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We evaluated how eutrophication and humification impacted nutrient cycles and the efficiency of carbon transfer in the planktonic food webs of 12 lakes in north-eastern Poland that differed in trophic state. Our results indicated that the effectiveness of carbon transfer between phytoplankton and zooplankton varied from 0.0005% to 0.14%, which is much lower than the theoretical 10%. The highest efficiency of carbon transfer occurred in the mesotrophic lakes due to the higher hypolimnetic zooplankton production, while the lowest efficiency was observed in the dystrophic lakes and in one eutrophic lake that was dominated by cyanobacteria. Inedible algae (e.g. Gonyostomum semen) and cyanobacteria appeared to be the main factors reducing the efficiency of the transfer of matter in pelagic food webs. The results of our study showed that plankton communities are a key component of the nutrient cycle in freshwater food webs. Phytoplankton were a very effective nitrogen sink, and in the mesotrophic lakes, up to 99% of the total nitrogen was sequestrated in phytoplankton. As a result, there was a depletion of inorganic nitrogen in the upper water layer. Furthermore, zooplankton were an important phosphorus sink, thus significantly influencing the nutrient cycles.

The freshwater raphidophyte Gonyostomum semen forms extensive summer blooms in northern European humic lakes. The development of these blooms might be facilitated by a lack of natural top-down control, as few zooplankton species are able to prey on these large algal cells (up to 100 μm) that expel trichocysts upon physical stress. In this study, we describe a small ciliate species (< 17 μm) that preys on G. semen by damaging the cell membrane until cytoplasm and organelles spill out. Sequencing of clonal cultures of the ciliate tentatively identified it as the prostomatid species Urotricha pseudofurcata . Grazing experiments illustrated that feeding by U. cf. pseudofurcata can significantly reduce cell concentrations of the microalga. However, differences in cell size and growth rate between two investigated ciliate strains resulted in noticeably different grazing pressure. Environmental sequencing data from five different lakes supported potential interactions between the two species. Urotricha cf. pseudofurcata might, thus, play an important role in aquatic ecosystems that are regularly dominated by G. semen , reducing the abundance of this bloom-forming microalga and enabling transfer of organic carbon to higher trophic levels.

The study of microbial diversity over time and space is fundamental to the understanding of their ecology and evolution. The underlying processes driving these patterns are not fully resolved but can be studied using population genetic approaches. Here we investigated the population genetic structure of Gonyostomum semen, a bloom-forming phytoplankton species, across two continents. The species appears to be expanding in Europe, whereas similar trends are not observed in the USA. Our aim was to investigate if populations of Gonyostomum semen in Europe and in the USA are genetically differentiated, if there is population genetic structure within the continents, and what the potential drivers of differentiation are. To this end, we used a novel method based on single-amplified genomes combined with Restriction-site Associated DNA sequencing that allows de novo genotyping of natural single-cell isolates without the need for culturing. We amplified over 900 single-cell genomes from 25 lake populations across Europe and the USA and identified two distinct population clusters, one in Europe and another in the USA. Low genetic diversity in European populations supports the hypothesized recent expansion of Gonyostomum semen on this continent. Geographic population structure within each continent was associated with differences in environmental variables that may have led to ecological divergence of population clusters. Overall, our results show that single-amplified genomes combined with Restriction-site Associated DNA sequencing can be used to analyze microalgal population structure and differentiation based on single-cell isolates from natural, uncultured samples.

Microalgae are fundamentally important organisms for global ecosystem functioning with high potential in biotechnology and its applications. The knowledge of their nuclear DNA content has become a prerequisite for many areas of microalgal research. Due to common presence of various pigments, secondary metabolites and complex cell walls, the nuclear DNA content estimation using flow cytometry (FCM) is, however, often laborious or even impossible with the currently used protocols. In this study the performance of six nuclei isolation protocols was compared on various problematic microalgae using FCM. The nuclei isolation methods involved osmotic bursting of cells, razor blade chopping of fresh biomass and two newly introduced protocols, razor blade chopping of desiccated biomass and bead beating. These techniques also involved the use of two different nuclei isolation solutions, Otto I + II solutions, and LB01 buffer. Performance of the particular protocols differed greatly, depending on the used nuclei isolation solution and microalgal group. The most successful method was a newly adopted chopping of desiccated biomass in LB01 buffer. This method seems more appropriate for nuclei isolation in filamentous microalgae; on the other hand, bead beating appears to be more suitable for nuclei isolation in solitarily living algae. Using the optimal protocol for a given species, their nuclear DNA content was estimated, resulting in first DNA content estimates for four investigated taxa (Chlamydomonas noctigama, Gonyostomum semen, Microglena sp. and Stigeoclonium sp.). The estimated DNA content spanned from 0.15 to 32.52 pg.

