Integrating depth-dependent protist dynamics and microbial interactions in spring succession of a freshwater reservoir.

BACKGROUND: Protists are essential contributors to eukaryotic diversity and exert profound influence on carbon fluxes and energy transfer in freshwaters. Despite their significance, there is a notable gap in research on protistan dynamics, particularly in the deeper strata of temperate lakes. This study aimed to address this gap by integrating protists into the well-described spring dynamics of Rímov reservoir, Czech Republic. Over a 2-month period covering transition from mixing to established stratification, we collected water samples from three reservoir depths (0.5, 10 and 30 m) with a frequency of up to three times per week. Microbial eukaryotic and prokaryotic communities were analysed using SSU rRNA gene amplicon sequencing and dominant protistan groups were enumerated by Catalysed Reporter Deposition-Fluorescence in situ Hybridization (CARD-FISH). Additionally, we collected samples for water chemistry, phyto- and zooplankton composition analyses. RESULTS: Following the rapid changes in environmental and biotic parameters during spring, protistan and bacterial communities displayed swift transitions from a homogeneous community to distinct strata-specific communities. A prevalence of auto- and mixotrophic protists dominated by cryptophytes was associated with spring algal bloom-specialized bacteria in the epilimnion. In contrast, the meta- and hypolimnion showcased a development of a protist community dominated by putative parasitic Perkinsozoa, detritus or particle-associated ciliates, cercozoans, telonemids and excavate protists (Kinetoplastida), co-occurring with bacteria associated with lake snow. CONCLUSIONS: Our high-resolution sampling matching the typical doubling time of microbes along with the combined microscopic and molecular approach and inclusion of all main components of the microbial food web allowed us to unveil depth-specific populations' successions and interactions in a deep lentic ecosystem.

Seasonal Variability of Cultivable Nitrate-Reducing and Denitrifying Bacteria and Functional Gene Copy Number in Fresh Water Lake.

This study describes the seasonal course of denitrifying and nitrate-reducing bacteria in a dimictic mesotrophic lake (Lake Scharmützelsee, Brandenburg, Germany) within a three-year period from 2011 to 2013. The bacterial cell numbers were quantified by the fluorescence microscopy, most probable number (MPN) and PCR-dependent quantification of the chromosomal 16S rDNA and of the nirS and nirK gene copy number. The highest seasonal differences (up to three orders of magnitudes) have been measured using MPN in the epilimnion. This variation was not reflected by PCR-dependent approaches or direct microscopical enumeration. At adverse conditions (low temperature and/or low nitrate concentrations), the differences between MPN and gene copy numbers increased by up to five orders of magnitudes and decreased to one magnitude at favourable environmental conditions. These results can be explained best by an increasing ratio of viable but not cultivable (VBNC) cells or dead cells at impairing conditions. In the hypolimnion, the courses of MPN and nir gene copy numbers were similar. This can be explained by a higher feeding pressure and therefore smaller amounts of dormant cells. In the pelagial in general, the total cell numbers enumerated by either microscopical or molecular approaches were similar. In the sediment, more than 99% of the DNA was obviously not related to viable bacteria but was rather DNA in dead cells or adsorbed to particle surfaces.

Anoxia begets anoxia: A positive feedback to the deoxygenation of temperate lakes.

