Protozoan community structure in a fractal soil environment.

Protozoan abundance was quantified, and 365 protozoan species were recorded, in 150 soil samples from an upland grassland in Scotland. Across the entire size range (2-200 pm) protozoan species richness varied by a factor of two, whereas abundance increased by a factor of 20 with decreasing body size. As the soil had fractal structure, the relatively flat species curve can be explained if spatial heterogeneity determines species number--for in a fractal environment, heterogeneity will be the same at all spatial scales. Community structure appeared to approach a temporary steady-state about six days after re-hydration of dried soil. A simple model based on combining the fractal character of increasing habitat area at smaller spatial scales, with the weight-specific energy requirements of protozoa, provided theoretical curves of abundance and biovolume on body size which provide a reasonable fit to real data. We suggest two possibilities--that the apparent competence of the theoretical model is fortuitous and the product of poorly understood dynamic elements of the trophic structure in the community; or that key elements of protozoan community structure in a fractal soil environment may be largely explained in terms of habitat space and energy requirements.

True chemotaxis in oxygen gradients of the sulfur-oxidizing bacterium Thiovulum majus.

Observations of free-swimming Thiovulum majus cells show that these bacteria exhibit a phobic response as well as true chemotaxis in oxygen gradients. Both phenomena of their chemotactic behavior are integrated into a single model of helical klinotaxis, which is demonstrated by computer simulations.

How dinoflagellates swim.

Dinoflagellates possess two flagella; usually these are directed perpendicular to one another constituting a transversal flagellum and a longitudinal, trailing flagellum, respectively. The transversal flagellum causes the cell to rotate around its length axis. The trailing flagellum is responsible for the translation of the cell; due to its asymmetric insertion it also causes a rotation of the cell around an axis perpendicular to the longitudinal axis. Together, these two rotational components result in a helical swimming path. Cells can vary the two rotational components independently as well as the translational velocity. With these three degrees of freedom, cells can vary the parameters of their helical swimming paths for steering. Dinoflagellates use this mechanism for orientation in chemical concentration gradients ("helical klinotaxis").

Artificial Cyanobacterial Mats: Growth, Structure, and Vertical Zonation Patterns.

The formation of cyanobacterial mats (originally induced by incubation of sediment cores in which metazoans and most other eukaryotes had been removed) was followed over approximately 2.6 years. The thickness of the mats increased at a rate of 2-3 mm per year because of accumulation of empty cyanobacterial sheaths and as a result of carbonate deposition; the fraction of living biomass remained relatively constant over at least 2 years, but there was a slow accumulation of nonliving organic C ( approximately 1 mmol yr(-1)). Biota composition (dominated by five types of filamentous cyanobacteria, unicellular cyanobacteria, diatoms, anoxygenic phototrophs, and heterotrophic bacteria) and vertical zonation patterns in the upper 2-3 mm of the mats were also almost constant over time. Using transmission electron microscopy and stereological analysis it was possible to quantify the vertical distribution of major groups of organisms.

Bio-optical Characteristics and the Vertical Distribution of Photosynthetic Pigments and Photosynthesis in an Artificial Cyanobacterial Mat.

