Comparing the benthic nitrogenase activity and diazotrophic community assembly of three large river-connected freshwater lakes in eastern China.

Biological nitrogen fixation (BNF) is a crucial process that provides bioavailable nitrogen and supports primary production in freshwater lake ecosystems. However, the characteristics of diazotrophic community and nitrogenase activity in freshwater lake sediments remain poorly understood. Here, we investigated the diazotrophic communities and nitrogenase activities in the sediments of three large river-connected freshwater lakes in eastern China using 15N-isotope tracing and nifH sequencing. The sediments in these lakes contained diverse nitrogenase genes that were phylogenetically grouped into Clusters I and III. The diazotrophic communities in the sediments were dominated by stochastic processes in Hongze Lake and Taihu Lake, which had heterogeneous habitats and shallower water depths, while in Poyang Lake, which had deeper water and a shorter hydraulic retention time, the assembly of the diazotrophic community in the sediments was dominated by homogeneous selection processes. Temperature and water depth were also found the key environmental factors affecting the sediment diazotrophic communities. Sediment nitrogenase activities varied in the three lakes and within distinct regions of an individual lake, ranging from 0 to 14.58 nmol/(kg·hr). Nitrogenase activity was significantly correlated with ferric iron, total phosphorus, and organic matter contents. Our results suggested that freshwater lake sediment contain high diversity of nitrogen-fixing microorganisms with potential metabolic diversity, and the community assembly patterns and nitrogenase activities varied with the lake habitat.

Aquibaculum sediminis sp. nov., a halotolerant bacteria isolated from salt lake sediment.

An aerobic, Gram-stain negative bacterium was isolated from sediment samples of Barkol salt lake in Hami City, Xinjiang Uygur Autonomous Region, China, with the number EGI_FJ10229T. The strain is ellipse-shaped, oxidase-negative, catalase-positive, and has white, round, smooth, opaque colonies on marine 2216 E agar plate. Growth occurs at 4.0-37.0 â„ƒ (optimal:30.0 â„ƒ), pH 7.0-9.0 (optimal: pH 8.0) and NaCl concentration of 0-8.0% (optimal: 3.0%). Phylogenetic analysis based on 16S rRNA gene and genome sequences indicated that the isolated strain should be assigned to the genus Aquibaculum and was most closely related to Aquibaculum arenosum CAU 1616 T. Average nucleotide identity (ANI) and Average amino-acid identity (AAI) values between the type species of the genus Aquibaculum and other related type species were lower than the threshold values recommended for bacterial species. The genomic DNA G + C content of EGI_FJ10229T was 65.41%. The major polar lipids were diphosphatidylglycerol, phosphatidylmethylethanolamine, phosphatidylcholine, phosphatidylethanolamine and unidentified phospholipid. The major fatty acids (> 5%) were C19:0 cyclo ω8c (42.0%) and C18:1 ω7c (33.78%). The respiratory quinone identified was Q-10. Differential phenotypic and genotypic characteristics of this strain and species of genus Aquibaculum showed that the strain should be classified as representing a new species belonging to this genus, for which the name Aquibaculum sediminis sp. nov. is proposed. The type strain of the proposed novel species is EGI_FJ10229T (= KCTC 8570 T = GDMCC 1.4598 T).

Lake Bacterial Communities in North Patagonian Andes: The Effect of the Nothofagus pumilio Treeline.

One of the most noticeable environmental discontinuities in mountains is the transition that exists in vegetation below and above the treeline. In the North Patagonian Andean lakes (between 900 and 1950 m a.s.l.), we analyzed the bacterial community composition of lakes in relation to surrounding vegetation (erected trees, krummholz belt, and bare rocks), dissolved organic carbon (DOC), and total dissolved nutrients (nitrogen, TDN and phosphorus, TDP). We observed a decrease in DOC, TDP, and TDN concentrations with altitude, reflecting shifts in the source inputs entering the lakes by runoff. Cluster analysis based on bacterial community composition showed a segregation of the lakes below treeline, from those located above. This first cluster was characterized by the cyanobacteria Cyanobium PCC-6307, while in the krummholz belt and bare rocks, bacterial communities were dominated by Actinobacteria hgcl-clade and Proteobacteria (Sandarakinorhabdus and Rhodovarius), with the presence of pigments such as actinorhodopsin, carotenoids, and bacteriochlorophyll a. The net relatedness index (NRI), which considers the community phylogenetic dispersion, showed that lakes located on bare rocks were structured by environmental filtering, while communities of lakes below treeline were structured by species interactions such as competition. Beta-diversity was higher among lakes below than among lakes located above the treeline. The contribution of species turnover was more important than nestedness. Our study brings light on how bacterial communities may respond to changes in the surrounding vegetation, highlighting the importance of evaluating different aspects of community structure to understand metacommunity organization.

