Host manipulation of a freshwater crustacean (Gammarus roeseli) by an acanthocephalan parasite (Polymorphus minutus) in a biological invasion context

Several gammarid species serve as intermediate hosts for the acanthocephalan parasite Polymorphus minutus. This parasite influences gammarid behaviour in order to favour transmission to its ultimate host, generally a bird. We investigated this host manipulation in Gammarus roeseli, a gammarid species introduced in France 150 years ago which now coexists with several exotic species from different origins. In the field, vertical distribution of G. roeseli revealed a higher proportion of infected individuals close to the water's surface and the size distribution of infected gammarids revealed predation pressure on infected individuals. However, under laboratory conditions both infected and non-infected individuals remained benthic. The addition of a second gammarid, Dikerogammarus villosus, to the experimental device involved a vertical displacement of infected G. roeseli towards the water's surface. Dikerogammarus villosus, originating from the Ponto-Caspian basin, can be considered as an aggressive predator. The substitution of D. villosus with Atyaephyra desmarestii, a planktivore decapod, did not alter the gammarids' distribution, with both infected and uninfected G. roeseli staying benthic. Thus, biotic interactions between D. villosus and G. roeseli represent selective pressure encouraging the expression of manipulated behaviour in infected amphipods. Through manipulation, P. minutus was found to increase the survival of infected G. roeseli when faced with non-host predators and to make it more vulnerable to predation by the parasite's definitive host.     

Intra- and inter-specific variations in egg survival and brood development time for Austrian populations of Gammarus fossarum and G. roeseli (Crustacea: Amphipoda)

SUMMARY. 1. Egg survival (ES, percentage of eggs hatched in vitro ), reproductive success (RS, percentage of live young released from the brood pouch) and brood development lime ( d , days) in four populations of Gammarus fossarum and two populations of Gammarus roeseli were studied, in the laboratory at water temperatures of 2.0–26.1°C. Intraspecific differences between populations were not significant, but interspecific differences were found between the two species.
2. In natural stream populations, the reproductive period of G. fossarum lasted from December to September, that of G. roeseli from March to September.
3. In the experimental temperature range 2–26°C, 73% of the total number (771) of G. fossarum females and 69% of 469 G. roeseli females were ovigerous. Of these, 45% of G. fossarum and 43% of G. roeseli females successfully released live young from their brood pouches.
4. For G. fossarum , the optimum temperatures were 11.4°C for ES, where 76% of the eggs hatched, and 11.8°C for RS, where 77% of the females released live young from their brood pouches. For G. roseli , the optimum temperatures were 13.5°C for ES (51% hatched) and 14.0°C for RS (76% released). Over 50% of eggs hatched at temperatures of 3.6–19.2°C in G. fossarum and at 1 1.9–15.1°C in G. roeseli . Development time increased from 12 days at 21.9°C to 251 days at 2.0°C in G. fossarum , and from 10 days at 24.1°C to 212 days at 4.1°C in G. roeseli .
5. interspecific differences between the effects of water temperature on ES, RS and d are used to explain the different distributional patterns of G. fossarum and G. roeseli in central European running water systems. assuming that other physico-chemical variables are suitable for both species.     

Is the host or the parasite the most locally adapted in an amphipod-acanthocephalan relationship? A case study in a biological invasion context

Manipulative endoparasites with complex life cycles can alter their intermediate host immunity and behaviour in ways that increase survival probability within the host body cavity and enhance successful transmission to the definitive host. These parasitic manipulations are variable among and within parasite species and may result from co-evolutionary processes, in which the parasite is constrained for adaptation to the local intermediate host. Hence, arrival of a new host species in a local host population may promote local parasite maladaptation. This study tested the occurrence of local adaptation in two distantly located populations of the acanthocephalan parasite Pomphorhynchus laevis and its effect on the immunity and behaviour of its gammarid intermediate host Gammarus roeseli. This was done in France (an area for which G. roeseli is a recent invader) and Hungary (an area from which G. roeseli was believed to be native). As expected, we found no alteration in G. roeseli's immune defence and behaviour associated with infection by P. laevis in localities, where the gammarid is invasive. Unexpectedly, we found similar results in Hungarian populations, where the parasite was even more exposed to the host immune response. Whilst these results suggest maladaptation of the parasite to the gammarid in both countries, they also suggest that the gammarid host might be locally adapted to the parasite. Genetic analyses were performed on both the parasite and the host and the results suggest that the two subsets of populations we studied harbour rather isolated host-parasite systems, both probably deriving from a common ancestral population. We propose that G. roeseli is also of recent acquisition in Hungary, and that a recent co-evolutionary history between P. laevis and G. roeseli in association with a long generation time in the parasite has constrained parasite adaptations in Europe or even favoured host adaptation to the parasite.     

