Future research prospects highlighted include the use of macroalgae as a means to prevent, mitigate, and control (PCM) HABs and the process by which multi-disciplinary studies involving molecular approaches (omics), remote in situ detection, artificial intelligence, and mega-data analyses might be used to develop refined and realistic HAB forecasting platforms. Collectively, this review demonstrates the significant evolution of HAB science since the 20th century in China and demonstrates that while HABs in China are complex and widespread, recent and on-going discoveries make the development of detailed understanding and effective measures to mitigate the negative effects of HABs a hopeful outcome in the coming years.

Nanoparticles, an Emerging Control Method for Harmful Algal Blooms: Current Technologies, Challenges, and Perspectives

Harmful algal blooms (HABs) are a global concern because they harm aquatic ecosystems and pose a risk to human health. Various physical, chemical, and biological approaches have been explored to control HABs. However, these methods have limitations in terms of cost, environmental impact, and effectiveness, particularly for large water bodies. Recently, the use of nanoparticles has emerged as a promising strategy for controlling HABs. Briefly, nanoparticles can act as anti-algae agents via several mechanisms, including photocatalysis, flocculation, oxidation, adsorption, and nutrient recovery. Compared with traditional methods, nanoparticle-based approaches offer advantages in terms of environmental friendliness, effectiveness, and specificity. However, the challenges and risks associated with nanoparticles, such as their toxicity and ecological impact, must be considered. In this review, we summarize recent research progress concerning the use of nanoparticles to control HABs, compare the advantages and disadvantages of different types of nanoparticles, discuss the factors influencing their effectiveness and environmental impact, and suggest future directions for research and development in this field. Additionally, we explore the causes of algal blooms, their harmful effects, and various treatment methods, including restricting eutrophication, biological control, and disrupting living conditions. The potential of photocatalysis for generating reactive oxygen species and nutrient control methods using nanomaterials are also discussed in detail. Moreover, the application of flocculants/coagulants for algal removal is highlighted, along with the challenges and potential solutions associated with their use. This comprehensive overview aims to contribute to the development of efficient and sustainable strategies for controlling HAB control.

The dispersal of dinoflagellate cyst caused by international ships under repair conditions: a potential invasion risk to the Yangtze River Estuary, China

The foreign dinoflagellate cysts in ships' ballast water tank sediments (BWTS) can be viable for a long time under hostile storage conditions. It is crucial to understand the detailed mechanisms of harmful biological invasions in estuary ecosystems. To study the relationship between the abundance of dinoflagellate cysts and environmental factors, cyst assemblages were analyzed in 7 sediment samples, collected from one international commercial ship that arrived in Shanghai in August 2020. Twenty-three dinoflagellate cyst taxa were identified in 5 groups, including autotrophic (9) and heterotrophic (14) species. Distribution of dinoflagellate cysts in the different ballast water tanks is heterogeneous. Dinoflagellate cysts in BWTS of the repaired ship were dominated by Scrippsiella acuminata, Protoperidinium leonis, Protoperidinium oblongum, Lingulodinium polyedra, Alexandrium tamarense/A. catenella, Protoperidinium pentagonum, and Protoperidinium subinerme. The abundance of the dinoflagellate cysts in each tank ranged from 80.69 to 330.85 cysts g-1 DS (dry sediment). Multivariate statistical analyses revealed that the variation in cysts from different tanks had positive correlations with total nitrogen (TN), total phosphorus (TP), and pH and negative correlations with total organic carbon (TOC) except for sample TK5. For germination of dinoflagellate cysts in BWTS, 12 species were germinated in 40 days and cysts of potentially toxic dinoflagellate species were more abundant than those of non-toxic species. Results show that potentially viable and harmful/toxic dinoflagellate cysts are present in BWTS of ships arriving in Shanghai, China. Consequently, knowledge obtained in this study can be valuable for further managing of potential biological invasion of the Yangtze River Estuary.