Algal blooms occur frequently in lakes and oceans and the causes and consequences of those are often studied. In this study, we focus on a less well known type of algal bloom by the freshwater raphidophyte Gonyostomum semen . This species’ abundance and occurrence is increasing, especially in brown water lakes, the most abundant lake type in the boreal zone. The aim of the study was to investigate which environmental factors are associated with G . semen by statistical evaluation of field data of 95 Swedish lakes over five years. Although we found G . semen to be associated with dark waters it was, contrary to our expectations, mainly high concentrations of iron, and only to a lesser extent high TOC (total organic carbon) concentrations, that were associated with blooms of G . semen . In addition, high phosphorus concentrations and low pH also appear to facilitate G . semen blooms. We suggest that browning of lakes caused by increased iron concentrations may decrease net heterotrophy by fostering heavy algal blooms, i.e. the opposite to commonly assumed effects of increased DOM (dissolved organic matter).

The ability to identify drivers responsible for algal community shifts is an important aspect of environmental issues. The lack of long-term datasets, covering periods prior to these shifts, is often limiting our understanding of drivers responsible. The freshwater alga, Gonyostomum semen (Raphidophyceae), has significantly increased distribution and mass occurrences in Scandinavian lakes during the past few decades, often releasing a skin irritating slime that causes discomfort for swimmers. While the alga has been extensively studied, long-term data from individual lakes are often absent or greatly limited and drivers behind this species’ success are still not clear. However, if specific and persistent taxa biomarkers for G. semen could be detected in dated sediment cores, long-term data would be improved and more useful. To test for biomarkers, we examined the pigment composition of several G. semen strains in culture. Further, dated sediment core samples from Lake Lundebyvann, Norway, were used to test the pigments’ suitability as biomarkers in paleolimnological studies. Modifications to a common analysis allowed for the successful detection of the polar xanthophyll heteroxanthin and the non-polar chlorophyll a, as well as several other algal pigments by using high performance liquid chromatography-photometric diode arrays (HPLC-PDA). Heteroxanthin was confirmed by liquid chromatography-mass spectrometry (LC-MS) and detected by HPLC-PDA in all examined G. semen strains, along with chlorophyll a. Using HPLC-PDA, we also identified and confirmed the presence of the biomarker, xanthophyll heteroxanthin, in sediment core samples up to 60 years of age. The specificity of this xanthophyll was also tested by examining a wide range of algal strains from common Norwegian phytoplankton species. Heteroxanthin was not detected in any species commonly occurring in significant amounts in Norwegian lakes. We therefore conclude that heteroxanthin is a suitable pigment biomarker for G. semen and that this pigment can be successfully used for paleolimnological studies.

Physical defenses and grazer avoidance of the bloom-forming microalga Gonyostomum semen may reduce the direct coupling between phytoplankton and higher trophic levels and result in an increased importance of alternative basal food resources such as bacteria and heterotrophic protozoans. To assess the importance of algal and heterotrophic food resources for zooplankton during G. semen blooms and the effects of zooplankton diets on a higher consumer, we analyzed the fatty acid composition of zooplankton and the invertebrate predator Chaoborus flavicans from eight lakes along a gradient in the predominance of G. semen relative to other algae and the duration of G. semen blooms. The proportion of fatty acids of bacterial origin increased significantly along the G. semen gradient in all consumers studied. In addition, the proportion of polyunsaturated fatty acids (PUFA) decreased in cladocerans. These results suggest that heterotrophic pathways can compensate for a reduced trophic coupling between phytoplankton and filter-feeding zooplankton. The lower PUFA content in cladoceran prey from lakes at the higher end of the G. semen gradient did not affect the PUFA content of the predator C. flavicans, suggesting selective assimilation and retention of PUFA and/or feeding on other, more PUFA-rich prey.

