Long-term Microcosm Experiment Research Articles

Long-Term Stability of Bacterial Associations in a Microcosm of Ostreococcus tauri (Chlorophyta, Mamiellophyceae).

Phytoplankton–bacteria interactions rule over carbon fixation in the sunlit ocean, yet only a handful of phytoplanktonic–bacteria interactions have been experimentally characterized. In this study, we investigated the effect of three bacterial strains isolated from a long-term microcosm experiment with one Ostreococcus strain (Chlorophyta, Mamiellophyceae). We provided evidence that two Roseovarius strains (Alphaproteobacteria) had a beneficial effect on the long-term survival of the microalgae whereas one Winogradskyella strain (Flavobacteriia) led to the collapse of the microalga culture. Co-cultivation of the beneficial and the antagonistic strains also led to the loss of the microalga cells. Metagenomic analysis of the microcosm is consistent with vitamin B12 synthesis by the Roseovarius strains and unveiled two additional species affiliated to Balneola (Balneolia) and Muricauda (Flavobacteriia), which represent less than 4% of the reads, whereas Roseovarius and Winogradskyella recruit 57 and 39% of the reads, respectively. These results suggest that the low-frequency bacterial species may antagonize the algicidal effect of Winogradskyella in the microbiome of Ostreococcus tauri and thus stabilize the microalga persistence in the microcosm. Altogether, these results open novel perspectives into long-term stability of phytoplankton cultures.

Effects of fertilizer and weed species richness on soil nematode community in a microcosm field experiment

Both fertilizer and plant species richness may affect the soil nematode community. However, the influence of fertilizer and weed species richness interaction is unclear. Nematode abundance and biodiversity in wheat and weed plots soil treated with nitrogen, phosphate and potassium fertilizer addition and weed species richness (0, 1, 2 and 4 weed levels) were investigated in a long-term microcosm experiment established in 2010 at Kaifeng, China. The results demonstrated that fertilizer treatment increased the abundance of total nematode, bacterivores, and plant parasites whereas it decreased the total genera number, the Shannon—Wiener diversity index (H′), Margalef richness index (SR), the total maturity index (ΣMI), and structure index (SI), and degraded the structure of the nematode community. In contrast, weed species richness increased these ecological indices and enhanced the structure of the nematode community. Principal component analysis (PCA) indicated that the fertilizer’s effect was more significant than weed species richness. Redundancy analysis (RDA) demonstrated that fertilizer affected soil nematode mainly through increasing soil available phosphorus by 4.71 times and ammonium nitrogen content by 74.03%; weed species richness increased the diversity indices of soil nematode mainly through enhancing soil moisture by 2.07%. Our results suggest weed species richness may relieve the negative effect of fertilizer on the diversity of soil nematode community.

Diversity of Bacteria in Lakes with Different Chlorophyll Content and Investigation of Their Respiratory Activity through a Long-Term Microcosm Experiment

Bacterial community structure and metabolism are critical factors for ecosystem functioning since they affect remineralization of nutrients and carbon flow. We used Illumina sequencing of 16SrRNA V3-V4 regions to investigate whether bacterial assemblage composition differs between four samples from two lakes in the geographic region of Epirus (Greece) characterized by distinct oligotrophic to eutrophic/hypereutrophic conditions as revealed by chlorophyll-a values. We found high similarity (>60%) for bacterial assemblages recovered from the two lakes when eutrophic/hypereutrophic conditions prevailed. Distinct bacterial communities appeared in oligotrophic and mesotrophic waters. Low temperature was occasionally an important factor in shaping the bacterial community. In parallel, microcosm experiments were performed to estimate respiration rates of bacterioplankton at in situ temperature and under a 2 °C temperature increase scenario. Differently assembled communities were found to display similar rates except under hypereutrophic conditions when respiration increased significantly, leading to hypoxic conditions. Temperature increase did not affect respiration rates. Overall this study indicated a clear differentiation of bacterial communities between sites of different trophic state. However, different communities responded similarly under a specific range of chlorophyll-a values and resisted small scale temperature perturbations. Different results were found for hypereutrophic conditions and this has implications for ecosystems functioning, given the increasing occurrence of eutrophication events.

Mercury distribution and speciation in biochar particles reacted with contaminated sediment up to 1030 days: A synchrotron-based study

A previous long-term microcosm experiment showed mercury (Hg) in the aqueous phase of contaminated sediment was effectively stabilized through the addition of biochar. The present study focuses on the application of synchrotron-related methods to evaluate the distribution and speciation of Hg in the biochar particles reacted for 235, 387, and 1030 days. The study provided more information on Hg stabilization mechanisms in addition to the information obtained by the previous studies. Confocal micro-X-ray fluorescence imaging (CMXRFI) and micro-X-ray fluorescence (micro-XRF) maps show that mercury co-exists with S, Cu, Fe, Mn, and Zn on the surface and inside the particles of biochar. Extended X-ray absorption fine structure (EXAFS) modeling shows that Hg is in an oxide form on the surface of an iron (hydro)oxide particle from fresh sediment and in Hg-sulfide forms in biochar samples. S X-ray absorption near-edge structure (XANES) analyses show that sulfide is present within the biochar particles. After amendment with biochars, a fraction of the Hg originally present in unstable forms (dissolvable, HgO, colloidal, nano, etc.) in the sediment was likely stabilized as less soluble Hg-sulfide phases on the surface or within the biochar particle. These results suggest Hg accumulation by the biochar particles renders it less potential for transport and bioavailability.