Declining oxygen concentrations in the deep waters of lakes worldwide pose a pressing environmental and societal challenge. Existing theory suggests that low deep-water dissolved oxygen (DO) concentrations could trigger a positive feedback through which anoxia (i.e., very low DO) during a given summer begets increasingly severe occurrences of anoxia in following summers. Specifically, anoxic conditions can promote nutrient release from sediments, thereby stimulating phytoplankton growth, and subsequent phytoplankton decomposition can fuel heterotrophic respiration, resulting in increased spatial extent and duration of anoxia. However, while the individual relationships in this feedback are well established, to our knowledge, there has not been a systematic analysis within or across lakes that simultaneously demonstrates all of the mechanisms necessary to produce a positive feedback that reinforces anoxia. Here, we compiled data from 656 widespread temperate lakes and reservoirs to analyze the proposed anoxia begets anoxia feedback. Lakes in the dataset span a broad range of surface area (1-126,909 ha), maximum depth (6-370 m), and morphometry, with a median time-series duration of 30 years at each lake. Using linear mixed models, we found support for each of the positive feedback relationships between anoxia, phosphorus concentrations, chlorophyll a concentrations, and oxygen demand across the 656-lake dataset. Likewise, we found further support for these relationships by analyzing time-series data from individual lakes. Our results indicate that the strength of these feedback relationships may vary with lake-specific characteristics: For example, we found that surface phosphorus concentrations were more positively associated with chlorophyll a in high-phosphorus lakes, and oxygen demand had a stronger influence on the extent of anoxia in deep lakes. Taken together, these results support the existence of a positive feedback that could magnify the effects of climate change and other anthropogenic pressures driving the development of anoxia in lakes around the world.

Surface-enhanced Raman spectroscopy enables confirmatory detection of dyes on hair submerged in hypolimnion water for up to twelve weeks.

Difficulties in the localization of bodies of homicidal or drowning victims in natural water result in their submergence for weeks if not months. Water insects and microbes drastically change the body's appearance, which significantly changes the determination of a victim's identity. DNA analysis is commonly used for identifying the decedent; however, this PCR-based approach is time-consuming and destructive of the evidence. Considering that nearly half of the people in the world dye their hair with a variety of permanent and semi-permanent dyes, one can expect that confirmatory identification of dyes on the body's hair can be used to shed light on the victim's identity. A growing body of evidence suggests that surface-enhanced Raman spectroscopy (SERS) can be used to detect and identify hair dyes. In this study, we investigated the extent to which SERS could be used to detect black and blue, permanent and semi-permanent dyes on hair submerged in hypolimnion water for up to twelve weeks. We found that SERS enabled 100% accurate identification of analyzed dyes on hair submerged in hypolimnion water for up to 8 weeks, whereas, on average, 87% accurate identification of the hair dyes could be achieved on hair exposed for 10 weeks and 50% for hair exposed 12 weeks in hypolimnion water. We also found that the aqueous environment caused progressive fading of some dyes, whereas other dyes showed substantial spectral transformations after prolonged submergence. Finally, we found that changes in the intensity of vibrational bands of dyes could be used to predict the duration of submergence of colored hair in hypolimnion water.

Lake Toba stratification study with physical, chemical, and isotopic parameter approach.

In general, lakes provide various ecosystem services that support biotic habitats and human life. Lake Toba, the largest caldera volcanic lake in the world, has been functioning as a tourism destination and for fish farming, a fresh water source, and for power supplies. The lake has maximum depth of about 505 m. Stratification in the lake water column especially in a tropical region like Indonesia is a common phenomenon. Stratification of the lake is one important factor which will influence the next stage in the biological processes and water quality of the lake. The objective of the current study was to analyze and elucidate the stratification of Lake Toba through the variations of physical, chemical, and isotopic parameters. Water temperature, dissolved oxygen concentrations, water chemical contents, and isotopic parameters were observed periodically from 2016 until 2019. Fourteen sampling points evenly distributed across the surface of the lake were predetermined to represent the North, South, East, and West of the lake. A conductivity, temperature, and depth (CTD) device and Baro-divers were used to measure temperature and conductivity at various depths in the water column for each sampling point. The water samples for isotopic and chemical parameter measurements were collected from water depths of 0, 20, 40, 60, 80, and 100 m using a horizontal transparent acrylic water sampler at each sampling point. Some results from isotope analysis showed that all the water at all levels along the water column had undergone evaporation. Although there were some slight fluctuations, the chemical composition of the lake water was relatively homogenous up to 100 m of depth. The chemical pattern suggested that there was no other secondary process that altered the chemical composition of the lake water, thus the lake water and river water had the same facies. The stratification of Lake Toba was found to be permanent. The depth of the hypolimnion layer was consistently at about 80 m below the surface. However, the depth of the upper layer, the epilimnion, was strongly affected by the climate at the surface of the lake.