Zonations of photosynthesis and photopigments in artificial cyanobacterial mats were studied with (i) oxygen and pH microsensors, (ii) fiber-optic microprobes for field radiance, scalar irradiance, and PSII fluorescence, and (iii) a light microscope equipped with a spectrometer for spectral absorbance and fluorescence measurements. Our analysis revealed the presence of several distinct 1-2 mm thick cyanobacterial layers mixed with patches of anoxygenic photosynthetic bacteria. Strong attenuation of visible light confined the euphotic zone to the uppermost 3 mm of the mat, where oxygen levels of 3-4 times air saturation and a pH peak of up to pH 8.8 were observed under saturating irradiance (413 µmol photon m(-2) s(-1)). Oxygen penetration was 5 mm in light and decreased to 1 mm in darkness. Volumetric oxygen consumption in the photic and aphotic zones of illuminated mat was 5.5 and 2.9 times higher, respectively, than oxygen consumption in dark incubated mats. Scalar irradiance reached 100-150% of incident irradiance in the upper 0.5 mm of the mat due to intense scattering in the matrix of cells, exopolymers, and carbonate precipitates. In deeper mat layers scalar irradiance decreased nearly exponentially, and highest attenuation coefficients of 6-7 mm(-1) were found in cyanobacterial layers, where photosynthesis and photopigment fluorescence also peaked. Visible light was attenuated >100 times more strongly than near infrared light. Microscope spectrometry on thin sections of mats allowed detailed spectral absorbance and fluorescence measurements at defined positions relative to the mat surface. Besides strong spectral signals of cyanobacterial photopigments (Chl a and phycobiliproteins), the presence of both green and purple photosynthetic bacteria was evident from spectral signals of Bchl a and Bchl c. Microprofiles of photopigment absorbance correlated well with microdistributions of phototrophs determined in an accompanying study.

Motile chemosensory behaviour of phagotrophic protists: mechanisms for and efficiency in congregating at food patches.

Phagotrophic protists are capable of congregating at point sources of food within a few minutes, from distances of up to several cm in the case of ciliates, or several mm in the case of microflagellates. This is exemplified by four ciliate species and a heterotrophic flagellate. Congregation is accomplished by the combined effect of more than one type of chemosensory motile behaviour including "kinetic responses", "temporal-gradient sensing", and "helical klinotaxis". The results are discussed in terms of microscale patchiness in nature.

Modelling of microscale patch encounter by chemotactic protozoa.

A model of protozoan chemotaxis, based on the rate of change of chemoreceptor occupancy, was used to analyse the efficiency of chemotaxis in a variety of situations. Simulated swimming behaviour replicated patterns observed experimentally. These were classified into three forms of chemosensory behaviour; run-tumble, steered turning, and helical klinotaxis. All three could be simulated from a basic model of chemotaxis by modifying memory times and rotational velocities. In order to steer during helical klinotaxis, the cell must have a short term memory for responding to a signal within a fraction of the time period of the helix. Steered turning was identified as a form where cells react to negative changes in concentration by steering around the turn to swim back up the gradient. All 3 forms were quite effective for encountering targets within the response radius. A response to negative changes in concentration, experienced when the cell is moving away from a target, was found to be important in the absence of periodic changes in swimming direction. The frequency of patch encounter at a fixed density was calculated to be roughly proportional to swimming speed. On the basis of the model, cells are only able to sense point sources within a radius of a few mm. However, even a response radius of 1 mm is enough to increase encounter probability of otherwise minute targets by 2 orders of magnitude. The mean time for patch encounter was calculated to be an exponential function of the mean distance between patches. This results in a very sharp threshold at approximately 6 cm, above which they are not encountered by protozoa within time periods of several days.

Microscale nutrient patches in planktonic habitats shown by chemotactic bacteria

Are nutrients available to microbial communities in micropatches long enough to influence growth and competition? And what are the sources of such patches? To answer these questions, the swimming behavior of chemotactic bacteria in seawater samples was examined. Clusters of bacteria formed in conjunction with cell lysis and excretion by protozoa. These point sources of nutrients spread into spherical patches a few millimeters in diameter and sustained swarms of bacteria for about 10 minutes. Within that time, a large proportion of the nutrients was encountered by bacteria, chemotactic and nonchemotactic alike. Chemotaxis is advantageous for bacteria using patches over a certain size.

Protozoan diversity: converging estimates of the global number of free-living ciliate species.

