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Species belonging to the dinoflagellate genus Prorocentrum are known to cause red tides or harmful algal blooms. To understand the dynamics of a Prorocentrum sp., its growth and mortality due to predation need to be assessed. However, there are only a few Prorocentrum spp. for which heterotrophic protist predators have been reported. We explored feeding by the common heterotrophic dinoflagellates Gyrodinium dominans, Oxyrrhis marina, Pfiesteria piscicida, Oblea rotunda, and Polykrikos kofoidii and the naked ciliate Strombidinopsis sp. (approx. 90 µm cell length) on the planktonic species Prorocentrum triestinum, P. cordatum, P donghaiense, P rhathymum, and P micans as well as the benthic species P lima and P hoffmannianum. All heterotrophic protists tested were able to feed on the planktonic prey species. However, O. marina and O. rotunda did not feed on P lima and P hoffmannianum, while G. dominans, P kofoidii, and Strombidinopsis sp. did. The growth and ingestion rates of G. dominans and P kofoidii on one of the seven Prorocentrum spp. were significantly different from those on other prey species. G. dominans showed the top three highest growth rates when it fed on P. triestinum, P. cordatum, and P. donghaiense, however, P. kofoidii had negative growth rates when fed on these three prey species. In contrast, P kofoidii had a positive growth rate only when fed on P hoffmannianum. This differential feeding on Prorocentrum spp. between G. dominans and P kofoidii may provide different ecological niches and reduce competition between these two common heterotrophic protist predators.

Microalgae are unicellular organisms and commonly present in the euphotic zone of marine ecosystems. From the western coast of Mauritius, three strains of Prorocentrum species were isolated from macrophytes and cultured under standard laboratory conditions. Morphologies were examined by light, fluorescence, and scanning electron microscopy, and phylogenetic analyses were based on partial large subunit LSU rDNA (D1-D2) and ITS1-5.8S-ITS2 (ITS) regions. Three Prorocentrum species, including the P. fukuyoi complex, P. rhathymum, and P. lima complex, were identified. The antimicrobial activities were assayed against potential human pathogenic bacterial strains. The highest zone of inhibition was recorded for intracellular and extracellular protein extracts of Prorocentrum rhathymum against Vibrio parahaemolyticus. The polysaccharide extracts of the Prorocentrum fukuyoi complex had a higher zone of inhibition (24 ± 0.4 mm) against MRSA at a minimum concentration of 0.625 μg/mL. The extracts from the three Prorocentrum species had different levels of activity against the pathogens used, and this can be of scientific interest in the search for antibiotics from natural marine sources.

The dinoflagellates Karenia mikimotoi and Prorocentrum donghaiense are both important causative species of harmful algal blooms (HABs) in the East China Sea. The ichthyotoxic K. mikimotoi, which occasionally leads to large-scale HABs in the East China Sea, is difficult to be discriminated from other morphologically similar species in the family Kareniaceae by light microscope observation. To improve the accuracy and efficiency in detection of K. mikimotoi, a real-time quantitative PCR (qPCR) assay was developed in this study. The qPCR assay has high specificity and sensitivity, which allows the detection of K. mikimotoi at the lower detection limit of one cell. The qPCR assay target K. mikimotoi, together with another qPCR assay previously developed for P. donghaiense, was applied to study a bloom of dinoflagellates in the coastal waters of Fujian province from April 25 to June 11 in 2019. Cells of K. mikimotoi were detected in about half of the samples, and the maximum abundance was lower than 30 cells L−1. Abundance of P. donghaiense cells (maximum abundance above 106 cells L−1) were determined using both qPCR assay and light microscope cell counting, and the results of the two methods were consistent with each other. The qPCR assays of the blooming dinoflagellates offer reliable and accurate approaches for the detection of HABs species.

Marine toxins (MTs) are a group of structurally complex natural products with unique toxicological and pharmacological activities. In the present study, two common shellfish toxins, okadaic acid (OA) (1) and OA methyl ester (2), were isolated from the cultured microalgae strain Prorocentrum lima PL11. OA can significantly activate the latent HIV but has severe toxicity. To obtain more tolerable and potent latency reversing agents (LRAs), we conducted the structural modification of OA by esterification, yielding one known compound (3) and four new derivatives (4–7). Flow cytometry-based HIV latency reversal activity screening showed that compound 7 possessed a stronger activity (EC50 = 46 ± 13.5 nM) but was less cytotoxic than OA. The preliminary structure–activity relationships (SARs) indicated that the carboxyl group in OA was essential for activity, while the esterification of carboxyl or free hydroxyls were beneficial for reducing cytotoxicity. A mechanistic study revealed that compound 7 promotes the dissociation of P-TEFb from the 7SK snRNP complex to reactivate latent HIV-1. Our study provides significant clues for OA-based HIV LRA discovery.

