Temperature × light interaction and tolerance of high water temperature in the planktonic freshwater flagellates Cryptomonas (Cryptophyceae) and Dinobryon (Chrysophyceae)

How does increasing temperature affect the toxicity of bisphenol A on Cryptomonas ovata and its consumer Daphnia magna?

The global rise in plastic production has led to significant plastic deposition in aquatic ecosystems, releasing chemical compounds as plastics degrade. Among these, bisphenol A (BPA) is a major global concern due to its endocrine-disrupting effects and widespread presence in aquatic environments. Furthermore, the toxicity of BPA on aquatic organisms can be modulated by global change stressors such as temperature, which plays an essential role in the metabolism of organisms, including the degradation and accumulation of toxic compounds. In this study, we aimed to understand how temperature can modulate the toxic effect of BPA on a phytoplankton species (Cryptomonas ovata) and how this effect can be transferred to its herbivorous consumer (Daphnia magna). To do this, we first determined the sensitivity of C. ovata over a BPA gradient (0-10 mg L-1). Subsequently, we experimentally determined how the increase in temperature (+5ºC) could modify the toxic effect of BPA on the physiology, metabolism and growth of the phytoplankton. Finally, we investigated how this effect transferred to the growth rate of D. magna through food. Our results show a negative effect of BPA on C. ovata from 5 mg BPA L-1, affecting its photosynthetic yield of photosystem II, net primary production, respiration, and growth. This effect was accelerated when the temperature was higher. Additionally, the growth rate of D. magna also decreased when fed on C. ovata grown in the presence of BPA and high temperature. Our results indicate that high temperature can accelerate the toxic effects of BPA on organisms located at the base of the food web and this effect could be transferred to higher levels through food.

Photophysiology of the haploid form of the cryptophyte Teleaulax amphioxeia

Cryptophytes are abundant and ubiquitous microalgae that constitute a major plastid source for kleptoplastidic ciliates and dinoflagellates. Despite their ecological significance, the understanding of their light preferences and photophysiology remains limited. Here, we provide a comprehensive study of the response of the haploid strain Teleaulax amphioxeia (Cr10EHU) to varying light irradiance. This strain is capable of growing under a wide range of irradiance levels, notably by finely tuning the different pigments bound to the membrane light-harvesting proteins. Analysis of the luminal phycoerythrin content revealed remarkable flexibility, with phycoerythrin emerging as a pivotal protein facilitating acclimation to varying light levels. Detailed ultrastructure examinations unveiled that this adaptability was supported by the synthesis of large thylakoidal vesicles, likely enhancing the capture of green photons efficiently under low light, a phenomenon previously undocumented. Teleaulax amphioxeia Cr10EHU effectively regulated light utilization by using a cryptophyte state transition-like process, with a larger amplitude observed under high growth irradiance. Furthermore, our results revealed the establishment of growth irradiance-dependent non-photochemical quenching of fluorescence, likely inducing the dissipation of excess light. This study underscores the particularities and the significant photoadaptability of the plastid of the haploid form of T. amphioxeia. It constitutes a comprehensive photophysiological characterization of the Cr10EHU strain that paves the way for future studies of the kleptoplastidy process.

Metabarcoding reveals potentially mixotrophic flagellates and picophytoplankton as key groups of phytoplankton in the Elbe estuary

In estuaries, phytoplankton are faced with strong environmental forcing (e.g. high turbidity, salinity gradients). Taxa that appear under such conditions may play a critical role in maintaining food webs and biological carbon pumping, but knowledge about estuarine biota remains limited. This is also the case in the Elbe estuary where the lower 70 km of the water body are largely unexplored. In the present study, we investigated the phytoplankton composition in the Elbe estuary via metabarcoding. Our aim was to identify key taxa in the unmonitored reaches of this ecosystem and compare our results from the monitored area with available microscopy data. Phytoplankton communities followed distinct seasonal and spatial patterns. Community composition was similar across methods. Contributions of key classes and genera were correlated to each other (p < 0.05) when obtained from reads and biovolume (R2 = 0.59 and 0.33, respectively). Centric diatoms (e.g. Stephanodiscus) were the dominant group - comprising on average 55 % of the reads and 66-69 % of the biovolume. However, results from metabarcoding imply that microscopy underestimates the prevalence of picophytoplankton and flagellates with a potential for mixotrophy (e.g. cryptophytes). This might be due to their small size and sensitivity to fixation agents. We argue that mixotrophic flagellates are ecologically relevant in the mid to lower estuary, where, e.g., high turbidity render living conditions rather unfavorable, and skills such as phagotrophy provide fundamental advantages. Nevertheless, further findings - e.g. important taxa missing from the metabarcoding dataset - emphasize potential limitations of this method and quantitative biases can result from varying numbers of gene copies in different taxa. Further research should address these methodological issues but also shed light on the causal relationship of taxa with the environmental conditions, also with respect to active mixotrophic behavior.

