Feeding characteristics of two tintinnid ciliate species on phytoplankton including harmful species: effects of prey size on ingestion rates and selectivity

Rising temperature more strongly promotes low-abundance Paramecium to remove Microcystis and degrade microcystins

Driven by global warming and eutrophication, Microcystis blooms have posed a severe threat to freshwater ecosystems, especially their derived pollutants cause serious harm to aquatic organisms, thus it is urgent to develop an effective strategy to eliminate nuisance Microcystis. Some protozoa can efficiently graze on toxic Microcystis aeruginosa and degrade cyanotoxins, and play a vital role in regulating harmful cyanobacteria. In the process of protozoa feeding on harmful algae, both temperature and protozoa population density are critical factors that affect the consequences of harmful M. aeruginosa population dynamics. In this study, we first found that Paramecium multimicronucleatum has strong ability to feed on M. aeruginosa, and then studied the interactive effects between temperature and initial density of P. multimicronucleatum on controlling M. aeruginosa. Results showed that increasing temperature accelerated the elimination of M. aeruginosa by P. multimicronucleatum, e.g. the time for M. aeruginosa elimination at 32 °C was shortened to 3.5-4 days. The higher temperatures (26, 29, and 32 °C) were more conducive to improve the efficiency of controlling M. aeruginosa by P. multimicronucleatum with low initial density (10 inds mL^-1). Furthermore, P. multimicronucleatum can rapidly degrade microcystins, and the degradation ratio approximately 100% at 32 °C after 6 days. This is the first study to discover that P. multimicronucleatum can high efficiently graze on M. aeruginosa and has a much higher grazing rate (3.5-5.5 × 10⁴Microcystis Paramecium^-1 d^-1) than other protozoa. These findings contribute to the establishment of a new feasible method for the biological control of M. aeruginosa, and provide a theoretical guidance for the practical application of P. multimicronucleatum in the removal of M. aeruginosa.

Microplastics interfere with mixotrophic Ochromonas eliminating toxic Microcystis

Microplastics with different sizes exist widely in fresh waters, which may affect the interspecific dynamics between predator and prey. The flagellate Ochromonas gloeopara can efficiently eliminate Microcystis aeruginosa and degrade microcystins, which shows great potential for controlling harmful Microcystis. In order to evaluate the effects of microplastics on O. gloeopara eliminating Microcystis, we designed an experiment of O. gloeopara feeding on Microcystis under different sizes (0.07 and 3 μm) and concentrations (0, 0.4, 0.8, 1.6, and 2.0 mg L^-1) of microplastics. The results showed that maximum abundance of M. aeruginosa decreased significantly with addition of microplastics, regardless of the size and concentration. O. gloeopara can ingest the microplastics and suffer from their adverse effects. The maximum abundance of O. gloeopara decreased with enhancing concentrations of 3 μm microplastics during the process of O. gloeopara eliminating M. aeruginosa, whereas 0.07 μm microplastics did not affect the growth of O. gloeopara obviously. During the period of exposure under microplastics, clearance rate of O. gloeopara on M. aeruginosa decreased with the increasing concentrations of microplastics. Specially, 3 μm microplastics had a stronger reduction on clearance rate of O. gloeopara. The time to M. aeruginosa extinction was prolonged with the increasing concentrations of microplastics in both sizes. Comparatively speaking, 3 μm microplastics had a stronger delayed effect on the removal of Microcystis. These findings suggest that microplastics can interfere with protozoa eliminating toxic Microcystis, which may aggravate their adverse impacts on aquatic ecosystem.

Are tintinnids picky grazers: Feeding experiments on a mixture of mixotrophic dinoflagellates and implications for red tide dynamics

To understand and predict the outbreak of red tides, which are often dominated by mixotrophic dinoflagellates (MTDs), the effects of "top-down" control by co-occurring predators on red-tide MTDs should be taken into consideration. We studied the numerical and functional responses of the tintinnid ciliate Favella ehrenbergii feeding on two red-tide MTDs, Scrippsiella trochoidea and Heterocapsa triquetra, under single and mixed prey conditions. Our results suggest that a mixed diet could support a better growth of predators compared to a monodiet. In addition, the predators preferred to graze S. trochoidea in the mixed diets, suggesting that predators may switch their feeding preference. The grazing by tintinnid predators could potentially inhibit the outbreaks of red tides dominated by MTDs. The findings in this study provide basic data and new insights for understanding the complex predator-prey relationships in marine microbial food webs, and the dynamics of red tides dominated by MTDs.

