Harmful Algal Blooms have been increasing globally in extension and impacts on public health, aquaculture industry, fisheries, and ecosystems such as oxygen depletion, reduction in water quality. Among all Harmful Algal Bloom-causing species, Dinoflagellates are the most important contributors, as about 75% of reported Harmful Algal Blooms are caused by Dinoflagellates. Dinoflagellates have a number of characteristic features and are one of the most important primary producers and a vital component of Coral Reef symbiotic system. While Harmful Algal Bloom events may be caused by a variety of environmental and autecological factors such as illumination, water temperature, nutrients availability, growth rate, vertical migration, and special life history, as feedbacks, Harmful Algal Blooms may cause many negative effects on the ecosystems that can be viewed from different levels (from ecosystem, community to sub-cellular and molecular levels) and aspects (physical, chemical, biological, public health, and economic). In general, previous studies on the negative effects of Harmful Algal Blooms have been mainly focused on fisheries, aquaculture, and Human public health, relatively fewer studies, however, have investigated the effects of Harmful Algal Blooms on the level of community, and even fewer using high throughput metagenomic approach, such as phytoplankton community diversity, community structure, function and stability. This oddness was at least partly due to the limitations in how to obtain comprehensive lists of species and identify species of small sizes, simple or similar morphologies, low abundances, and to process numerous samples efficiently. Conventional methods for identifying and quantifying phytoplankton species from field samples generally involved in the use of light microscopy, and sometimes were aided with flow cytometry and alike, pigment analysis, however they all have limitations in identifying and quantifying those species of highly small sizes, simple or similar morphologies, low abundances, and novel taxa that have not been described. With the development of molecular approaches, high-throughput gene sequencing (e.g., 18S and 28S rRNA genes) have recently been increasingly applied to environmental samples to conquer these limitations. The well-developed, high-throughput sequencing allows us to deeply sequence environmental samples and to sensitively and accurately identify species, and thus detect slight changes at the community level.
In a paper published in the journal Acta Oceanologica Sinica in April 2020, Huan Wang of the Key Laboratory of Marine Ecology and Environmental Sciences at the Institute of Oceanology of the Chinese Academy of Sciences, and the University of Chinese Academy of Sciences, Zhangxi Hu, Zhaoyang Chai, and Yunyan Deng, of the Key Laboratory of Marine Ecology and Environmental Sciences at the Institute of Oceanology of the Chinese Academy of Sciences, the Laboratory for Marine Ecology and Environmental Science at the Pilot National Laboratory for Marine Science and Technology, and the Center for Ocean Mega-Science of the Chinese Academy of Sciences, Zifeng Zhan of the Department of Marine Organism Taxonomy and Phylogeny at the Institute of Oceanology of the Chinese Academy of Sciences, and Ying Zhong Tang, also of the Key Laboratory of Marine Ecology and Environmental Sciences at the Institute of Oceanology of the Chinese Academy of Sciences, the Laboratory for Marine Ecology and Environmental Science at the Pilot National Laboratory for Marine Science and Technology, and the Center for Ocean Mega-Science of the Chinese Academy of Sciences, present the results of a study which investigated the effects of blooms of a common Harmful Algal Bloom-causing Dinoflagellate in China, Prorocentrum donghaiense, on the Dinoflagellate sub-community level, which the bloom species belongs to, in terms of species richness and other biodiversity indices by applying a high-throughput amplicon sequencing approach.
Wang et al. applied a pair of particularly designed primers targeting the large subunit rRNA gene to sequencing the samples taken before, during, and after Prorocentrum donghaiense blooming from the Sansha Bay in Fujian Province, China. They also measured other variables including cell density of Prorocentrum donghaiense, chlorophyll content, nutrients (total nitrogen, nitrate, nitrite, ammonium , total phosphorus, phosphate, and silicate), salinity, and temperature to examine the interactions among these variables,Prorocentrum donghaiense blooms, and the Dinoflagellate community succession.
