Community composition of lacustrine small eukaryotes in hyper-eutrophic conditions in relation to top-down and bottom-up factors

Small eukaryotes (0.2-5 microm) in hyper-eutrophic conditions were described using terminal restriction fragment length polymorphism and cloning-sequencing, and were related to environmental variables both by an experimental approach and by a temporal field study. In situ analysis showed marked temporal variations in the dominant terminal restriction fragments (T-RFs), which were related to environmental variables such as nutrient concentrations and metazooplankton composition. To monitor the responses of the small-eukaryote community to top-down (absence or presence of planktivorous fish) and bottom-up (low or high nitrogen and phosphorus addition) effects, a cross-classified design mesocosm experiment was used. Depending on the type of treatment, we recorded changes in the diversity of T-RFs, as well as modifications in phylogenetic composition. Centroheliozoa and Cryptophyta were found in all types of treatment, whereas Chlorophyta were specific to enclosures receiving high nutrient loadings, and were associated either with LKM11 and 'environmental sequences'. Cercozoa and Fungi were not detected in enclosures receiving high nutrient loadings and fishes. Our results showed that resources and top-down factors are both clearly involved in shaping the structure of small eukaryotes, not only autotrophs but also heterotrophs, via complex interactions and trophic cascades within a microbial loop, notably in response to nutrient loading.     

Effects of UVB radiation on marine and freshwater organisms: a synthesis through meta-analysis

Ultraviolet-B (UVB) radiation is a global stressor with potentially far-reaching ecological impacts. In the first quantitative analysis of the effects of UVB on aquatic organisms, we used meta-analytic techniques to explore the effects of UVB on survival and growth in freshwater and marine systems. Based on the large body of literature on the effects of UVB in aquatic systems, we predicted that UVB would have different effects in different habitats, experimental venues, trophic groups and life history stages. Contrary to our predictions, we found an overall negative effect of UVB on both survival and growth that crossed life histories, trophic groups, habitats and experimental venues. UVB had larger negative effects on growth in embryos compared with later life history stages. Despite the overall negative effect of UVB, effect sizes varied widely. In the survival analyses, no relationship between mean effect size and taxonomic groups or levels of exposure to UVB was detected. In the growth analyses, a larger negative effect on protozoans was observed. Our analyses suggest that the effects of UVB in aquatic systems are large and negative but highly variable between organisms. Variation in susceptibility may have important implications for population and community structure.     

Succession and regulation factors of small eukaryote community composition in a lacustrine ecosystem (Lake Pavin)

The structure and dynamics of small eukaryotes (cells with a diameter less than 5 microm) were studied over two consecutive years in an oligomesotrophic lake (Lake Pavin in France). Water samples were collected at 5 and 30 m below the surface; when the lake was stratified, these depths corresponded to the epilimnion and hypolimnion. Changes in small-eukaryote structure were analyzed using terminal restriction fragment length polymorphism (T-RFLP) and cloning and sequencing of the 18S rRNA genes. Terminal restriction fragments from clones were used to reveal the dominant taxa in T-RFLP profiles of the environmental samples. Spumella-like cells (Chrysophyceae) did not dominate the small eukaryote community identified by molecular techniques in lacustrine ecosystems. Small eukaryotes appeared to be dominated by heterotrophic cells, particularly Cercozoa, which represented nearly half of the identified phylotypes, followed by the Fungi-LKM11 group (25%), choanoflagellates (10.3%) and Chrysophyceae (8.9%). Bicosoecida, Cryptophyta, and ciliates represented less than 9% of the community studied. No seasonal reproducibility in temporal evolution of the small-eukaryote community was observed from 1 year to the next. The T-RFLP patterns were related to bottom-up (resources) and top-down (grazing) variables using canonical correspondence analysis. The results showed a strong top-down regulation of small eukaryotes by zooplankton, more exactly, by cladocerans at 5 m and copepods at 30 m. Among bottom-up factors, temperature had a significant effect at both depths. The concentrations of nitrogenous nutrients and total phosphorus also had an effect on small-eukaryote dynamics at 5 m, whereas bacterial abundance and dissolved oxygen played a more important structuring role in the deeper zone.     