Incubation and grazing effects on spirotrich ciliate diversity inferred from molecular analyses of microcosm experiments

We used an experimental approach of analyzing marine microcosms to evaluate the impact of both predation (top-down) and food resources (bottom-up) on spirotrich ciliate communities. To assess the diversity, we used two molecular methods–denaturing gradient gel electrophoresis (DGGE) and high-throughput sequencing (HTS). We carried out two types of experiments to measure top-down (adult copepods as predators) and bottom-up effects (phytoplankton as food resources) on the spirotrich ciliates. We observed both strong incubation effects (untreated controls departed from initial assessment of diversity) and high variability across replicates within treatments, particularly for the bottom-up experiments. This suggests a rapid community turn-over during incubation and differential susceptibility to the effects of experimental manipulation. Despite the variability, our analyses reveal some broad patterns such as (1) increasing adult copepod predator abundance had a greater impact on spirotrich ciliates than on other microbial eukaryotes; (2) there was no evidence for strong food selection by the dominant spirotrich ciliates.

Removal of two pathogenic scuticociliates Miamiensis avidus and Miamiensis sp. using cells or culture filtrates of the dinoflagellate Alexandrium andersonii

Scuticociliatosis, which is caused by parasitic protistan pathogens known as scuticociliates, is one of the most serious diseases in marine aquaculture worldwide. Thus, elimination of these ciliates is a primary concern for scientists and managers in the aquaculture industry. To date, formalin and other toxic chemicals have been used as anti-scuticociliate agents, but issues regarding their secondary effects often arise. Consequently, development of safer methods is necessary. To find out a safe method of controlling scuticociliate populations in aqua-tanks or small-scale natural environments, cultures of 14 phototrophic dinoflagellates were tested to determine whether they were able to control populations of the common scuticociliates Miamiensis avidus and Miamiensis sp. isolated from Korean waters. Among the dinoflagellates tested, both cells and culture filtrates of Alexandrium andersonii effectively killed M. avidus and Miamiensis sp. The minimal concentration of cells and equivalent culture filtrates of A. andersonii to kill all M. avidus cells within 48h of incubation was ca. 2500 and 4500 cells ml^-1, respectively; whereas those needed to kill all Miamiensis sp. cells were ca. 1000 and 4500 cells ml^-1, respectively. It was estimated that 1m³ of the stock culture containing 20,000A. andersonii cells ml^-1 could eliminate all M. avidus cells in 7m³ of waters within the aqua-tanks on land and all Miamiensis sp. cells in 19m³ of waters within 48h. None of the brine shrimp Artemia salina nauplii incubated with concentrations of 50-4500A. andersonii cells ml^-1 for 24h was dead. Furthermore, none of the flounder Paralichthys olivaceus juveniles incubated with a mean concentration of ca. 2280A. andersonii cells ml^-1 for 96h was dead. Therefore, A. andersonii cultures may be used as a safe biological method for controlling populations of scuticociliates and can replace toxic formalin. The results of this study provided the basis for developing the method to control scuticociliate populations and understanding interactions between scuticociliates and phototrophic dinoflagellates in marine ecosystems.

Cyttarocylis ampulla, a polymorphic tintinnid ciliate of the marine plankton

Tintinnid species are traditionally distinguished via lorica features. Recently, sequencing has revealed polymorphism, i.e., genetically identical individuals with distinct lorica morphologies. One such polymorphic species is Cyttarocylis ampulla; individuals can display lorica morphologies of formally different species of Cyttarocylis and Petalotricha, well-represented in the literature. We compiled and analysed a global database of species records to determine if there is a main form and if different morphotypes have distinct temporal or spatial distributions. The two genera show very similar widespread distributions but with some statistical evidence of spatial segregation. Examining co-occurrence among the common 'species' we found most were rarely found alone, only 6-14% of the records for all species except for 2 forms: C. eucecryphalus and P. ampulla reported alone in 34% and 43%, respectively, of their records. We identify them as the main forms and analysed data of global distributions, spatial distribution across the Mediterranean in summer and winter and temporal distributions from a site in the Adriatic. The two main forms show frequent co-occurrence, similar lack of strong seasonality and widespread geographic distributions. We tentatively conclude that the different lorica morphologies may only reflect conditions of high temporally variability such as quantities and composition of prey. Directions for further research are suggested.