Halostreptopolyspora alba gen. nov., sp. nov., a halophilic actinobacterium isolated from saline soil of Xinjiang, Northwest of China.

A Gram-stain-positive, aerobic, moderate halophilic actinobacterium, designated strain YIM 96095T, was isolated from a saline soil sample collected from Aiding Lake, Xinjiang, North-western China. Phylogenetic analysis based on 16S rRNA gene sequences revealed that the isolate belonged to the family Nocardiopsidaceae, formed a distinct subclade, and was most closely related to Lipingzhangella halophila DSM 102030T and Allosalinactinospora lopnorensis DSM 45697T with sequence identity values of 95.8 and 95.1%, respectively. Optimal growth occurred at 37 °C, pH 7.0-8.0 and with 5-16% (w/v) NaCl, with well-developed, non-fragmented substrate mycelia and single-, double-, or triple-wrinkled spore(s) on the mature aerial hyphae. The chemical analysis presented meso-diaminopimelic acid as the diagnostic diamino acid of the cell-wall peptidoglycan, and glucose, galactose and rhamnose as the major whole-cell sugars, and iso-C15 : 0 and anteiso-C15 : 0 as the major fatty acids. The phospholipids were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylcholine, unidentified phospholipids and unidentified glycolipid. The menaquinones were MK-10(H8), MK-10(H6) and MK-9(H10). Its G+C content was 69.7 mol% in the determined genome sequence. Based on phenotypic, chemotaxonomic and phylogenetic characteristics, a novel genus and species named Halostreptopolyspora alba gen. nov., sp. nov. is proposed for isolate YIM 96095T (=KCTC 49266T=CGMCC 4.7636T).

The effect of urbanization on planktonic and biofilm bacterial communities in different water bodies of the Danube River in Hungary.

Freshwaters play an essential role in providing ecosystem services worldwide, however, the water quality of different water bodies is strongly influenced by human activities such as urbanization, industry and agriculture. In this study, water and biofilm samples were collected from the main channel of the Danube River upstream and downstream of a metropolitan, from a regulated side arm within an urbanized area, and from two differently separated oxbow lakes located in nature conservation areas. The taxonomic diversity of bacterial communities was revealed by 16S rRNA gene-based amplicon sequencing using Illumina MiSeq platform. The results showed that all samples were dominated by phyla Pseudomonadota, Actinobacteriota and Bacteroidota. The bacterial community structures, however, clearly differentiated according to planktonic and epilithic or epiphytic habitats, as well as by riverine body types (main channel, side arm, oxbow lakes). The taxonomic diversity of biofilm communities was higher than that of planktonic ones in all studied habitats. Human impacts were mainly reflected in the slowly changing biofilm composition compared to the planktonic ones. Genera with pollution tolerance and/or degradation potential, such as Acinetobacter, Pseudomonas and Shewanella were mainly detected in biofilm communities of the highly urbanized section of the river side arm.

The Fungal Side of the Story: Saprotrophic- vs. Symbiotrophic-Predicted Ecological Roles of Fungal Communities in Two Meromictic Hypersaline Lakes from Romania.