Polymorphus minutus affects antitoxic responses of gammarus roeseli exposed to cadmium

The acanthocephalan parasite Polymorphus minutus is a manipulator of its intermediate host Gammarus roeseli, which favours its transmission to the final host, a water bird. In contaminated environments, G. roeseli have to cope with two stresses, i.e. P. minutus infection and pollutants. As P. minutus survival relies on its host's survival, we investigated the influence of P. minutus on the antitoxic defence capacities and the energy reserves of G. roeseli females after cadmium exposure. In parallel, malondialdehyde, a toxic effect biomarker, was measured in G. roeseli females and in P. minutus. The results revealed that infected females displayed higher cell damage than uninfected ones, despite an apparent increase in reduced glutathione and metallothionein production. In fact, the increase of these antitoxic systems could be counterbalanced by carotenoid intake by the parasite, so that the overall defence system seemed less efficient in infected females than in uninfected ones. In addition, we demonstrated that cadmium induced cell damage in P. minutus, probably linked with cadmium accumulation in the parasite. Altogether, we observed a paradoxical pattern of responses suggesting that P. minutus increases cadmium toxicity in G. roeseli females although (i) it tends to increase several host antitoxic defence capacities and (ii) it bears part of the pollutant, as reflected by cell damage in the parasite.     

Pollution-induced heat shock protein expression in the amphipod Gammarus roeseli is affected by larvae of Polymorphus minutus (Acanthocephala)

The relationship between the exposure of organisms to chemicals and subsequent alterations in various biochemical processes (commonly referred to as biomarkers) is of growing importance in environmental and ecotoxicological research. However, parasites which also affect the physiological homeostasis of their hosts, and thus may alter biomarker reactions, are usually ignored in environmental research. To address this deficit, we have used the host-parasite system Gammarus roeseli naturally infected with cystacanths of the acanthocephalan Polymorphus minutus to investigate whether infection of gammarids with parasites alters their heat shock protein response following exposure to palladium (Pd). After 24 days of metal exposure relative levels of heat shock protein hsp70 were analysed in the tissues of parasites and intermediate hosts. Simultaneously, the metal accumulation in gammarids and parasites was determined. As none of the infected gammarids showed hsp70 levels at the end of the Pd exposure (either exposed or not), infected and unparasitized G. roeseli were exposed to heat. Again, only uninfected gammarids showed a temperature-dependent increase in hsp70 levels. Interestingly, although the intermediate hosts showed no hsp70 response, exposure to Pd and heat results in increasing hsp70 in the parasites within in the haemocoel of G. roeseli. Heat experiments with isolated cystacanths also showed increasing hsp70 levels in P. minutus with temperature. Concerning uninfected G. roeseli, exposure to Pd and heat causes a hsp70 response. Pd concentrations were found to be higher in the larval parasites than in the gammarids. This result clearly contradicts previous results, as high metal accumulation was so far only described from adult acanthocephalans. Our findings provide experimental evidence that parasites alter the biomarker responses of their host and that the infection status of test animals is extremely important for ecotoxicological studies.     

Comparative study of mathematical models for the relationship between water temperature and brood development time of Gammarus fossarum and G. roeseli (Crustacea: Amphipoda)

SUMMARY. 1. The relationship between water temperature ( T°C ) and brood development time (d days for embryonic development time plus 'post-hatch time') was investigated experimentally using animals from four populations of Gammarus fossarum (populations 1–4) and two populations of G. roeseli (populations 5 and 6) in Austrian streams.
2. Eleven mathematical models were examined as suitable functions to describe the relationship between d and T . The models were compared with respect to their goodness-of-fit and suitability for further quantitative, statistical analyses of intra- and interspecific differences between natural populations of G. fossarum and G. roeseli .
3. Statistical analyses were performed with logarithmically transformed values of d , in order to provide homogeneous variances between the different constant temperatures used. The following three-parameter model was found to be the most suitable for the data:
This equation was fitted in its linear form:      

Vertical Migration in the Sediment-Dwelling Sulfur Bacteria Thioploca spp. in Overcoming Diffusion Limitations

