Allelopathy prevents competitive exclusion and promotes phytoplankton biodiversity

It has been hypothesized that allelopathy can prevent competitive exclusion and promote phytoplankton diversity in aquatic ecosystems, where numerous species coexist on a limited number of resources. However, experimental proof‐of‐principle is not available to support this hypothesis. Here we present the first experimental evidence to support this hypothesis by demonstrating that allelopathy promotes the coexistence of two phytoplankton species, Ankistrodesmus falcatus and Oscillatoria sp., that compete for a single limiting nutrient. By performing long‐term competition experiments in nitrate‐limited continuous cultures, and by describing the population dynamics using a mechanistic model, we demonstrate that when allelopathy comes into play, one of the following outcomes is possible depending on the relative initial abundances of the species: dominance of the stronger competitor for nitrate (the non‐allelopathic species), oscillatory coexistence, or dominance of the weaker competitor (the allelopathic species). Our model analysis revealed that sustained oscillatory coexistence of the two species would be a common outcome of this experiment. Our study confirms for the first time, based on laboratory experiments combined with mechanistic models, that allelopathy can alter the predicted outcome of inter‐specific competition in a nutrient‐limited environment and increase the potential for the coexistence of more species than resources, thereby contributing to the identification of endogenous mechanisms that explain the extreme diversity of phytoplankton communities.     

The outcome of competition between the two chrysomonads Ochromonas sp. and Poterioochromonas malhamensis depends on pH

We investigated the effect of pH on the competition of two closely related chrysomonad species, Poterioochromonas malhamensis originating from circumneutral Lake Constance, and Ochromonas sp. isolated from a highly acidic mining lake in Austria (pH ∼2.6). We performed pairwise growth experiments between these two species at four different pH ranging from 2.5 to 7.0. Heterotrophic bacteria served as food for both flagellates. Results were compared to growth rates measured earlier in single species experiments over the same pH range. We tested the hypothesis that the acidotolerant species benefits from competitive release under conditions of acid stress. The neutrophilic strain numerically dominated over the acidotolerant strain at pH 7.0, but was the inferior competitor at pH 2.5. At pH 3.5 and 5.0 both strains coexisted. Surprisingly, P. malhamensis prevailed over Ochromonas sp. under moderately acidic conditions, i.e. at the pH where growth rates of the latter peaked when grown alone. Since bacterial food was not limiting, resource competition is improbable. It appears more likely that P. malhamensis ingested cells of its slightly smaller competitor. Adverse effects mediated via allelopathy, either directly on the competing flagellate or indirectly by affecting its bacterial food, might also have affected the outcome of competition.     

Context‐dependent induction of allelopathy in plants under competition

Some plants use allelopathy to compete against neighbouring plants, and the ability to induce allelopathic compound production in response to competition is hypothesized to be adaptive, as plants can save costs of metabolite production in the absence of competitors. However, whether plants induce allelopathy has rarely been explored so far. We studied the inducibility of polyacetylenes – putative allelopathic compounds in Solidago altissima – in response to competition. Polyacetylenes were found in natural soil surrounding S. altissima patches within the range of concentration known to inhibit competitor growth. Individual S. altissima plants with higher polyacetylene concentration in roots suppressed the growth of the competitor plants more, suggesting that root polyacetylene levels proximate plants’ allelopathic capacity. Competition induced polyacetylenes in a context‐dependent manner: Whereas introduced Japanese and Australian populations of S. altissima had higher constitutive concentration of polyacetylenes than the native North American populations, inducibility was observed only in Australian plants, where the population is still at an early stage of invasion. Also, induction became more prominent under nutrient depletion, where enhanced allelopathy may be particularly beneficial for suppressing a competitor's exploitative capacity. Finally, we found weak evidence for a tradeoff between constitutive and induced polyacetylenes. The observed patterns suggest that allelopathic plants could respond to competition by inducing allelochemical production, but the benefit of such plasticity may vary across time and space. Shifts in competitor communities in introduced range over time may shape plant's plastic responses to competition, while variation in resource availability may alter competitive environment to influence the degree to which plants induce allelopathy.     