Over three-quarters of Earth's surface exhibits extreme environments where life thrives under harsh physicochemical conditions. While prokaryotes have often been investigated in these environments, only recent studies have revealed the remarkable adaptability of eukaryotes, in particular fungi. This study explored the mycobiota of two meromictic hypersaline lakes, Ursu and Fara Fund, in Transylvania (Romania). The intrinsic and extrinsic fungal diversity was assessed using amplicon sequencing of environmental DNA samples from sediments, water columns, surrounding soils, and an associated rivulet. The fungal communities, illustrated by the 18S rRNA gene and ITS2 region, exhibited contrasting patterns between the lakes. The ITS2 region assessed better than the 18S rRNA gene the fungal diversity. The ITS2 data showed that Ascomycota was the most abundant fungal group identified in both lakes, followed by Aphelidiomycota, Chytridiomycota, and Basidiomycota. Despite similar α-diversity levels, significant differences in fungal community structure were observed between the lakes, correlated with salinity, total organic carbon, total nitrogen, and ammonium. Taxonomic profiling revealed depth-specific variations, with Saccharomycetes prevalent in Ursu Lake's deeper layers and Lecanoromycetes prevalent in the Fara Fund Lake. The functional annotation using FungalTraits revealed diverse ecological roles within the fungal communities. Lichenized fungi were dominant in Fara Fund Lake, while saprotrophs were abundant in Ursu Lake. Additionally, wood and soil saprotrophs, along with plant pathogens, were more prevalent in the surrounding soils, rivulet, and surface water layers. A global overview of the trophic relations in each studied niche was impossible to establish due to the unconnected graphs corresponding to the trophic interactions of the analyzed fungi. Plotting the unweighted connected subgraphs at the genus level suggests that salinity made the studied niches similar for the identified taxa. This study shed light on the understudied fungal diversity, distribution, and ecological functions in hypersaline environments.

Comprehensive characterization and resistome analysis of Antarctic Pseudomonas migulae strain CAS19.

Although traditionally considered pristine, Antarctica faces an increasing threat of antibiotic resistance due to human intervention. Here, we obtained a bacterial isolate, the CAS19 strain, from a lake water sample from Ardley Island, Antarctica and characterized it comprehensively. The CAS19 was a psychrotrophic and neutrophilic/alkalitolerant bacterium thriving at temperatures from 15 to 33 °C and pH levels from 6.0 to 9.0. Besides the production of siderophore and indole acetic acid, it also exhibited proteolytic and lipolytic activities. It was identified as Pseudomonas migulae by multilocus (16S rRNA, gyrB, rpoB and rpoD) sequence analysis, and its genome was 6.5 Mbps in length, had 59% GC content, and contained 5,821 coding sequences. The CAS19 was resistant to several antibiotics, including trimethoprim, penicillin, vancomycin, and erythromycin, confirmed by RT-qPCR analysis, with a notable increase in dfr (63-fold), bla (461-fold), vanW (31.7-fold) and macA (24.7-fold) expressions upon antibiotic exposure. Additionally, CAS19 exhibited resistance to heavy metals with an order of Cr(III) = Cu(II) > Ni(II) > Zn(II) > Cd(II), and showed diesel fuel (5%) degradation capacity. Cold-related genes cspA_2 and cspD were overexpressed at 4 and 15 °C, consistent with the cold adaptation mechanism. In conclusion, for the first time an Antarctic P. migulae isolate has been characterized in detail, uncovering a rich resistome repertoir that might be associated with anthropogenic disturbances.

Polymorphospora lycopeni sp. nov., a lycopene-producing actinomycetes isolated from lakeside soil sample of Baiyangdian.

A novel actinomycetes producing lycopene, designated HBU208002T, was isolated from a lakeside soil sample collected in Baiyangdian, located in Xiong'an New Area of China, and its taxonomic position was investigated by a polyphasic approach. Phylogenetic analysis based on 16S rRNA gene sequence comparisons revealed that the strain HBU208002T fell within the genus Polymorphospora and was closely related to Polymorphospora rubra JCM 14904T (99.73% identity). However, the average nucleotide identity (ANI), digital DNA-DNA hybridization (dDDH) and average amino acid identity (AAI) values between the strain HBU208002T and P. rubra JCM 14904T were 91.78, 44.7 and 91.6%, respectively, which were lower than the ANI (95-96%), DDH (>70%) and AAI (>95%) thresholds of prokaryotic microbial defined species. The predominant fatty acids of the strain HBU208002T were iso-C16:0, C17:1 ω8c. The menaquinones of the strain HBU208002T were MK-8(H8) and MK9(H4), while those of P. rubra JCM 14904T were MK-10(H6), MK-10(H4), MK-9(H6) and MK-9(H4). Meanwhile, some phenotypic characterizations and antibacterial activities distinguished the strain HBU208002T from P. rubra JCM 14904T. The strain HBU208002T exhibited inhibitory effects on Fusarium graminearum, Fusarium verticillioides and Botrytis cinerea, but P. rubra JCM 14904T had no activity. Therefore, the strain HBU208002T should be assigned as representing a novel species of the genus Polymorphospora, for which the name Polymorphospora lycopeni was proposed. The type strain is HBU208002T (=KCTC49833T = GDMCC4.236T).