The study area, Sansha Bay, is located to the northeast of the city of Ningde, one of Fujian Province’s major aquaculture centres on the East China Sea, where there have been observed highly frequent Harmful Algal Blooms caused by Prorocentrum donghaiense, Karenia mikimotoi, and, occasionally, other species. There were about 161 Harmful Algal Bloom incidents during 2001–2010 and 65 events between 2011 and 2015, with Prorocentrum donghaiense being the main causative species.
Locations of sampling sites in the Sansha Bay, Ningde, China. Wang et al. (2020).
From March to July, 2016, Wang et al. conducted six cruises and collected a total of 50 samples, which covered pre-, during, and post-bloom periods. Four or five sampling sites were selected in the study area. 31 March was prior to the bloom, 22 April, 3 May, and 13 May were during the bloom period (based on cell counts of Prorocentrum donghaiense), with 3 May coinciding the peak of a bloom, and 31 May 31 and 19 July were being post-bloom.
Wang et al define a bloom as to chlorophyll a concentration higher than 5 μg per litre of water and a dominant specie concentration of over 20 000 cells per millilitre; this being an arbitrarily chosen measure in the absence of any commonly accepted standard of cell density to define a bloom.
Water temperature and salinity were measured on site using a hand-held thermometer and a refractometer. Water samples were taken from 0.5 m below the surface and transferred into 5 L polyethylene bucket. Plankton samples for DNA extraction were collected by filtering 1.5 L water through a hydrophilic polycarbonate membrane (47 mm diameter, 0.4 μm pore size) with duplicates, put into an icebox and then cooled to –20°C immediately after arriving the laboratory and then stored at –80°C until DNA extraction. Water samples were also fixed with Lugol’s iodine solution (final concentration, 2%) for counting cells of Prorocentrum donghaiense using plankton counting chamber under an inverted light microscope.
The Dinoflagellate Prorocentrum donghaiense, was the dominant species in Sansha Bay during the 2016 bloom season. Lu et al. (2005).
During the six cruises from March to July of 2016, Prorocentrum donghaiense reached the maximum cell density of 430 000 cells per millilitre on 3 May. The cell density of Prorocentrum donghaiense was 270 cells per millilitre on 31 March (pre-blooms) and the lowest cell density of 83 cells per millilitre was on 19 July (after the blooms). During the bloom period of late April to early May, Prorocentrum donghaiense abundance ranged from 300 to 430 000 cells per millilitre. However, among the sampling sites, Prorocentrum donghaiense cell density varied significantly, with Site B or Site C having significantly higher abundance than that of Site A.
The chlorophyll a level ranged from 0.3 to 26.8 μgper litre, with the highest observed at Site D on 3 May, coninciding with an observed bloom of Prorocentrum donghaiense was (with a cell density of 50 000 cells per millilitre). There existed a significant positive correlation between Prorocentrum donghaiense cell density and chlorophyll a concentration, indicating chlorophyll was one of, but not the only, major contributors of phytoplankton biomass. Strikingly, it is noteworthy that for the sample taken from site B on 3 May, there was a discrepancy between chlorophyll a and the cell abundance of Prorocentrum donghaiense, which Wang et al. think was possibly due to a lower chlorophyll a content per cell for Prorocentrum donghaiense relative to that of other phytoplankton species such as Diatoms and Green Microalgae because of the very small-sized cells and pigment composition of Prorocentrum donghaiense. In addition, the extremely high abundance of Prorocentrum donghaiense during the blooming period (e.g., early May) also led to a decrease in the abundance of other phytoplankton with higher chlorophyll a contents per cell.
Water temperature ranged from 13.9°C to 29.5°C during the sampling period. No significant correlation was observed between water temperature and chlorophyll a concentration, nor between temperature and Prorocentrum donghaiense cell density. Wang et al. observed no correlation between Prorocentrum donghaiense cell density and ammonium or phosphate concentrations, but Prorocentrum donghaiense cell density correnlated negatively with nitrate and nitrite (i.e. the concentrations of these nutrients dropped during the bloom peaks), and positively with total nitrogen, total phosphorus and silicate (the concentrations of which went up during peak blooms), suggesting that nitrogen and phosphorus are driving factors for Prorocentrum donghaiense blooms. The ratios of dissolved inorganic nitrogen (nitrate, nitrite, and ammonia) to dissolved inorganic phosphorus (phosphate) in the surface water tended to decrease along with the development and maintenance of bloom. On 13 May this ratio reached the minimum observed.