Allelopathic effects of the Baltic cyanobacteria Nodularia spumdigena, Aphanizomenon flos-aquae and Anabaena lemmermannii on algal monocultures

Allelopathy, the release of extracellular compounds that inhibit the growth of other microorganisms, may be one factor contributing to the formation and/or maintenance of cyanobacterial blooms. We investigated the allelopathic effects of three cyanobacterial species (Nodularia spumigena, Aphanizomenon flos-aquae and Anabaena lemmermannii) that frequently form mass-occurrences in the Baltic Sea. We exposed monocultures of three phytoplankton species (Thalassiosira weissflogii, Rhodomonas sp. and Prymnesium parvum) to cell-free filtrates of the three cyanobacteria, and quantified allelopathic effects with cell counts. We also investigated the role of the growth phase of cyanobacteria in their allelopathy, by comparing the effects of an exponential and a stationary phase culture of N. spumigena. All tested cyanobacteria inhibited the growth of Rhodomonas sp., but none of them affected P. parvum. The effects on T. weissflogii were more variable, and they were amplified by repeated filtrate additions compared to a single filtrate addition. N. spumigena was more allelopathic in exponential than in stationary growth phase, whereas the culture filtrate was more hepatotoxic in stationary phase. Hepatotoxins were thus probably not involved in the allelopathic effects, which is also indicated by the allelopathic properties of the non-toxic A. flos-aquae and A. lemmermannii. The results demonstrate that the common Baltic cyanobacteria affect some coexisting phytoplankton species negatively. Allelopathy may therefore play a role in interspecific competition and contribute to cyanobacterial bloom maintenance.     

Conditional allelopathic potential of temperate lianas

Allelopathy is often treated as an innate characteristic of a species rather than a phenotypically plastic trait that can vary with environmental conditions. Lianas are a highly competitive, phenotypically plastic life form that typically occur in both shaded and unshaded environments. As such, we hypothesized that temperate lianas may conditionally change allocation to allelopathic chemicals in response to light availability though the expected direction of change is unclear. Shading may reduce resource availability and therefore reduce allocation to allelochemicals, induce allelopathy as a competitive mechanism, or may not be related to allelopathy. To test the conditionality of allelopathy, sun and shade leaves of five common liana species (Toxicodendron radicans, Parthenocissus quinquefolia, Celastrus orbiculatus, Lonicera japonica, and Vitis vulpina) were collected from a young deciduous forest in New Jersey, USA, and tested with laboratory bioassays to detect allelopathic potential. All liana species showed allelopathic potential, and three species exhibited induction of increased allelopathic potential in shaded environments. The two species that were not shade induced are late successional lianas that persist for long periods in forest canopies. In contrast, the inducible lianas were early successional species that typically decline with canopy closure. This research indicates that lianas have the potential to be allelopathic and allelopathic potential conditionally responds to shading only for species that would normally be excluded from the forest canopy. As early successional lianas are present throughout forest regeneration in a range of light environments, allelopathic plasticity may increase their success by differentially allocating resources based on environmental conditions.     

Absence of negative allelopathic effects of cylindrospermopsin and microcystin-LR on selected marine and freshwater phytoplankton species

Cyanobacterial toxins have been regarded by some researchers as allelopathic substances that could modulate the growth of competitors. Nevertheless, often the concentrations of toxins used are too high to be considered ecologically relevant. In this work we tested the hypothesis that microcystin-LR (MC-LR) and cylindrospermopsin (CYN) at ecologically relevant concentrations have no allelopathic effects on some species of phytoplankton. Extracts containing the toxins as well as pure MC-LR and CYN toxins were used to assess their effects on the growth rates of Nannochloropsis sp., Chlamydomonas reinhardtii, and Chlorella vulgaris. Cyanobacterial crude extracts induced more pronounced effects on growth rates than pure toxins. Microcystis aeruginosa and Aphanizomenon ovalisporum crude extracts containing MC-LR and CYN at 0.025–2.5 mg l^?1 stimulated growth rates of microalgae, whereas A. ovalisporum crude extracts containing 2.5 mg l^?1 of CYN strongly inhibited growth rates of microalgae after 4 and 7 days of exposure. MC-LR and CYN at environmentally occurring concentrations were unable to affect negatively the growth of microalgae, and therefore these molecules may play roles other than allelopathy in natural ecosystems.     

