Functional ecology of planktonic ciliates: Measuring mortality rates in response to starvation.

Population dynamics of aquatic ciliates are controlled "bottom-up" via food supply and "top-down" by grazing and parasitism. While intrinsic growth rates of ciliates under saturating food conditions have been studied in some detail, mortality rates induced by starvation have received little attention thus far. To this end, we examined the response of three algivorous freshwater ciliate species to starvation using three different optical methods. Two of these methods, i.e. ciliate mortality rates (δ) estimated from (i) numerical response experiments and (ii) the rate of decline (ROD) in cell numbers, investigated the response of the ciliate population using conventional light microscopy. The third method, imaging cytometry using a FlowCAM instrument, monitored single cells during the starvation experiment. Like light microscopy, the FlowCAM approach estimated δ based on ROD in the experimental containers. However, imaging cytometry also measured the relative cellular chlorophyll a content in the ciliates' food vacuoles as a proxy for the nutritional status of the cells. The linear decline of the cellular chl. a yielded an independent estimate of δ that was similar to δ calculated from ROD. Additionally, the FlowCAM measurements revealed a high degree of phenotypic plasticity of the ciliates when exposed to starvation.

Functional ecology of the ciliate Glaucomides bromelicola, and comparison with the sympatric species Bromeliothrix metopoides.

We investigated the ecology and life strategy of Glaucomides bromelicola (family Bromeliophryidae), a very common ciliate in the reservoirs (tanks) of bromeliads, assessing its response to food quality and quantity and pH. Further, we conducted competition experiments with the frequently coexisting species Bromeliothrix metopoides (family Colpodidae). In contrast to B. metopoides and many other colpodean ciliates, G. bromelicola does not form resting cysts, which jeopardizes this ciliate when its small aquatic habitats dry out. Both species form bactivorous microstomes and flagellate-feeding macrostomes. However, only G. bromelicola has a low feeding threshold and is able to adapt to different protist food. The higher affinity to the local bacterial and flagellate food renders it the superior competitor relative to B. metopoides. Continuous encystment and excystment of the latter may enable stable coexistence of both species in their natural habitat. Both are tolerant to a wide range of pH (4-9). These ciliates appear to be limited to tank bromeliads because they either lack resting cysts and vectors for long distance dispersal (G. bromelicola) and/or have highly specific food requirements (primarily B. metopoides).

Temperature-dependent resistance to starvation of three contrasting freshwater ciliates.

We investigated the temperature-dependent response to starvation of three contrasting freshwater ciliates (Ciliophora). The cyst-forming algivorous species Meseres corlissi and the bactivorous species Glaucomides bromelicola, which cannot form cysts, co-occur in the reservoirs (tanks) of tree bromeliads. The mixotrophic species Coleps spetai is common in many lakes. We hypothesized that the ciliates' different traits and life strategies would affect their survival rates and temperature sensitivity under food depleted conditions. We measured the decline of the ciliate populations in microcosm experiments at different temperatures for several days. We used an imaging flow cytometer to size the ciliates and documented their morphological and physiological changes in response to starvation. We found that the cyst-forming species had the highest mortality rates but may endure long-term starvation by encystment. The sympatric, non-encysting species suffered the lowest mortality rates and could survive for more than three weeks without food. The mixotrophic species had intermediate mortality rates but showed the highest phenotypic plasticity in response to starvation. A significant fraction of the C. spetai population appeared unaffected by starvation, suggesting that the endosymbionts provided some resources to the host cells. The mean mortality rate per day of all three species increased with temperature by 0.09 °C-1.

Molecular Data Reveal a Cryptic Diversity in the Genus Urotricha (Alveolata, Ciliophora, Prostomatida), a Key Player in Freshwater Lakes, With Remarks on Morphology, Food Preferences, and Distribution.

