The feeding apparatus of a chitinivorous ciliate

The chitinivorous ciliate Ascophrys, an ectosymbiont of the shrimp Palaemon serratus, is enclosed by a thick cyst wall except for a ventral hiatus exposing a circular area of exoskeleton to the interior of the cyst. The exoskeleton underlying the cyst wall remains intact, but the circular area of exoskeleton is dissolved enzymatically and ingested. The feeding ciliate forms a cavity in the exoskeleton into which it sinks. Its complex oral apparatus resembles a pump encircled by cytoplasm containing Golgi and high concentrations of coated vesicles that join pellicular pores between cilia. The ingestive apparatus is formed of microtubular lamellae that originate in the midplane of the body, descend toward a coated membrane on the surface, and ascend again as a lamellar lining to a complex food tube that ends in the middle of the body surrounded by food vacuoles. The cytoplasm enclosed between the descending lamellae and the food tube is crowded with membrane organelles that recycle as food vacuole membranes at the coated membrane. We hypothesize that vacuoles containing dissolved exoskeleton are drawn up into the oral tube and are released into the cytoplasm at the terminus of the tube, where their contents are concentrated and excess vacuolar membrane collapsed into membrane organelles.     

Mitosis-like macronuclear division in a ciliate

The macronucleus of a small marine ciliate of the genus Protocrucia consists of a cluster of ten vesicles which give rise to 20 distinct chromosomal elements in the course of prophase-like condensation stages. Size differences of vesicles and chromosomes are cytological indications of their genetic individuality. In an anaphase-like stage, the chromosomal elements are separated in two daughter groups which re-form 10 vesicles each. The micronucleus divides simultaneously. The existence of a precisely functioning mode of chromosome distribution is also indicated by DNA measurements. Since the macronucleus contains much more DNA than the micronucleus, the macronuclear chromosomes are thought to be oligotenic. This hypothesis is supported by the much larger size of the macronuclear chromosomes. In contrast to other modes of macronuclear division known so far, this ciliate has retained some essential features of mitosis.     

A comparative study of digestion in a raptorial ciliate and in the facultative carnivorous form of a filter feeding ciliate

Digestion in the carnivorous form (giant) of the filter feeding ciliateOxytricha bifaria and in the obligatory carnivorous, raptorial feeding ciliateLitonotus lamella were studied and compared at the ultrastructural level. It was found that whenO. bifaria, shifts from the normal, bacterivorous to the gigantic carnivorous form, it modifies its morphology, acquiring new and more effective feeding devices but maintaining unaltered the digestion pattern and mode of food absorption. This takes place through pinocytotic activity at the food vacuolar membrane. The digestive process inLitonotus is far more rapid: within 10 min the vacuolar membrane disappears; in this way the cytoplasm of the prey, not yet completely digested, is mixed with that of the predator and a pinocytotic mechanism does not seem to he necessary for nutrient assimilation. In general, results demonstrate that filter feeders and raptorial ciliates differ not only in their food intake mechanism, but also in the pattern and the timing of their digestive process, even when they ingest similar kinds of food.     

Competition-colonization trade-offs in a ciliate model community

There is considerable theoretical evidence that a trade-off between competitive and colonization ability enables species coexistence. However, empirical studies testing for the presence of a competition–colonization (CC) trade-off and its importance for species coexistence have found mixed results. In a microcosm experiment, we looked for a CC trade-off in a community of six benthic ciliate species. For each species, we measured the time needed to actively disperse to and colonize an empty microcosm. By measuring dispersal rates and growth rates of the species, we were able to differentiate between these two important components of colonization ability. Competitive ability was investigated by comparing species’ growth with or without a competitor in all pairwise species combinations. Species significantly differed in their colonization abilities, with good colonizers having either high growth rates or high dispersal rates or both. Although species showed a clear competitive hierarchy, competitive and colonization ability were uncorrelated. The weakest competitors were also the weakest colonizers, and the strongest competitor was an intermediate colonizer. However, some of the inferior competitors had higher colonization abilities than the strongest competitor, indicating that a CC trade-off may enable coexistence for a subset of the species. Absence of a community-wide CC trade-off may be based on the lack of strong relationships between the traits underlying competitive and colonization ability. We show that temporal effects and differential resource use are alternative mechanisms of coexistence for the species that were both slow colonizers and poor competitors.     