Activated carbon may have undesired side effects for testing allelopathy in invasive plants

Activated carbon has become a widely used tool to investigate root-mediated allelopathy of plants, especially in plant invasion biology, because it adsorbs and thereby neutralizes root exudates. Allelopathy has been a controversially debated phenomenon for years, which revived in plant invasion biology as one possible reason for the success of invasive plants. Noxious plant exudates may harm other plants and provide an advantage to the allelopathic plant. However, root exudates are not always toxic, but may stimulate the microbial community and change nutrient availability in the rhizosphere. In a greenhouse experiment, we investigated the interacting effects of activated carbon, arbuscular mycorrhiza and plant competition between the invasive Senecio inaequidens and the native Artemisia vulgaris. Furthermore, we tested whether activated carbon showed any undesired effects by directly affecting mycorrhiza or soil chemistry. Contrary to the expectation, S. inaequidens was a weak competitor and we could not support the idea that allelopathy was involved in the competition. Activated carbon led to a considerable increase in the aboveground biomass production and reduced the infection with arbuscular mycorrhiza of both plant species. We expected that arbuscular mycorrhiza promotes plant growth by increasing nutrient availability, but we found the contrary when activated carbon was added. Chemical analyses of the substrate showed, that adding activated carbon resulted in a strong increase in plant available phosphate and in a decrease of the C_organic/N_total ratio, both of which suggest stimulated microbial activity. Thus, activated carbon not only reduced potential allelopathic effects, but substantially changed the chemistry of the substrate. These results show that activated carbon should be handled with great care in ecological experiments on allelopathy because of possible confounding effects on the soil community.     

从种内、种间化感作用分析入侵种薇甘菊及其近邻种的非协同进化趋势

为预测入侵植物与其邻近物种间的非协同进化趋势,分别提取薇甘菊(Mikania micrantha)及其近邻种五爪金龙(Ipomoea cairica)、葛藤(Pueraria lobata)和鸡矢藤(Paederia scandens)的叶片水提液处理幼苗,对薇甘菊及其近邻种的种内、种间化感作用进行研究。结果表明,薇甘菊对3种近邻种的化感作用呈现不同趋势(SE=0.50, 0.61,–0.16),但种内存在较强的化感促进作用(SE=0.61),说明其种内化感竞争较弱;葛藤的种内化感促进作用(SE=0.32)弱于薇甘菊,种间的化感促进作用与薇甘菊相似(SE=0.52, 0.50);五爪金龙种内化感促进作用(SE=0.06)弱于薇甘菊和葛藤(SE=0.32),种间促进作用(SE=0.24)弱于薇甘菊(SE=0.61);鸡矢藤的种内、种间化感作用均表现为抑制作用(SE=–0.18,–0.07),说明种内、种间化感竞争较强。野外调查表明4种植物分布的频度和多度均表现为薇甘菊葛藤五爪金龙鸡矢藤,这与化感竞争力结果一致。可见,种内与种间化感作用在入侵植物与近邻植物的综合竞争力中扮演着极为重要的作用,这为入侵种与近邻种之间的非协同进化趋势指明了方向。     

Light energy partitioning,photosynthetic efficiency and biomass allocation in invasive <Emphasis Type="Italic">Prunus serotina</Emphasis> and native <Emphasis Type="Italic">Quercus petraea</Emphasis> in relation to light environment,competition and allelopathy