First insights into the prokaryotic community structure of Lake Cote, Costa Rica: Influence on nutrient cycling.

Prokaryotic diversity in lakes has been studied for many years mainly focusing on community structure and how the bacterial assemblages are driven by physicochemical conditions such as temperature, oxygen, and nutrients. However, little is known about how the composition and function of the prokaryotic community changes upon lake stratification. To elucidate this, we studied Lake Cote in Costa Rica determining prokaryotic diversity and community structure in conjunction with physicochemistry along vertical gradients during stratification and mixing periods. Of the parameters measured, ammonium, oxygen, and temperature, in that order, were the main determinants driving the variability in the prokaryotic community structure of the lake. Distinct stratification of Lake Cote occurred (March 2018) and the community diversity was compared to a period of complete mixing (March 2019). The microbial community analysis indicated that stratification significantly altered the bacterial composition in the epi-meta- and hypolimnion. During stratification, the Deltaproteobacteria, Chloroflexi, Bacteroidetes, Nitrospirae, and Euryarchaeota were dominant in the hypolimnion yet largely absent in surface layers. Among these taxa, strict or facultative anaerobic bacteria were likely contributing to the lake nitrogen biogeochemical cycling, consistent with measurements of inorganic nitrogen measurements and microbial functional abundance predictions. In general, during both sampling events, a higher abundance of Alphaproteobacteria, Betaproteobacteria, Actinobacteria, and Cyanobacteria was found in the oxygenated layers. Lake Cote had a unique bacterial diversity, with 80% of Amplicon Sequence Variant (ASV) recovered similar to unclassified/uncultured strains and exhibits archetypal shallow lake physicochemical but not microbial fluctuations worthy of further investigation. This study provides an example of lake hydrodynamics impacts to microbial community and their function in Central American lakes with implications for other shallow, upland, and oligotrophic lake systems.

A case study of thermal and chemical stratification in a drinking water reservoir.

Thermal stratification has an important impact on the cycling of reservoir water quality. Uneven vertical distribution of water quality factors, such as dissolved oxygen and nutrients, occurs during seasonal stratification, which creates chemical stratification. Typically, thermocline and chemocline characteristics vary across different reservoirs. In this study, a field study was conducted in the Zhoucun Reservoir (ZCR), China, to investigate the variation in water quality during periods of thermal stratification. The results revealed the maximum water depth as 15 m at 122 mamsl, which is relatively shallow for stratified reservoirs. However, an extremely high thermocline strength index (TSI) was recorded, which indicates that the thermal stratification of ZCR water is very stable, contrary to our common understanding that stratification is more obvious at greater water depths. The chemical stratification strength index (IC-i) was compared with the TSI and the relative water column stability (RWCS). The TSI had a high correlation with IC-i, which explains the high stability of ZCR chemical stratification. Moreover, it highlights the suitability of TSI for characterising the chemical stability of reservoir stratification. Finally, this study also found that the formation and disappearance of thermal and chemical stratification in the ZCR were synchronous. As stratification directly influences a reservoir water quality regime, this study may be a helpful reference for optimising water quality management.

Patterns of Tetracapsuloides bryosalmonae infection of three salmonid species in large, deep Norwegian lakes.

Proliferative kidney disease (PKD), caused by the myxozoan endoparasite Tetracapsuloides bryosalmonae, is of serious ecological and economical concern to wild and farmed salmonids. Wild salmonid populations have declined due to PKD, primarily in rivers, in Europe and North America. Deep lakes are also important habitats for salmonids, and this work aimed to investigate parasite presence in five deep Norwegian lakes. Kidney samples from three salmonid species from deep lakes were collected and tested using real-time PCR to detect PKD parasite presence. We present the first detection of T. bryosalmonae in European whitefish in Norway for the first time, as well as the first published documentation of the parasite in kidneys of Arctic charr, brown trout and whitefish in four lakes. The observed prevalence of the parasite was higher in populations of brown trout than of Arctic charr and whitefish. The parasite was detected in farmed, but not in wild, charr in one lake. This suggests a possible link with a depth of fish habitat and fewer T. bryosalmonae-infected and PKD-affected fish. Towards a warmer climate, cold hypolimnion in deep lakes may act as a refuge for wild salmonids, while cold deep water may be used to control PKD in farmed salmonids.