In order to investigate the environmental requirements of the filamentous sulfur bacteria Thioploca spp., we tested the chemotactic responses of these sedimentary microorganisms to changes in oxygen, nitrate, and sulfide concentrations. A sediment core with a Thioploca mat, retrieved from the oxygen-minimum zone on the Chilean shelf, was incubated in a recirculating flume. The addition of 25 (mu)mol of nitrate per liter to the seawater flow induced the ascent of the Thioploca trichomes (length, up to 70 mm) in their mostly vertically oriented gelatinous sheaths. The upper ends of the filaments penetrated the sediment surface and protruded 1 to 3 mm into the flowing water before they bent downstream. By penetrating the diffusive boundary layer, Thioploca spp. facilitate efficient nitrate uptake in exposed trichome sections that are up to 30 mm long. The cumulative length of exposed filaments per square centimeter of sediment surface was up to 92 cm, with a total exposed trichome surface area of 1 cm(sup2). The positive reaction to nitrate overruled a negative response to oxygen, indicating that nitrate is the principal electron acceptor used by Thioploca spp. in the anoxic environment; 10-fold increases in nitrate fluxes after massive emergence of filaments strengthened this hypothesis. A positive chemotactic response to sulfide concentrations of less than 100 (mu)mol liter(sup-1) counteracted the attraction to nitrate and, along with phobic reactions to oxygen and higher sulfide concentrations, controlled the vertical movement of the trichomes. We suggest that the success of Thioploca spp. on the Chilean shelf is based on the ability of these organisms to shuttle between the nitrate-rich boundary layer and the sulfidic sediment strata.     

Effects of temperature on persistence times of native and invasive gammarid species in the stomachs of Cottus gobio

Gastric lavage was used to investigate the effects of temperature on persistence time of two amphipod species, one native Gammarus pulex and one invasive Gammarus roeseli , in the stomachs of bullhead Cottus gobio . Persistence time was strongly influenced by temperature and prey type, such that G. pulex species degraded faster than G. roeseli .     

Effect of multiple parasitic infections on the tolerance to pollutant contamination

The horizontally-transmitted acanthocephalan parasite Polymorphus minutus and the vertically-transmitted microsporidian parasite Dictyocoela roeselum have both been shown to influence on the antitoxic responses of mono-infected Gammarus roeseli exposed to cadmium. The present study investigates the effect of this co-infection on the antitoxic defence responses of naturally infected females exposed to cadmium stress. Our results revealed that, depending on the cadmium dose, bi-infection induced only slight, significant increased cell damage in G. roeseli as compared to non-infection. In addition, the antitoxic defence pattern of cadmium-exposed bi-infected hosts was similar to the pattern of cadmium-exposed D. roeselum-infected hosts. Reduced glutathione concentrations, carotenoid levels and γ-glutamylcystein ligase activity decreased, while metallothionein concentrations increased. This similar pattern indicates that host physiology can be controlled to some extent by microsporidia under stress conditions. It supports the hypothesis of a disruption of acanthocephalan effects in the presence of microsporidia. However, the global negative effects of bi-infection on host condition should be tested on more biological models, since competition between parasites depends on life history trade-off.     

Parasite-induced changes in the diet of a freshwater amphipod: field and laboratory evidence

Trophically transmitted parasites are likely to strongly influence food web-structure. The extent to which they change the trophic ecology of their host remains nevertheless poorly investigated and field evidence is lacking. This is particularly true for acanthocephalan parasites whose invertebrate hosts can prey on other invertebrates and contribute to leaf-litter breakdown. We used a multiple approach combining feeding experiments, neutral lipids and stable isotopes to investigate the trophic ecology of the freshwater amphipod Gammarus roeseli parasitized by the bird acanthocephalan Polymorphus minutus. Infected compared to uninfected amphipods consumed as many dead isopods, but fewer live isopods and less leaf material. Infection had no influence on the total concentration of neutral lipids. Contrary to what we expected based on laboratory findings, the nitrogen isotope signature, which allows for the estimation of consumer's trophic position, was not influenced by infection status. Conversely, the carbon isotope signature, which is used to identify food sources, changed with infection and suggested that the diet of infected G. roeseli includes less perilithon (i.e. fixed algae on rocks, stones) but more terrestrial inputs (e.g. leaf material) than that of uninfected conspecifics. This study shows evidence of changes in the trophic ecology of P. minutus-infected G. roeseli and we stress the need to complement feeding experiments with field data when investigating top-down effects of infection in an opportunistic feeder which adapts its diet to the available food sources.     

