Simultaneous Evaluation of Life Cycle Dynamics between a Host Paramecium and the Endosymbionts of Paramecium bursaria Using Capillary Flow Cytometry

Size dependent uptake and trophic transfer of polystyrene microplastics in unicellular freshwater eukaryotes

Microplastics (MP) have become a well-known and widely investigated environmental pollutant. Despite the huge amount of new studies investigating the potential threat posed by MP, the possible uptake and trophic transfer in lower trophic levels of freshwater ecosystems remains understudied. This study aims to investigate the internalization and potential trophic transfer of fluorescent polystyrene (PS) beads (0.5 μm, 3.6 × 108 particles/mL; 6 μm, 2.1 × 105 particles/mL) and fragments (<30 μm, 5 × 103 particles/mL) in three unicellular eukaryotes. This study focuses on the size-dependent uptake of MP by two freshwater Ciliophora, Tetrahymena pyriformis, Paramecium caudatum and one Amoebozoa, Amoeba proteus, serving also as predator for experiments on potential trophic transfer. Size-dependent uptake of MP in all three unicellular eukaryotes was shown. P. caudatum is able to take up MP fragments up to 27.7 μm, while T. pyriformis ingests particles up to 10 μm. In A. proteus, small MP (PS0.5μm and PS6μm) were taken up via pinocytosis and were detected in the cytoplasm for up to 14 days after exposure. Large PS-MP (PS<30μm) were detected in A. proteus only after predation on MP-fed Ciliophora. These results indicate that A. proteus ingests larger MP via predation on Ciliophora (PS<30μm), which would not be taken up otherwise. This study shows trophic transfer of MP at the base of the aquatic food web and serves as basis to study the impact of MP in freshwater ecosystems.

The impacts of host association and perturbation on symbiont fitness

Symbiosis can benefit hosts in numerous ways, but less is known about whether interactions with hosts benefit symbionts-the smaller species in the relationship. To determine the fitness impact of host association on symbionts in likely mutualisms, we conducted a meta-analysis across 91 unique host-symbiont pairings under a range of spatial and temporal contexts. Specifically, we assess the consequences to symbiont fitness when in and out of symbiosis, as well as when the symbiosis is under suboptimal or varying environments and biological conditions (e.g., host age). We find that some intracellular symbionts associated with protists tend to have greater fitness when the symbiosis is under stressful conditions. Symbionts of plants and animals did not exhibit this trend, suggesting that symbionts of multicellular hosts are more robust to perturbations. Symbiont fitness also generally increased with host age. Lastly, we show that symbionts able to proliferate in- and outside host cells exhibit greater fitness than those found exclusively inside or outside cells. The ability to grow in multiple locations may thus help symbionts thrive. We discuss these fitness patterns in light of host-driven factors, whereby hosts exert influence over symbionts to suit their own needs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13199-024-00984-6.

Cadmium accumulation and isotope fractionation in typical protozoa Tetrahymena: A new perspective on remediation of Cd pollution in wastewater

Cadmium (Cd) pollution threatens water safety and human health, which has raised serious public concern. Tetrahymena is a model protozoan, possessing the potential to remediate Cd contaminated water given the rapid expression of thiols. However, the mechanism of Cd accumulation in Tetrahymena has not been well understood, which hinders its application in environmental remediation. This study elucidated the pathway of Cd accumulation in Tetrahymena using Cd isotope fractionation. Our results showed that Tetrahymena preferentially absorb light Cd isotopes, with Δ^114/110Cd_{Tetrahymena-solution} = -0.20 ± 0.02‰ ∼ - 0.29 ± 0.02‰, which implies that the intracellular Cd is probably in the form of Cd-S. The fractionation generated by Cd complexation with thiols is constant (Δ^114/110Cd_{Tetrahymena-remaining solution} ∼ -0.28 ± 0.02‰), which is not affected by the concentrations of Cd in intracellular and culture medium, nor by the physiological changes in cells. Furthermore, the detoxification process of Tetrahymena results in an increase in cellular Cd accumulation from 11.7% to 23.3% with the elevated Cd concentrations in batch Cd stress culture experiments. This study highlights the promising application of Cd isotope fractionation in Tetrahymena for the remediation of heavy metal pollution in water.