Species of the ciliate genus Urotricha are key players in freshwater plankton communities. In the pelagial of lakes, about 20 urotrich species occur throughout an annual cycle, some of which play a pivotal role in aquatic food webs. For example, during the phytoplankton spring bloom, they consume a remarkable proportion of the algal production. In ecological studies, urotrich ciliates are usually merely identified to genus rank and grouped into size classes. This is unsatisfying considering the distinct autecological properties of individual species and their specific spatial and temporal distribution patterns. As a basis for future research, we characterized in detail four common urotrich morphotypes, i.e., specimens identified as U. furcata and tentatively as U. agilis, U. pseudofurcata, and U. castalia, using state-of-the-art methods. We used an integrative polyphasic approach, in which morphological studies (in vivo observation, silver staining methods, scanning electron microscopy) were linked with a molecular approach exploiting four different gene fragments as taxonomic DNA barcodes with different resolution potential (SSU rDNA, ITS-1, ITS-2, hypervariable V4 and V9 regions of the SSU rDNA). We shed light on the diversity of urotrich ciliates as well as on their global distribution patterns, and annual cycles. Additionally, we coupled individual species occurrences and environmental parameters, and subsequently modeled the distribution and occurrence, using logistic regressions. Furthermore, for one strain putatively identified as U. castalia, we ascertained the optimal cultivation media and food preferences. Thereby, our comprehensive view on these important freshwater ciliates that frequently occur in environmental high throughput sequencing datasets worldwide will allow future studies to better exploit protistan plankton data from lakes.

Bromeliothrix metopoides, a boom and bust ciliate (Ciliophora, Colpodea) from tank bromeliads.

We investigated the recently described colpodid ciliate Bromeliothrix metopoides in a series of laboratory experiments to reveal the environmental factors that constrain this species to its peculiar habitat, i.e. the tanks of bromeliads. Our results demonstrated that the various life stages of this ciliate (bacterivorous theronts and microstome trophonts, flagellate-feeding macrostomes) have specific demands in terms of food quality and quantity. Bromeliothrix required a high food threshold (>1.4 mg CL(-1)) in order to thrive. Food quality also affected resting cyst formation of B. metopoides when the experimental containers dried out. Its maximum growth rates (µmax=4.71 d(-1), i.e., 6.8 doublings d(-1)) belong to the highest ones recorded thus far for free-living ciliates. The pH niche of B. metopoides was relatively wide (pH ∼4 to >9) under optimal food conditions. However, its high sensitivity to unfavourable environmental conditions let the population collapse within several hours. We conclude that B. metopoides is a boom and bust ciliate that is specifically adapted to its peculiar habitat but virtually unviable in other environments.

Systematics and species-specific response to pH of Oxytricha acidotolerans sp. nov. and Urosomoida sp. (Ciliophora, Hypotricha) from acid mining lakes.

We investigated the morphology, phylogeny of the 18S rDNA, and pH response of Oxytricha acidotolerans sp. nov. and Urosomoida sp. (Ciliophora, Hypotricha) isolated from two chemically similar acid mining lakes (pH~2.6) located at Langau, Austria, and in Lusatia, Germany. Oxytricha acidotolerans sp. nov. from Langau has 18 frontal-ventral-transverse cirri but a very indistinct kinety 3 fragmentation so that the assignment to Oxytricha is uncertain. The somewhat smaller species from Lusatia has a highly variable cirral pattern and the dorsal kineties arranged in the Urosomoida pattern and is, therefore, preliminary designated as Urosomoida sp. The pH response was measured as ciliate growth rates in laboratory experiments at pH ranging from 2.5 to 7.0. Our hypothesis was that the shape of the pH reaction norm would not differ between these closely related (3% difference in their SSU rDNA) species. Results revealed a broad pH niche for O. acidotolerans, with growth rates peaking at moderately acidic conditions (pH 5.2). Cyst formation was positively and linearly related to pH. Urosomoida sp. was more sensitive to pH and did not survive at circumneutral pH. Accordingly, we reject our hypothesis that similar habitats would harbour ciliate species with virtually identical pH reaction norm.