Transarterial wall oxygen gradients at the deployment site of an intra-arterial stent in the rabbit

Intimal hyperplasia, common at the deployment site of an intra-arterial stent, may be caused by artery wall hypoxia. The purpose of this study was to determine the effect of an intra-arterial stent on artery wall oxygen concentrations. Transarterial wall oxygen gradients were measured by microelectrode at stent deployment sites in New Zealand White rabbits. Thinned artery walls and decreased oxygen tensions were noted throughout the artery wall immediately following stent deployment with a return toward control values at 28 days. Angioplasty alone had no acute effect on artery wall oxygen concentrations. Larger stent deployment diameters yielded acutely lower artery wall oxygen tensions. Using a linear one-dimensional model for the oxygen profile, we noted that stent deployment resulted in acute changes in oxygen consumption in the inner artery wall that rapidly returned to control values. Changes were noted without differences in blood pressure or arterial blood oxygen concentrations. Oxygen delivery to and consumption within the artery wall are altered by intra-arterial stent deployment. A role for artery wall hypoxia in artery wall pathology at the deployment site of an intra-arterial stent is supported by these findings.     

Linking denitrifier community structure and prevalent biogeochemical parameters in the pelagial of the central Baltic Proper (Baltic Sea)

The oxic-anoxic interface of the water column of the Gotland Basin (central Baltic Sea) is characterised by defined biogeochemical gradients and is hypothesised to be a zone of pronounced denitrification. Our aim was to analyse the composition and distribution of pelagic denitrifying microorganisms in relation to the physico-chemical gradients in the water column. PCR-amplified nirS genes--coding for dissimilatory nitrite reductase--were analysed as functional markers by terminal restriction fragment length polymorphism and cloning. The overall nirS diversity was low, with the lowest levels found at the oxic-anoxic interface. Only a few terminal restriction fragments dominated the denitrifier communities throughout the water column, and these could be assigned to several new Baltic Sea clusters that were revealed by phylogenetic analysis. The novel clusters were separated in two groups corresponding to the oxygen concentrations within specific layers of the water column. Gradients of prevalent biogeochemical parameters (H(2)S, NH(4) (+), NO(3) (-) and O(2)) largely determined the composition of the nirS-type denitrifier communities within the water column of the Gotland Basin.     

Analysis of nonlinear coupled diffusion of oxygen and lactic acid in intervertebral discs

The transport of oxygen and lactate (i.e., lactic acid) in the human intervertebral disc was investigated accounting for the measured coupling between species via the pH level in the tissue. Uncoupled cases were also analyzed to identify the extent of the effect of such coupling on the solute gradients across the disc. Moreover, nonlinear lactic production rate versus lactic concentration and oxygen consumption rate versus oxygen concentration were considered. The nonlinear coupled diffusion equations were solved using an in-house finite element program and an axisymmetric model of the disc with distinct nucleus and anulus regions. A pseudotransient approach with a backward integration scheme was employed to improve convergence. Coupled simulations influenced the oxygen concentration and lactic acid concentration throughout the disc, in particular the gradient of concentrations along the disc mid-height to the nucleus-anulus boundary where the solutes reached their most critical values; minimum for the oxygen tension and maximum for the lactate. Results suggest that for realistic estimates of nutrient and metabolite gradients across the disc, it could be important to take into account the coupling between the rates of synthesis and overall local metabolite/nutrient concentration.     

Uptake and transformation of metals and metalloids by microbial mats and their use in bioremediation