Impact of a large dam on reproduction of a non-migratory teleost species, Acestrorhynchus lacustris (Characiformes: Acestrorhynchidae) / Impacto de uma grande barragem na reprodução de uma espécie de teleósteo não migradora, Acestrorhynchus lacustris (Characiformes: Acestrorhynchidae)

The release of water from the reservoir hypolimnion, lower concentration of oxygen and the anthropogenic regulation of the river flow, could affect the reproduction of fish, especially migratory species. However, little is known about the effects of these changes in water on non-migratory species. In this sense, the reproduction of Acestrohynchus lacustris was evaluated in two sections of São Francisco River, Minas Gerais, Brazil. Section 1, located immediately downstream from Três Marias Dam (18°09'31.65"S and 45°13'36.00"W) and section 2, located at the confluence of the São Francisco and the Abaeté Rivers (18°02'47.78"S and 45°1057.95"W). For this, we obtained the physico-chemical parameters of water of each study section. Additionally, biometric data and biological indices ofall specimens were measured. Fecundity and follicles diameters were measured in females. Temperature, dissolvedoxygen and flow showed lower values in section 1. Fish captured in this section, had lower values of GSI in bothsexes, and females presented decreased values of fecundity and follicles diameter. This species showed reproductiveactivity in the two sections analyzed, however, in section 1 where the temperature and dissolved oxygen presentedsignificant lower values, the reproductive capacity of A. lacustris, was negatively affected.

A liberação de água do hipolímnio do reservatório, baixa concentração de oxigênio e a regulação antropogênica do fluxo do rio podem afetar a reprodução de peixes, principalmente espécies migradoras. No entanto, pouco se sabe sobre os efeitos dessas mudanças nas condições da água em espécies não migradoras. A reprodução de Acestrohynchus lacustris foi avaliada em duas seções do rio São Francisco. Seção 1, localizada imediatamente a jusante da barragem de Três Marias e seção 2, localizada na confluência dos rios São Francisco e Abaeté. Para isso, foram obtidos os parâmetros físico-químicos da água de cada seção do estudo. Além disso, dados biométricos e índices biológicos de todos os peixes capturados foram obtidos. Adicionalmente, nas fêmeas foram medidos os diâmetros dos folículos vitelogênicos e a fecundidade. Temperatura, oxigênio dissolvido e fluxo apresentaram valores mais baixos na seção 1. Os peixes capturados nesta seção apresentaram menores valores de IGS em ambos os sexos, e as fêmeas apresentaram menores valores de fecundidade e diâmetro dos folículos. Essa espécie apresentou atividade reprodutiva nas duas seções analisadas, porém, na seção 1, onde os parâmetros da água apresentam piores condições para o processo reprodutivo de peixes, a capacidade reprodutiva de A. lacustris foi afetada negativamente.

Impact of a large dam on reproduction of a non-migratory teleost species, Acestrorhynchus lacustris (Characiformes: Acestrorhynchidae)

Abstract The release of water from the reservoir hypolimnion, lower concentration of oxygen and the anthropogenic regulation of the river flow, could affect the reproduction of fish, especially migratory species. However, little is known about the effects of these changes in water on non-migratory species. In this sense, the reproduction of Acestrohynchus lacustris was evaluated in two sections of São Francisco River, Minas Gerais, Brazil. Section 1, located immediately downstream from Três Marias Dam (18°0931.65S and 45°1336.00W) and section 2, located at the confluence of the São Francisco and the Abaeté Rivers (18°0247.78S and 45°1057.95W). For this, we obtained the physico-chemical parameters of water of each study section. Additionally, biometric data and biological indices of all specimens were measured. Fecundity and follicles diameters were measured in females. Temperature, dissolved oxygen and flow showed lower values in section 1. Fish captured in this section, had lower values of GSI in both sexes, and females presented decreased values of fecundity and follicles diameter. This species showed reproductive activity in the two sections analyzed, however, in section 1 where the temperature and dissolved oxygen presented significant lower values, the reproductive capacity of A. lacustris, was negatively affected.