Glacier Retreat Induces Contrasting Shifts in Bacterial Biodiversity Patterns in Glacial Lake Water and Sediment : Bacterial Communities in Glacial Lakes.

Glacial lake ecosystems are experiencing rapid changes due to accelerated glacier retreat. As glaciers recede, their influence on downstream habitats diminishes, potentially affecting the biodiversity of glacial lake microbial communities. However, there remains a knowledge gap regarding how bacterial biodiversity patterns in glacial lakes are altered by diminishing glacial influence. Here, we investigated shifts in bacterial communities in paired water and sediment samples collected from seven glacial lakes on the Tibetan Plateau, using a space-for-time substitution approach to understand the consequences of glacier retreat. Our findings reveal that bacterial diversity in lake water increases significantly with a higher glacier index (GI), whereas sediment bacterial diversity exhibits a negative correlation with GI. Both the water and sediment bacterial communities display significant structural shifts along the GI gradient. Notably, reduced glacial influence decreases the complexity of bacterial co-occurrence networks in lake water but enhances the network complexity in sediment. This divergence in diversity and co-occurrence patterns highlights that water and sediment bacterial communities respond differently to changes in glacial influence in these lake ecosystems. This study provides insights into how diminishing glacial influence impacts the bacterial biodiversity in glacial lake water and sediments, revealing contrasting patterns between the two habitats. These findings emphasize the need for comprehensive monitoring to understand the implications of glacier retreat on these fragile ecosystems.

Novel genomic and phenotypic traits of polyhydroxyalkanoate-producing bacterium ZZQ-149, the type strain of Halomonas qinghailakensis.

BACKGROUND: Polyhydroxyalkanoates (PHAs) are optimal potential materials for industrial and medical uses, characterized by exceptional sustainability, biodegradability, and biocompatibility. These are primarily from various bacteria and archaea. Bacterial strains with effective PHA formation capabilities and minimal production cost form the foundation for PHA production. Detailed genomic analysis of these PHA-generating bacteria is vital to understand their PHA production pathways and enhance their synthesis capability. RESULTS: ZZQ-149, a halophilic, PHA-producing bacterium, was isolated from the sediment of China's Qinghai Lake. Here, we decoded the full genome of ZZQ-149 using Single Molecule Real Time (SMRT) technology based on PacBio RS II platform, coupled with Illumina sequencing platforms. Physiological, chemotaxonomic traits, and phylogenetic analysis based on 16 S rRNA gene and single copy core genes of ninety-nine Halomonas type strains identified ZZQ-149 as the type strain of Halomonas qinghailakensis. Furthermore, a low average nucleotide identity (ANI, < 95%) delineated the genetic differences between ZZQ-149 and other Halomonas species. The ZZQ-149 genome, with a DNA G + C content of 52%, comprises a chromosome (3, 798, 069 bps) and a plasmid (6, 107 bps). The latter encodes the toxin-antitoxin system, BrnT/BrnA. Through comprehensive genome sequencing and analysis, we identified multiple PHA-synthesizing enzymes and an unprecedented combination of eight PHA-synthesizing pathways in ZZQ-149. CONCLUSIONS: Being a halophilic, PHA-producing bacterium, ZZQ-149 exhibits potential as a high PHA producer for engineered bacteria via genome editing while ensuring low-cost PHA production through continuous, unsterilized fermentation.

Molecular characterization, virulence and antimicrobial and biocidal susceptibility of selected bacteria isolated from the cloaca of nestling ospreys (Pandion haliaetus) from Mono Lake, California, USA.