Role of host ciliate Paramecium bursaria mitochondria and trichocysts for symbiotic Chlorella variabilis attachment beneath the host cell cortex

Symbiotic Chlorella variabilis is encased in the perialgal vacuole (PV) membrane of ciliate Paramecium bursaria. The PV membrane is stably anchored below the host cell cortex by adhesion to host mitochondria. Host trichocysts, which are defensive organelles against predators, are present in the mitochondria and PV membrane vicinity. The mechanism by which PV attaches beneath the host cell cortex remains unknown. When P. bursaria is centrifuged at high speed, the symbiotic algae are displaced from the host cell cortex and concentrate at the posterior end. When centrifugation is stopped, the dislocated algae reattach beneath the host cell cortex with fast cytoplasmic streaming. The densities of mitochondria and trichocysts before and after centrifugation were compared using indirect immunofluorescence microscopy with monoclonal antibodies. Almost all trichocysts were shed by high-speed centrifugation, but dislocated algae could reattach even in the absence of trichocysts. In contrast, host mitochondria were unaffected in localization and number, and the dislocated algae also reattached. These findings suggest trichocysts are unnecessary for algal relocalization and that mitochondria are colocalized with the algae. However, many mitochondria were also present in the cell's anterior region without symbiotic algae. Therefore, not all areas with mitochondria contained algae, but there was an algal localization bias within the host cell.

Method for Stress Assessment of Endosymbiotic Algae in Paramecium bursaria as a Model System for Endosymbiosis

Endosymbiosis between heterotrophic host and microalga often breaks down because of environmental conditions, such as temperature change and exposure to toxic substances. By the time of the apparent breakdown of endosymbiosis, it is often too late for the endosymbiotic system to recover. In this study, I developed a technique for the stress assessment of endosymbiotic algae using Paramecium bursaria as an endosymbiosis model, after treatment with the herbicide paraquat, an endosymbiotic collapse inducer. Microcapillary flow cytometry was employed to evaluate a large number of cells in an approach that is more rapid than microscopy evaluation. In the assay, red fluorescence of the chlorophyll reflected the number of endosymbionts within the host cell, while yellow fluorescence fluctuated in response to the deteriorating viability of the endosymbiont under stress. Hence, the yellow/red fluorescence intensity ratio can be used as an algal stress index independent of the algal number. An optical evaluation revealed that the viability of the endosymbiotic algae within the host cell decreased after treatment with paraquat and that the remaining endosymbionts were exposed to high stress. The devised assay is a potential environmental monitoring method, applicable not only to P. bursaria but also to multicellular symbiotic units, such as corals.

Endosymbiotic Chlorella variabilis reduces mitochondrial number in the ciliate Paramecium bursaria

Extant symbioses illustrate endosymbiosis is a driving force for evolution and diversification. In the ciliate Paramecium bursaria, the endosymbiotic alga Chlorella variabilis in perialgal vacuole localizes beneath the host cell cortex by adhesion between the perialgal vacuole membrane and host mitochondria. We investigated whether host mitochondria are also affected by algal endosymbiosis. Transmission electron microscopy of host cells showed fewer mitochondria beneath the algae-bearing host cell cortex than that of alga-free cells. To compare the density and distribution of host mitochondria with or without symbiotic algae, we developed a monoclonal antibody against Paramecium mitochondria. Immunofluorescence microscopy with the monoclonal antibody showed that the mitochondrial density of the algae-bearing P. bursaria was significantly lower than that of the alga-free cells. The total cell protein concentration of alga-free P. bursaria cells was approximately 1.8-fold higher than that of algae-bearing cells, and the protein content of mitochondria was significantly higher in alga-free cells than that in the algae-bearing cells. These results corresponded with those obtained by transmission electron and immunofluorescence microscopies. This paper shows that endosymbiotic algae affect reduced mitochondrial number in the host P. bursaria significantly.

SymbiQuant: A Machine Learning Object Detection Tool for Polyploid Independent Estimates of Endosymbiont Population Size

Quantifying the size of endosymbiont populations is challenging because endosymbionts are typically difficult or impossible to culture and commonly polyploid. Current approaches to estimating endosymbiont population sizes include quantitative PCR (qPCR) targeting endosymbiont genomic DNA and flow-cytometry. While qPCR captures genome copy number data, it does not capture the number of bacterial cells in polyploid endosymbiont populations. In contrast, flow cytometry can capture accurate estimates of whole host-level endosymbiont population size, but it is not readily able to capture data at the level of endosymbiotic host cells. To complement these existing approaches for estimating endosymbiont population size, we designed and implemented an object detection/segmentation tool for counting the number of endosymbiont cells in micrographs of host tissues. The tool, called SymbiQuant, which makes use of recent advances in deep neural networks includes a graphic user interface that allows for human curation of tool output. We trained SymbiQuant for use in the model aphid/Buchnera endosymbiosis and studied Buchnera population dynamics and phenotype over aphid postembryonic development. We show that SymbiQuant returns accurate counts of endosymbionts, and readily captures Buchnera phenotype. By replacing our training data with data composed of annotated microscopy images from other models of endosymbiosis, SymbiQuant has the potential for broad application. Our tool, which is available on GitHub, adds to the repertoire of methods researchers can use to study endosymbiosis at the organismal, genome, and now endosymbiotic host tissue or cell levels.

