Research into molecular machinery of eukaryotic unicellular organisms (protists) gives a unique opportunity to unveil mechanisms of intricate cellular processes and complete life cycles at the level of a single cell as a whole organism. Knowledge of protistan molecular biology and life cycle shifts as adaptations to varying physical-chemical features of the environment helps resolve important issues like evaluation of the balance between external abiotic triggers and internal molecular drivers of cells development. Understanding molecular mechanisms of these processes is crucial for prognostic modeling of population growth of the ecologically and economically important protists like toxic species forming harmful algal blooms (HABs) that currently expand worldwide. Here, we review basic features of bloom-forming mixotrophic dinoflagellates as drivers of their excess proliferation and HABs. We unveil key components of molecular machinery involved in life cycle regulation of dinoflagellates and highlight the deficiency/redundancy of nitrogen- and phosphorus-containing substrates as modulators of life cycles, diversity and nutrition strategies, including mixotrophy and kleptoplasty. We discuss linkages between cellular and molecular biology, omics and ecological modeling, since merging research approaches of these disciplines is a promising perspective for future progress of molecular biology within the framework of translational aquatic ecology.

The impact of nitrogen on harmful algal blooms (HABs) and functions of biota in marine ecosystems under eutrophication is a topical issue of growing importance. The article aimed at describing the diversity of planktonic bloom-forming dinoflagellates in the SW Baltic Sea coastal waters under variable eutrophication. The analysis of 44 year-long database revealed 82 dinoflagellate species and demonstrated diversity patterns of ten common bloom-forming species, including seven mixotrophs from the genera Prorocentrum, Dinophysis, and Ceratium, under variable eutrophication evaluated using total nitrogen (TN) content in water. Based on the Intermediate Disturbance Hypothesis (IDH), we presumed those coastal waters with total nitrogen concentrations that are optimal to dinoflagellates to host greater taxonomic diversity compared to areas with non-optimum TN content. The results showed that the highest dinoflagellate species richness was associated with much lower TN concentrations than the optimum values for these species. Thus, our findings disagreed with the IDH. We suggested and discussed possible reasons of this inconsistency, including algal growth rates and disturbance frequency. We also updated the classification of eutrophication levels in the Baltic Sea based on the distribution of TN content and diversity of HAB-forming dinoflagellates. The results can contribute to predictive assessment of HABs under growing eutrophication.

Kleptoplastidy is a nutrition mode in which cells of protists and some multicellular organisms acquire, maintain, and exploit chloroplasts of prey algae cells as photosynthesis reactors. It is an important aspect of the mixotrophic feeding strategy, which plays a role in the formation of harmful algae blooms (HABs). We developed a new mathematical model, in which kleptoplastidy is regarded as a mechanism of enhancing mixotrophy of protists. The model is constructed using three thought (theoretical) experiments and the concept of biological time. We propose to measure the contribution of kleptoplastidy to mixotrophy using a new ecological indicator: the kleptoplastidy index. This index is a function of two dimensionless variables, one representing the ratio of photosynthetic production of acquired chloroplasts versus native chloroplasts, and the other representing the balance between autotrophic and heterotrophic feeding modes. The index is tested by data for the globally distributed, bloom-forming potentially toxic mixotrophic dinoflagellates Prorocentrum cordatum. The model supports our hypothesis that kleptoplastidy can increase the division rate of algae significantly (by 40%), thus boosting their population growth and promoting blooms. The proposed model can contribute to advancements in ecological modeling aimed at forecasting and management of HABs that deteriorate marine coastal environments worldwide.

The analysis of species composition and quantitative characteristics of phototrophic unicellular eukaryotes (protists) and prokaryotes (cyanobacteria) of phytoplankton communities in the Guinea-Bissau zone of the central-eastern Atlantic Ocean during the cold season of 2013 was carried out for the first time.In total, 189 phytoplankton species from seven taxonomic divisions were identified; the majority of permanent species was represented by dinoflagellates.Based on the results of multivariate statistical analysis, three phytoplankton communities were distinguished in the study region: neritic, secondary ecotone type, and distant neritic community.The highest phytoplankton biomass values were recorded in the neritic community in the shelf zone and were confined to freshened, nutrients-saturated upwelling waters.The average values of the phytoplankton abundance and biomass in the Guinea-Bissau coastal area corresponded to the mesotrophic conditions and were nearly thrice as high as in the Moroccan zone.