In the present study, the presence of the Enterobacterales, Staphylococcus spp., Mammaliicoccus spp., and Enterococcus spp. in cloacal samples of nestling ospreys (Pandion haliaetus), a fish-eating specialist, from Mono Lake, California, USA was examined by a multiphasic approach, including antimicrobial and biocide susceptibility testing, genotyping, and whole genome sequencing of selected isolates. The most commonly detected species was Escherichia coli, followed by Mammaliicoccus sciuri, Staphylococcus delphini, Enterococcus faecalis, Enterococcus faecium, Hafnia alvei, Klebsiella pneumoniae, Citrobacter braakii and single isolates of Edwardsiella tarda, Edwardsiella albertii, Klebsiella aerogenes, Plesiomonas shigelloides and Staphylococcus pseudintermedius. Multi-drug resistance (MDR) was observed in two E. coli isolates and in an Enterococcus faecium isolate. The MDR blaCTX-M-55-positive E. coli belonged to the pandemic clone ST58. The results of the present study suggest that nestling ospreys are exposed to MDR bacteria, possibly through the ingestion of contaminated fish. Ospreys may be good biosentinels for the presence of these microorganisms and antibiotic resistance in the local environment and the risk for other wildlife, livestock and humans.

Assessment of Water Quality Parameters in Lake Hayq, Northeastern Ethiopia.

Lake Hayq is one of the highland lakes of Ethiopia that furnishes very important ecosystem services, fishing, tourism, transportation, drinking water, livestock watering, and irrigation. However, the lake ecosystem is being degraded by pollution, siltation, and excessive growth of macrophytes, buffer zone degradation, overfishing, and climate variability. Therefore, this study was conducted to assess the physicochemical, heavy metals, and biological water quality parameters of Lake Hayq. Physiochemical (pH, water temperature, conductivity, TDS, total alkalinity, dissolved oxygen, Ca2+, Mg2+, Na+, K+, NH4+, NH3, NO2 -, NO3 -, CO3 -, HCO3 -, SO4 2-, PO4 3-, SiO2, and total phosphorus), heavy metals (Pb, Fe, Zn, Cr, Cu2+, Mn2+, and Ni), and biological (BOD5 and total coliforms) water quality parameters were analyzed both in situ and ex situ. The physicochemical parameters were measured using portable water quality measuring multimeters, the heavy metal analysis was done using the Atomic Absorption Spectrometer, the BOD5 was measured using a BOD5 meter, and the total coliform analysis was done using the spread plate technique. The collected data were analyzed using multivariate, two-way ANOVA to see the mean difference among sampling sites and seasons through the application of SPSS 16. Most of the water quality parameters of Lake Hayq have met the WHO standards for recreation, aquatic life, and drinking water quality. However, some parameters, such as Pb, BOD5, and total coliforms, were above WHO water quality permissible limits. Therefore, ecohydrological (nature-based) waste treatment methods such as macrophyte restoration in buffer zones and ecofriendly farming activities should be practiced to minimize the contamination of the lake.

Yunchengibacter salinarum gen. nov., sp. nov., a novel bacterium of the family Kordiimonadaceae isolated from sediment in Yuncheng salt lake.

A Gram-stain-negative, aerobic, motile and rod-shaped bacterium, the color of the bacterial colony ranges from light yellow to yellow, designated YC-2023-2T, was isolated from sediment sample of Yuncheng salt lake. Growth occurred at 15-45℃ (optimum 37℃), pH 6.0-9.0 (optimum pH 7.0-8.0) and with 0-8.0% NaCl (w/v, optimum 2.0%). The phylogenetic analysis based on 16S rRNA gene sequences showed that strain YC-2023-2T belonged to the family Kordiimonadaceae. The closely related members were Gimibacter soli 6D33T (92.38%), Kordiimonas lipolytica M41T (91.88%), Eilatimonas milleporae DSM 25217T (91.88%) and Kordiimonas gwangyangensis JCM 12864T (91.84%). The genome of strain YC-2023-2T was 2957513 bp, and the genomic DNA G+C content was 63.91%. The main respiratory quinone was Q-10 and the major fatty acids (>10%) were iso-C15:0, C16:0, C19:0 cyclo ω8c, Summed Feature 8 (C18:1 ω6c or C18:1 ω7c) and Summed Feature 9 (iso-C17:1 ω9c or C16:0 10-methyl). The major polar lipids consisted of phosphatidylethanolamine, diphosphatidylglycerol, phosphatidylglycerol, unidentified glycolipid, unidentified lipid, and two unidentified aminolipids. Based on the phylogenetic, phenotypic and chemotaxonomic characteristics, strain YC-2023-2T is proposed to represent a novel species of a novel genus named Yunchengibacter salinarum gen. nov., sp. nov., within the family Kordiimonadaceae. The type strain is YC-2023-2T (= GDMCC 1.4502T = KCTC 8546T).