This study addressed whether competition under different light environments was reflected by changes in leaf absorbed light energy partitioning, photosynthetic efficiency, relative growth rate and biomass allocation in invasive and native competitors. Additionally, a potential allelopathic effect of mulching with invasive Prunus serotina leaves on native Quercus petraea growth and photosynthesis was tested. The effect of light environment on leaf absorbed light energy partitioning and photosynthetic characteristics was more pronounced than the effects of interspecific competition and allelopathy. The quantum yield of PSII of invasive P. serotina increased in the presence of a competitor, indicating a higher plasticity in energy partitioning for the invasive over the native Q. petraea, giving it a competitive advantage. The most striking difference between the two study species was the higher crown-level net CO₂ assimilation rates (A_crown) of P. serotina compared with Q. petraea. At the juvenile life stage, higher relative growth rate and higher biomass allocation to foliage allowed P. serotina to absorb and use light energy for photosynthesis more efficiently than Q. petraea. Species-specific strategies of growth, biomass allocation, light energy partitioning and photosynthetic efficiency varied with the light environment and gave an advantage to the invader over its native competitor in competition for light. However, higher biomass allocation to roots in Q. petraea allows for greater belowground competition for water and nutrients as compared to P. serotina. This niche differentiation may compensate for the lower aboveground competitiveness of the native species and explain its ability to co-occur with the invasive competitor in natural forest settings.     

Effects of macroalgae Ulva pertusa (Chlorophyta) and Gracilaria lemaneiformis (Rhodophyta) on growth of four species of bloom-forming dinoflagellates

The effects of fresh thalli and culture medium filtrates from two species of marine macroalgae, Ulva pertusa Kjellm (Chlorophyta) and Gracilaria lemaneiformis (Bory) Dawson (Rhodophyta), on growth of marine microalgae were investigated in co-culture under controlled laboratory conditions. A selection of microalgal species were used, all being identified as bloom-forming dinoflagellates: Prorocentrum donghaiense Lu sp., Alexandrium tamarense (Lebour) Balech, Amphidinium carterae Hulburt and Scrippsiella trochoide (Stein) Loeblich III. Results showed that the fresh thalli of either U. pertusa or G. lemaneiformis significantly inhibited the microalgal growth, or caused mortality at the end of the experiment. However, the overall effects of the macroalgal culture filtrates on the growth of the dinoflagellates were species-specific (inhibitory, stimulatory or none) for different microalgal species. Results indicated an allelopathic effect of macroalga on the co-cultured dinoflagellate. We then took P. donghaiense as an example to further assess this hypothesis. The present study was carried out under controlled conditions, thereby excluded the fluctuation in light and temperature. Nutrient assays showed that nitrate and phosphate were almost exhausted in G. lemaneiformis co-culture, but remained at enough high levels in U. pertusa co-culture, which were well above the nutrient limitation for the microalgal growth, when all cells of P. donghaiense were killed in the co-culture. Daily f/2 medium enrichment greatly alleviated the growth inhibition on P. donghaiense in G. lemaneiformis co-culture, but could not eliminate it. Other environmental factors, such as carbonate limitation, bacterial presence and the change of pH were also not necessary for the results. We thus concluded that allelopathy was the most possible reason leading to the negative effect of U. pertusa on P. donghaiense, and the combined roles of allelopathy and nutrient competition were essential for the effect of G. lemaneiformis on P. donghaiense.     

The allelopathic effects of Festuca paniculata depend on competition in subalpine grasslands

Allelopathy is recognized as an important process in plant–plant interactions, but how it affects plant communities growing in competitive conditions has not been assessed. This article investigates whether the allelopathic effect of Festuca paniculata is modified by competition between target plants in subalpine grasslands. We hypothesized that plants growing in mixed stands will be more affected by allelochemicals than the same species in monoculture. At Lautaret pass (Northern French Alps), a pot experiment was designed. We used leachates from donor pots (Treatments: 1. Bare soil, 2. F. paniculata clipped, and 3. F. paniculata unclipped) to water target pots (Treatments: 1. Control (soil only), 2. Dactylis glomerata, 3. Agrostis capillaris, and 4. D. glomerata and A. capillaris). Target plants were cultivated during one growing season. The effects of leachates from donor pots and interspecific competition in target pots were evaluated by measuring the final biomass of plants. Soil fertility was controlled in all target pots by measuring NO₃ ^?, NH₄ ⁺, N, and C % of the soil. Effect of target treatment under bare soil : Both D. glomerata and A. capillaris grew better in monocultures than in mixture. Effect of donor treatment on monocultures : Under bare soil, D. glomerata grew better than under F. paniculata leachates. By contrast, A. capillaris did not respond to donor pot treatment. Effect of donor treatment on mixtures: However, when both species were cultivated together under F. paniculata leachates, the biomass of D. glomerata was similar to that in monoculture under bare soil. Differences in sensitivity to allelopathy reversed the impact of interspecific competition: A. capillaris facilitated D. glomerata under allelopathy, which made allelopathy of F. paniculata on D. glomerata inefficient. The complexity of overlapping mechanisms of plant–plant interactions are highlighted by this semi-natural experiment. In subalpine grasslands, allelopathy not only limits the growth of neighboring plants, but it may also modify community assembly by affecting other plant–plant interactions such as competition. This study contributes to explore the way allelopathy interacts with other plant–plant interactions in natural systems.     