Examining the Relationship Between the Testate Amoeba Hyalosphenia papilio (Arcellinida, Amoebozoa) and its Associated Intracellular Microalgae Using Molecular and Microscopic Methods

Symbiotic relationships between heterotrophic and phototrophic partners are common in microbial eukaryotes. Among Arcellinida (Amoebozoa) several species are associated with microalgae of the genus Chlorella (Archaeplastida). So far, these symbioses were assumed to be stable and mutualistic, yet details of the interactions are limited. Here, we analyzed 22 single-cell transcriptomes and 36 partially-sequenced genomes of the Arcellinida morphospecies Hyalosphenia papilio, which contains Chlorella algae, to shed light on the amoeba-algae association. By characterizing the genetic diversity of associated Chlorella, we detected two distinct clades that can be linked to host genetic diversity, yet at the same time show a biogeographic signal across sampling sites. Fluorescence and transmission electron microscopy showed the presence of intact algae cells within the amoeba cell. Yet analysis of transcriptome data suggested that the algal nuclei are inactive, implying that instead of a stable, mutualistic relationship, the algae may be temporarily exploited for photosynthetic activity before being digested. Differences in gene expression of H. papilio and Hyalosphenia elegans demonstrated increased expression of genes related to oxidative stress. Together, our analyses increase knowledge of this host-symbiont association and reveal 1) higher diversity of associated algae than previously characterized, 2) a transient association between H. papilio and Chlorella with unclear benefits for the algae, 3) algal-induced gene expression changes in the host.

Importance of protozoa Tetrahymena in toxicological studies: A review

Tetrahymena is a single-cell eukaryotic organism present in all aquatic environments and can easily be maintained in laboratory conditions in a cost-effective manner. This review gives a brief description of the physiology of Tetrahymena, culture handling, and maintenance of Tetrahymena species. The review article focuses on various toxicological bioassays at different biological organizational (biochemical, individual, population, and community) levels. Furthermore, some techniques such as single cell gel electrophoresis (SCGE) and microcalorimetry assay are also available to investigate the effect of xenobiotics on the integrity of DNA and metabolic state of Tetrahymena species respectively. The article also discusses how the general physiology, behavioural activities and different organelles of Tetrahymena could be useful in toxicological studies. The strength and limitations of Tetrahymena over other model organisms are also discussed. This article also provides suggestions to overcome some problems related to toxicity assessment. Various aspects associated with variability in results, toxicity endpoints, characteristics of organisms and responses against xenobiotic substances (old and new emerging toxicants) are considered.

Endosymbiotic Green Algae in Paramecium bursaria: A New Isolation Method and a Simple Diagnostic PCR Approach for the Identification

Paramecium bursaria is a single-celled model organism for studying endosymbiosis among ciliates and green algae. Most strains of P. bursaria bear either Chlorella variabilis or Micractinium conductrix as endosymbionts. Both algal genera are unicellular green algae characterized by cup-shaped chloroplasts containing a single pyrenoid and reproduction by autospores. Due to their size and only few morphological characteristics, these green algae are very difficult to discriminate by microscopy only. Their cultivation is laborious and often unsuccessful, but we developed a three-step isolation method, which provided axenic cultures of endosymbionts. In addition to the time-consuming isolation, we developed a simple diagnostic PCR identification method using specific primers for C. variabilis and M. conductrix that provided reliable results. One advantage of this approach was that the algae do not have to be isolated from their host. For a comparative study, we investigated 19 strains of P. bursaria from all over the world (new isolates and available laboratory strains) belonging to the five known syngens (R1–R5). Six European ciliate strains belonging to syngens R1 and R2 bore M. conductrix as endosymbiont whereas C. variabilis was discovered in syngens R1–R5 having worldwide origins. Our results reveal the first evidence of C. variabilis as endosymbiont in P. bursaria in Europe.