Mikołaj Kokociński

The global rise of harmful cyanobacterial blooms (HCBs) in freshwater systems is driven by anthropogenic pressure and climate change, yet limited information exists on how these factors shape HCBs and their phycosphere-associated bacterial communities (PBCs). We examined the influence of eutrophication (nutrient concentrations and land use), climate zone, and seasonality on phytoplankton and PBCs across three freshwater bodies: hypertrophic Lake Amatitlán with urban and agricultural catchments, mesotrophic Laguna del Pino with a forested/agricultural catchment (both tropical, Guatemala), and eutrophic Sulejów Reservoir (temperate, Poland). Surface water was collected (2024) during the dry season (Guatemala) and summer (Poland). Phytoplankton was analyzed via qPCR and microscopy; bacterioplankton via 16S rRNA sequencing; and microcystins via HPLC. Lake Amatitlán was dominated by toxic Microcystis exceeding World Health Organization recreational microcystin thresholds, especially in the agricultural catchment. Sulejów Reservoir showed a seasonal bloom of Microcystis , Aphanizomenon , and Snowella . In contrast, the diatom Cyclotella dominated lower-nutrient Laguna del Pino. PBCs included Flavobacterium , Pseudomonas , and Limnohabitans in nutrient-rich waters, whereas Polynucleobacter , often associated with good water quality, was abundant in Laguna del Pino. Nutrient enrichment was associated with cyanobacterial dominance and PBC structure in tropical systems, while nutrient availability and temperature were associated with bloom dynamics in the temperate system.

Objective: In recent decades, the invasive cyanobacteria Raphidiopsis raciborskii and Raphidiopsis mediterranea have expanded their distribution globally, particularly in temperate regions. Understanding the ecological drivers of Raphidiopsis distribution is imperative to addressing the challenges associated with these species. Here, we aimed to characterize the distribution and biomass of R. raciborskii and R. mediterranea across 112 lakes in Poland and Lithuania in relation to local and regional factors.Research design and methods: Integrated water samples were collected from 102 Polish and 10 Lithuanian lakes from different regions for phytoplankton and chemical analyses. The lakes varied in surface area, and exhibited diverse mixing regimes, trophic states, and morphometries. Phytoplankton was identified and quantified using a Fuchs-Rosenthal or Nageotte chamber. Additionally, we characterized the degree of human pressures the climatic constraints experienced by each lake.Results: R. raciborskii occurrence has increased in eastern regions of Poland but biomass is relatively low compared to western Poland, likely due to lower air temperatures and nutrient concentrations, especially phosphorus. In contrast, R. mediterranea only occurred in a small number of lakes in Poland, and in a single lake in Lithuania, with no relation to measured local and regional variables.Conclusions: Our study shows contrasting patterns in the distribution of two invasive cyanobacteria species in Europe, highlighting the importance of climate and nutrients to the distribution of R. raciborskii, the most widespread species, and providing relevant information for decision making and conservation strategies.

Cyanobacteria, particularly Raphidiopsis raciborskii, are known for their harmful blooms, posing significant ecological and economic challenges in aquatic ecosystems. This study investigates the strain‐specific adaptive mechanisms of five R. raciborskii strains (CS‐505, CS‐506, UAM/DH‐KmRr, UAM/DH‐BiRr, and UAM/DH‐MRr) under chill/light stress (low temperature and high light intensity), focusing on growth, pigment composition, photosynthetic activity, gene expression, and toxin production. Results revealed that growth rates were significantly reduced under stress for all strains, with the Polish strain UAM/DH‐MRr showing the highest resilience. Stress conditions caused a reduced chlorophyll a level and increased carotenoid/Chl a ratio, suggesting photoprotective adaptations. Photosynthetic quantum efficiency (Fv/Fm) declined during chill/light but was recovered post‐stress, while electron transport rates (rETR) were not significantly altered. Enhanced respiration and increased photosynthetic oxygen evolution were observed, particularly in CS‐506 and UAM/DH‐MRr. Principal component analysis (PCA) highlighted pigment content and parameters related to photosynthetic electron transport as key factors influencing response; no variable‐specific groupings were observed among strains regarding the broad range of parameters. Gene expression analyses revealed divergent transcriptional regulation of genes involved in photosynthesis and cylindrospermopsin (CYN) biosynthesis, with toxic CS‐505 upregulating cyr genes under stress, while CS‐506 down‐regulated them. Intracellular CYN levels decreased under stress, yet cell quotas increased in CS‐505, which warrants further investigation. Our results indicate the complexity of strain‐specific responses to chill/light stress and highlight the importance of understanding physiological mechanisms for predicting and managing cyanobacterial blooms in diverse climates.