Resumo A liberação de água do hipolímnio do reservatório, baixa concentração de oxigênio e a regulação antropogênica do fluxo do rio podem afetar a reprodução de peixes, principalmente espécies migradoras. No entanto, pouco se sabe sobre os efeitos dessas mudanças nas condições da água em espécies não migradoras. A reprodução de Acestrohynchus lacustris foi avaliada em duas seções do rio São Francisco. Seção 1, localizada imediatamente a jusante da barragem de Três Marias e seção 2, localizada na confluência dos rios São Francisco e Abaeté. Para isso, foram obtidos os parâmetros físico-químicos da água de cada seção do estudo. Além disso, dados biométricos e índices biológicos de todos os peixes capturados foram obtidos. Adicionalmente, nas fêmeas foram medidos os diâmetros dos folículos vitelogênicos e a fecundidade. Temperatura, oxigênio dissolvido e fluxo apresentaram valores mais baixos na seção 1. Os peixes capturados nesta seção apresentaram menores valores de IGS em ambos os sexos, e as fêmeas apresentaram menores valores de fecundidade e diâmetro dos folículos. Essa espécie apresentou atividade reprodutiva nas duas seções analisadas, porém, na seção 1, onde os parâmetros da água apresentam piores condições para o processo reprodutivo de peixes, a capacidade reprodutiva de A. lacustris foi afetada negativamente.

Impact of a large dam on reproduction of a non-migratory teleost species, Acestrorhynchus lacustris (Characiformes: Acestrorhynchidae) / Impacto de uma grande barragem na reprodução de uma espécie de teleósteo não migradora, Acestrorhynchus lacustris (Characiformes: Acestrorhynchidae)

The release of water from the reservoir hypolimnion, lower concentration of oxygen and the anthropogenic regulation of the river flow, could affect the reproduction of fish, especially migratory species. However, little is known about the effects of these changes in water on non-migratory species. In this sense, the reproduction of Acestrohynchus lacustris was evaluated in two sections of São Francisco River, Minas Gerais, Brazil. Section 1, located immediately downstream from Três Marias Dam (18°09'31.65"S and 45°13'36.00"W) and section 2, located at the confluence of the São Francisco and the Abaeté Rivers (18°02'47.78"S and 45°10'57.95"W). For this, we obtained the physico-chemical parameters of water of each study section. Additionally, biometric data and biological indices of all specimens were measured. Fecundity and follicles diameters were measured in females. Temperature, dissolved oxygen and flow showed lower values in section 1. Fish captured in this section, had lower values of GSI in both sexes, and females presented decreased values of fecundity and follicles diameter. This species showed reproductive activity in the two sections analyzed, however, in section 1 where the temperature and dissolved oxygen presented significant lower values, the reproductive capacity of A. lacustris, was negatively affected.