Plant-mediated effects of elevated ultraviolet-B radiation on peat microbial communities of a subarctic mire

Elevated ultraviolet‐B (UVB) radiation has been reported to have few effects on plants but to alter the soil microbial community composition. However, the effects on soil microorganisms have to be mediated via plants, because direct radiation effects are only plausible on the uppermost millimeters of soil. Here, we assessed secondary effects of UVB on soil microbes. The responses in the dominant plant Eriophorum russeolum, peat pore water and microbial communities in the peat were recorded at a subarctic mire in the middle of the third growing season under field exposure simulating 20% depletion in the ozone layer. The UVB treatment significantly reduced the sucrose and the total soluble sugar (sucrose+glucose+fructose) concentration of the plant leaves while increasing the sucrose concentration in the belowground storage organ rhizome. The starch concentration of the leaves was also slightly reduced by elevated UVB. In the plant roots, carbohydrate concentrations remained unaffected but the total phenolics concentration increased under elevated UVB. We suggest that the simultaneously observed decrease in bacterial growth rate and the altered bacterial community composition are due to UVB‐induced changes in the plant photosynthate allocation and potential changes in root exudation. There were no effects of elevated UVB on microbial biomass, peat pore water or nutrient concentrations in the peat. The observed responses are in line with the previously reported lower ecosystem dark respiration under elevated UVB, and they signify that the changed plant tissue quality and lower bacterial activity are likely to reduce decomposition.     

Genetic diversity of small eukaryotes from the coastal waters of Nansha Islands in China

Population structures and genetic diversity of the small eukaryotic plankton from the coastal waters of the Nansha Islands in China were investigated. Two genes libraries using 18S rDNA of the marine small eukaryotes were constituted, and 323 clones were identified within alveolates (more than 43%), acanthareas, viridiplantaes, and stramenopiles. Many novel clones were detected in the two libraries, including two groups of alveolates and two clades related to both acanthareas and polycystineas. Several sequences unrelated to any other known eukaryotes may represent early branches in the phylogenetic tree. Our results reveal that there is a high diversity and abundance of small eukaryotes in the marine regions of China.     

Short‐term dynamics of diversity patterns: evidence of continual reassembly within lacustrine small eukaryotes

The short‐term variation in the community structure of freshwater small eukaryotes (0.2–5 μm) was investigated in a mesotrophic lake every 2–3 days over one summer by coupling three molecular methods: 454 amplicon pyrosequencing, qPCR and TSA‐FISH. The pyrosequencing approach unveiled a much more extensive small‐eukaryotic diversity (991 OTUs) than has been described previously. The vast majority of the diversity described was represented by rare OTUs (≤ 0.01% of reads) belonging primarily to Cryptomycota, Dikarya and photosynthetic organisms, which were never detected as abundant in any of the samples. The small eukaryote community was characterized by a continual and important reassembly. These rearrangements involved the 20 ‘core taxa’ (≥ 1% of reads), and, were essentially due to a handful of OTUs that were detected in intermediate abundance (0.01–1% of reads) and sporadically in dominant taxa. Putative bacterivorous (Ciliophora and Cercozoa) as well as parasitic and saprotrophic taxa (Perkinsozoa and Cryptomycota) were involved in these changes of diversity. A putative infection of microalgae by a lacustrine perkinsozoan was also reported for the first time in this study. Open questions regarding both the patterns that govern the rapid small eukaryote reassemblies and the possible biogeography of these organisms arise from this study.     

Succession and Regulation Factors of Small Eukaryote Community Composition in a Lacustrine Ecosystem (Lake Pavin)

The structure and dynamics of small eukaryotes (cells with a diameter less than 5 μm) were studied over two consecutive years in an oligomesotrophic lake (Lake Pavin in France). Water samples were collected at 5 and 30 m below the surface; when the lake was stratified, these depths corresponded to the epilimnion and hypolimnion. Changes in small-eukaryote structure were analyzed using terminal restriction fragment length polymorphism (T-RFLP) and cloning and sequencing of the 18S rRNA genes. Terminal restriction fragments from clones were used to reveal the dominant taxa in T-RFLP profiles of the environmental samples. Spumella-like cells (Chrysophyceae) did not dominate the small eukaryote community identified by molecular techniques in lacustrine ecosystems. Small eukaryotes appeared to be dominated by heterotrophic cells, particularly Cercozoa, which represented nearly half of the identified phylotypes, followed by the Fungi-LKM11 group (25%), choanoflagellates (10.3%) and Chrysophyceae (8.9%). Bicosoecida, Cryptophyta, and ciliates represented less than 9% of the community studied. No seasonal reproducibility in temporal evolution of the small-eukaryote community was observed from 1 year to the next. The T-RFLP patterns were related to bottom-up (resources) and top-down (grazing) variables using canonical correspondence analysis. The results showed a strong top-down regulation of small eukaryotes by zooplankton, more exactly, by cladocerans at 5 m and copepods at 30 m. Among bottom-up factors, temperature had a significant effect at both depths. The concentrations of nitrogenous nutrients and total phosphorus also had an effect on small-eukaryote dynamics at 5 m, whereas bacterial abundance and dissolved oxygen played a more important structuring role in the deeper zone.     