The impacts of extreme coastal events (ECEs), such as marine inundations or extreme wind events, on lake ecosystems vary widely from minimal to catastrophic. Accurately predicting the response of a specific system remains challenging due to a limited understanding of the attributes that drive the resilience of lakes. In an attempt to better understand the possible impacts of ECEs on shallow brackish lakes, we employed a paleolimnological approach to reconstruct the responses of Broad Pond to four ECEs identifiable from clear sedimentary markers and dated to ca. 1740, 1790, 1862, and 1993 CE. We aimed to evaluate the overall impacts of ECEs on Broad Pond and their specific effects on various hydrobiont groups. We investigated the sediment core covering the last ca. 400 years by examining terrestrial and aquatic pollen, spores and nonpollen palynomorphs, cladocerans, chironomids, and diatoms. Overall, Broad Pond exhibited resilient responses to ECEs, as indicated by mostly limited compositional turnovers in our proxy records. Statistically significant relationships between species composition and ECEs were observed only for diatoms. The only prolonged change identified is the spread of previously near-absent chironomids in the aftermath of a slight lake refreshment likely associated with the ca. 1740 CE event. This event was also followed by a short-lived (ca. 15 years) five-fold increase in the abundance of Scenedesmus that could have been triggered by the effects of the ECE on nutrient availability. The impact of the remaining three ECEs was discernible only in the diatom record, consistently showing a decline in two calcifobic and oligotrophic species, Achnanthidium petersenii and Platessa oblongella, also evident for the ca. 1740 CE event. The relatively minor ecosystem alteration induced by ECEs in Broad Pond lacks a single explanation and requires consideration of multiple site- and event-specific factors.

Cyanobacterial blooms are increasingly common during winters, especially when they are mild. The goal of this study was to determine the summer and winter phytoplankton community structure, cyanotoxin presence, and toxigenicity in a eutrophic lake susceptible to cyanobacterial blooms throughout the year, using classical microscopy, an analysis of toxic cyanometabolites, and an analysis of genes involved in biosynthesis of cyanotoxins. We also assessed whether cyanobacterial diversity in the studied lake has changed compared to what was reported in previous reports conducted several years ago. Moreover, the bloom-forming cyanobacterial strains were isolated from the lake and screened for cyanotoxin presence and toxigenicity. Cyanobacteria were the main component of the phytoplankton community in both sampling times, and, in particular, Oscillatoriales were predominant in both summer (Planktothrix/Limnothrix) and winter (Limnothrix) sampling. Compared to the winter community, the summer community was denser; richer in species; and contained alien and invasive Nostocales, including Sphaerospermopsis aphanizomenoides, Raphidiopsis raciborskii, and Raphidiopsis mediterranea. In both sampling times, the blooms contained toxigenic species with genetic determinants for the production of cylindrospermopsin and microcystins. Toxicological screening revealed the presence of microcystins in the lake in summer but no cyanotoxins in the winter period of sampling. However, several cyanobacterial strains isolated from the lake during winter and summer produced anabaenopeptins and microcystins. This study indicates that summer and winter blooms of cyanobacteria in the temperate zone can differ in biomass, structure, and toxicity, and that the toxic hazards associated with cyanobacterial blooms may potentially exist during winter.