Hyperexpansion of genetic diversity and metabolic capacity of extremophilic bacteria and archaea in ancient Andean lake sediments.

BACKGROUND: The Andean Altiplano hosts a repertoire of high-altitude lakes with harsh conditions for life. These lakes are undergoing a process of desiccation caused by the current climate, leaving terraces exposed to extreme atmospheric conditions and serving as analogs to Martian paleolake basins. Microbiomes in Altiplano lake terraces have been poorly studied, enclosing uncultured lineages and a great opportunity to understand environmental adaptation and the limits of life on Earth. Here we examine the microbial diversity and function in ancient sediments (10.3-11 kyr BP (before present)) from a terrace profile of Laguna Lejía, a sulfur- and metal/metalloid-rich saline lake in the Chilean Altiplano. We also evaluate the physical and chemical changes of the lake over time by studying the mineralogy and geochemistry of the terrace profile. RESULTS: The mineralogy and geochemistry of the terrace profile revealed large water level fluctuations in the lake, scarcity of organic carbon, and high concentration of SO42--S, Na, Cl and Mg. Lipid biomarker analysis indicated the presence of aquatic/terrestrial plant remnants preserved in the ancient sediments, and genome-resolved metagenomics unveiled a diverse prokaryotic community with still active microorganisms based on in silico growth predictions. We reconstructed 591 bacterial and archaeal metagenome-assembled genomes (MAGs), of which 98.8% belonged to previously unreported species. The most abundant and widespread metabolisms among MAGs were the reduction and oxidation of S, N, As, and halogenated compounds, as well as aerobic CO oxidation, possibly as a key metabolic trait in the organic carbon-depleted sediments. The broad redox and CO2 fixation pathways among phylogenetically distant bacteria and archaea extended the knowledge of metabolic capacities to previously unknown taxa. For instance, we identified genomic potential for dissimilatory sulfate reduction in Bacteroidota and α- and γ-Proteobacteria, predicted an enzyme for ammonia oxidation in a novel Actinobacteriota, and predicted enzymes of the Calvin-Benson-Bassham cycle in Planctomycetota, Gemmatimonadota, and Nanoarchaeota. CONCLUSIONS: The high number of novel bacterial and archaeal MAGs in the Laguna Lejía indicates the wide prokaryotic diversity discovered. In addition, the detection of genes in unexpected taxonomic groups has significant implications for the expansion of microorganisms involved in the biogeochemical cycles of carbon, nitrogen, and sulfur. Video Abstract.

Taxonomic and functional metagenomic assessment of a Dolichospermum bloom in a large and deep lake south of the Alps.

Untargeted genetic approaches can be used to explore the high metabolic versatility of cyanobacteria. In this context, a comprehensive metagenomic shotgun analysis was performed on a population of Dolichospermum lemmermannii collected during a surface bloom in Lake Garda in the summer of 2020. Using a phylogenomic approach, the almost complete metagenome-assembled genome obtained from the analysis allowed to clarify the taxonomic position of the species within the genus Dolichospermum and contributed to frame the taxonomy of this genus within the ADA group (Anabaena/Dolichospermum/Aphanizomenon). In addition to common functional traits represented in the central metabolism of photosynthetic cyanobacteria, the genome annotation uncovered some distinctive and adaptive traits that helped define the factors that promote and maintain bloom-forming heterocytous nitrogen-fixing Nostocales in oligotrophic lakes. In addition, genetic clusters were identified that potentially encode several secondary metabolites that were previously unknown in the populations evolving in the southern Alpine Lake district. These included geosmin, anabaenopetins, and other bioactive compounds. The results expanded the knowledge of the distinctive competitive traits that drive algal blooms and provided guidance for more targeted analyses of cyanobacterial metabolites with implications for human health and water resource use.