A total of 800 185 valid sequence reads of Dinoflagellates with an average length of about 400 bp were generated from the 50 samples collected. By clustering the unique sequences at 97% similarity level, these dinoflagellate sequences were grouped into 560 Operational Taxonomic Units (clusters of organisms, grouped by DNA sequence similarity of a specific taxonomic marker gene; a pragmatic proxy for species in the absence of a full taxonomic study), with the number of Operational Taxonomic Units ranging from 39 to 304 per sample. The highest richness was observed in a sample taken at site C on 19 July 2016 (after bloom) and the lowest richness was observed at site A on 3 May 2016 (during bloom). Operational Taxonomic Unit richness decreased during the blooming period from 22 April to 13 May, and then increased with the disappearance of bloom from 31 May to 17 July.
Metagenomic analysis revealed changes in the abundance of Operational Taxonomic Units classifiable to various taxonomic levels, including shifts in dominant genera and species on date basis. The top 20 most abundant genera and species of each sample showed that 26 of the 50 samples were dominated by Prorocentrum (76.6%–99.6%), while during the before-blooming period, the Dinoflagellate community was dominated by an Operational Taxonomic Unit that could not be well identified to any currently accepted genus of Dinoflagellates (4.9%–79.6% dominance). All samples taken on 22 April, 3 May, and 13 May except for two taken on 13 May at site A were from the blooming area and dominated by Prorocentrum donghaiense. The samples from 13 May at site A were froma non-blooming area and dominated by Levanderina fissa. After the blooming period, most samples taken on 19 July were dominated by Levanderina fissa (25.0%–69.3% dominance).
The Dinoflagellate Levanderina fissa was the dominant species in Sansha Bay in 2016 after the end of the bloom season. Moestrup et al. (2014).
Wang et al.'s study demonstrated that the bloom of Prorocentrum donghaiense affected the structure of Dinoflagellate sub-community of the total phytoplankton by reducing the species richness and diversity. The Dinoflagellate community during the blooming period differed significantly from those before and after blooming periods. The species composition of Dinoflagellate community changed with transition stages of the Prorocentrum donghaiense bloom. For instance, the Dinoflagellate community was dominated by a species that has not been well described, Prorocentrum donghaiense, and Levanderina fissa for the periods of before, during, and after blooming, respectively. These results supported Wang et al.'s hypothesis that Prorocentrum donghaiense blooms would reduce the diversity of the Dinoflagellate community and alter the community structure.
Investigations on the effect of Harmful Algal Blooms on species diversity and community succession have been comparatively rare, particularly so for that using high throughput metagenomic approach. An earlier study investigated abundance and composition of phytoplankton populations during different bloom stages of Gymnodinium breve off the North Carolina coast, and found that total phytoplankton abundance increased regardless of Gymnodinium breve abundance. Further, that study discovered that the cell densities of some groups increased but others decreased, which is in contrast to Wang et al.'s results, possibly because the Gymnodinium breve bloom was not monospecific. About 127 phytoplankton species were identified microscopically from all water samples from North Carolina, which was a relatively low number in comparison to Wang et al.'s work targeting on Dinoflagellates only.
However, a more recent study, using high-throughput pyrosequencing approach also but targeting on a broader spectrum of microorganisms, demonstrated that microbial community structure is strongly linked to the bloom progression of Alexandrium catenella in the Nauset Marsh System on Cape Cod, Massachusetts. Multiple aspects of that study are consistent with Wang et al.'s results, such as that a decrease in diversity of the entire community of plankton during the bloom of Alexandrium catenella and reflects complex interactions among taxa comprising the phycosphere environment.