The ambivalence of planktonic invaders has been studied using the example of ecosystem effects of the invasive cladoceran Cercopagis pengoi (Ostroumov, 1891) in the Vistula Lagoon of the Baltic Sea. The influence of the invader on the taxonomic structure and productivity of plankton is studied. It is discovered that, in the long term, the predation pressure of C. pengoi on zooplankton has decreased; the abundance, biomass, and production of the dominant species of Rotifera, Cladocera, and Copepoda also declined. The ne-gative impact of C. pengoi on the planktonic community is reflected in a reduction in the food supply of juvenile Baltic herring and other planktivorous fish. The wide ecological niche of Cercopagis contributes to the expansion of its range, which can lead to a reduction in populations of phytophagous crustaceans, increased harmful algae blooms, and the further deterioration of the food supply of fish.

Harmful algal blooms (HABs) and their consequences cause multiple devastating effects in various freshwater, brackish and marine ecosystems. However, HAB species at moderate population densities have positive ecological roles as primary producers of organic matter and food for zooplankton and fish. They also enhance benthic-pelagic coupling and participate in the biogeochemical cycles. The consequences of HABs are transported across the conventional environmental boundaries by numerous cascade effects in the food webs and beyond. Meanwhile, forecasts of bloom events are still limited, largely because of scarcity of reliable information on ecological niches of the bloom-forming algae. To fill up this knowledge gap, this study focused on dinoflagellates, a diverse group of mostly photosynthesizing protists (unicellular eukaryotes) capable of mixotrophy, since they play a key role in primary production and formation of blooms in marine and brackish waters worldwide. In this study, ecological niches of 17 abundant bloom-forming dinoflagellate species from coastal regions of the southern Baltic Sea were identified for the first time. It was hypothesized that wider ecological niches ensure more frequent dinoflagellate blooms compared to the species with narrower niches. This hypothesis was verified using the long-term (44 years) database on phytoplankton abundance and physical-chemical characteristics of the environment. It were analyzed 4534 datasets collected from 1972 to 2016. Fourteen abiotic parameters (water temperature, salinity, Secchi depth, pH, Chl a, and concentration of basic nutrients) were considered as ecological niche dimensions. The Principal Component Analysis presented the dissolved inorganic nitrogen, total nitrogen, Chl a, and temperature as principal niche dimensions of dinoflagellates. The algal bloom criteria were refined. It was for the first time proved statistically that HAB frequency of dinoflagellate species robustly correlated with the width of their ecological niches.

Cyanobacterial blooms occur regularly during summer and fall in various freshwater and brackish environments worldwide causing harm to aquatic flora, fauna, and water quality in natural ecosystems as well as in aquaculture. They also interfere with industrial fisheries, tourism, and human health. Meanwhile, up to date limited knowledge exists about these detrimental phenomena in brackish waters, and patterns of their expansion across the freshwater-to-marine continuum remain unclear. In this study, we analyzed long-term (1972-2016) dynamics of cyanobacterial harmful algal blooms (cyanoHABs) in the brackish southern Baltic Sea coastal waters, refined the cyanoHAB criteria, and for the first time measured ecological niches of the dominant bloom-forming cyanobacteria. The ecological niches of selected cyanoHAB species were determined considering a set of abiotic parameters as niche dimensions: water temperature, salinity, and concentrations of basic nutrients (PO43-, SiO44-, dissolved inorganic nitrogen - DIN, total phosphorus - TP, total nitrogen - TN, and TN/TP ratio), with special emphasis on salinity. We hypothesized that in highly variable brackish environments, the cyanobacterial species with broader ecological niches occur and bloom more often compared to those with narrower niches. For testing this hypothesis, we evaluated the effects of ecological niche width on the occurrence of the dominant cyanoHAB species and frequency of their blooms. The results revealed a pronounced statistically significant (p < 0.05) positive correlation between salinity-niche width and frequency of cyanoHABs. Moreover, the occurrence of cyanoHAB species with wider DIN and PO43- concentration-niches was substantially higher than that of the species with narrower nutrient-concentration niches. The effect of temperature-niche width on the occurrence of cyanoHAB species, on the contrary, was negligible and statistically insignificant. The latter result concurs with the fact that most of these cyanobacteria are highly thermophilic: on average, they bloom at temperatures above 21 degrees C, and their realized temperature niches in the southern Baltic coastal waters are relatively narrow.

Actual problems of planktonology, their detailed consideration and in-depth analysis at the modern scientific level are of great importance both for fundamental research and for generalizations in the field of aquatic ecology, and for solving the most important tasks of applied hydrobiology, fisheries and environmental protection. In this regard, in September 2022, scientists from Kaliningrad gathered domestic and foreign planktonologists for the fourth time on the Baltic coast to participate in the scientific forum "Actual problems of planktonology". The most striking results and the latest achievements were presented at the forum in the field of studying species richness, functional diversity, systematics, distribution features and specifics of trophic relationships of planktonic invertebrates and algoflora. Special attention was paid to harmful phytoplankton blooms and the disclosure of ecological, physico-chemical and molecular-cellular mechanisms of these dangerous phenomena, as well as the importance of planktonic organisms in bioindication, the role of invasive species in aquatic ecosystems and anthropogenic impact on populations and communities of planktonic organisms.