The invasive and nuisance microalga Gonyostomum semen has recently expanded its geographical distribution and increased its biomass in Sweden and Finland. Temperatures, total organic carbon (TOC), water colour and phosphorous are main factors suggested as drivers for its success. Already in the 1980s there were indications of the same patterns also in Norway, and in this study we further examine these observations based on the hypotheses that (1) G. semen has continued its increase in abundance in Norway since then, (2) after settling in a lake, G. semen may increase its biomass, provided a simultaneous change in climatic factors. We use existing data from Norwegian monitoring databases for the study and additional water samples to investigate distribution. G. semen was detected by microscopy and quantitative PCR, while trends over time in G. semen biomass and possible explanatory variables were analysed by simple regression. We show that G. semen has increased its distribution in Norway since the 1980s, geographically and to new lakes. In eight of the nine lakes studied, the proportion of G. semen biomass in lake phytoplankton has increased with time. These changes seem to be explained only to some degree by temperature, TOC, water colour and phosphorous.

Elevated concentrations of dissolved organic carbon (DOC) promote browning of lakes, which alters the physicochemical properties of water and ecosystem functioning. However, browning‐induced changes in basal production of polyunsaturated fatty acids from the n‐3 and n‐6 families (n‐3 and n‐6 PUFA) in lakes are not fully understood. The concentrations of PUFA, which are micronutrients required to maintain growth and reproduction of consumers, have been suggested to either rise or decline in seston as a response to lake browning. Elevated DOC concentrations may also promote bacterial biomass and production and thus potentially increase the concentration of bacterial fatty acids (BAFA) in seston. We analyzed phytoplankton, bacteria and heterotroph biomasses, the taxonomic composition of phytoplankton, and the concentrations and mass fractions of bioseston PUFA, BAFA, eicosapentaenoic, and docosahexaenoic acids in ten boreal lakes in eastern Finland, with DOC concentration ranging from 2.8 to 18.7 mg/L. Our results showed that the abundance of PUFA in seston depended on the responses of phytoplankton biomass and community composition to lake browning. Lake browning increased seston PUFA and BAFA concentrations (expressed as µg/L) but not the contents (expressed as µg/mg bioseston C). Although low DOC lakes had a favorable phytoplankton community (in terms of PUFA content), the phytoplankton biomass in these oligotrophic lakes was so low that the concentration of PUFA remained low compared to high DOC lakes. The increasing concentration of PUFA in bioseston along the DOC gradient was mainly due to the increasing biomass of nuisance alga Gonyostomum semen. However, Gonyostomum may be too large for small‐sized zooplankton to ingest, and thus, the trophic transfer of PUFA may be impaired. The trajectories for lake browning and the basal production of PUFA also may depend on the source of carbon and associated nutrient loading; DOC and nutrient loading from agricultural areas may promote cyanobacteria dominance and decrease PUFA availability in lakes, while DOC runoff from more acidic and nutrient‐poor peatlands may promote Gonyostomum dominance and increase seston PUFA concentration.

Biological invasions often cause major perturbations in the environment and are well studied among macroorganisms. Less is known about invasion by free-living microbes. Gonyostomum semen (Raphidophyceae) is a freshwater phytoplankton species that has increased in abundance in Northern Europe since the 1980's and has expanded its habitat range. In this study, we aimed to determine the genetic population structure of G. semen in Northern Europe and to what extent it reflects the species' recent expansion. We sampled lakes from 12 locations (11 lakes) in Norway, Sweden and Finland. Multiple strains from each location were genotyped using Amplified Fragment Length Polymorphism (AFLP). We found low differentiation between locations, and low gene diversity within each location. Moreover, there was an absence of genetic isolation with distance (Mantel test, p = 0.50). According to a Bayesian clustering method all the isolates belonged to the same genetic population. Together our data suggest the presence of one metapopulation and an overall low diversity, which is coherent with a recent expansion of G. semen.