An earlier study on freshwater and brackish water ecosystems in in Fenno–Scandia demonstrated that the diversity of phytoplankton communities is the best predictor for resource use efficiency (e.g., nutrients) of phytoplankton and factors reducing phytoplankton diversity may have direct detrimental effects on the amount and predictability of aquatic primary production.
While environmental variables such as temperature, turbulence, and nutrient levels are generally the primary forces shaping the community structure and driving Harmful Algal Blooms (see the discussion below), a bloom can be a vital driving force by its own for the transition of phytoplankton community structure due to the biological features of the blooming species. For example, most Harmful Algal Bloom-causing species have been demonstrated to be allelopathic (harmfull) to other co-occurring phytoplankton species via releasing allelochemicals which surpress their growth. A blooming species generally can squeeze the living space of other species via fast growth, which will consequently reduce the nutrient and space availability to competitors.
Wang et al.'s results showed that Prorocentrum donghaiense abundance, nitrate and silicate concentrations were the three most important environmental factors affecting the Dinoflagellate community. Prorocentrum donghaiense abundance was correlated negatively to nitrate, nitrite, phosphate, ammonium, temperature and salinity, but positively to total nitrogen, total phosphorus, chlorophyl a and silicate. Although Dinoflagellates do not need silicate for growth, Wang et al.'s results showed that it appeared to be one of those important factors in shaping the Dinoflagellate community, which might be indirectly caused via the effects of on the transition of Diatom community during the sampling period. The ratio of dissolved inorganic nitrogen to dissolved inorganic phosphorus tended to decrease along with the development and maintenance of blooms, and increase along with disappearance of blooms. At the beginning of the survey (31 March), the cell density of Prorocentrum donghaiense was comparatively low (270 cells per millilitre), and the dissolved inorganic nitrogen to dissolved inorganic phosphorus ratio was 18–22, which was more suitable for the growth of Prorocentrum donghaiense, while, during the blooming period, the ratio showed a downward trend in general, possibly due to the different absorption rates for different nutrients by the bloom-forming organism. This trend indicates a faster absorption rate of dissolved inorganic nitrogen by Prorocentrum donghaiense and consequently a larger effect of dissolved inorganic nitrogen on the growth of Prorocentrum donghaiense, compared to phosphate. On 13 May, the ratio reached the minimum, indicating a limiting level of dissolved inorganic nitrogen to Prorocentrum donghaiense growth. Furthermore, it was observed there were significant negative correlations between total phosphorus and diversity, indicating that total phosphorus also stimulated the growth or bloom of Prorocentrum donghaiense. However, phosphate did not exhibit a significant correlation with diversity, indicating the utilisation or uptake of phosphorus by Prorocentrum donghaiense was not linearly correlated with the ambient concentration of phosphate
Wang et al.'s analysis revealed that, in addition to nutrients, temperature and salinity also made contributions to the transition of the Dinoflagellate community, which is in contrast to earlier studies where temperature and salinity were two key environmental factors associated with changes in Bacterial and Archaeal community structure but not with variations in Eukaryotic community. While it is well understandable that temperature acted as an important factor, the apparent correlation between salinity and Prorocentrum donghaiense and the Dinoflagellate community might be a good indication of nutrient input from freshwater runoff.
In summary, Wang et al.'s investigation observed that blooms of Prorocentrum donghaiense negatively affected biodiversity in the Dinoflagellate sub-community. The results showed that the Dinoflagellate community during the blooming period of Prorocentrum donghaiense differed significantly from the community before and after the blooming period. Wang et al.'s analyses indicated that Prorocentrum donghaiense abundance was the most important factor affecting the Dinoflagellate community, which strongly indicates that the bloom of Prorocentrum donghaiense played a vital role in shaping the Dinoflagellate community structure, possibly via processes such as allelopathy, nutrient and space competition, and fast growth itself. Although these results were not beyond their anticipation, Wang et al. believe the work provides meaningful and solid evidence for the negative effects of Harmful Algal Blooms on the plankton community and coastal ecosystem based on a comprehensive series of field sampling and high throughput pyrosequencing.
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