Harmful algal blooms (HABs) in aquatic ecosystems constitute one of the topical research issues in modern planktonology.Currently, HABs have intensified in many freshwater basins and marine coastal regions of the world because of the increasing anthropogenic loads enhanced by climate change.Aftereffects of these events reduce water quality and deteriorate the environment, fisheries and aquaculture, flora, fauna, and human health.Harmful blooms are most prominently expressed in coastal ecosystems and semi-closed brackish-water seas.However, HAB patterns in the highly variable brackish coastal environments are seriously understudied; therefore, research and forecasting of these events is still hindered.The article presents an overview of recent studies using multidisciplinary approach, which allows for applying modern molecular-genetic, cellular-biological, biochemical and biophysical techniques, bioinformatics, and methods of aquatic ecology for obtaining the most urgently needed new biological data and their inclusion into prognostic mathematical models.These topical issues were addressed in details and widely discussed at the IV All-Russian Conference "Frontiers in Plankton Research" held in Svetlogorsk (Kaliningrad Region, Russia) on 25-30 September 2022.

The article is devoted to harmful blooms of potentially toxic planktonic dinoflagellates, which do not synthesize toxins, but are capable of releasing secondary metabolites the high concentration of which in the water column is detrimental to aquatic animals and dangerous to human health. Modern ideas about the key ecological, physicochemical, cellular, and molecular genetic prerequisites for the occurrence of "red tides " of dinoflagellates are analyzed. Effective adaptation strategies of potentially toxic dinoflagellates are considered based on the example of species of the genus Prorocentrum, characterized by significant intrapopulation heterogeneity, mixotrophic metabolism, and wide ecological plasticity in coastal waters of the Baltic Sea.

Despite a large array of knowledge about triggers and drivers of phytoplankton dynamics in the sea coastal waters there is still no comprehensive understanding of what combinations of hydro-physical, hydro-chemical and biotic characteristics induce harmful algal blooms (HABs), which nowadays expand globally under the conditions of warm weather, high irradiance, and reduced turbulent water mixing in the absence of wind. Recent laboratory experiments highlighted the importance of abiotic (physico-chemical) stability as a generic environmental feature responsible for various modes of plankton dynamics, including chaotic behavior. In contrast to experimental studies, until recently abiotic stability in natural estuarine and coastal ecosystems was considered a rare phenomenon, which is particularly difficult to detect and measure. In this study, we aimed at measuring stability of the major physico-chemical parameters in a coastal marine ecosystem and assessing its impact on the algal proliferation. We hypothesized that abiotic stability in natural environments can eventually facilitate HABs. To test this hypothesis, we used the database collected in the southern Baltic Sea coastal waters between 1988 and 2016. We analyzed long-term trends in phytoplankton composition and abundance, with the focus on bloom-forming dinoflagellates. Fifteen dinoflagellate bloom events were registered during the three decades of study; nearly all blooms were dominated by the Prorocentrum species, most often by the mixotrophic P. cordatum. Stability of eleven abiotic characteristics (water temperature and salinity, pH, Secchi depth, concentrations of chlorophyll a, total phosphorus, total nitrogen, nitrite, nitrate, ammonium, and TN/TP ratio) was measured using the ‘synchronized’ and ‘shifted-data’ approaches. Our results verified the proposed research hypothesis and demonstrated that the dinoflagellate blooms were usually bound to higher stability of water temperature, pH, and concentrations of nitrogen compounds. Thus, we showed that abiotic stability, but not just the critical absolute values of the environmental parameters, can induce HABs. However, high adaptability and plasticity of feeding strategies employed by the dominant mixotrophic dinoflagellates hamper the exposure of clear correlations between stability of nutrients concentration and the magnitude of blooms. These findings allow assuming that the ‘paradox of chaos’ – the phenomenon, which was originally discovered in the abiotically stable experimental ecosystems, can also manifest itself in natural sea coastal waters, demonstrating how the complex biotic interactions drive plankton dynamics and promote HABs in the absence of external abiotic triggers.