Drivers and mechanisms of harmful algal blooms across hydrologic extremes in hypereutrophic grand lake st marys (Ohio).

Grand Lake St. Marys (GLSM) is a large, shallow, hypereutrophic lake situated in an agricultural watershed with high-nutrient, non-point source runoff. The resulting harmful algal blooms (HABs) are typically dominated by Planktothrix, which can produce microcystin, a potent cyanobacterial toxin that has varied in concentration over the past decade. Some drivers of bloom biomass and toxicity in GLSM are described, but recent years (2019-2022) have exhibited anomalous combinations of winter ice cover and spring runoff, suggesting that additional factors contribute to variability in HAB severity and toxicity. 2020 and 2022 were typical water years, with normal tributary runoff volumes occurring primarily in late winter and spring after either little to no ice cover (2019-2020) or heavy/prolonged ice cover (2021-2022). However, 2021 exhibited prolonged winter ice and low winter/spring runoff. 2020 and 2022 were typical bloom years, with near monoculture, Planktothrix-dominated biomass (11 to 405 µg/L total chlorophyll) and high total concentrations of microcystins (<0.3 to 65 µg/L). However, the first half of 2021 exhibited lower biomass (18 to 65 µg/L chlorophyll a) and toxin concentrations (0.4 to 2.0 µg/L). While biomass returned to bloom levels when external tributary loading increased, ammonium uptake and regeneration rates and microcystin concentrations remained low throughout 2021 (in contrast to other years). Overall, potential ammonium uptake rates strongly correlated with chlorophyll and microcystin concentrations (Bayesian R2 = 0.59; 95% CI = 0.44 to 0.65). Phytoplankton diversity was higher in 2021 than other years, especially in spring/early summer, with increased dinoflagellates and diatoms in spring, followed by a mixed cyanobacterial assemblage in summer. These results suggest that lower external nutrient loads can drive immediate positive impacts on water quality, such as reduced HAB biomass and toxicity and higher phytoplankton diversity, even in hypereutrophic, shallow lakes.

A novel strategy of bloom forming cyanobacteria Microcystis sp. in response to phosphorus deficiency: Using non-phosphorus lipids substitute phospholipids.

Despite significant reductions in phosphorus (P) loads, lakes still experience cyanobacterial blooms. Little is known regarding cellular P regulation in response to P deficiency in widely distributed bloom causing species such as Microcystis. In this study, we investigated changes in P containing and non-P lipids contents and their ratios concomitantly with the determinations of expression levels of genes encoding these lipids in cultural and field Microcystis samples. In the culture, the content of phosphatidylglycerol (PG) decreased from 2.1 µg g-1 in P replete control to 1.2 µg g-1 in P-deficient treatment, while non-P lipids, like sulfoquinovosyldiacylglycerol (SQDG) and monogalactosyldiacylglycerol (MGDG), increased dramatically from 13.6 µg g-1 to 142.3 µg g-1, and from 0.9 µg g-1 to 16.74 µg g-1, respectively. The expression of the MGDG synthesis gene, mgdE, also increased under low P conditions. Significant positive relationships between soluble reactive phosphorus (SRP) and ratios of P-containing lipids (PG) to non-P lipids, including SQDG, MGDG and digalactosyldiacylglycerol (DGDG) (P < 0.05) were observed in the field investigations. Both cultural and field data indicated that Microcystis sp. might increase non-P lipids proportion to lower P demand when suffering from P deficiency. Furthermore, despite lipid remodeling, photosynthetic activity remained stable, as indicated by comparable chlorophyll fluorescence and Fv/Fm ratios among cultural treatments. These findings suggested that Microcystis sp. may dominate in P-limited environments by substituting glycolipids and sulfolipids for phospholipids to reduce P demand without compromising the photosynthetic activity. This effective strategy in response to P deficiency meant a stricter P reduction threshold is needed in terms of Microcystis bloom control.

Spatiotemporal dynamics of 2-methylisoborneol produced by filamentous cyanobacteria and associated driving factors in Lake Taihu, China.