Phytoplankton may serve as a key entry for methylmercury (MeHg) into aquatic food webs however very few studies have quantified the bioconcentration of MeHg in marine phytoplankton from seawater, particularly for non-diatoms. Experiments using 203Hg to measure MeHg uptake rates and concentration factors in six marine phytoplankton species belonging to different algal classes were conducted and the influence of light, temperature, and nutrient conditions on MeHg bioaccumulation were determined. All algal species greatly concentrated MeHg out of seawater, with volume concentration factors (VCFs) ranging from 0.2 × 10⁵ to 6.4 × 10⁶. VCFs were directly related to cellular surface area-to-volume ratios. Most of the cellular MeHg was found in the cytoplasm. Temperature, light, and nutrient additions did not directly affect MeHg uptake in most species, with the exception that the dinoflagellate Prorocentrum minimum displayed significantly greater uptake per cell at 18°C than at 4°C, suggesting an active uptake for this species. Passive transport seemed to be the major pathway for most phytoplankton to acquire MeHg and was related to the surface area-to-volume ratio of algal cells. Environmental conditions that promoted cell growth resulted in more total MeHg associated with cells, but with lower concentrations per unit biomass due to biodilution. The very high bioconcentration of MeHg in marine phytoplankton is by far the largest bioconcentration step in marine food chains and variations in algal uptake may account for differences in the amount of MeHg that ultimately builds up in different marine ecosystems.

Incubation experiments with axenic cultures of four common phytoplankton species of the genera Chaetoceros, Skeletonema, Prorocentrum, and Micromonas were performed to test for the production of fluorescent dissolved organic matter (FDOM) by marine phytoplankton. Our results prove that the four species exuded both fluorescent protein-like and marine humic—like materials in variable amounts, with more production by the diatoms Chaetoceros sp. and Skeletonema costatum and less by Prorocentrum minimum. Whereas the exudation of protein-like substances by healthy phytoplankton cells has been recognized, the in sity production of marine humic-like substances is still a matter of debate. Using axenic cultures, we demonstrate unequivocally that phytoplankton can directly contribute to the autochthonous production of colored humic-like substances in the ocean. Extrapolation of these findings to the field indicates that about 20% of the marine humic—like substances produced in the highly productive coastal upwelling system of the Ría de Vigo could originate from growing phytoplankton. Therefore, the exudation of FDOM by marine phytoplankton should be considered in future studies of the dynamics of colored DOM in marine systems.

We investigated a possible reduction in CO2 uptake rate by phototrophic red tide dinoflagellates arising from mixotrophy. We measured the daily ingestion rates of Prorocentrum minimum by Prorocentrum micans over 5 days in 10 L experimental bottles, and the uptake rates of total dissolved inorganic carbon (C sub( T)) by a mixture of P micans and P minimum (mixotrophic growth), and for the predator P micans (phototrophic growth; control) and prey P minimum (phototrophic growth; control) alone. To account for the effect of pH on the phototrophic growth rates of P micans and P minimum, measurements of C sub( T) and pH in the predator and prey control bottles were continued until the pH reached the same level (pH 9.5) as that in the experimental bottles on the final day of incubation. The measured total C uptake rate by the mixture of P micans and P minimum changed from 123 to 161 mu mol C. kg super( -1) d super( -1) over the course of the experiment, and was lower than the C sub( T) uptake rates shown by P micans and P minimum in the predator and prey control bottles, respectively, which changed from 132 to 176 pmol C sub( T) kg super( -1) d super( -1) over the course of the experiment. The reduction in total CT uptake rate arising from the mixotrophy of P micans was 7-31% of the daily C sub( T) uptake rate seen during photosynthesis. The results suggest that red tide dinoflagellates take up less C sub( T) during mixotrophy.