Summary Constructed microbial mats, used for studies on the removal and transformation of metals and metalloids, are made by combining cyanobacteria inoculum with a sediment inoculum from a metal-contaminated site. These mats are a heterotrophic and autotrophic community dominated by cyanobacteria and held together by slimy secretions produced by various microbial groups. When contaminated water containing high concentrations of metals is passed over microbial mats immobilized on glass wool, there is rapid removal of the metals from the water. The mats are tolerant of high concentrations of toxic metals and metalloids, such as cadmium, lead, chromium, selenium and arsenic (up to 350 mg L^–1). This tolerance may be due to a number of mechanisms at the molecular, cellular and community levels. Management of toxic metals by the mats is related to deposition of metal compounds outside the cell surfaces as well as chemical modification of the aqueous environment surrounding the mats. The location of metal deposition is determined by factors such as redox gradients, cell surface micro-environments and secretion of extra-cellular bioflocculents. Metal-binding flocculents (polyanionic polysaccharides) are produced in large quantities by the cyanobacterial component of the mat. Steep gradients of redox and oxygen exist from the surface through the laminated strata of microbes. These are produced by photosynthetic oxygen production at the surface and heterotrophic consumption in the deeper regions. Additionally, sulfur-reducing bacteria colonize the lower strata, removing and utilizing the reducing H₂S, rather than water, for photosynthesis. Thus, depending on the chemical character of the microzone of the mat, the sequestered metals or metalloids can be oxidized, reduced and precipitated as sulfides or oxides. For example precipitates of red amorphous elemental selenium were identified in mats exposed to selenate (Se-VI) and insoluble precipitates of manganese, chromium, cadmium, cobalt, and lead were found in mats exposed to soluble salts of these metals. Constructed microbial mats offer several advantages for use in the bioremediation of metal-contaminated sites. These include low cost, durability, ability to function in both fresh and salt water, tolerance to high concentrations of metals and metalloids and the unique capacity of mats to form associations with new microbial species. Thus one or several desired microbial species might be integrated into mats in order to design the community for specific bioremediation applications.     

Carotenoids of two freshwater amphipod species (Gammarus pulex and G. roeseli) and their common acanthocephalan parasite Polymorphus minutus

Carotenoid compositions of two freshwater Gammarus species (Crustacea: Amphipoda) and of their common acanthocephalan parasite Polymorphus minutus were characterized. The effect of carotenoid uptake by the parasite was addressed by comparing the carotenoid content of uninfected and infected female hosts. Using high-pressure liquid chromatography (HPLC), co-chromatography of reference pigments and electron ionization mass spectrometry of collected HPLC fractions (EI-MS), several xanthophylls and non-polar compounds were identified. Seven kinds of carotenoids, mainly xanthophylls, were identified in gammarids. Astaxanthin was predominant, amounting to 40 wt.% of total carotenoid in both uninfected G. pulex and G. roeseli. By contrast, we found only non-polar compounds with a predominance of esterified forms of astaxanthin in P. minutus larvae. No significant effect of infection on carotenoid content was evidenced in G. pulex and G. roeseli females. Our study highlights the use of a Matrix Solid Phase Dispersion as an efficient extraction method of both xanthophylls and non-polar pigments in small samples, including lipid-rich ones as P. minutus parasite. We discuss on the presumptive pathway leading to the formation of free astaxanthin in gammarids via hydroxy compounds, and on the accumulation of esters of astaxanthin in parasites.     

Nile water as a cause of Eastern Mediterranean sapropel formation: Evidence for and against

During the late Pleistocene, sapropels (layers of organic-carbon rich sediment) formed throughout the entire Eastern Mediterranean Basin in close association with glacial/interglacial transitions. The current theory for the mechanism of sapropel formation involves a density stratification of the water column, due to the invasion of a large quantity of low-saline water, which resulted in oxygen depletion of the bottom waters. Most workers believe that this low-salinity water was glacial meltwater that entered the Mediterranean via the Black Sea and a series of interconnected glacial lakes, but the suggestion also has been made that the freshwater originated from the Nile River. In this study the oxygen isotope values of planktonic foraminifera,Globigerinoides ruber, have been examined in six gravity cores and one piston core from the southern Levantine Basin, and compared with the oxygen isotope records ofG. ruber from other areas of the Eastern Mediterranean. This study deals mainly with the latest sapropel which was deposited approximately 7000 to 9000 years ago. Results indicate that Nile discharge probably does reduce salinities somewhat in the immediate area surrounding the mouth of the Nile, but this water is rapidly mixed with the highly saline waters of the easternmost Mediterranean.Using a mixing equation and surface water salinity limitations, an approximate oxygen isotope balance of surface waters was calculated for the time of latest sapropel deposition. This calculation shows that neither Nile River discharge nor Black Sea input (nor both together) are large enough to account for the large-scale oxygen isotope depletion associated with latest sapropel deposition in the Eastern Mediterranean. This suggests that part of the isotopic change at Termination I is probably due to increased surface water salinities during the last glacial maximum. In addition, evidence from the timing of sapropel 1 deposition and the dissolved oxygen balance indicates that deposition of the latest sapropel is associated with increased surface water production of biogenic material, as much as three times higher than that of present day.     