Metabarcoding analysis identifies high diversity of harmful algal bloom species in the coastal waters of the Beibu Gulf

Harmful algal blooms (HABs) have occurred more frequently in recent years. In this study, to investigate their potential impact in the Beibu Gulf, short-read and long-read metabarcoding analyses were combined for annual marine phytoplankton community and HAB species identification. Short-read metabarcoding showed a high level of phytoplankton biodiversity in this area, with Dinophyceae dominating, especially Gymnodiniales. Multiple small phytoplankton, including Prymnesiophyceae and Prasinophyceae, were also identified, which complements the previous lack of identifying small phytoplankton and those unstable after fixation. Of the top 20 phytoplankton genera identified, 15 were HAB-forming genera, which accounted for 47.3%-71.5% of the relative abundance of phytoplankton. Based on long-read metabarcoding, a total of 147 OTUs (PID > 97%) belonging to phytoplankton were identified at the species level, including 118 species. Among them, 37 species belonged to HAB-forming species, and 98 species were reported for the first time in the Beibu Gulf. Contrasting the two metabarcoding approaches at the class level, they both showed a predominance of Dinophyceae, and both included high abundances of Bacillariophyceae, Prasinophyceae, and Prymnesiophyceae, but the relative contents of the classes varied. Notably, the results of the two metabarcoding approaches were quite different below the genus level. The high abundance and diversity of HAB species were probably due to their special life history and multiple nutritional modes. Annual HAB species variation revealed in this study provided a basis for evaluating their potential impact on aquaculture and even nuclear power plant safety in the Beibu Gulf.

A combined approach detected novel species diversity and distribution of dinoflagellate cysts in the Yellow Sea, China

Resting cysts of dinoflagellates seed harmful algal blooms (HABs) and their geographic expansion, which makes it fundamentally important to obtain comprehensive inventories of dinoflagellate resting cysts in HABs-prone regions. The Yellow Sea (YS) of China has observed numerous outbreaks of dinoflagellate HABs with some novel species recorded recently indicating an underestimated HABs-causing species diversity. We report our investigation of dinoflagellate cysts of YS via an approach combining metabarcoding sequencing and single-cyst morpho-molecular identification, which identified many novel cyst species and a significant controlling effect of the Yellow Sea Cold Water Mass on cyst composition. The metabarcoding and single cyst-based sequencing detected 11 cyst species never being unambiguously reported in China, 10 never reported as cyst producers, and 3 HABs-causing species never reported from YS. Our detections of many potentially toxic or HABs-causative, particularly novel, cysts and distribution pattern provide important insights into the risks and ecology of dinoflagellate HABs.

Characterizing the Status of Energetic Metabolism of Dinoflagellate Resting Cysts under Mock Conditions of Marine Sediments via Physiological and Transcriptional Measurements

Similar to the seeds of higher plants, resting cysts, a non-motile, benthic, and dormant stage in the life history of many dinoflagellate species, play vital roles via germination in the seasonal dynamics and particularly the initiation of harmful algal blooms (HABs) of dinoflagellates. It is thus crucial for resting cysts to balance between the energetic catabolism for viability maintenance and the energy preservation for germination during their dormancy. Despite this importance, studies on how resting cysts of dinoflagellates accomplish energetic metabolism in marine sediment have been virtually absent. In this study, using the cosmopolitan HABs-causing species Scrippsiella acuminata as a representative, we measured the transcriptional activity of the most efficient pathway of the energy catabolism tricarboxylic acid (TCA) cycle, cell viability (via neutral red staining), and the cellular ATP content of resting cysts under a set of mock conditions in marine sediments (e.g., 4 °C, darkness, and anoxia) for a maximum period of one year. Based on the correlation analyses among the expression levels of genes, cyst viability, and ATP content, we revealed that the TCA cycle was still a crucial pathway of energetic catabolism for resting cysts under aerobic conditions, and its expression was elevated at higher temperatures, light irradiation, and the early stage of dormancy. Under anaerobic conditions, however, the TCA cycle pathway ceased expression in resting cysts, as also supported by ATP measurements. Our results have laid a cornerstone for the comprehensive revelation of the energetic metabolism and biochemical processes of dormancy of resting cysts in marine sediments.