Preferential feeding by the ciliates Chilodonella and Tetrahymena spp. and effects of these protozoa on bacterial biofilm structure and composition

Protozoa are important components of microbial food webs, but protozoan feeding preferences and their effects in the context of bacterial biofilms are not well understood. The feeding interactions of two contrasting ciliates, the free-swimming filter feeder Tetrahymena sp. and the surface-associated predator Chilodonella sp., were investigated using biofilm-forming bacteria genetically modified to express fluorescent proteins. According to microscopy, both ciliates readily consumed cells from both Pseudomonas costantinii and Serratia plymuthica biofilms. When offered a choice between spatially separated biofilms, each ciliate showed a preference for P. costantinii biofilms. Experiments with bacterial cell extracts indicated that both ciliates used dissolved chemical cues to locate biofilms. Chilodonella sp. evidently used bacterial chemical cues as a basis for preferential feeding decisions, but it was unclear whether Tetrahymena sp. did also. Confocal microscopy of live biofilms revealed that Tetrahymena sp. had a major impact on biofilm morphology, forming holes and channels throughout S. plymuthica biofilms and reducing P. costantinii biofilms to isolated, grazing-resistant microcolonies. Grazing by Chilodonella sp. resulted in the development of less-defined trails through S. plymuthica biofilms and caused P. costantinii biofilms to become homogeneous scatterings of cells. It was not clear whether the observed feeding preferences for spatially separated P. costantinii biofilms over S. plymuthica biofilms resulted in selective targeting of P. costantinii cells in mixed biofilms. Grazing of mixed biofilms resulted in the depletion of both types of bacteria, with Tetrahymena sp. having a larger impact than Chilodonella sp., and effects similar to those seen in grazed single-species biofilms.

High-level congruence of Myrionecta rubra prey and Dinophysis species plastid identities as revealed by genetic analyses of isolates from Japanese coastal waters

We analyzed cryptophyte nucleomorph 18S rRNA gene sequences retained in natural Myrionecta rubra cells and plastid 16S rRNA gene and psbA sequences retained in natural cells of several Dinophysis species collected from Japanese coastal waters. A total of 715 nucleomorph sequences obtained from 134 M. rubra cells and 564 plastid 16S rRNA gene and 355 psbA sequences from 71 Dinophysis cells were determined. Almost all sequences in M. rubra and Dinophysis spp. were identical to those of Teleaulax amphioxeia, suggesting that M. rubra in Japanese coastal waters preferentially ingest T. amphioxeia. The remaining sequences were closely related to those of Geminigera cryophila and Teleaulax acuta. Interestingly, 37 plastid 16S rRNA gene sequences, which were different from T. amphioxeia and amplified from Dinophysis acuminata and Dinophysis norvegica cells, were identical to the sequence of a D. acuminata cell found in the Greenland Sea, suggesting that a widely distributed and unknown cryptophyte species is also preyed upon by M. rubra and subsequently sequestered by Dinophysis. To confirm the reliability of molecular identification of the cryptophyte Teleaulax species detected from M. rubra and Dinophysis cells, the nucleomorph and plastid genes of Teleaulax species isolated from seawaters were also analyzed. Of 19 isolates, 16 and 3 clonal strains were identified as T. amphioxeia and T. acuta, respectively, and no sequence variation was confirmed within species. T. amphioxeia is probably the primary source of prey for M. rubra in Japanese coastal waters. An unknown cryptophyte may serve as an additional source, depending on localities and seasons.