Genetic diversity in populations of a freshwater ciliate

RAPD fingerprinting with nine different primers revealed that all of 18 E. aediculatus isolates from nine ponds and streams in western Germany, France and the U.S.A. were genetically different. The extent of genetic similarity between genotypes from different waters did not show a significant relationship with the geographical distance among habitats, although genotypes isolated from the same habitat showed a higher genetic similarity than genotypes isolated from different habitats. Phylogenetic analyses of RAPD patterns indicate a separation of E. aediculatus strains into subgroups within one species, but all strains were genetically more similar to one another than to strains from two other Euplotes species. Crossings of the different E. aediculatus strains revealed they belonged to seven mating types of one gene pool. The high genetic diversity observed is explained by a frequent occurrence of conjugation in the studied populations.     

Cytomembrane differentiation in a ciliate,Tetrahymena pyriformis

Summary Two special kinds of smooth surfaced differentiations of the endoplasmic reticulum (ER) of the ciliate Tetrahymena pyriformis are described. (A) A novel type of cytomembrane structure is represented by localized bifacial regions in which one side of the cisterna is studded with ribosomes, flexible in outline and of a cytomembraneous ultrastructure and the other side has a smooth, straight profile and a plasma membrane-like triple-layered appearance. Such smooth patches of predominantly rough ER-cisternae have a tendency to pair with a separation of ca. 250 Å. The micrographs suggest a participation of such patches in the formation of vesicles and/or dictyosomes. (B) Tubular structures, including those with microtubular as well as with macrotubular (300–650 Å) diameters, can be in continuity with ER profiles. Possible origins and functions of these tubular forms are discussed.The work was supported in part by the Deutsche Forschungsgemeinschaft.The authors are indebted to Miss Sigrid Krien for skilful technical assistance as well as to Drs. Ch. Bracker, D. J. Morré (both Purdue University, Lafayette, U.S.A), and H. Falk (this institute) for helpful discussions.     

Chemoattraction of a bactivorous ciliate to bacteria surface compounds

The locomotory response to cell surface compounds extracted from two prey species,Vibrio natriegens andVibrio neries, was tested for a bacterivorous ciliate,Pseudochnilembus marinus Thompson 1966. Chemoattraction of the ciliate to the surface compounds stabilized in agarose baits was not equal for the two prey species. Fractionation of the extracts suggested the attractive substance was a high molecular weight compound. The expression of the differential response was dependant on the physiological condition and prior prey species exposure of the ciliate test population. The recognition and response to material normally found on the surface of prey cells supports evidence for the involvement of chemical sensing of gradients of prey particles and dissolved compounds of prey origin in the natural swimming behavior of bacterivorous ciliates. The prey species-specific reactions and influence of ciliate physiological state on chemosensory response suggest ciliate-bacteria interactions may be more complex than preciously assumed.     

Sequence-specific fragmentation of macronuclear DNA in a holotrichous ciliate

Macronuclear DNA from the protozoan G. chattoni, a holotrichous ciliate, was analyzed. Most, if not all, of the macronuclear DNA is subchromosomal, ranging in size from above 100 kb down to 2.1 kb, with molecules in the lower molecular weight range being resolvable by gel electrophoresis into reproducible, specific, discrete size classes. A prominent class of linear 9.3 kb molecules consists of single free rRNA genes. Upon denaturation and partial renaturation, a high percentage of total macronuclear DNA was found as single-stranded circles. Sequence analysis showed that a minimum of 38 tandem repeats of the sequence CCCCAA is present in inverted orientation at each end of most or all Glaucoma macronuclear DNA molecules, including the rDNA. This sequence must therefore be recognized during site-specific fragmentation of chromosomes in macronuclear development.     