毛竹浸提液对苦槠幼苗生长的化感效应

为探讨毛竹(Phyllostachys edulis)扩张过程中潜在的化感作用,选择苦槠(Castanopsis sclerophylla(Lindl)Schott)为研究对象。采用水浸提的方法,用毛竹茎叶、枯落物和土壤3部分浸提液浇灌苦槠幼苗,以蒸馏水处理作为对照,对比分析质量浓度分别为0.1、0.05、0.02 g/mL的3个浓度梯度浸提液处理下苦槠幼苗生长指标及各项光合生理指标的差异。结果表明,毛竹浸提液对苦槠幼苗苗高、地径和叶绿素相对含量的影响大体上呈现高浓度抑制低浓度促进的双重浓度效应。不同来源毛竹浸提液的化感效应不尽相同,土壤浸提液对苦槠幼苗生长和光合生理均呈现抑制作用,而茎叶、枯落物浸提液低浓度时为促进作用。毛竹潜在的化感作用,在其扩张过程中可能会干扰森林主要树种更新,从而对森林群落产生威胁。     

The interspecific relationships of seaweeds and their role in the formation of communities of Ahnfeltia tobuchiensis (Kanno et Matsubara, 1932) Makienko, 1970 (Rhodophyta)

The character and intensity of interspecific relationships between common species of an Ahnfeltia bed community (Ahnfeltia tobuchiensis, Ahnfeltiopsis flabelliformis, Chondrus armatus, and Ptilota filicina) were investigated in relation to light levels and concentrations of dissolved inorganic nitrogen and phosphorus compounds. Growth and photosynthetic rates of seaweeds were measured in mono and co-cultures, as well as in a medium containing the metabolites of attendant species. Under low light conditions, A. tobuchiensis showed the highest competitive ability among the species we studied. The experimental results indicated both negative (exploitative competition, allelopathic inhibition of production processes of a competitor) and positive (stimulation of growth and photosynthesis of accompanying species) relationships between seaweeds. Increased nutrient concentrations and light levels resulted in a substantial reduction in the intensity of interspecific relationships. The dynamic system of positive and negative relationships may regulate the A. tobuchiensis community structure; the intensity of interspecific relationships depends on the availability of resources such as light and nutrients.     

Seed-to-seed potential allelopathic effects between <Emphasis Type="Italic">Ligularia virgaurea</Emphasis> and native grass species of Tibetan alpine grasslands

Allelopathy is an important process in plant communities, but the role of seed allelopathy in natural ecosystems remains poorly understood. In the present study, we examined the potential allelopathic effects of Ligularia virgaurea (a dominant species in degraded Tibetan grasslands) seeds on the germination of four native grass species (Festuca sinensis, Agrostis gigantean, Bromus inermis, and Elymus nutans). The results showed that L. virgaurea seeds can have potential allelopathic effects on seed germination, mean time to germination and root growth rates of native grass species. We further demonstrate that these effects are driven by a water-soluble seed leachate. Species with smaller seeds were generally more sensitive than larger seeded species. The results suggest that seed-to-seed allelopathic potential may be an important mechanism driving the dominance of L. virgaurea in degraded alpine grasslands on the Tibetan Plateau. Further studies are required to demonstrate effects of seed-to-seed potential allelopathy in a field setting.     

