The toxic dinoflagellate Karenia mikimotoi has been well‐known for causing large‐scale and dense harmful algal blooms (HABs) in coastal waters worldwide and serious economic loss in aquaculture and ...fisheries and other adverse effects on marine ecosystems. Whether K. mikimotoi forms resting cysts has been a puzzling issue regarding to the mechanisms of bloom initiation and geographic expansion of this species. We provide morphological and molecular confirmation of sexually produced thin‐walled resting cysts by K. mikimotoi based on observations of laboratory cultures and their direct detection in marine sediments. Light and scanning electron microscopy evidences for sexual reproduction include attraction and pairing of gametes, gamete fusion, formation of planozygote and thin‐walled cyst, and the documentation of the thin‐walled cyst germination processes. Evidence for cysts in marine sediments was in three aspects: positive PCR detection of cysts using species‐specific primers in the DNA extracted from whole sediments; fluorescence in situ hybridization detection of cysts using FISH probes; and single‐cell PCR sequencing for cysts positively labeled with FISH probes. The existence of sexually produced, thin‐walled resting cysts by K. mikimotoi provides a possible mechanism accounting for the initiation of annually recurring blooms at certain regions and global expansion of the species during the past decades.
In numerous studies, researchers have explored the interactions between fungi and their hosting biota in terrestrial systems, while much less attention has been paid to the counterpart interactions ...in aquatic, and particularly marine, ecosystems. Despite the growing recognition of the potential functions of fungi in structuring phytoplankton communities, the current insights were mostly derived from phytoplankton hosts, such as diatoms, green microalgae, and cyanobacteria. Dinoflagellates are the second most abundant group of phytoplankton in coastal marine ecosystems, and they are notorious for causing harmful algal blooms (HABs). In this study, we used high-throughput amplicon sequencing to capture global snapshots of specific fungal assemblages associated with laboratory-cultured marine dinoflagellate. We investigated a total of 13 clonal cultures of the dinoflagellate
that were previously isolated from 5 geographic origins and have been maintained in our laboratory from several months to more than 14 years. The total recovered fungal microbiome, which consisted of 349 ASVs (amplicon sequencing variants, sequences clustered at a 100% sequence identity), could be assigned to 4 phyla, 18 classes, 37 orders, 65 families, 97 genera, and 131 species. The fungal consortium displayed high diversity and was dominated by filamentous fungi and ascomycetous and basidiomycetous yeasts. A core set of three genera among all the detected fungi was constitutively present in the
strains isolated from geographically distant regions, with the top two most abundant genera,
and
, capable of using hydrocarbons as the sole or major source of carbon and energy. In addition, fungal taxa previously documented as endophytes in other hosts were also found in all tested strains of
. Because host-endophyte interactions are highly variable and strongly case-dependent, these fungal taxa were not necessarily genuine endosymbionts of
; instead, it raised the possibility that dinoflagellates could potentially serve as an alternative ecological niche for the colonization of fungal endophytes. Our findings lay the foundation for further investigations into the potential roles or functions of fungi in the regulation of the growth dynamics and HABs of marine dinoflagellates in the field.
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
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.
Light has significant effect on the growth and development of Saccharina japonica, but there are limited reports on blue light mediated physiological responses and molecular mechanism. In this study, ...high-throughput paired-end RNA-sequencing (RNA-Seq) technology was applied to transcriptomes of S. japonica exposed to blue light and darkness, respectively. Comparative analysis of gene expression was designed to correlate the effect of blue light and physiological mechanisms on the molecular level.
RNA-seq analysis yielded 70,497 non-redundant unigenes with an average length of 538 bp. 28,358 (40.2%) functional transcripts encoding regions were identified. Annotation through Swissprot, Nr, GO, KEGG, and COG databases showed 25,924 unigenes compared well (E-value <10(-5)) with known gene sequences, and 43 unigenes were putative BL photoreceptor. 10,440 unigenes were classified into Gene Ontology, and 8,476 unigenes were involved in 114 known pathways. Based on RPKM values, 11,660 (16.5%) differentially expressed unigenes were detected between blue light and dark exposed treatments, including 7,808 upregulated and 3,852 downregulated unigenes, suggesting S. japonica had undergone extensive transcriptome re-orchestration during BL exposure. The BL-specific responsive genes were indentified to function in processes of circadian rhythm, flavonoid biosynthesis, photoreactivation and photomorphogenesis.
Transcriptome profiling of S. japonica provides clues to potential genes identification and future functional genomics study. The global survey of expression changes under blue light will enhance our understanding of molecular mechanisms underlying blue light induced responses in lower plants as well as facilitate future blue light photoreceptor identification and specific responsive pathways analysis.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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.