Pesticide responses of Arctic and temperate microalgae differ in relation to ecophysiological characteristics

Polar ecosystems play an important role in global primary production. Microalgae have adaptations that enable them to live under low temperature environments where irradiance and day length change drastically. Their adaptations, leading to different ecophysiological characteristics relative to temperate species, could also alter their sensitivity to pollutants such as pesticides. This study's objective was to understand how different ecophysiological characteristics influence the response of Arctic phytoplankton to pesticides in relation to the responses of their temperate counterparts. Ecophysiological endpoints were related to growth, cell biovolume, pigment content, photosynthetic activity, photoprotective mechanisms (NPQ, antioxidant enzyme activities), and reactive oxygen species (ROS) content. The Arctic species Micromonas polaris was more resistant to atrazine and simazine than its temperate counterpart Micromonas bravo. However, the other Arctic species Chaetoceros neogracilis was more sensitive to these herbicides than its temperate counterpart Chaetoceros neogracile. With respect to two other pesticide toxicity, both temperate microalgae were more sensitive to trifluralin, while Arctic microalgae were more sensitive to chlorpyrifos (insecticide). All differences could be ascribed to differences in the eco-physiological features of the two microalgal groups, which can be explained by cell size, pigment content, ROS content and protective mechanisms (NPQ and antioxidant enzymes).

Phytoplankton communities in temporary ponds under different climate scenarios

Temporary water bodies, especially vernal pools, are the most sensitive aquatic environments to climate change yet the least studied. Their functioning largely depends on their phytoplankton community structure. This study aimed to determine how temperature and photoperiod length (by simulating inundation in different parts of the year under five climate scenarios) affect the succession and structure of phytoplankton communities soon after inundation. Photoperiod was the most important factor affecting phytoplankton species richness, total abundance and the abundance of taxonomic groups in the course of succession. A long photoperiod (16 h) and a moderate temperature (16 °C) in vernal pool microcosms (late spring inundation after a warm snowless winter) were the most favourable conditions for phytoplankton growth (especially for the main taxonomic groups: chlorophytes and cryptophytes) and species richness. With short photoperiods (inundation in winter) and low temperatures, the communities transformed towards diatoms, euglenoids and cyanobacteria. In line with our predictions, a high temperature (25 °C) favoured a decline in phytoplankton species diversity. Our study shows that climate change will result in seasonal shifts in species abundance or even in their disappearance and, finally, in potential strong changes in the biodiversity and food webs of aquatic ecosystems in the future.

Container volume may affect growth rates of ciliates and clearance rates of their microcrustacean predators in microcosm experiments

We studied the effect of volume in small containers (microcosms) on five common planktonic freshwater ciliates and three zooplankton species, namely Daphnia sp., the calanoid copepod Eudiaptomus sp., and the cyclopoid copepod Cyclops sp. We measured ciliate specific growth rates and their loss rates due to microcrustacean predation in short-term experiments. We hypothesized that container volume ranging from 10 to 200 mL would not affect the activity of our prey and predator species. We found that the response to volume was species-specific; growth rates of three ciliate species were sensitive to volume. However, the volume effect was not unequivocal because different timing of the microcosm experiments (block effects) may have caused random bias due to varying morphological and/or physiological conditions of the ciliates and their predators. For predator clearance rate, the volume effect was insignificant in the filter-feeding Daphnia and Eudiaptomus but was significant for the predatory copepod Cyclops, which was hampered in the smallest experimental containers. Total crustacean clearance rates averaged over all treatments appeared unaffected by predator species, while ciliate species significantly affected the results. Our growth and clearance rates are close to previous findings with the same or similar planktonic prey and predator species.