Impact of imidacloprid on life-cycle development of Coccinella septempunctata in laboratory microcosms

Long-term effects of a single application of imidacloprid on ladybird beetle, Coccinella septempunctata L., were studied in indoor laboratory microcosms, starting with the 2nd instar larvae of C. septempunctata but covering the full life cycle. The microcosms comprised enclosures containing a pot with soil planted with broad bean plants and black bean aphid, Aphis craccivora Koch, as food. Exposure doses (0.85–13.66g a.i. ha−1) in the long-term microcosm experiment were based on a preliminary short-term (72h) toxicity test with 2nd instar larvae. The measurement endpoints used to calculate NOERs (No Observed Effect application Rates) included development time, hatching, pupation, adult emergence, survival and number of eggs produced. Furthermore, for these endpoints ER50 (application rate causing 50 percent effect) and LR50 (application rate causing 50 percent mortality) values were calculated when possible. The single imidacloprid application affected survival (lowest LR50 4.07g a.i. ha−1; NOER 3.42g a.i. ha−1), egg production (ER50 26.63g a.i. ha−1) and egg hatching (NOER 6.83g a.i. ha−1). Statistically significant treatment-related effects on the whole development duration, pupation and adult emergence could not be demonstrated (NOER≥13.66g a.i. ha−1). The lowest L(E)R50 values and NOERs derived from the laboratory microcosm test with C. septempunctata are lower than the reported field application rates of imidacloprid (15–60g a.i. ha−1) in cotton cultivation in China, suggesting potential risks to beneficial arthropods.

The effect of earthworms (Lumbricus rubellus) and simulated tillage on soil organic carbon in a long-term microcosm experiment

In a long-term (2.4 years) laboratory experiment, we studied carbon (C) loss from microcosms in which intact litter was placed on the soil surface with or without earthworms (Lumbricus rubellus) or was fragmented and mechanically mixed into the soil to simulate the tillage. Two soil and leaf litter combinations common in post-mining sites near Sokolov (Czech Republic) were used: clay with alder (Alnus glutinosa) and sand with willow (Salix caprea).During the first 20 weeks, respiration was highest with surface litter with earthworms, lowest with mechanical mixing, and intermediate with surface litter minus earthworms. From week 20–80, respiration did not differ among the treatments. From week 80–126, respiration was highest when fragmented litter was mechanically mixed into soil. These results applied to both combinations of soil and litter types. At week 126, C stock was highest with surface-applied litter minus earthworms but most of the litter remained on the soil surface. In the mineral soil, the active C pool was significantly lower with surface-applied litter minus earthworms than in the other treatments. Based on microbial biomass and ergosterol content, microcosms with surface-applied litter minus earthworms were dominated by fungi while those with earthworms or litter that was mechanically mixed into soil were dominated by bacteria. Overall, the results indicate that C sequestration in soil is greater when litter is mixed into the soil by earthworms than by mechanical mixing.

The relative importance of competition and predation in environment characterized by resource pulses--an experimental test with a microbial community.

BackgroundResource availability and predation are believed to affect community dynamics and composition. Although the effects of resource availability and predation on prey communities are usually studied in isolation, these factors can also have interactive effects, especially since the outcome of competition under shared predation is expected to depend on resource availability. However, there are few experimental studies that test the interactive roles of resources and predation on dynamics of more complex multispecies communities. Here, we examine the importance of competition and predation on microbial community dynamics in a resource pulse environment.ResultsWe manipulated resource availability and predation simultaneously in a microbial microcosm experiment, where a bacterial community was exposed to the protozoan predator Tetrahymena thermophila in three different resource concentrations (low, intermediate and high). The prey community consisted of three heterotrophic bacterial species: Bacillus cereus, Serratia marcescens and Novosphingobium capsulatum, all feeding on a shared plant detritus medium. In fresh culture media, all species grew in all resource concentrations used. However, during experiments without any addition of extra resources, the existing resources were soon depleted to very low levels, slowing growth of the three bacterial species. Prior to the microcosm experiment, we measured the competitive ability and grazing resistance, i.e. reduced vulnerability to predation, of each prey species. The three species differed in allocation patterns: in general, N. capsulatum had the best competitive abilities and B. cereus had good grazing resistance abilities. In the long-term microcosm experiment, N. capsulatum dominated the community without predation and, with predation, B. cereus was the dominant species in the intermediate and high resource environments.ConclusionsShort-term, single-species assays revealed significant differences in the allocation of competitive and defensive traits among the prey species. Based on these differences, we were, to some extent, able to predict how the long-term community structure, e.g. species dominance, is modified by the resource availability and predation interaction in pulsed resource environments. Our results are consistent with theoretical predictions and also highlight the importance of interactive effects of resource competition and predation, suggesting that these factors should not be studied in isolation.