Phylogenetic Differences in Attached and Free-Living Bacterial Communities in a Temperate Coastal Lagoon during Summer,Revealed via High-Throughput 16S rRNA Gene Sequencing

Most of what is known about coastal free-living and attached bacterial diversity is based on open coasts, with high particulate and nutrient riverine supply, terrestrial runoffs, and anthropogenic activities. The Magdalen Islands in the Gulf of St. Lawrence (Canada) are dominated by shallow lagoons with small, relatively pristine catchments and no freshwater input apart from rain. Such conditions provided an opportunity to investigate coastal free-living and attached marine bacterial diversity in the absence of confounding effects of steep freshwater gradients. We found significant differences between the two communities and marked temporal patterns in both. Taxonomic richness and diversity were greater in the attached than in the free-living community, increasing over summer, especially within the least abundant bacterial phyla. The highest number of reads fell within the SAR 11 clade (Pelagibacter, Alphaproteobacteria), which dominated free-living communities. The attached communities had deeper phylum-level diversity than the free-living fraction. Distance-based redundancy analysis indicated that the particulate organic matter (POM) concentration was the main variable separating early and late summer samples with salinity and temperature changes also significantly correlated to bacterial community structure. Our approach using high-throughput sequencing detected differences in free-living versus attached bacteria in the absence of riverine input, in keeping with the concept that marine attached communities are distinct from cooccurring free-living taxa. This diversity likely reflects the diverse microhabitats of available particles, implying that the total bacterial diversity in coastal systems is linked to particle supply and variability, with implications for understanding microbial biodiversity in marine systems.     

Marine microbial community structure assessed from combined metagenomic analysis and ribosomal amplicon deep-sequencing

The microbial taxonomic composition of the three domains of life in two coastal plankton samples was assessed by random total community metagenomic sequencing and PCR-based rDNA amplicon deep-sequencing in order to compare the resulting diversity and investigate possible limitations and complementarities of each method. The various universal primer sets, used to amplify different hypervariable rDNA regions, revealed the same major high-level taxonomic groups in Bacteria and unicellular Eukaryota, and showed a scarce Archaea apparent richness. However, significant differences were found between the different primer sets (p-value < 0.05, with the Kolmogorov–Smirnov test), regarding both operational taxonomic unit (OTU) richness and relative abundance of the major high-level taxonomic groups detected. Based on the metagenomic approach, the phylum Bacteroidetes dominated the prokaryotic community, followed by Proteobacteria, while the detected eukaryotic unicellular taxa belonged to the groups of Alveolata, Fungi, Chlorophyta, Stramenopiles and Phaeophyceae. These groups were found to carry genes typically found in microbial communities, which are linked to DNA, RNA and protein metabolism and the synthesis of nucleotides, amino acids, carbohydrates and vitamins. Although our findings suggest that the total community metagenomic approach can provide a more comprehensive picture of the planktonic microbial community structure, a number of issues associated with this approach emerged. These issues include the still relatively high cost compared to amplicon sequencing, the possible low coverage of the full marine diversity, the insufficiency of databases for other gene markers than the small subunit gene, and the bias towards bacterial sequences because of their higher abundance relative to eukaryotes in marine environments.     