Deposition of Cryptosporidium oocysts in streambeds

The transfer of Cryptosporidium oocysts from the surface water to the sediment beds of streams and rivers influences their migration in surface waters. We used controlled laboratory flume experiments to investigate the deposition of suspended Cryptosporidium parvum oocysts in streambeds. The experimental results demonstrate that hydrodynamic interactions between an overlying flow and a sediment bed cause oocysts to accumulate in the sediments and reduce their concentrations in the surface water. The association of C. parvum with other suspended sediments increased both the oocysts' effective settling velocity and the rate at which oocysts were transferred to the sediment bed. A model for the stream-subsurface exchange of colloidal particles, including physical transport and physicochemical interactions with sediment grains, accurately represented the deposition of both free C. parvum oocysts and oocysts that were attached to suspended sediments. We believe that these pathogen-sediment interactions play an important role in regulating the concentrations of Cryptosporidium in streams and rivers and should be taken into consideration when predicting the fate of pathogens in the environment.     

Deep water stratification in deep caldera lakes Ikeda, Towada, Tazawa, Kuttara, Toya and Shikotsu

From six deep caldera lakes in Japan, namely lakes Ikeda, Towada, Tazawa, Toya, Kuttara and Shikotsu (in Japanese referred to as Ikedako, Towadako, Tazawako, Toyako, Kuttarako and Shikotsuko), fine resolution profiles of temperature, electrical conductivity, dissolved oxygen and pH have been measured. Measurements were conducted just after deep circulation from the end of March to the end of May 2005. Lake Ikeda and Lake Towada did not undergo a complete turnover. Both showed meromictic features with a clear interface separating the recirculated mixolimnion from the deeper monimolimnion. Lake Tazawa and Lake Toya showed a complete turnover in winter 2004/05, while in Lake Kuttara and Lake Shikotsu, a deep water body remained throughout winter due to pressure effects on the temperature of maximum density. Although these deep waters were never fully recycled into the mixolimnion, they presented themselves as well supplied with oxygen and well circulated within themselves. Where overturn had not been complete small gradients in the oxygen profile, pH profiles and electrical conductivity profiles could be detected. However only in Lake Towada were concentrations of dissolved substances and gradients high enough to have decisive impact on the circulation pattern.     

Benthic-pelagic flux rates on mussel beds: tunnel and tidal flume methodology compared

Material flux rates in an intertidal mussel bed were measured synchronously over two tidal cycles in June 1989 with Benthic Ecosystem Tunnels and a double lane flume. The tunnels enclosed the near bottom water, whereas the flume canalized the total water column. One tunnel was set up in a mussel bed and another one in an adjacent sand bottom as a control. The flume enclosed a mussel lane and a sand lane. In the tunnel and in the flume the mussel bed revealed ammonium and phosphate discharge. At the same time, phytoplankton, dominated byPhyaeocystis globosa, was taken up intensively. These flux rates showed the same tendency but they were higher in the flume than in the tunnel. Different tendencies and flux rates for oxygen and particulate organic matter (POC, PN) were found in flume and tunnel. These differences demonstrate the importance of water column processes regarding the material exchange of a mussel bed. Tunnels enclose smaller bodies of water and are therefore expected to detect even small effects of the benthos on the passing water. In flumes, benthic influence may be diluted over the entire water column but conditions are more natural. The use of flumes is restricted to shallow waters while tunnels have the potential to be used at any depth.     

The development of concentration gradients in a suspension of chemotactic bacteria

When a suspension of bacterial cells of the speciesBacillus subtilis is placed in a chamber with its upper surface open to the atmosphere complex bioconvection patterns are observed. These arise because the cells: (1) are denser than water; and (2) usually swim upwards, so that the density of an initially uniform suspension becomes greater at the top than the bottom. When the vertical density gradient becomes large enough, an overturning instability occurs which ultimately evolves into the observed patterns. The reason that the cells swim upwards is that they are aerotactic, i.e. they swim up gradients of oxygen, and they consume oxygen. These properties are incorporated in conservation equations for the cell (N) and oxygen (C) concentrations, and these are solved in the pre-instability phase of development whenN andC depend only on the vertical coordinate and time. Numerical results are obtained for both shallow- and deep-layer chambers, which are intrinsically different and require different mathematical and numerical treatments. It is found that, for both shallow and deep chambers, a thin boundary layer, densely packed with cells, forms near the surface. Beneath this layer the suspension becomes severely depleted of cells. Furthermore, in the deep chamber cases, a discontinuity in the cell concentration arises between this cell-depleted region and a cell-rich region further below, where no significant oxygen concentration gradients develop before the oxygen is fully consumed. The results obtained from the model are in good qualitative agreement with the experimental observations.