Shell biofilm‐associated nitrous oxide production in marine molluscs: processes, precursors and relative importance

Emission of the greenhouse gas nitrous oxide (N2 O) from freshwater and terrestrial invertebrates has exclusively been ascribed to N2 O production by ingested denitrifying bacteria in the anoxic gut of the animals. Our study of marine molluscs now shows that also microbial biofilms on shell surfaces are important sites of N2 O production. The shell biofilms of Mytilus edulis, Littorina littorea and Hinia reticulata contributed 18-94% to the total animal-associated N2 O emission. Nitrification and denitrification were equally important sources of N2 O in shell biofilms as revealed by (15) N-stable isotope experiments with dissected shells. Microsensor measurements confirmed that both nitrification and denitrification can occur in shell biofilms due to a heterogeneous oxygen distribution. Accordingly, ammonium, nitrite and nitrate were important drivers of N2 O production in the shell biofilm of the three mollusc species. Ammonium excretion by the animals was found to be sufficient to sustain N2 O production in the shell biofilm. Apparently, the animals provide a nutrient-enriched microenvironment that stimulates growth and N2 O production of the shell biofilm. This animal-induced stimulation was demonstrated in a long-term microcosm experiment with the snail H. reticulata, where shell biofilms exhibited the highest N2 O emission rates when the animal was still living inside the shell.

Secondary production of a zoobenthic community under metal stress

Little is known about the influence of toxicants on the function of freshwater sediments. To better understand these effects, a long-term microcosm experiment was carried out with cadmium (Cd) as the model pollutant (50 and 400 mg Cd kg−1 dw). In a seven-month study the effect of Cd was examined on secondary production of the zoobenthos (higher taxonomic level) and specifically of the nematode community (species level). Production of almost all taxa decreased under low Cd stress, with rotifers as the only taxon that was able to thrive under this condition. High Cd stress resulted in a decrease in secondary production of all groups with strong differences between taxa. Nematode production likewise decreased, with strongest effects in the higher Cd concentration. Interestingly, at the end of the study, several bacteria-feeding species had benefited from the low Cd stress, probably due to their rapid development in relation to other species and/or the high bacterial density under this condition. Taken together, the results of this study provide insight into secondary production of sediment communities and the important effects of a toxicant thereon.

The fate of catechol in soil as affected by earthworms and clay

The effect of endogeic earthworms ( Octolasion tyrtaeum) and the availability of clay (Montmorillonite) on the mobilization and stabilization of uniformly 14C-labelled catechol mixed into arable and forest soil was investigated in a short- and a long-term microcosm experiment. By using arable and forest soil the effect of earthworms and clay in soils differing in the saturation of the mineral matrix with organic matter was investigated. In the short-term experiment microcosms were destructively sampled when the soil had been transformed into casts. In the long-term experiment earthworm casts produced during 7 days and non-processed soil were incubated for three further months. Production of CO 2 and 14CO 2 were measured at regular intervals. Accumulation of 14C in humic fractions (DOM, fulvic acids, humic acids and humin) of the casts and the non-processed soil and incorporation of 14C into earthworm tissue were determined. Incorporation of 14C into earthworm tissue was low, with 0.1 and 0.44% recovered in the short- and long-term experiment, respectively, suggesting that endogeic earthworms preferentially assimilate non-phenolic soil carbon. Cumulative production of CO 2-C was significantly increased in casts produced from the arable soil, but lower in casts produced from the forest soil; generally, the production of CO 2-C was higher in forest than in arable soil. Both soils differed in the pattern of 14CO 2-C production; initially it was higher in the forest soil than in the arable soil, whereas later the opposite was true. Octolasion tyrtaeum did not affect 14CO 2-C production in the forest soil, but increased it in the arable soil early in the experiment; clay counteracted this effect. Clay and O. tyrtaeum did not affect integration of 14C into humic fractions of the forest soil. In contrast, in the arable soil O. tyrtaeum increased the amount of 14C in the labile fractions, whereas clay increased it in the humin fraction. The results indicate that endogeic earthworms increase microbial activity and thus mineralization of phenolic compounds, whereas clay decreases it presumably by binding phenolic compounds to clay particles when passing through the earthworm gut. Endogeic earthworms and clay are only of minor importance for the fate of catechol in soils with high organic matter, clay and microbial biomass concentrations, but in contrast affect the fate of phenolic compounds in low clay soils.