A liberação de água do hipolímnio do reservatório, baixa concentração de oxigênio e a regulação antropogênica do fluxo do rio podem afetar a reprodução de peixes, principalmente espécies migradoras. No entanto, pouco se sabe sobre os efeitos dessas mudanças nas condições da água em espécies não migradoras. A reprodução de Acestrohynchus lacustris foi avaliada em duas seções do rio São Francisco. Seção 1, localizada imediatamente a jusante da barragem de Três Marias e seção 2, localizada na confluência dos rios São Francisco e Abaeté. Para isso, foram obtidos os parâmetros físico-químicos da água de cada seção do estudo. Além disso, dados biométricos e índices biológicos de todos os peixes capturados foram obtidos. Adicionalmente, nas fêmeas foram medidos os diâmetros dos folículos vitelogênicos e a fecundidade. Temperatura, oxigênio dissolvido e fluxo apresentaram valores mais baixos na seção 1. Os peixes capturados nesta seção apresentaram menores valores de IGS em ambos os sexos, e as fêmeas apresentaram menores valores de fecundidade e diâmetro dos folículos. Essa espécie apresentou atividade reprodutiva nas duas seções analisadas, porém, na seção 1, onde os parâmetros da água apresentam piores condições para o processo reprodutivo de peixes, a capacidade reprodutiva de A. lacustris foi afetada negativamente.

Inducing an extended naturally complete mixing period in a stratified reservoir via artificial destratification.

Naturally complete mixing (i.e., ΔT <1 °C across the entire water column) driven by convection in winter is an ideal state for maintaining good water quality, as it spontaneously redistributes dissolved oxygen (DO) over the entire water column and prevents hypolimnetic anoxia and associated pollution. A complete mixing duration is quite short under natural mixing conditions, whereas artificial destratification systems can artificially induce an earlier occurrence of complete mixing, thereby prolonging the span of the naturally complete mixing period by several months. Based on multi-year in situ water quality measurements and meteorological data during natural and artificial mixing periods, this study evaluates the effects of water-lifting aerators (WLAs) and climatic factors on convective mixing processes and their duration. WLA-supplied turbulent kinetic energy (TKE) and WLA-induced hypolimnion warming significantly decrease the water stability and extend the naturally complete mixing period for 2.6-fold. The results indicate that an optimal WLA implementation should immediately achieve complete mixing when surface mixing occurs in autumn. By evaluating the influence of WLAs and other factors on convective mixing, this study provides insights for successful destratification system operations (i.e., WLAs) to replenish oxygen concentrations across the water column and minimize operating costs by taking advantage of climatic conditions. Although our study focuses on WLA-induced mixing, these observations can be applicable to other destratification systems in most of the stratified reservoirs and lakes.

Iron and manganese fluxes across the sediment-water interface in a drinking water reservoir.

The development of low dissolved oxygen (DO) concentrations in the hypolimnion of drinking water reservoirs during thermal stratification can lead to the reduction of oxidized, insoluble iron (Fe) and manganese (Mn) in sediments to soluble forms, which are then released into the water column. As metals degrade drinking water quality, robust measurements of metal fluxes under changing oxygen conditions are critical for optimizing water treatment. In this study, we conducted benthic flux chamber experiments in summer 2018 to directly quantify Fe and Mn fluxes at the sediment-water interface under different DO and redox conditions of a eutrophic drinking water reservoir with an oxygenation system (Falling Creek Reservoir, Vinton, VA, USA). Throughout the experiments, we monitored DO, oxidation-reduction potential (ORP), water temperature, and pH in the chambers and compared the metal fluxes in the chambers with time-series of fluxes calculated using a hypolimnetic mass balance method. Our results showed that metal fluxes were highly variable during the monitoring period and were sensitive to redox conditions in the water column at the sediment-water interface. The time-series changes in fluxes and relationship to redox conditions are suggestive of "hot moments", short time periods of intense biogeochemical cycling. Although the metal concentrations and fluxes are specific to this site, the approaches for examining relationships between metals, oxygen concentrations and overall redox conditions can be applied by water utilities to improve water quality management of Fe and Mn.

Diazotroph Genomes and Their Seasonal Dynamics in a Stratified Humic Bog Lake.