Deficiency of nitrogen but not phosphorus triggers the life cycle transition of the dinoflagellate Scrippsiella acuminata from vegetative growth to resting cyst formation

Nitrogen (N) and phosphorus (P) are essential elements for algal growth. When N and P are deficient, dinoflagellates will take a series of measures to achieve population continuation including formation of resting cysts, an important ecological strategy of dinoflagellates that plays a key role in the initiation and termination of harmful algal blooms (HABs). How the deficiency of N and P affects algal growth and cyst formation has been investigated in some dinoflagellate species, but how it affects the life cycle transition in dinoflagellates has been poorly understood. In this study, we further explored the effect of N and P deficiency on the algal growth and resting cyst production in the cosmopolitan HABs-causing species Scrippsiella acuminata via refining the N and P concentration gradients. Further, we tracked the expression patterns of one CyclinB and one CDK1 genes of S. acuminata at different growth stages under three deficiency concentrations (1/1000 dilutions of N, P, and both N and P). The results suggest that N deficiency always triggered the cyst formation but P deficiency mainly inhibited the vegetative growth instead of inducing cyst formation. We also observed the highest cyst production when S. acuminata was cultured in the f/2-Si medium that was a one-thousandth dilution of N and P (N∼ 0.882 μM; P∼ 0.0362 μM). Our results for the expressions of CyclinB and CDK1 were well consistent with the results of algal growth and cyst formation at different deficiencies of N and P in terms of that higher expressions of these two genes were corresponding to higher rates of vegetative cell growth, while their expressions in resting cysts maintained to be moderate but significantly lower than that in fast-growing vegetative cells. Although we are still not sure whether the changing expressions of the two genes did regulate the transition of life cycle (i.e. cyst formation), or happened as parallels to the expressions of other truly regulating genes, our observations are surely inspirational for further investigations on the genetic regulation of life cycle transition in dinoflagellates. Our work will provide clues to probe the physiological and molecular mechanisms underlying the nutrient deficiency-induced alternation between life cycle stages in dinoflagellates.

Removal of harmful algae in natural water by semiconductor photocatalysis- A critical review

Harmful Algal Blooms (HABs) have turned out to be a global occurrence owing to the detrimental phenomenon like eutrophication and global climate change caused by human activities. This newly emergent threat imposes a severe hazardous to public health, ecosystems and fishery-based economies. Rapid and exponential growth of certain delirious and toxic algal species shall be held causative to the formation of HABs. The potential disadvantages they pose, make it necessary the identification of efficient treatment methodologies. Photocatalysis has been identified as the most promising solution amongst all the identified and investigated methods, for the environmental and economic benefits beheld. Different treatment methodologies were evaluated and light has been thrown on the advantages beheld by photocatalysis over the other methods. Focus has been given to the different photocatalysts that have been so far put to use towards photocatalytic disinfection of HABs and algal toxins. This present study provides useful information on the application of the traditional and photocatalysis process for removal of HABs in water bodies. Moreover, the results revealed that photocatalysis method could cause potent inhibitory effect on growth of algae species and disrupted algal cells membranes to some extent. Finally, the conventional treatment techniques have been recognized to be insufficient for removal of HABs. However, the photocatalyst technology have been utilized mostly for the mineralization and neutralization of the algal pollutants without any harmful secondary pollutants.

Metabarcoding of harmful algal bloom species in sediments from four coastal areas of the southeast China

In the past three decades, harmful algal blooms (HAB) have become more frequent and widespread in southeast Chinese sea areas. Resting stages are regarded as the “seed bank” of algal blooms, and play an important role in initiating HABs. The distribution of resting stages in sediments especially those of HAB species can make good predictions about the potential risk of future blooms, however with limited reports. In this study, surface sediment samples were collected in the four sea areas along the southeast Chinese coasts, including Dafeng Port (DF) in the southern Yellow Sea, Xiangshan Bay (XS), Funing Bay (FN), and Dongshan Bay (DS) in the East China Sea. Diversity and community structure of eukaryotic microalgae in surface sediments were assessed by metabarcoding V4 region of the 18S rDNA, focusing on the distribution of HAB species. Biogenic elements including total organic carbon (TOC), total nitrogen (TN), total phosphorus (TP), biogenic silicon (BSi), and moisture content (MC) were analyzed. A total of 454 eukaryotic algal OTUs were detected, which belonged to 31 classes of 9 phyla. Altogether 149 algal species were detected in this study, and 59 taxa have been reported to form resting stages. Eukaryotic algal community was similar in XS, FN and DS of the East China Sea, which were predominated by dinoflagellates. However, algal community was different in DF of the Yellow Sea, and characterized by the dominance of chrysophytes and low OTU richness. The distribution of most abundant HAB species showed positive correlations with TN, BSi, and TOC, suggesting that eutrophication and consequent increase in diatom productivity may have a significant influence on the distribution of HAB species and facilitate the occurrence of HABs. Furthermore, HAB species occurred more abundantly and widely in FN. Our results suggest high potential risks of HABs in the southeast Chinese coast especially in Funing Bay.