Protozoa are the most abundant phagotrophs in the biosphere, but no scientific strategy has emerged that might allow accurate definition of the dimensions of protozoan diversity on a global scale. We have begun this task by searching for the common ground between taxonomy and ecology. We have used two methods - taxonomic analysis, and extrapolation from ecological datasets - to estimate the global species richness of free-living ciliated protozoa in the marine interstitial and freshwater benthos. The methods provide estimates that agree within a factor of two, and it is apparent that the species-area curves for ciliates must be almost flat (the slope z takes the very low value of 0.043 in the equation: [number of species] = [constant][area](z)). Insofar as independent ecological datasets can be extrapolated to show similiar, flat, species-area relations, and that these converge with an independent estimate from taxonomic analysis, we conclude that the great majority of freeliving ciliates are ubiquitous. This strengthens our recent claim that the global species richness of free-living ciliated protozoa is relatively low (∼3000).

A new polymorphic methanogen, closely related to Methanocorpusculum parvum, living in stable symbiosis within the anaerobic ciliate Trimyema sp.

A new anaerobic microbial consortium has been discovered: the partners are the ciliated protozoon Trimyema sp. and a single species of methanogen. The consortium has been maintained in culture for more than four years. Each ciliate contains up to 300 symbiotic bacteria; many are relatively small and irregularly disc-shaped, and these are distributed throughout the host's cytoplasm, whereas those which are attached to the ciliate's hydrogenosomes are significantly larger and profusely dentate. This attachment is interpreted as an adaptation to maximize capture by the bacteria of the H2 escaping from hydrogenosomes. The 16S rRNA gene of the symbionts has been partially sequenced, and fluorescent oligonucleotide probes have been constructed and used to detect the different morphotypes of the symbiont within the ciliate. The symbionts belong to a new species of archaeobacterium which is a close relative of the free-living methanogen Methanocorpusculum parvum.

Identification of sulphate-reducing ectosymbiotic bacteria from anaerobic ciliates using 16S rRNA binding oligonucleotide probes.

The identity of ectosymbiotic bacteria of some marine, free-living anaerobic ciliates (Metopus contortus, Caenomorpha levanderi and Parablepharisma sp.) was studied using fluorescent-dye-conjugated oligonucleotides complementary to short sequence elements of 16S ribosomal RNA. The ectosymbiotic bacteria of all species hybridized with a eubacterial probe and those of the two former mentioned species hybridized with a general probe for sulphate-reducing bacteria, but not to a probe specific for Desulfobacter. The results support indirect evidence suggesting that ectosymbiotic bacteria of anaerobic ciliates are sulphate-reducers which depend on host metabolites for substrates.

An anaerobic ciliate as a natural chemostat for the growth of endosymbiotic methanogens.

An anaerobic ciliate (Metopus palaeformis) with methanogen endosymbionts was grown in batch culture. The growth of both partners was monitored for four ciliate generations following re-feeding. Methanogens consistently accounted for approximately 1% of total consortium biovolume. Minor variations in ciliate and methanogen biovolume were closely correlated. Methanogens completed one to two generations per ciliate generation. The growth of methanogens in these ciliates is analogous to the growth of microbes in a chemostat, with methanogen growth rate rapidly converging on a rate that is equal to the 'dilution rate' provided by the expanding ciliate cell volume. Methanogen digestion was insignificant in growing ciliates and no other loss processes were recognised. We conclude that the growth rates of ciliates and methanogens are approximately equivalent. The only exception occurs in ciliates showing no net growth, when substrate supply within the ciliate is still sufficient to promote measurable growth of methanogens. The methanogens within each ciliate appear to divide simultaneously and irrespective of their size. This phenomenon is not apparently integrated with events in the host cell cycle. It may give the methanogens some selective advantage in securing their persistence in successive ciliate generations.

The biology of free-living anaerobic ciliates.