The article compares ecological requirements of the potentially toxic bloomforming dinoflagellates Prorocentrum cordatum and their grazers, the neritic calanoid copepods Acartia tonsa in the Baltic and Black seas.A preliminary analysis of the ecological niche characteristics of these two common invasive species allowed assuming the partial juxtaposition of their niches and speculating on the perspectives of this phenomenon for the invasion success and putative top-down control of harmful algal blooms in marine coastal ecosystems.

Despite the enticing discoveries of chaos in nature, triggers and drivers of this phenomenon remain a classical enigma which needs irrefutable empirical evidence. Here we analyze results of the yearlong replicated mesocosm experiment with multi-species plankton community that allowed revealing signs of chaos at different trophic levels in strictly controlled abiotic environment. In mesocosms without external stressors, we observed the “paradox of chaos” when biotic interactions (internal drivers) were acting as generators of internal abiotic triggers of complex plankton dynamics. Chaos was registered as episodes that vanished unpredictably or were substituted by complex behaviour of other candidates when longer time series were considered. Remarkably, episodes of chaos were detected even in the most abiotically stable conditions. We developed the Integral Chaos Indicator to validate the results of the Lyapunov exponent analysis. These findings are essential for modelling and forecasting behaviour of a variety of natural and other global systems.

The increasing inflow of nitrogen (N) substrates into marine nearshore ecosystems induces proliferation of harmful algal blooms (HABs) of dinoflagellates, such as potentially toxic invasive species Prorocentrum minimum. In this study, we estimated the influence of NO3-, NH4+ and urea on transcription levels and urea transporter dur3 and nitrate transporter nrt2 genes expression in these dinoflagellates. We identified dur3 and nrt2 genes sequences in unannotated transcriptomes of P. minimum and other dinoflagellates presented in MMETSP database. Phylogenetic analysis showed that these genes of dinoflagellates clustered to the distinct Glade demonstrating evolutionary relationship with the other known dur3 and nrt2 genes of microalgae. The evaluation of expression levels of dur3 and nrt2 genes by RT-qPCR revealed their sensitivity to input of the studied N sources. Dur3 expression levels were downregulated after the supplementation of additional N sources and were 1.7-2.6-fold lower than in the nitrate-grown culture. Nrt2 expression levels decreased 1.9-fold in the presence of NH4+. We estimated total RNA and DNA synthesis rates by the analysis of incorporation of 3H-thymidine and H-3-uridine in batch and continuous cultures. Addition of N compounds did not affect the DNA synthesis rates. Transcription levels increased up to 12.5-fold after the N supplementation in urea-limited treatments. Investigation of various nitrogen sources as biomarkers of dinoflagellate proliferation due to their differentiated impact on expression of dur3 and nrt2 genes and transcription rates in P. minimum cells allowed concluding about high potential of the studied parameters for future modeling of HABs under global N pollution. (C) 2019 Elsevier Ltd. All rights reserved.

The article presents the results of long-term monitoring studies on the abundance dynamics of invasive species Cercopagis pengoi (Ostroumov, 1891) in the Vistula (Kaliningradskii) Lagoon of the Baltic Sea and its impact on the structural and functional organization of zooplankton community. The data on the effect of the invader on the zooplankton taxonomic structure and productivity are presented. It was discovered that, in the long run, the general complexity of the plankton community increased, while the impact of C. pengoi on zooplankton as assessed by the Impact Index decreased. At the same time, the abundance and production of the dominant species of Rotifera, Cladocera, and Copepoda decreased. For prognostic purposes, the equation was applied to correlate the predation pressure of C. pengoi on the zooplankton community with the average abundance of this invader.

Biomarkers of temperature stress were studied as major characteristics crucial for the understanding complex processes that underlie the response of marine planktonic microorganisms to environmental factors and their sublethal effects. Using the potentially toxic dinoflagellates Prorocentrum minimum as a model object, the impact of temperature stress on viability, cell cycle, RNA synthesis and DNA replication in these protists was evaluated. It was shown by flow cytometry that stress evoked by a temperature increase from 25°C (control) to 37 or 42°C during 15 to 60 min did not cause any considerable alterations in the cell cycle, while cell death rate increased from ≤ 1% (control) to 2–12% at 37°C and 4–22% at 42°C. Along with a relatively low cell death rate, following a temperature increase to 37 and/or 42°C, P. minimum displayed the ability to boost the synthesis of DNA (1.7–1.9 and 1.2–1.6 times, respectively) and especially RNA (2.5–3.1 and 1.7–2.8 times, respectively) during the first 15–30 min after stress. At certain stages of the life cycle, this effect can be critical for maintaining the viability and normal development of the P. minimum population. The obtained results demonstrate that a significantly elevated synthesis of nucleic acids can serve as an indicator (biomarker) of sublethal environmental stress.