Ciliates and their picophytoplankton-feeding activity in a high-altitude warm-monomictic saline lake

The impact of feeding on autotrophic picoplankton (APP) on the ciliate composition of the assemblage was surveyed monthly along a depth gradient in the maar crater, athalassohaline, warm monomictic Lake Alchichica (Puebla, Mexico) from June 2003 to December 2005. Numbers of APP were evaluated from their autofluorescence. DAPI staining and the Fluorescently Labeled Bacteria technique were employed to count ciliates and estimate their feeding rates. A total of 38 taxa of ciliates have been identified using Quantitative Protargol Staining. Peritrichs followed by minute spirotrichs (particularly Halteria grandinella) often numerically dominated the ciliate assemblage and emerged as the most efficient APP feeders. A maximum of 54 ciliate cells ml(-1) was observed in the surface layer at the end of the mixing period, during the development of diatoms (Cyclotella alchichicana), the cyanobacterial bloom (Nodularia sp.) and its decay. Vorticellids (Pelagovorticella natans, Vorticella sp.) had the highest APP uptake (median 130 APP cil(-1) h(-1)). Mixotrophic Euplotes cf. daidaleos were important APP grazers near the oxycline. Scuticociliates (Cyclidium glaucoma, Uronema nigricans and an anaerobic cf. Isocyclidium globossum), were numerically dominant within the hypolimnetic assemblages and did not ingest APP. Generally, APP were not an important food source for the majority of the ciliate assemblage, being positively selected by a few species during the APP decay in aerobic and microaerobic conditions.

Interactions between Planktonic Microalgae and Protozoan Grazers1

For an algal bloom to develop, the growth rate of the bloom-forming species must exceed the sum of all loss processes. Among these loss processes, grazing is generally believed to be one of the more important factors. Based on numerous field studies, it is now recognized that microzooplankton are dominant consumers of phytoplankton in both open ocean and coastal waters. Heterotrophic protists, a major component of microzooplankton communities, constitute a vast complex of diverse feeding strategies and behavior which allow them access to even the larger phytoplankton species. A number of laboratory studies have shown the capability of different protistan species to feed and grow on bloom-forming algal species. Because of short generation times, their ability for fast reaction to short-term variation in food conditions enables phagotrophic protists to fulfill the function of a heterotrophic buffer, which might balance the flow of matter in case of phytoplankton blooms. The importance of grazing as a control of microalgae becomes most apparent by its failure; if community grazing controls initial stages of bloom development, there simply is no bloom. However, if a certain algal species is difficult to graze, e.g. due to specific defense mechanisms, reduced grazing pressure will certainly favor bloom development. The present contribution will provide a general overview on the interactions between planktonic microalgae and protozoan grazers with special emphasis on species-specific interactions and algal defense strategies against protozoan grazers.

Feeding by the Heterotrophic Dinoflagellate Oxyrrhis marina on the Red-Tide Raphidophyte Heterosigma akashiwo: a Potential Biological Method to Control Red Tides Using Mass-Cultured Grazers

As part of the development of a method to control the outbreak and persistence of red tides using mass-cultured heterotrophic protist grazers, we measured the growth and ingestion rates of cultured Oxyrrhis marina (a heterotrophic dinoflagellate) on cultured Heterosigma akashiwo (a raphidophyte) in bottles in the laboratory and in mesocosms (ca. 60 liter) in nature, and those of the cultured grazer on natural populations of the red-tide organism in mesocosms set up in nature. In the bottle incubation, specific growth rates of O. marina increased rapidly with increasing concentration of cultured prey up to ca. 950 ng C ml(-1) (equivalent to 9,500 cells ml(-1)), but were saturated at higher concentrations. Maximum specific growth rate (mumax), KGR (prey concentration sustaining 0.5 mumax) and threshold prey concentration of O. marina on H. akashiwo were 1.43 d(-1), 104 ng C ml(-1), and 8.0 ng C ml(-1), respectively. Maximum ingestion and clearance rates of O. marina were 1.27 ng C grazer(-1) d(-1) and 0.3 microl grazer(-1) h(-1), respectively. Cultured O. marina grew well effectively reducing cultured and natural populations of H. akashiwo down to a very low concentration within 3 d in the mesocosms. The growth and ingestion rates of cultured O. marina on natural populations of H. akashiwo in the mesocosms were 39% and 40%, respectively, of those calculated based on the results from the bottle incubation in the laboratory, while growth and ingestion rates of cultured O. marina on cultured H. akashiwo in the mesocosms were 55% and 36%, respectively. Calculated grazing impact by O. marina on natural populations of H. akashiwo suggests that O. marina cultured on a large scale could be used for controlling red tides by H. akashiwo near aquaculture farms that are located in small ponds, lagoons, semi-enclosed bays, and large land-aqua tanks to which fresh seawater should be frequently supplied.