The coevolution of two phytoplankton species on a single resource: Allelopathy as a pseudo-mixotrophy

Without the top-down effects and the external/physical forcing, a stable coexistence of two phytoplankton species under a single resource is impossible — a result well known from the principle of competitive exclusion. Here I demonstrate by analysis of a mathematical model that such a stable coexistence in a homogeneous media without any external factor would be possible, at least theoretically, provided (i) one of the two species is toxin producing thereby has an allelopathic effect on the other, and (ii) the allelopathic effect exceeds a critical level. The threshold level of allelopathy required for the coexistence has been derived analytically in terms of the parameters associated with the resource competition and the nutrient recycling. That the extra mortality of a competitor driven by allelopathy of a toxic species gives a positive feed back to the algal growth process through the recycling is explained. And that this positive feed back plays a pivotal role in reducing competition pressures and helping species succession in the two-species model is demonstrated. Based on these specific coexistence results, I introduce and explain theoretically the allelopathic effect of a toxic species as a ‘pseudo-mixotrophy’—a mechanism of ‘if you cannot beat them or eat them, just kill them by chemical weapons’. The impact of this mechanism of species succession by pseudo-mixotrophy in the form of alleopathy is discussed in the context of current understanding on straight mixotrophy and resource-species relationship among phytoplankton species.     

Canola cultivar performance in weed-infested field plots confirms allelopathy ranking from in vitro testing

Crop competition and allelopathy are two cultural control options for possible inclusion in cropping systems. This research aimed to identify superior allelopathic canola genotypes through a two-year field study. First year screening results of 312 diverse Brassica genotypes showed genotypes differed significantly in their ability to suppress weed infestations. Crop plant height was correlated with the competitive ability of several genotypes, while other genotypes showed good weed-suppressive ability despite being short. Thirty-six of the genotypes grown in the field had been previously assessed for their allelopathic ability to inhibit the growth of annual ryegrass (Lolium rigidum) seedlings using an in vitro technique. The highly allelopathic genotypes: Av-opal, Sardi603, Rivette and Atr-beacon performed well against annual ryegrass in the laboratory and also against other species, including Capsella bursa-pastoris, Sisymbrium orientale and Hordeum leporinum in the field. The weakly allelopathic Barossa and X-06-6-3725 genotypes performed poorly both in the laboratory studies and in the field. The following year, field testing of selected genotypes at two sowing dates further suggested that the most allelopathic genotypes in the laboratory bioassay were generally those that suppressed weed numbers and their biomass in the field. The late sowing time increased the natural weed pressure leading to a decrease in both canola grain yield and quality. Many of the highly allelopathic canola genotypes, which caused low weed populations in the field, had relatively low grain yield. This suggests that the allelopathic trait is independent of local adaptation and yields potential under weed-free conditions. Ideally, cultivars with both high allelopathy and high competitive ability would be most useful to help farmers maximise yield and control weeds. Selection for allelopathy in canola shows potential as a future non-chemical weed control option and requires further investigation.     

Plankton community and the relationship with the environment in saline lakes of Onon-Torey plain,Northeastern Mongolia

The plankton community of sixteen saline lakes located on Onon-Torey plain (Northeastern Mongolia) during the filling phase and the raising of the water level was investigated in July 2011. Thirty-five taxa of phytoplankton and thirty-one species of zooplankton were found. For phytoplankton, blue-green algae (Merismopedia elegans, Anabaenopsis elenkinii, Arthrospora fusiformis, Spirulina major, Lyngbya sp., Oscillatoria sp.) and green algae (Monoraphidium minutum, Tetrastrum komarekii, Ankyra ocellata, Oocystis sp.) were dominant. For zooplankton, Filinia longiseta, Brachionus plicatilis, B. variabilis, Hexarthra mira (Rotifera), Daphnia magna, Moina brachiata, M. mongolica (Cladocera), Arctodiaptomus bacillifer, Mixodiaptomus incrassatus, Metadiaptomus asiaticus (Copepoda) dominated. Mineralization, active hydrogen ratio, dissolved oxygen and water temperature were the main factors influencing the diversity, structure and distribution of plankton organisms in the steppe lakes during low water level. The RDA analysis for phytoplankton and zooplankton from different lakes was carried out for selected two groups which included lakes and a subset related species. The first group is of oligohaline and mesohaline lakes in which mostly green algae, rotifers and copepods inhabit. The second group is of mesohaline and polyhaline lakes with mainly blue-green algae, some crustaceans and rotifers inhabiting. High abundance and biomass of Spirulina major, Oscillatoria sp. and Brachionus variabilis were observed in lakes with high mineralization, pH and temperature.     