Pseudocochlodinium profundisulcus Resting Cysts Detected in the Ballast Tank Sediment of Ships Arriving in the Ports of China and North America and the Implications in the Species' Geographic Distribution and Possible Invasion

Over the past several decades, much attention has been focused on the dispersal of aquatic nonindigenous species via ballast tanks of shipping vessels worldwide. The recently reclassified dinoflagellate Pseudocochlodinium profundisulcus (previously identified as Cochlodinium sp., Cochlodinium geminatum, or Polykrikos geminatus) was not reported in China until 2006. However, algal blooming events caused by this organism have been reported almost every year since then in the Pearl River Estuary and its adjacent areas in China. Whether P. profundisulcus is an indigenous or an invasive species has thus become an ecological question of great scientific and practical significance. In this study, we collected the sediments from ballast tanks of ships arriving in the ports of China and North America and characterized dinoflagellate resting cysts via a combined approach. We germinated two dark brownish cysts from the tank of an international ship (Vessel A) arriving at the Jiangyin Port (China) into vegetative cells and identified them as P. profundisulcus by light and scanning electron microscopy and phylogenetic analyses for partial LSU rDNA sequences. We also identified P. profundisulcus cyst from the ballast tank sediment of a ship (Vessel B) arriving in the port of North America via single-cyst PCR and cloning sequencing, which indicated that this species could be transported as resting cyst via ship. Since phylogenetic analyses based on partial LSU rDNA sequences could not differentiate all sequences among our cysts from those deposited in the NCBI database into sub-groups, all populations from China, Australia, Japan, and the original sources from which the cysts in the two vessels arrived in China and North America were carried over appeared to share a very recent common ancestor, and the species may have experienced a worldwide expansion recently. These results indicate that P. profundisulcus cysts may have been extensively transferred to many regions of the world via ships’ ballast tank sediments. While our work provides an exemplary case for both the feasibility and complexity (in tracking the source) of the bio-invasion risk via the transport of live resting cysts by ship’s ballast tanks, it also points out an orientation for future investigation.

Geographic distribution and historical presence of the resting cysts of Karenia mikimotoi in the seas of China

The toxic dinoflagellate Karenia mikimotoi frequently forms harmful algal blooms (HABs) and thus causes massive kills of fish and shellfish in worldwide coastal waters, which has led to intensive investigations on multiple facets of the species. Following our recent discovery of K. mikimotoi forming resting cyst, a very possible mechanism for the inoculation of blooms and geographic expansion for this and many other HABs-causing species, here we report our detection of K. mikimotoi resting cysts in 125 surface sediment samples collected from the coastal waters (covering a latitude range from 18.29°N to 39.85°N) and 3 sediment cores (accumulated in 70‒100 years) collected from the East China Sea where are adjacent to the frequent blooming areas of K. mikimotoi. Via applications of quantitative real-time PCR (LSU rDNA-targeted), species-specific fluorescence in situ hybridization (FISH), and nested-PCR-and-sequencing to both types of the sediment samples that were pretreated with sodium polytungstate solution (SPT), we demonstrated that 1) K. mikimotoi cysts are widely present in surface sediments of the China seas (Bohai Sea (BS), Yellow Sea (YS), East China Sea (ECS), and South China Sea (SCS)), 2) the abundance of cysts is generally low (0 to 33 cysts in 32 g wet sediment), with that in the ECS and the SCS being higher than that in the YS and the BS, and the highest abundance was observed in sites of the ECS (e.g., Ningde, Fujian province) where the blooms of the species occurred frequently, as quantified by both methods, and 3) the cysts of K. mikimotoi have been present in the sediments of the ECS since 1970s, a short time prior to the first recorded bloom of K. mikimotoi in the SCS at 1980s. Our results not only demonstrated the wide geographic distribution of resting cyst of K. mikimotoi along the coast of China, but also proved a 50 years preservation of the cysts in the sediments of coastal area prone to forming frequent blooms. We consider our results have provided critical insights into the mechanisms of frequent bloom outbreaks and global distribution of K. mikimotoi in general, and particularly into the historical origin of K. mikimotoi in China. Further investigations are suggested to focus on on-site surveys for the cyst production and germination rates.