Many studies have demonstrated that some macroalgae (including the widely cultivated red alga
Gracilaria lemaneiformis
) can inhibit the growth of microalgae via allelopathy and resource competition, ...with the former as the major contributor. However, little is known currently about whether or not and how microalgae react as a feedback to the inhibitory effects or, more generally, the interactions between seaweed and microalgae in their co-culturing system. Here, we report a laboratory-based study on the interactions between the seaweed
Gracilaria lemaneiformis
and the common harmful algal blooms (HABs)-forming dinoflagellate
Akashiwo sanguinea
. We found that while both the fresh thalli of
G. lemaneiformis
and the extracts of fresh and dried
G. lemaneiformis
could significantly inhibit the growth of
A. sanguinea
, the dead cells of
A. sanguinea
“revenged”
G. lemaneiformis
via promoting the growth of bacteria and consequently slowed the growth of
G. lemaneiformis
, which was evidenced by the lowered pH, slowed nutrients consumption in the cultures, the elevated counts of bacteria, and the reduced biomass of
G. lemaneiformis
. Collectively, our results showed that while
G. lemaneiformis
could inhibit the growth of microalgae with allelopathy as a major contributor, the death of allelopathy-affected microalgae could promote bacterial growth, which sequentially inhibits the growth of
G. lemaneiformis
as a feedback.
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.
Identification of a core microbiome (a group of taxa commonly present and consistently abundant in most samples of host populations) is important to capture the key microbes closely associated with a ...host population, as this process may potentially contribute to further revealing their spatial distribution, temporal stability, ecological influence, and even impacts on their host’s functions and fitness. The naked dinoflagellate
Karlodinium veneficum
is a cosmopolitan and toxic species, which is also notorious in forming harmful algal blooms (HABs) and causing massive fish-kills. Here we reported the core microbiome tightly associated with 19 strains of
K. veneficum
that were originally isolated from 6 geographic locations along the coast of China and from an estuary of Chesapeake Bay, United States, and have been maintained in the laboratory for several months to over 14 years. Using high-throughput metabarcoding of the partial 16S rRNA gene amplicons, a total of 1,417 prokaryotic features were detected in the entire bacterial microbiome, which were assigned to 17 phyla, 35 classes, 90 orders, 273 families, and 716 genera. Although the bacterial communities associated with
K. veneficum
cultures displayed heterogeneity in feature (sequences clustered at 100% sequence similarity) composition among strains, a core set of 6 genera were found persistent in their phycospheres, which could contribute up to 74.54% of the whole bacterial microbiome. Three γ-proteobacteria members of the “core,” namely,
Alteromonas
,
Marinobacter
, and
Methylophaga
, were the predominant core genera and made up 83.25% of the core bacterial microbiome. The other 3 core genera,
Alcanivorax
,
Thalassospira
, and
Ponticoccus
, are reported to preferably utilize hydrocarbons as sole or major source of carbon and energy, and two of which (
Alcanivorax
and
Ponticoccus
) are recognized as obligate hydrocarbonoclastic bacteria (OHCB). Since OHCB generally present in extremely low abundance in marine water and elevate their abundance mostly in petroleum-impacted water, our detection in
K. veneficum
cultures suggests that the occurrence of obligate and generalist hydrocarbon-degrading bacteria living with dinoflagellates may be more frequent in nature. Our work identified a core microbiome with stable association with the harmful alga
K. veneficum
and opened a window for further characterization of the physiological mechanisms and ecological implications for the dinoflagellate-bacteria association.
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
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
subunit of ATP synthase gene (
), a key indicator for the status of cell's energetic metabolism. The full-length cDNA of
gene from
(
) 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
in resting cysts was generally much lower than that in vegetative cells, and (2) the
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
. 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.
Due to the vital importance of resting cysts in the biology and ecology of many dinoflagellates, a transcriptomic investigation on
was conducted with the aim to reveal the molecular processes and ...relevant functional genes regulating encystment and dormancy in dinoflagellates. We identified via RNA-seq 3,874 (out of 166,575) differentially expressed genes (DEGs) between resting cysts and vegetative cells; a pause of photosynthesis (confirmed via direct measurement of photosynthetic efficiency); an active catabolism including β-oxidation, glycolysis, glyoxylate pathway, and TCA in resting cysts (tested via measurements of respiration rate); 12 DEGs encoding meiotic recombination proteins and members of MEI2-like family potentially involved in sexual reproduction and encystment; elevated expressions in genes encoding enzymes responding to pathogens (chitin deacetylase) and ROS stress in cysts; and 134 unigenes specifically expressed in cysts. We paid particular attention to genes pertaining to phytohormone signaling and identified 4 key genes regulating abscisic acid (ABA) biosynthesis and catabolism, with further characterization based on their full-length cDNA obtained via RACE-PCR. The qPCR results demonstrated elevated biosynthesis and repressed catabolism of ABA during the courses of encystment and cyst dormancy, which was significantly enhanced by lower temperature (4 ± 1°C) and darkness. Direct measurements of ABA using UHPLC-MS/MS and ELISA in vegetative cells and cysts both fully supported qPCR results. These results collectively suggest a vital role of ABA in regulating encystment and maintenance of dormancy, akin to its function in seed dormancy of higher plants. Our results provided a critical advancement in understanding molecular processes in resting cysts of dinoflagellates.
PDF