Bacterial and eukaryotic microbial communities in urban water systems profiled via Illumina MiSeq platform

Microbial communities from a lake and river flowing through a highly dense urbanized township in Malaysia were profiled by sequencing amplicons of the 16S V3-V4 and 18S V9 hypervariable rRNA gene regions via Illumina MiSeq. Results revealed that Proteobacteria, Bacteroidetes, Actinobacteria and Firmicutes were the dominant prokaryotic phyla; whereas, eukaryotic communities were predominantly of the SAR clade and Opisthokonta. The abundance of Pseudomonas and Flavobacterium in all sites suggested the possible presence of pathogens in the urban water systems, supported by the most probable number (MPN) values of more than 1600 per 100 mL. Urbanization could have impacted the microbial communities as transient communities (clinical, water-borne and opportunistic pathogens) coexisted with common indigenous aquatic communities (Cyanobacteria). It was concluded that in urban water systems, microbial communities vary in their abundance of microbial phyla detected along the water systems. The influences of urban land use and anthropogenic activities influenced the physicochemical properties and the microbial dynamics in the water systems.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-020-02617-3.

Functional Ecology of Two Contrasting Freshwater Ciliated Protists in Relation to Temperature

We conducted microcosm experiments with two contrasting freshwater ciliates on functional traits (FTs) related to their growth rate (numerical response, NR) and ingestion rate (functional response, FR) over a range of ecologically relevant temperatures. Histiobalantium bodamicum and Vorticella natans are common planktonic ciliates but their abundance, swimming behavior, and temperature tolerance are different. In contrast to most sessile peritrich species, the motile V. natans is not strictly bacterivorous but also voraciously feeds upon small algae. We observed three main alterations in the shape of NR of both species with temperature, that is, change in the maximum growth rate, in the initial slope and in the threshold food level needed to sustain the population. Similarly, maximum ingestion rate, gross growth efficiency (GGE), and cell size varied with temperature and species. These findings caution against generalizing ciliate performance in relation to the ongoing global warming. Our results suggest that V. natans is the superior competitor to H. bodamicum in terms of temperature tolerance and bottom-up control. However, the abundance of V. natans is usually low compared to H. bodamicum and other common freshwater ciliates, suggesting that V. natans is more strongly top-down controlled via predation than H. bodamicum. The taxonomic position of V. natans has been debated. Therefore, to confirm species and genus affiliation of our study objects, we sequenced their small subunit ribosomal RNA (SSU rDNA) gene.

Conserving the Amazon River Basin: The case study of the Yahuarcaca Lakes System in Colombia

Colombia is the fourth contributor to the Amazon River Basin (ARB) by surface, and the third by mean annual runoff. The Yahuarcaca Lakes System (YLS), consisting of four large interconnected water bodies situated on the floodplain of Amazon River, was identified as one of the key areas for the conservation of freshwater biodiversity in the Colombian ARB. This review aimed to provide a general overview of YLS, present its environmental and biological features, identify main ecological and health threats, and propose mitigation strategies and future research prospects. A systematic search was performed using various databases. In summary, YLS harbors significant biodiversity and provides a number of ecological services for local communities, encompassing fish and drinking water supply and utilization of the floodplain for agriculture. Ensuring its sustainability requires attention from local and international authorities, collaboration with indigenous communities and future interdisciplinary research.

Combined Effects of Temperature, Irradiance, and pH on Teleaulax amphioxeia (Cryptophyceae) Physiology and Feeding Ratio For Its Predator Mesodinium rubrum (Ciliophora)1

The cryptophyte Teleaulax amphioxeia is a source of plastids for the ciliate Mesodinium rubrum and both organisms are members of the trophic chain of several species of Dinophysis. It is important to better understand the ecology of organisms at the first trophic levels before assessing the impact of principal factors of global change on Dinophysis spp. Therefore, combined effects of temperature, irradiance, and pH on growth rate, photosynthetic activity, and pigment content of a temperate strain of T. amphioxeia were studied using a full factorial design (central composite design 2³ *) in 17 individually controlled bioreactors. The derived model predicted an optimal growth rate of T. amphioxeia at a light intensity of 400 μmol photons · m^-2 · s^-1 , more acidic pH (7.6) than the current average and a temperature of 17.6°C. An interaction between temperature and irradiance on growth was also found, while pH did not have any significant effect. Subsequently, to investigate potential impacts of prey quality and quantity on the physiology of the predator, M. rubrum was fed two separate prey: predator ratios with cultures of T. amphioxeia previously acclimated at two different light intensities (100 and 400 μmol photons · m^-2 s^-1 ). M. rubrum growth appeared to be significantly dependent on prey quantity while effect of prey quality was not observed. This multi-parametric study indicated a high potential for a significant increase of T. amphioxeia in future climate conditions but to what extent this would lead to increased occurrences of Mesodinium spp. and Dinophysis spp. should be further investigated.