The combined effects of energy and disturbance on species richness in protist microcosms

The supply of energy, and the frequency of disturbance can both affect species diversity, often, although not always, producing unimodal diversity patterns along an energy or disturbance gradient. However, it has been suggested that diversity is a combined function of both factors, and the dynamic equilibrium model proposed by Huston (1979) is one formalization of this suggestion. This idea has received little direct testing. Here we carry out such a test using protist microcosms, in a factorial manipulation of six levels of energy (quantity of organic resource) and five levels of disturbance (frequency of temperature shock). Species richness responded to both energy supply and disturbance frequency, although not always unimodally. Patterns of response changed over time, and there was some evidence of an interaction between energy and disturbance at two of six time intervals. However, the specific form of this interaction differs from that predicted by the dynamic equilibrium model, and overall, the results provide little support for the model.     

Biomineralization of Fungal Hyphae with Calcite (CaCO3) and Calcium Oxalate Mono- and Dihydrate in Carboniferous Limestone Microcosms

The formation of biogenic fabrics in limestone by two fungi, Serpula himantioides and a polymorphic fungal isolate from limestone identified as a Cephalotrichum (syn. Doratomyces) sp., was investigated. The fungal cultures were grown in laboratory microcosms consisting of Carboniferous limestone and after 21 d incubation at 25°C, biomineralization of fungal filaments was observed. Environmental electron scanning microscopy (ESEM) and X-ray micro-analysis (EDXA) of crystalline precipitates on the hyphae of S. himantioides demonstrated that the secondary crystals exhibited different crystalline forms but were similar in elemental composition to the original limestone. Powder X-ray diffraction (XRD) of crystalline precipitates showed they were composed of a mixture of calcite (CaCO ₃ ) and calcium oxalate monohydrate (CaC ₂ O ₄ · H ₂ O). Analysis of crystals precipitated on the hyphae of the limestone isolate, using ESEM and EDXA, showed that the crystals exhibited similar morphological characteristics and elemental composition to the original limestone. XRD showed that they were composed solely of calcite (CaCO ₃ ) or of calcite with some calcium oxalate dihydrate (CaC ₂ O ₄ · 2H ₂ O). These results provide direct experimental evidence for the precipitation of calcite (CaCO ₃ ) and also secondary mycogenic minerals, on fungal hyphae in low nutrient calcareous environments, and suggest that fungi may play a wider role in the biogeochemical carbon cycle than has previously been appreciated.     

Warming reduces the effects of enrichment on stability and functioning across levels of organisation in an aquatic microbial ecosystem

Warming and nutrient enrichment are major environmental factors shaping ecological dynamics. However, cross‐scale investigation of their combined effects by linking theory and experiments is lacking. We collected data from aquatic microbial ecosystems investigating the interactive effects of warming (constant and rising temperatures) and enrichment across levels of organisation and contrasted them with community models based on metabolic theory. We found high agreement between our observations and theoretical predictions: we observed in many cases the predicted antagonistic effects of high temperature and high enrichment across levels of organisation. Temporal stability of total biomass decreased with warming but did not differ across enrichment levels. Constant and rising temperature treatments with identical mean temperature did not show qualitative differences. Overall, we conclude that model and empirical results are in broad agreement due to robustness of the effects of temperature and enrichment, that the mitigating effects of temperature on effects of enrichment may be common, and that models based on metabolic theory provide qualitatively robust predictions of the combined ecological effects of enrichment and temperature.     

Plasmid transfer between Pseudomonas spp. within epilithic films in a rotating disc microcosm

Abstract A microcosm using rotating slate discs in a chemostat was used to study bacterial population dynamics and genetic interactions in river epilithon. Populations of all introduced donor and recipient Pseudomonas spp. decreased with time but all the bacteria survived better on the slate discs than in the liquid phase. Conjugal transfer of an epilithic plasmid encoding mercury resistance (pQM1) occured with transfer frequencies of 1.4 × 10⁻⁶ to 3.6 × 10⁻³ per recipient, which were about 100-fold lower than in standard membrane filter mating experiments.     

Hot spots become cold spots: coevolution in variable temperature environments

Antagonistic coevolution between hosts and parasites is a key process in the genesis and maintenance of biological diversity. Whereas coevolutionary dynamics show distinct patterns under favourable environmental conditions, the effects of more realistic, variable conditions are largely unknown. We investigated the impact of a fluctuating environment on antagonistic coevolution in experimental microcosms of Pseudomonas fluorescens SBW25 and lytic phage SBWΦ2. High‐frequency temperature fluctuations caused no deviations from typical coevolutionary arms race dynamics. However, coevolution was stalled during periods of high temperature under intermediate‐ and low‐frequency fluctuations, generating temporary coevolutionary cold spots. Temperature variation affected population density, providing evidence that eco‐evolutionary feedbacks act through variable bacteria–phage encounter rates. Our study shows that environmental fluctuations can drive antagonistic species interactions into and out of coevolutionary cold and hot spots. Whether coevolution persists or stalls depends on the frequency of change and the environmental optima of both interacting players.     