Expression Patterns of the Heat Shock Protein 90 (Hsp90) Gene Suggest Its Possible Involvement in Maintaining the Dormancy of Dinoflagellate Resting Cysts

Heat shock protein 90 (Hsp90) is a highly conserved molecular chaperone functioning in cellular structural folding and conformational integrity maintenance and thus plays vital roles in a variety of biological processes. However, many aspects of these functions and processes remain to be fully elucidated, particularly for non-model organisms. Dinoflagellates are a group of eukaryotes that are exceedingly important in primary production and are responsible for the most harmful algal blooms (HABs) in aquatic ecosystems. The success of dinoflagellates in dominating the plankton community is undoubtedly pertinent to their remarkable adaptive strategies, characteristic of resting cyst production and broad tolerance to stresses of temperature and others. Therefore, this study was conducted to examine the putative roles of Hsp90 in the acclimation to temperature stress and life stage alterations of dinoflagellates. Firstly, we isolated the full-length cDNA of an Hsp90 gene (StHsp90) via RACE from the cosmopolitan HAB species Scrippsiella trochoidea and tracked its transcriptions in response to varied scenarios via real-time qPCR. The results indicated that StHsp90 displayed significant mRNA augment patterns, escalating during 180-min treatments, when the cells were exposed to elevated and lowered temperatures. Secondly, we observed prominently elevated StHsp90 transcriptions in the cysts that were stored at the cold and dark conditions compared to those in newly formed resting cysts and vegetative cells. Finally, and perhaps most importantly, we identified 29 entries of Hsp90-encoding genes with complete coding regions from a dinoflagellate-specific environmental cDNA library generated from marine sediment assemblages. The observed active transcription of these genes in sediment-buried resting cysts was fully supported by the qPCR results for the cold-stored resting cysts of S. trochoidea. Hsp90s expressions in both laboratory-raised and field-collected cysts collectively highlighted the possible involvement and engagement of Hsp90 chaperones in the resting stage persistence of dinoflagellates.

The Implication Inferred from the Expression of Small Heat-Shock Protein Genes in Dinoflagellate Resting Cysts Buried in Marine Sediment

Dinoflagellates are unicellular eukaryotic microalgae, occupying pivotal niches in aquatic ecosystems with great ecological, biological, and economic significance. Small heat shock proteins (sHsps) are the most omnipresent, but the least conserved, family of molecular chaperones found in all domains of life. Although their common name (small Hsp) implies to exclusively stress their heat shock-responsive function, many sHsps in fact engage in a variety of physiological processes, from cell growth and proliferation to embryogenesis, development, differentiation, apoptosis, and even to human disease prevention. Recent years have greatly expanded our understanding of sHsps in higher plants; however, comprehensive study aiming to delineate the composition and expression pattern of dinoflagellate sHsp gene family has not yet been performed. In this study, we constructed dinoflagellate-specific environmental cDNA library from marine sediment and sequenced using the third-generation sequencing technique. Screening of sHsp genes from the library returned 13 entries with complete coding regions, which were considered to be transcriptionally activated in the natural community of dinoflagellate resting cysts. All the 13 dinoflagellate sHsps consisted of a solely characteristic α-crystallin domain, covering 88–123 amino acid residues with the typical A-X-X-X-N-G-V-L motif, flanked by variable N- and C-terminal extensions. Multiple alignment revealed considerable amino acid divergence (~26.7% average similarity) among them. An unexpected close relationship was revealed between dinoflagellate and green algal sHsps in the phylogenetic tree, seemingly reflecting a close evolutionary relationship of these sHsps themselves. We confirmed that sHsp mRNAs are expressed during dormancy of the resting cyst assemblages of dinoflagellates that were buried in marine sediment, which raised the possibility that the sHsp expression is part of the machinery of maintaining the dormancy or/and the adaptation to ambient conditions of dinoflagellate resting cysts. Our results, although preliminary, gained an important glance on the universal presence of sHsps in dinoflagellates and their active expressions in the assemblage of resting cysts that were buried in the marine sediment. The essentiality of sHsps functioning in resting cysts necessitate more intensive and extensive investigations on all possible functions of Hsps in dinoflagellates, a group of protists with vital ecological and biological importance.