Aquatic N-fixation is generally associated with the growth and mass development of Cyanobacteria in nitrogen-deprived photic zones. However, sequenced genomes and environmental surveys suggest active aquatic N-fixation also by many non-cyanobacterial groups. Here, we revealed the seasonal variation and genomic diversity of potential N-fixers in a humic bog lake using metagenomic data and nif gene clusters analysis. Groups with diazotrophic operons were functionally divergent and included Cholorobi, Geobacter, Desulfobacterales, Methylococcales, and Acidobacteria. In addition to nifH (a gene that encodes the dinitrogenase reductase component of the molybdenum nitrogenase), we also identified sequences corresponding to vanadium and iron-only nitrogenase genes. Within the Chlorobi population, the nitrogenase (nifH) cluster was included in a well-structured retrotransposon. Furthermore, the presence of light-harvesting photosynthesis genes implies that anoxygenic photosynthesis may fuel nitrogen fixation under the prevailing low-irradiance conditions. The presence of rnf genes (related to the expression of H+/Na+-translocating ferredoxin: NAD+ oxidoreductase) in Methylococcales and Desulfobacterales suggests that other energy-generating processes may drive the costly N-fixation in the absence of photosynthesis. The highly reducing environment of the anoxic bottom layer of Trout Bog Lake may thus also provide a suitable niche for active N-fixers and primary producers. While future studies on the activity of these potential N-fixers are needed to clarify their role in freshwater nitrogen cycling, the metagenomic data presented here enabled an initial characterization of previously overlooked diazotrophs in freshwater biomes.

Widespread Dominance of Kinetoplastids and Unexpected Presence of Diplonemids in Deep Freshwater Lakes.

Kinetoplastid flagellates are generally abundant in the deep sea and recently they were even found to be dominant in the hypolimnion of a deep freshwater lake. Therefore, to understand the distribution of kinetoplastids in deep freshwater lakes, we have collected vertical samples from five lakes in Japan. The abundance of kinetoplastids was enumerated by Catalyzed Reporter Deposition-Fluorescence in situ Hybridization, and the diversity was determined by 18S amplicon sequencing using universal eukaryote and kinetoplastid-specific primers. Kinetoplastids were abundant in the deep waters of all the lakes, contributing up to 53.6% of total nanoeukaryotes. Despite this significant contribution, kinetoplastids remain undetected by amplicon sequencing using universal primers that are widely used in eukaryotic diversity studies. However, they were detected with specific primers, and the communities were characterized by both ubiquitous and lake-specific unique OTUs. Oligotyping of a ubiquitous and dominant OTU revealed the presence of lake-specific sequence types (oligotypes). Remarkably, we also detected diplonemids (a sister group of kinetoplastids and considered to be specific in the marine habitat) using kinetoplastid-specific primers, showing their presence in freshwaters. Underestimation of kinetoplastids and diplonemids using universal primers indicates that euglenozoan flagellates are overlooked in diversity studies worldwide. The present study highlighted the importance of kinetoplastids in the hypolimnion of deep lakes, thereby indicating their role in material cycling in deep waters.

The Broad Habitat Spectrum of the CL500-11 Lineage (Phylum Chloroflexi), a Dominant Bacterioplankton in Oxygenated Hypolimnia of Deep Freshwater Lakes.

CL500-11 (phylum Chloroflexi) is one of the most ubiquitous and abundant bacterioplankton lineages in deep freshwater lakes inhabiting the oxygenated hypolimnion. While metagenomics predicted possible eco-physiological characteristics of this uncultured lineage, no consensus on their ecology has so far been reached, partly because their niche is not clearly understood due to a limited number of quantitative field observations. This study investigated the abundance and distribution of CL500-11 in seven deep perialpine lakes using catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH). Samples were taken vertically (5-12 depths in each lake) and temporally (in two lakes) at the deepest point of the lakes located in Switzerland, Italy, and Austria with varying depth, trophic state, mixing regime, and water retention time. The results showed a dominance of CL500-11 in all the lakes; their proportion to total prokaryotes ranged from 4.3% (Mondsee) to 24.3% (Lake Garda) and their abundance ranged from 0.65 × 105 (Mondsee) to 1.77 × 105 (Lake Garda) cells mL-1. By summarizing available information on CL500-11 occurrence to date, we demonstrated their broad habitat spectrum, ranging from ultra-oligotrophic to meso-eutrophic lakes, while low abundances or complete absence was observed in lakes with shallow depth, low pH, and/or short water retention time (<1 year). Together with available metagenomic and geochemical evidences from literatures, here we reviewed potential substrates supporting growth of CL500-11. Overall, the present study further endorsed ubiquity and quantitative significance of CL500-11 in deep freshwater systems and narrowed the focus on their physiological characteristics and ecological importance.