Effects of temperature and ultraviolet radiation on microbial foodweb structure: potential responses to global change

1. A series of growth experiments were conducted with natural plankton communities from a lake and river in northern Quebec, to evaluate the response of microbial foodweb structure to changes in ambient temperature and solar ultraviolet radiation (UVR). 2. Bioassays were incubated for 6 days at two temperatures (10 and 20 °C) and three near-surface irradiance conditions [photosynthetically active radiation (PAR) + UVA + UVB, PAR + UVA, and PAR only). 3. The concentration of total bacteria showed no net response to temperature, but the percentage of actively respiring bacteria, as measured with the cellular redox probe 5-cyano-2,3-ditolyl tetrazolium chloride (CTC), was up to 57% higher at 20 °C relative to 10 °C. Chlorophyll a concentrations in the < 2 μm size fraction also reacted strongly to temperature, with a net increase of up to 61% over the temperature range of 10–20 °C. 4. The UVR effects were small or undetectable for most of the measured variables; however, the percentage of actively respiring bacteria was significantly inhibited in the presence of UVR at 20 °C, decreasing by 29–48% on day 6 in the lake experiments and by 59% on day 2 in one river experiment. 5. The results show differential sensitivity to temperature among organisms of microbial communities in subarctic freshwaters, and a resilience by the majority of micro-organisms to their present UVR conditions. Microbial foodwebs in northern freshwaters appear to be relatively unresponsive to short-term (days) changes in UVR. However, the observed responses to temperature suggest that climate change could influence community structure, with warmer temperatures favouring picoplanktonic phototrophs and heterotrophs, and a shift in nanoplankton species composition and size structure.     

Interaction matters: Bottom-up driver interdependencies alter the projected response of phytoplankton communities to climate change

Phytoplankton growth is controlled by multiple environmental drivers, which are all modified by climate change. While numerous experimental studies identify interactive effects between drivers, large-scale ocean biogeochemistry models mostly account for growth responses to each driver separately and leave the results of these experimental multiple-driver studies largely unused. Here, we amend phytoplankton growth functions in a biogeochemical model by dual-driver interactions (CO₂ and temperature, CO₂ and light), based on data of a published meta-analysis on multiple-driver laboratory experiments. The effect of this parametrization on phytoplankton biomass and community composition is tested using present-day and future high-emission (SSP5-8.5) climate forcing. While the projected decrease in future total global phytoplankton biomass in simulations with driver interactions is similar to that in control simulations without driver interactions (5%–6%), interactive driver effects are group-specific. Globally, diatom biomass decreases more with interactive effects compared with the control simulation (−8.1% with interactions vs. no change without interactions). Small-phytoplankton biomass, by contrast, decreases less with on-going climate change when the model accounts for driver interactions (−5.0% vs. −9.0%). The response of global coccolithophore biomass to future climate conditions is even reversed when interactions are considered (+33.2% instead of −10.8%). Regionally, the largest difference in the future phytoplankton community composition between the simulations with and without driver interactions is detected in the Southern Ocean, where diatom biomass decreases (−7.5%) instead of increases (+14.5%), raising the share of small phytoplankton and coccolithophores of total phytoplankton biomass. Hence, interactive effects impact the phytoplankton community structure and related biogeochemical fluxes in a future ocean. Our approach is a first step to integrate the mechanistic understanding of interacting driver effects on phytoplankton growth gained by numerous laboratory experiments into a global ocean biogeochemistry model, aiming toward more realistic future projections of phytoplankton biomass and community composition.     

Interaction of nitrogen status and UVB sensitivity in a temperate phytoplankton assemblage

The influence of nitrate supplementation and ultraviolet-B (UVB; 280–320 nm) enhancement was tested on a coastal phytoplankton assemblage from eastern Canada exposed to ambient or supplemental UVB irradiance, equivalent to a local 60% ozone depletion. During a 10 d-long mesocosm experiment, 24-h surface bag incubations were repeated three times with half the bags supplemented with nitrate, phosphate and silicate while the other half received only phosphate and silicate. At beginning and the end of these 24-h surface incubations, chlorophyll fluorescence measurements were performed and the abundance of photoprotective pigments diadinoxanthin and diatoxanhtin, of the PSII reaction center D1 protein and of the large subunit of the ribulose-1,5-biphosphate carboxylase/oxygenase enzyme (RuBisCO LSU) was determined. Results showed that as long as nutrients were abundant inside the mesocosms, phytoplankton exposed to supplemental UVB at the surface were able to maintain their maximal quantum yield of PSII fluorescence. As nutrients became limiting inside the mesosocoms, however, phytoplankton showed an increased sensitivity to supplemental UVB and suffered more net photodamage to the PSII reaction centers (seen as decreases in D1 protein abundance). Supplemental UVB also resulted in low abundance of RuBisCO LSU and exacerbated photoinhibition compared to the phytoplankton exposed to surface ambient irradiance. Supplementing nitrate during this nutrient deficient period limited the inactivation of the PSII reaction centers and lowered photoinhibition. Nitrate supplementation had no clear effect on the abundance of the D1 protein but it helped the community to maintain a greater abundance of RuBisCO LSU. Overall, results from this study suggest that the sensitivity of the RuBisCO enzyme to the combined effects of supplemental UVB and nitrate limitation can influence the tolerance of PSII to UVB stress.     