Anaerobic ciliates are incapable of using oxidative phosphorylation in their energy metabolism and they are more or less sensitive to oxygen. All anaerobic ciliates possess mitochondria-like organelles (with a double outer membrane and often a few cristae) but these do not contain typical mitochondrial enzymes (e.g., cytochromes, cytochrome oxidase). In some species these organelles are capable of fermenting pyruvate into acetate and H2 and they are then referred to as hydrogenosomes. At least six orders of ciliates include anaerobic species. It is concluded that the evolution of anaerobic forms has taken place independently within different taxonomic groups and that hydrogenosomes are modified mitochondria. Many anaerobic ciliates harbour ecto- or endosymbiotic bacteria. Several ciliate species which produce hydrogen as a metabolic waste product harbour endosymbiotic methanogenic bacteria; in some cases this symbiosis represents a mutualistic relationship in which the host controls the life cycle of the symbionts and gains from their presence in terms of growth rate and growth efficiency. Many marine anaerobic ciliates harbour ectosymbiotic bacteria, but the nature of these bacteria and the significance of the association is not yet understood. The present paper reviews what is known about the biology of anaerobic ciliates with special emphasis on free-living forms, including a discussion of their habitats and their role in the microbial communities of anoxic environments.

Respiration rates in heterotrophic, free-living protozoa.

Published estimates of protozoan respiratory rates are reviewed with the object of clarifying their value in ecological studies. The data show a surprisingly large variance when similarly sized cells or individual species are compared. This is attributed to the range of physiological states in the cells concerned. The concept of basal metabolism has little meaning in protozoa. During balanced growth, energy metabolism is nearly linearly proportional to the growth rate constant; at the initiation of starvation, metabolic rate rapidly declines. Motility requires an insignificant fraction of the energy budget of protozoans. For growing cells, metabolic rate is approximately proportional to weight(0.75) and the data fall nearly exactly on a curve extrapolated from that describing the respiration rates of poikilotherm metazoans as a function of body weight. It is conceivable that protozoan species exist with lower maximum potential growth and metabolic rates than those predicted from cell volume and the equations derived from the available data. However, the lack of information concerning the state of the cells studied prevents verification of this idea. Laboratory measurements of protozoan respiratory rates have no predictive value for protozoa in nature other than delimiting a potential range. For small protozoans, this range may, on an individual basis, represent a factor of 50.

Suspension feeding in ciliated protozoa: Functional response and particle size selection.

The quantitative uptake of latex beads of different sizes and of live cells by 14 species of ciliates was studied. The functional response (uptake rate as function of food particle concentration) can be fitted to a hyperbolic function and this can be explained in terms of the function of the mouth apparatus. Each species shows a distinct size spectrum of particles which are retained and ingested. These size spectra may be explained by mouth morphology, and particle size selection may play a role for niche separation of coexisting ciliates. Most bacterivorous holotrich ciliates retain particles down to 0.2µm and in one case down to 0.1µm; they retain particles between 0.3 and 1µm most efficiently. The spirotrich ciliates investigated do not retain particles smaller than 1-2µm.

Suspension feeding in ciliated protozoa: Feeding rates and their ecological significance.

The quantitative uptake of suspended particles has been studied in 14 species of ciliated protozoa in terms of the maximum rate of water cleared at low particle concentrations and of the maximum ingestion rate at high particle concentrations. The results, supported by data from the literature, show that ciliates which feed on larger particles (> 1-5µm) compare favorably with metazoan suspension feeders with respect to the ability to concentrate dilute suspensions of particles. Species specialized on smaller food particles (0.2-1µm), the size range of most bacteria in natural environments, require a higher concentration of particles. Bacterial population densities which can sustain ciliate growth are found in sediments, waters rich in organic material, and in the early successional stages of decomposing organic material. This is not the case in open waters in general where bacterivorous ciliates cannot play a role as grazers of bacteria.

Symbiotic Cellulose Degradation in Green Turtles, Chelonia mydas L.

A postgastric, fermentative breakdown of structural plant tissue was demonstrated for green turtles. About 90% cellulose was hydrolyzed. Bacterial and protozoan numbers compared with those of the rumen.