Evolution of density‐dependent movement during experimental range expansions

Range expansions and biological invasions are prime examples of transient processes that are likely impacted by rapid evolutionary changes. As a spatial process, range expansions are driven by dispersal and movement behaviour. Although it is widely accepted that dispersal and movement may be context‐dependent, for instance density‐dependent, and best represented by reaction norms, the evolution of density‐dependent movement during range expansions has received little experimental attention. We therefore tested current theory predicting the evolution of increased movement at low densities at range margins using highly replicated and controlled range expansion experiments across multiple genotypes of the protist model system Tetrahymena thermophila. Although rare, we found evolutionary changes during range expansions even in the absence of initial standing genetic variation. Range expansions led to the evolution of negatively density‐dependent movement at range margins. In addition, we report the evolution of increased intrastrain competitive ability and concurrently decreased population growth rates in range cores. Our findings highlight the importance of understanding movement and dispersal as evolving reaction norms and plastic life‐history traits of central relevance for range expansions, biological invasions and the dynamics of spatially structured systems in general.     

Evolution under pH stress and high population densities leads to increased density-dependent fitness in the protist Tetrahymena thermophila

Abiotic stress is a major force of selection that organisms are constantly facing. While the evolutionary effects of various stressors have been broadly studied, it is only more recently that the relevance of interactions between evolution and underlying ecological conditions, that is, eco-evolutionary feedbacks, have been highlighted. Here, we experimentally investigated how populations adapt to pH-stress under high population densities. Using the protist species Tetrahymena thermophila, we studied how four different genotypes evolved in response to stressfully low pH conditions and high population densities. We found that genotypes underwent evolutionary changes, some shifting up and others shifting down their intrinsic rates of increase (r₀). Overall, evolution at low pH led to the convergence of r₀ and intraspecific competitive ability (α) across the four genotypes. Given the strong correlation between r₀ and α, we argue that this convergence was a consequence of selection for increased density-dependent fitness at low pH under the experienced high density conditions. Increased density-dependent fitness was either attained through increase in r₀, or decrease of α, depending on the genetic background. In conclusion, we show that demography can influence the direction of evolution under abiotic stress.     

Ecosystem function and species loss – a microcosm study

There are too many kinds of organisms to be able to study and manage each, yet the loss of a single species can sometimes unravel an ecosystem. Such `fusewire species'– critical in the same sense that an electrical fuse can cut out a whole circuit – would be a rewarding focus for research and management effort. However, this approach can only be effective if these `fusewires' represent but a small proportion of the number of species in the system.  

Aim

To demonstrate methods for measuring what proportion of the species in a system are critical to ecosystem function.  

Methods

The prevalence of fusewire species was measured in manipulative experiments on an aquatic microcosm.  

Results

No single genus deletion caused changes in key characteristics of the system.  

Main conclusions

Comparison of these results with other published studies shows that the proportion of critical fusewire species varies amongst different ecosystems. The oxidation pond microcosms were shown to contain no single species indispensable to system function. They appear to be ill-suited to a management strategy which focuses on priority eukaryote species. However, a single study provides no evidence that this result is general or even typical of other kinds of ecosystems; it is presented here as an empirical model. Other methods of investigation are available; they are less experimentally rigorous but more practical. These could provide important guidance in planning an approach to management in a particular ecosystem.     

Fate of explosives and their metabolites in bioslurry treatment processes

Microcosm tests simulating bioslurry reactors with 40% soilcontent, containing high concentrations of TNT and/or RDX,and spiked with either [¹⁴C]-TNT or[¹⁴C]-RDX were conducted to investigate the fate ofexplosives and their metabolites in bioslurry treatment processes.RDX is recalcitrant to indigenous microorganisms in soil andactivated sludge under aerobic conditions. However, soilindigenous microorganisms alonewere able to mineralize 15% of RDX to CO₂ underanaerobic condition, and supplementation of municipal anaerobicsludge as an exogenous source of microorganismssignificantly enhanced the RDX mineralization to 60%. RDXmineralizing activity of microorganisms in soil and sludge wassignificantly inhibited by the presence of TNT. TNTmineralization was poor (< 2%) and was not markedlyimproved by the supplement ofaerobic or anaerobic sludge. Partitioning studies of[¹⁴C]-TNT in the microcosmsrevealed that the removal of TNTduring the bioslurry process was due mainly to thetransformation of TNT and irreversiblebinding of TNT metabolites onto soil matrix. In the case ofRDX under anaerobic conditions,a significant portion (35%) of original radioactivity wasalso incorporated into the biomass andbound to the soil matrix.