Effects of N and P supply on phytoplankton in Bizerte Lagoon (western Mediterranean)

Enriched bottle experiments were conducted in situ during winter (January and February) and summer (July and August) 2001 to examine the effects of nutrient enrichments (+ N, + P and + NP) on phytoplankton in Bizerte Lagoon, Tunisia. Chlorophyll a (Chl a), ranging from 3.05 μg L⁻¹ in winter to 4.52 μg L⁻¹ in summer, was dominated by the small size-faction (<5 μm) during both seasons. However, the contribution of the large size-fraction (5-200 μm) to Chl a increased from winter (26%) to summer (37%). Similarly, the carbon biomass of the 5-200 μm algae increased during the July/August period that was characterised by the high proliferation of several diatom taxa. In winter, N was the limiting element for phytoplankton growth. Its addition alone (+ N) or with P (+ NP) increased both the <5 μm and 5-200 μm Chl a concentrations. There was no change in the phytoplankton size structure, with the small cells dominating the final algal biomass in all treatments after 5 days. In summer, N and P limited the phytoplankton, but small and large algae exhibited diverse responses to different nutrient enrichments: addition of P increased the Chl a only in the 5-200 μm fraction, the + N treatment enhanced both size classes, and the NP fertilisation mostly stimulated the biomass of large cells. Consequently, the N and P addition in summer was followed by a significant change in the phytoplankton size structure, since both size-fractions contributed equally to the final Chl a biomass. Within the 5-200 μm algal community, various taxa had diverse responses to the nutrient supply during both seasons, leading to a change in the final community composition. The autotrophic flagellates appeared to grow well under N-deficient conditions. In contrast, diatom growth and biomass were mostly stimulated by the N enrichment while dinoflagellates exhibited the highest increase in their growth and biomass with P fertilisation. Our results suggest that the increasing anthropogenic supply of nutrients in the lagoon may influence algal dynamics as well as productivity in different ways depending on the nutrient composition.     

Archaea in boreal Swedish lakes are diverse,dominated by Woesearchaeota and follow deterministic community assembly

Despite their key role in biogeochemical processes, particularly the methane cycle, archaea are widely underrepresented in molecular surveys because of their lower abundance compared with bacteria and eukaryotes. Here, we use parallel high-resolution small subunit rRNA gene sequencing to explore archaeal diversity in 109 Swedish lakes and correlate archaeal community assembly mechanisms to large-scale latitudinal, climatic (nemoral to arctic) and nutrient (oligotrophic to eutrophic) gradients. Sequencing with universal primers showed the contribution of archaea was on average 0.8% but increased up to 1.5% of the three domains in forest lakes. Archaea-specific sequencing revealed that freshwater archaeal diversity could be partly explained by lake variables associated with nutrient status. Combined with deterministic co-occurrence patterns this finding suggests that ecological drift is overridden by environmental sorting, as well as other deterministic processes such as biogeographic and evolutionary history, leading to lake-specific archaeal biodiversity. Acetoclastic, hydrogenotrophic and methylotrophic methanogens as well as ammonia-oxidizing archaea were frequently detected across the lakes. Archaea-specific sequencing also revealed representatives of Woesearchaeota and other phyla of the DPANN superphylum. This study adds to our understanding of the ecological range of key archaea in freshwaters and links these taxa to hypotheses about processes governing biogeochemical cycles in lakes.     