Morpho-molecular description of a new HAB species, Pseudocochlodinium profundisulcus gen. et sp. nov., and its LSU rRNA gene based genetic diversity and geographical distribution

Harmful algal blooms (HABs) caused by an unknown dinoflagellate species have frequently occurred in the Pearl River Estuary, China Since 2006. These blooms were associated with severe water discoloration and economic losses, ranging from several km² to 300 km² with the maximum recorded cell density being 2.77 × 10⁷ cells⋅L^-1. This unknown dinoflagellate species was initially identified as Cochlodinium geminatum and subsequently reclassified as Polykrikos geminatus. However, after reviewing the original descriptions for Cochlodinium geminatum sensu Schütt (1895) and the genus Polykrikos, we considered this species is incongruent with their original descriptions. Further morphological examinations and particularly phylogenetic analyses based on the SSU and partial LSU rRNA genes of isolates and resting cysts from China and Japan prompted us to consider it a new species of a new genus. This new species was proposed to be Pseudocochlodinium profundisulcus gen. et sp. nov., based on its open comma-shaped apical structure complex (ASC), cingulum encircling the cell less than one and a half turns, a deep sulcus with a torsion of a half turn, either single cell or cell chain consisting of two cells with the same number of nuclei and zooids, the resting cyst bearing lobed ornaments, and the evolutionary distances from Polykrikos (and others) on the phylogenetic trees constructed using the concatenated SSU and partial LSU rRNA gene sequences. Metabarcoding investigation of surface sediment samples collected in China revealed that the species to be widely present along the entire Chinese coast with the highest abundance in the South China Sea. Further re-analysis of the Tara Oceans metabarcoding dataset targeting the SSU rRNA gene V9 domain suggested a global distribution of this new genus. Phylogenetic analyses on 46 OTUs (average length: ∼552 bases) of its LSU rRNA gene sequences (mainly D1-D2 domains) obtained from surface sediment samples revealed intraspecific genetic diversity of this species. Interestingly, based on the different distributions and the abundance of these OTUs along the coast of China, this species appeared to have expanded its distribution from the South China Sea to the northern Yellow Sea, or preferred a warm water habitat. We consider that the present work improves the taxonomy and provides important insights into the biogeography of Pseudocochlodinium profundisulcus.⋅.

Probing the Energetic Metabolism of Resting Cysts under Different Conditions from Molecular and Physiological Perspectives in the Harmful Algal Blooms-Forming Dinoflagellate Scrippsiella trochoidea

Energetic metabolism is essential in maintaining the viability of all organisms. Resting cysts play important roles in the ecology of dinoflagellates, particularly for harmful algal blooms (HABs)-causative species. However, the energetic metabolism underlying the germination potency maintenance of resting cysts of dinoflagellate have been extremely scarce in studies from physiological and, particularly, molecular perspectives. Therefore, we used the cosmopolitan Scrippsiella trochoidea as a representative of HABs-forming and cyst-producing dinoflagellates in this work to obtain novel insights into the molecular mechanisms, regulating the energetic metabolism in dinoflagellate resting cysts, under different physical condition. As the starting step, we established a cDNA subtractive library via suppression subtractive hybridization (SSH) technology, from which we screened an incomplete sequence for the β subunit of ATP synthase gene (β-F1-ATPase), a key indicator for the status of cell's energetic metabolism. The full-length cDNA of β-F1-ATPase gene from S.trochoidea (Stβ-F1-ATPase) was then obtained via rapid amplification of cDNA ends (RACE) (Accession: MZ343333). Our real-time qPCR detections, in vegetative cells and resting cysts treated with different physical conditions, revealed that (1) the expression of Stβ-F1-ATPase in resting cysts was generally much lower than that in vegetative cells, and (2) the Stβ-F1-ATPase expressions in the resting cysts under darkness, lowered temperature, and anoxia, and during an extended duration of dormancy, were significantly lower than that in cysts under the condition normally used for culture-maintaining (a 12 h light:12 h dark cycle, 21 °C, aerobic, and newly harvested). Our detections of the viability (via Neutral Red staining) and cellular ATP content of resting cysts, at the conditions corresponding to the abovementioned treatments, showed that both the viability and ATP content decreased rapidly within 12 h and then maintained at low levels within the 4-day experimentation under all the three conditions applied (4 °C, darkness, and anoxia), which are well in accordance with the measurements of the transcription of Stβ-F1-ATPase. These results demonstrated that the energy consumption of resting cysts reaches a low, but somehow stable, level within a short time period and is lower at low temperature, darkness, and anoxia than that at ambient temperature. Our work provides an important basis for explaining that resting cysts survive long-term darkness and low temperature in marine sediments from molecular and physiological levels.