Microbial Community Changes Elicited by Exposure to Cyanobacterial Allelochemicals

An increasing body of evidence points out that allelopathy may be an important process shaping microbial communities in aquatic ecosystems. Cyanobacteria have well-documented allelopathic properties, mainly derived from the evaluation of the activity of allelopathic extracts or pure compounds towards monocultures of selected target microorganisms. Consequently, little is known regarding the community dynamics of microorganisms associated with allelopathic interactions. In this laboratory-based study, a Microcystis spp.-dominated microbial community from a freshwater lake was exposed, for 15 days, to exudates from the cyanobacterium Oscillatoria sp. strain LEGE 05292 in laboratory conditions. This cyanobacterium is known to produce the allelochemicals portoamides, which were among the exuded compounds. The community composition was followed (by means of polymerase chain reaction followed by denaturing gradient gel electrophoresis and microscopic analyses) and compared to that of a non-exposed situation. Following exposure, clear differences in the community structure were observed, in particular for cyanobacteria and unicellular eukaryotic taxa. Interestingly, distinct Microcystis genotypes present in the community were differentially impacted by the exposure, highlighting the fine-scale dynamics elicited by the exudates. These results support a role for cyanobacterial allelochemicals in the structuring of aquatic microbial communities.     

Competition for Sulfate and Ethanol Among Desulfobacter, Desulfobulbus, and Desulfovibrio Species Isolated from Intertidal Sediments

Competition for sulfate and ethanol among Desulfobacter, Desulfobulbus, and Desulfovibrio species isolated from estuarine sediments was studied in energy-limited chemostats. Desulfovibrio baculatus was the most successful competitor for limiting amounts of sulfate and ethanol, followed by Desulfobulbus propionicus. The success of Desulfovibrio baculatus was dependent on the availability of sufficient iron. Of the three species studied, Desulfobacter postgatei was the least successful competitor for limiting amounts of sulfate. Although stimulating the growth of Desulfobacter postgatei, addition of Ca-saturated illite particles to culture media did not affect the outcome of competition for sulfate. Thus, under sulfate limitation acetate accumulated. This phenomenon was briefly discussed in relation to the flow of electrons during anaerobic mineralization in marine and estuarine sulfate-limited sediments.     

Effects of the novel allelochemical ethyl 2-methylacetoacetate from the reed (<Emphasis Type="Italic">Phragmitis australis</Emphasis> Trin) on the growth of several common species of green algae

Bioassays were performed to investigate the effects of the novel allelochemical, ethyl 2-methylacetoacetate (EMA), isolated from the reed (Phragmitis australis) on the growth of three common species of algae; Scenedesmus obliquus, Selenastrum capricornutum and Chlamydomonas reinhardtii. The results demonstrated that EMA has three quite different types of effect on these three species of algae. The growth of S. capricornutum was significantly inhibited by EMA during the whole cultivation period. The EC₅₀ values of EMA on S. capricornutum was 0.6 mg L⁻¹(7 days). However, the inhibitory effect of EMA on S. obliquus was apparent during the first 4 days of batch cultivation and then the inhibitory effect disappeared, and a stimulating effect was observed instead. The EC₅₀ value of EMA on S. obliquus was 0.43 mg L⁻¹(4 days). In addition, following the addition of EMA, the cells of S. obliquus and S. capricornutum became significantly larger than the normal untreated one and the algal cells changed morphologically. The microstructure of the algal cells was disrupted by the addition of EMA. There was no significant inhibition of the growth of C. reinhardtii by EMA, but cell motility was affected.