Impacts of alien plant invasion on native plant communities are mediated by functional identity of resident species,not resource availability

Alien plant invasion and nutrient enrichment as a result of anthropogenic landscape modification seriously threaten native plant community diversity. It is poorly understood, however, whether these two disturbances interact with the functional identity of recipient native plants to drive community change. We performed a mesocosm experiment to examine whether the interactive effects of invasion by a stoloniferous turf‐grass Stenotaphrum secundatum and nutrient enrichment vary across different plant growth forms of an endangered coastal plant community. Communities contained 18 species (drawn without replacement from a pool of 31 species) with either runner, tufted or woody growth forms. Species were well‐established and reproductively mature prior to S. secundatum introduction. Species growth (% cover), reproductive output, soil temperature and light availability were monitored for two growing seasons. Invasion and nutrient enrichment (two levels: ‘natural control’ and ‘enriched’) had no effect on species richness, community composition, reproductive output, soil temperature or light penetration. There was no interactive effect of nutrients and invasion on community productivity (i.e. final biomass), such that invasion caused a reduction in community biomass at both natural and enriched nutrient levels. This was driven only by reduced biomass of functionally‐similar native runner species, which share similar root morphologies and nutrient‐acquisition strategies with S. secundatum. Our study indicates that impacts of invasion are dependent upon the functional identity of species within recipient communities, not the availability of resources. This shows that management cannot buffer invader effects by manipulating resource availability. Revegetation strategies should target functionally‐similar natives for replacement following invader control.     

Effects of Photochemical Transformations of Dissolved Organic Matter on Bacterial Metabolism and Diversity in Three Contrasting Coastal Sites in the Northwestern Mediterranean Sea during Summer

The effects of phototransformation of dissolved organic matter (DOM) on bacterial growth, production, respiration, growth efficiency, and diversity were investigated during summer in two lagoons and one oligotrophic coastal water samples from the Northwestern Mediterranean Sea, differing widely in DOM and chromophoric DOM concentrations. Exposure of 0.2-μm filtered waters to full sun radiation for 1 d resulted in small changes in optical properties and concentrations of DOM, and no changes in nitrate, nitrite, and phosphate concentrations. After exposure to sunlight or dark (control) treatments, the water samples were inoculated with the original bacterial community. Phototransformation of DOM had contrasting effects on bacterial production and respiration, depending on the water’s origin, resulting in an increase of bacterial growth efficiency for the oligotrophic coastal water sample (120%) and a decrease for the lagoon waters (20 to 40%) relative to that observed in dark treatments. We also observed that bacterial growth on DOM irradiated by full sun resulted in changes in community structure of total and metabolically active bacterial cells for the three locations studied when compared to the bacteria growing on un-irradiated DOM, and that changes were mainly caused by phototransformation of DOM by UV radiation for the eutrophic lagoon and the oligotrophic coastal water and by photosynthetically active radiation (PAR) for the mesoeutrophic lagoon. These initial results indicate that phototransformation of DOM significantly alters both bacterial metabolism and community structure in surface water for a variety of coastal ecosystems in the Mediterranean Sea. Further studies will be necessary to elucidate a more detailed appreciation of potential temporal and spatial variations of the effects measured.     

Sheep-urine-induced changes in soil microbial community structure

Soil microbial communities play an important role in nutrient cycling and nutrient availability, especially in unimproved soils. In grazed pastures, sheep urine causes local changes in nutrient concentration which may be a source of heterogeneity in microbial community structure. In the present study, we investigated the effects of synthetic urine on soil microbial community structure, using physiological (community level physiological profiling, CLPP), biochemical (phospholipid fatty acid analysis, PLFA) and molecular (denaturing gradient gel electrophoresis, DGGE) fingerprinting methods. PLFA data suggested that synthetic urine treatment had no significant effect on total microbial (total PLFA), total bacterial or fungal biomass; however, significant changes in microbial community structure were observed with both PLFA and DGGE data. PLFA data suggested that synthetic urine induced a shift towards communities with higher concentrations of branched fatty acids. DGGE banding patterns derived from control and treated soils differed, due to a higher proportion of DNA sequences migrating only to the upper regions of the gel in synthetic urine-treated samples. The shifts in community structure measured by PLFA and DGGE were significantly correlated with one another, suggesting that both datasets reflected the same changes in microbial communities. Synthetic urine treatment preferentially stimulated the use of rhizosphere-C in sole-carbon-source utilisation profiles. The changes caused by synthetic urine addition accounted for only 10-15% of the total variability in community structure, suggesting that overall microbial community structure was reasonably stable and that changes were confined to a small proportion of the communities.     