[Response of the Water Quality of a Stratified Reservoir to an Extreme El Niño Event During Summer].

Global warming can intensify the El Niño phenomenon that recurs every 2-7 years, which will lead to a great interannual variability of climate and may induce the deterioration of the water quality of reservoirs. To study the influence of the extreme El Niño events on the water quality of stratified reservoirs during summer, field surveys were conducted in Zhoucun Reservoir and its inflow rivers from May to August in a normal year (2012) and a strong El Niño year (2015). Temporal variations of physical and chemical index were investigated during monitoring. The results showed that the Zhoucun Reservoir was stratified during the study period. The precipitation in the summer of the normal year was significantly higher than that in the El Niño year at the same period. In the summer of the normal year, the water level increased from 124.26 m to 127.14 m and the hypolimnion thickness increased by 3.1 m. However, in 2015, the rapid decrease of the water level from May to August (from 121.65 m to 119.46 m) led to the decrease of the hypolimnion thickness (by 3.2 m). The inflow rivers belonged to surface current and its nutrients concentrations were obviously higher than those in the epilimnion. The inflow nutrients loads increased significantly in the summer of the normal year, as a result, total nitrogen increased from 1.00 mg·L-1 to 2.06 mg·L-1, nitrate increased from 0.19 mg·L-1 to 1.28 mg·L-1, and total phosphorus increased from 0.023 mg·L-1 to 0.088 mg·L-1 in the lacustrine zone of the reservoir. In contrast, the nutrients concentrations changed little in the summer of the El Niño year due to the decrease in runoff. Nonetheless, the reducing pollutants concentrations of the hypolimnion in the El Niño year were significantly higher than those in the normal year, which may be due to the temporal variations of hypolimnion thicknesses. The maximum concentrations of iron, manganese, ammonium and sulfide in the summer of the El Niño year were 0.38, 1.36, 2.36 and 1.67 mg·L-1, respectively. All these index exceeded the standards for surface water Class Ⅲ. We conclude that the extreme El Niño event has an apparent influence on the nutrients concentrations in the epilimnion and the pollutants concentrations in the hypolimnion in Zhoucun Reservoir.

Methanotrophy under Versatile Conditions in the Water Column of the Ferruginous Meromictic Lake La Cruz (Spain).

Lakes represent a considerable natural source of methane to the atmosphere compared to their small global surface area. Methanotrophs in sediments and in the water column largely control methane fluxes from these systems, yet the diversity, electron accepting capacity, and nutrient requirements of these microorganisms have only been partially identified. Here, we investigated the role of electron acceptors alternative to oxygen and sulfate in microbial methane oxidation at the oxycline and in anoxic waters of the ferruginous meromictic Lake La Cruz, Spain. Active methane turnover in a zone extending well below the oxycline was evidenced by stable carbon isotope-based rate measurements. We observed a strong methane oxidation potential throughout the anoxic water column, which did not vary substantially from that at the oxic/anoxic interface. Both in the redox-transition and anoxic zones, only aerobic methane-oxidizing bacteria (MOB) were detected by fluorescence in situ hybridization and sequencing techniques, suggesting a close coupling of cryptic photosynthetic oxygen production and aerobic methane turnover. Additions of nitrate, nitrite and to a lesser degree iron and manganese oxides also stimulated bacterial methane consumption. We could not confirm a direct link between the reduction of these compounds and methane oxidation and we cannot exclude the contribution of unknown anaerobic methanotrophs. Nevertheless, our findings from Lake La Cruz support recent laboratory evidence that aerobic methanotrophs may be able to utilize alternative terminal electron acceptors under oxygen limitation.