Unusual characteristics of ciliate actins

Actin is a cytoskeletal protein that is ubiquitous in eukaryotes, hence the corresponding genes and proteins have been isolated from numerous organisms as different as animals, plants, fungi and protozoa. Several atomic models are available for the monomeric as well as the filamentous form, and more than 70 proteins that bind actin and control filament dynamics have been isolated from diverse eukaryotes. Moreover, the function and dynamics of the actin cytoskeleton in several eukaryotic systems have been depicted in depth. Unlike other protozoa, such as amoeba, actin is not an abundant protein in ciliates, whose cytoskeleton is mainly composed of microtubular arrays. Ciliate actin has been studied in several species, and it was established early on that this ciliate protein is very different from that of other eukaryotes. Similarly, the actin-binding proteins studied in ciliates display great differences with those of other eukaryotes. Consequently, ciliate actin has been considered as "unconventional," and this review focuses on molecular data leading to this conclusion.     

Interpreting soil ciliate biodiversity

The results of soil ciliate frequency studies of 47 field studies, from 12 ecosystem types, were combined with recent concepts and observations to assess the importance of soil ciliate biodiversity in ecosystem functioning. A few species typically furnish most of the individuals; increases in biodiversity, produced by less common species, alters soil ciliate communities. Soil ciliates were grouped according to position on the r/K continuum, with polyhymenophoran species as K, and colpodids as r organisms; and according to degree of soil affinity. Grasslands and hardwood forests were characterized by large numbers of K and intermediate species, whereas pine forests and more stressed ecosystems (e.g., arable lands, deserts) had more colpodid and fewer total species. Within these systems, certain species may become prominent, or be absent, suggesting that many soil ciliates exhibit widely overlapping niches, and that while a soil might function with a few species, high diversity allows systems to respond to changing seasons and climate.     

Seasonal development of hypolimnetic ciliate communities in a eutrophic pond

Abstract The ciliated protozoan communities in the hypolimnion of a highly produtive pond were investigated over two years. Three physiological groups could be distinguished: stratified water column; (b) anaerobic ciliates with endosymbiotic methanogens; and (c) anaerobes without endosymbiotic methanogens. Both groups of anaerobes were confined to the anoxic zone of the hypolimnion. Community biomass was dominated by microaerobic ciliates which had on average 20 times larger cells than anaerobic ciliates. Abundance and biomass of microaerobic ciliates decreased over the summer, while anaerobic ciliates increased. This reflected a spatial shift in the availability of inorganic nutrients and, as a result, of ciliate food from the epi- and metalimnion to the hypolimnion. The low biomass production of anaerobic ciliates was consistent with the low theoretical growth efficiency of anaerobic metabolism. Ciliate species displayed characteristic spatial and seasonal distribution patterns within the water column which were similar in both years investigated. Spatial and temporal distribution was mainly governed by two factors: (1) the distribution of dissolved oxygen; and (2) the availability of food. Distribution patterns were not related to chemical gradients other than the oxygen gradient, but they were correlated with the distribution of major food sources.     

The bloom-forming ciliate Mesodinium rubrum harbours a single permanent endosymbiont

Whether the red tide Mesodinium rubrum contains a permanent cryptophyte symbiont or whether it only sequesters chloroplasts from cryptophyte prey was addressed using electron microscopy and the dynamics of photosynthesis, chloroplasts and nuclei. Mesodinium rubrum contains a branched cryptophyte symbiont consisting of many chloroplasts, mitochondria, nucleomorphs, an endoplasmic reticulum and one nucleus. The volume of the symbiont constitutes 36% of the consortium and it is separated from its host by a single-cell membrane. The chloroplasts of Mesodium are larger and morphologically different from two Teleaulax species that served as prey. The symbiont nucleus is also much larger than Teleaulax nuclei. Although M. rubrum is functionally a phototroph, sustained growth beyond two to four generations requires ingestion of prey, but less than one prey cell per generation suffices for maximum growth. This suggests that either the ciliate or its symbiont needs an essential growth factor for continuous growth.