Microbial eukaryote plankton communities of high‐mountain lakes from three continents exhibit strong biogeographic patterns

Microbial eukaryotes hold a key role in aquatic ecosystem functioning. Yet, their diversity in freshwater lakes, particularly in high‐mountain lakes, is relatively unknown compared with the marine environment. Low nutrient availability, low water temperature and high ultraviolet radiation make most high‐mountain lakes extremely challenging habitats for life and require specific molecular and physiological adaptations. We therefore expected that these ecosystems support a plankton diversity that differs notably from other freshwater lakes. In addition, we hypothesized that the communities under study exhibit geographic structuring. Our rationale was that geographic dispersal of small‐sized eukaryotes in high‐mountain lakes over continental distances seems difficult. We analysed hypervariable V4 fragments of the SSU rRNA gene to compare the genetic microbial eukaryote diversity in high‐mountain lakes located in the European Alps, the Chilean Altiplano and the Ethiopian Bale Mountains. Microbial eukaryotes were not globally distributed corroborating patterns found for bacteria, multicellular animals and plants. Instead, the plankton community composition emerged as a highly specific fingerprint of a geographic region even on higher taxonomic levels. The intraregional heterogeneity of the investigated lakes was mirrored in shifts in microbial eukaryote community structure, which, however, was much less pronounced compared with interregional beta‐diversity. Statistical analyses revealed that on a regional scale, environmental factors are strong predictors for plankton community structures in high‐mountain lakes. While on long‐distance scales (>10 000 km), isolation by distance is the most plausible scenario, on intermediate scales (up to 6000 km), both contemporary environmental factors and historical contingencies interact to shift plankton community structures.     

UV effects that come and go: a global comparison of marine benthic community level impacts

Ambient UV radiation has substantially increased during the last decades, but its impact on marine benthic communities is hardly known. The aim of this study was to globally compare and quantify how shallow hard‐bottom communities are affected by UV during early succession. Identical field experiments in 10 different coastal regions of both hemispheres produced a consistent but unexpected pattern: (i) UV radiation affected species diversity and community biomass in a very similar manner, (ii) diversity and biomass were reduced to a larger extent by UVA than UVB radiation, (iii) ambient UV levels did not affect the composition of the communities, and (iv) any UV effects disappeared during species succession after 2–3 months. Thus, current levels of UV radiation seem to have small, predictable, and transient effects on shallow marine hard‐bottom communities.     

Characterizing man-made and natural modifications of microbial diversity and activity in coastal ecosystems

The impacts of growing coastal pollution and habitat alteration accompanying human encroachment are of great concern at the microbial level, where much of the ocean's primary production and biogeochemical cycling takes place. Coastal ecosystems are also under the influence of natural perturbations such as major storms and flooding. Distinguishing the impacts of natural and human stressors is essential for understanding environmentally-induced change in microbial diversity and function. The objective of this paper is to discuss the applications and merits of recently developed molecular, ecophysiological and analytical indicators and their utility in examining anthropogenic and climatic impacts on the structure and function of coastal microbial communities. The nitrogen-limited Neuse River Estuary and Pamlico Sound, North Carolina are used as examples of ecosystems experiencing both anthropogenic (i.e., accelerating eutrophication) and climatic stress (increasing frequencies of tropical storms and hurricanes). Additional examples are derived from a coastal monitoring site (LEO) on the Atlantic coast of New Jersey and Galveston Bay, on the Gulf of Mexico. In order to assess structure, function, and trophic state of these and other coastal ecosystems, molecular (DNA and RNA-based) characterizations of the microbial taxa involved in carbon, nitrogen and other nutrient transformations can be combined with diagnostic pigment-based indicators of primary producer groups. Application of these methods can reveal process-level microbial community responses to environmental variability over a range of scales. Experimental approaches combined with strategic monitoring utilizing these methods will facilitate: (a) understanding organismal and community responses to environmental change, and (b) synthesizing these responses in the context of ecosystem models that integrate physical, chemical and biotic variability with environmental controls. This revised version was published online in August 2006 with corrections to the Cover Date.