Harmful blooms of Prorocentrum donghaiense occur annually in the phosphorus-scarce coastal waters of the East China Sea (ECS). The enzymatic activities of alkaline phosphatase (AP) and its regulation by external phosphorus were studied during a P. donghaiense bloom in this area. The AP characteristics of P. donghaiense was further compared with Prorocentrum minimum and Prorocentrum micans in monocultures with both bulk and single-cell enzyme-labeled fluorescence AP assays. Concentrations of dissolved inorganic phosphorus (DIP) varied between 0.04 and 0.73 µmol l⁻¹, with more than half recording stations registering concentrations below 0.10 µmol l⁻¹. Concentrations of dissolved organic phosphorus (DOP) were comparable or even higher than those of DIP. P. donghaiense suffered phosphorus stress and expressed abundant AP, especially when DIP was lower than 0.10 µmol l⁻¹. The AP activities showed a negative correlation with DIP but a positive correlation with DOP. The AP activities were also regulated by internal phosphorus pool. The sharp increase in AP activities was observed until cellular phosphorus was exhausted. Most AP of P. donghaiense was located on the cell surface and some were released into the water with time. Compared with P. minimum and P. micans, P. donghaiense showed a higher AP affinity for organic phosphorus substrates, a more efficient and energy-saving AP expression quantity as a response to phosphorus deficiency. The unique AP characteristic of P. donghaiense suggests that it benefits from the efficient utilization of DOP, and outcompete other species in the phosphorus-scarce ECS.

Tetrodotoxins (TTXs) are traditionally associated with the occurrence of tropical Pufferfish Poisoning. In recent years, however, TTXs have been identified in European bivalve mollusc shellfish, resulting in the need to assess prevalence and risk to shellfish consumers. Following the previous identification of TTXs in shellfish from southern England, this study was designed to assess the wider prevalence of TTXs in shellfish from around the coast of the UK. Samples were collected between 2014 and 2016 and subjected to analysis using HILIC-MS/MS. Results showed the continued presence of toxins in shellfish harvested along the coast of southern England, with the maximum concentration of total TTXs reaching 253 µg/kg. TTX accumulation was detected in Pacific oysters (Crassostrea gigas), native oysters (Ostrea edulis) common mussels (Mytilus edulis) and hard clams (Mercenaria mercenaria), but not found in cockles (Cerastoderma edule), razors (Ensis species) or scallops (Pecten maximus). Whilst the highest concentrations were quantified in samples harvested during the warmer summer months, TTXs were still evident during the winter. An assessment of the potential causative factors did not reveal any links with the phytoplankton species Prorocentrum cordatum, instead highlighting a greater level of risk in areas of shallow, estuarine waters with temperatures above 15 °C.

Ocean-related global changes have altered phytoplankton community structure, especially the diatom-dinoflagellate competition, which further influences ecosystem structure and functions. The pivotal ecological roles of diatoms and dinoflagellates are strongly related with their biochemical composition, but quantitative comparisons of biochemical changes between diatoms and dinoflagellates under variable environments are still limited. We investigated responses of lipid biomarkers (sterols and fatty acids (FAs)) to different temperatures (12, 18 and 24 °C), nitrogen and phosphorus concentrations and their molar ratios (N:P ratios) of 10:1, 24:1 and 63:1 in marine diatom Phaeodactylum tricornutum and dinoflagellate Prorocentrum minimum. Over these wide ranges of temperature and nutrient conditions, sterol and FA profiles were relatively stable in the two species. For carbon-normalized contents of major lipid biomarkers, warming caused non-significant changes in the diatoms but an increase (up to 153%) in the dinoflagellates. Eutrophication caused an overall decrease (up to 53%) in major lipid biomarker contents in the diatoms but an overall increase (up to 77%) in the dinoflagellates. In contrast, imbalanced N:P ratios caused an overall increase (up to 64%) in major lipid biomarker contents in the diatoms but an overall decrease (up to 53%) in the dinoflagellates. Taken together, our results showed that the contents of major sterols and polyunsaturated FAs would change non-significantly in the diatom P. tricornutum, while those in the dinoflagellate P. minimum would increase (ca. 9–48%) under future ocean warming, eutrophication and imbalanced N:P ratios. This study expands our knowledge on lipid-based indicators of phytoplankton under changing environments, which by systematically linking with several other aspects of food quality will help to understand ecological consequences of diatom-dinoflagellate community changes.