The proliferation of filamentous cyanobacteria in lakes can result in the generation of odor-causing compounds, predominantly 2-methylisoborneol (2-MIB), which pose odor-related challenges. In an effort to elucidate the spatiotemporal dynamics of 2-MIB and related influencing factors in East Lake Taihu, monthly investigations were undertaken from April 2022 to March 2023. In addition to the monthly survey, a whole-lake survey was conducted during the high-temperature period from July to September. The monthly survey revealed a distinct unimodal fluctuation in the concentration of 2-MIB in East Lake Taihu, with an average concentration at 297.0 ng/L during the high-temperature period. During the high-temperature period, the filamentous cyanobacterial communities detected in East Lake Taihu consisted primarily of species belonging to genera Leptolyngbya, Oscillatoria, Planktothricoides, and Pseudanabaena. However, no significant correlations were found between their densities and 2-MIB concentration. In addition, the mic gene was predominantly detected in genera Pseudanabaena and Planktothricoides, with the latter being the primary contributor to 2-MIB production. Furthermore, a succession of cyanobacteria capable of producing 2-MIB was detected, with water temperature and radiation intensity being identified as the primary driving factors. The temporal variation of 2-MIB concentration within East Lake Taihu during the whole year was primarily modulated by factors such as water temperature, water transparency, dissolved oxygen, and chlorophyll-a. During the high-temperature period, the 2-MIB concentration in the alga-dominated zone of East Lake Taihu was approximately 1.7 times greater than that in the macrophyte-dominated zone, with nutrient and transparency being identified as the main influencing factors. Consequently, our findings are of great significance for monitoring the sources and variation of 2-MIB in shallow lakes, providing a scientific foundation and theoretical guidance for odor management.

Risk assessment of toxic cyanobacterial blooms in recreational waters: A comparative study of monitoring methods.

Toxic cyanobacterial blooms impose a health risk to recreational users, and monitoring of cyanobacteria and associated toxins is required to assess this risk. Traditionally, monitoring for risk assessment is based on cyanobacterial biomass, which assumes that all cyanobacteria potentially produce toxins. While these methods may be cost effective, relatively fast, and more widely accessible, they often lead to an overestimation of the health risk induced by cyanotoxins. Monitoring methods that more directly target toxins, or toxin producing genes, may provide a better risk assessment, yet these methods may be more costly, usually take longer, or are not widely accessible. In this study, we compared six monitoring methods (fluorometry, microscopy, qPCR of 16S and mcyE, ELISA assays, and LC-MS/MS), of which the last three focussed on the most abundant cyanotoxin microcystins, across 11 lakes in the Netherlands during the bathing water season (May-October) of 2019. Results of all monitoring methods significantly correlated with LC-MS/MS obtained microcystin levels (the assumed 'golden standard'), with stronger correlations for methods targeting microcystins (ELISA) and microcystin genes (mcyE). The estimated risk levels differed substantially between methods, with 78 % and 56 % of alert level exceedances in the total number of collected samples for fluorometry and microscopy-based methods, respectively, while this was only 16 % and 6 % when the risk assessment was based on ELISA and LC-MS/MS obtained toxin concentrations, respectively. Integrating our results with earlier findings confirmed a strong association between microcystin concentration and the biovolume of potential microcystin-producing genera. Moreover, using an extended database consisting of 4265 observations from 461 locations across the Netherlands in the bathing water seasons of 2015 - 2019, we showed a strong association between fluorescence and the biovolume of potentially toxin-producing genera. Our results indicate that a two-tiered approach may be an effective risk assessment strategy, with first a biomass-based method (fluorometry, biovolume) until the first alert level is exceeded, after which the risk level can be confirmed or adjusted based on follow-up toxin or toxin gene analyses.

Coassembly and binning of a twenty-year metagenomic time-series from Lake Mendota.

The North Temperate Lakes Long-Term Ecological Research (NTL-LTER) program has been extensively used to improve understanding of how aquatic ecosystems respond to environmental stressors, climate fluctuations, and human activities. Here, we report on the metagenomes of samples collected between 2000 and 2019 from Lake Mendota, a freshwater eutrophic lake within the NTL-LTER site. We utilized the distributed metagenome assembler MetaHipMer to coassemble over 10 terabases (Tbp) of data from 471 individual Illumina-sequenced metagenomes. A total of 95,523,664 contigs were assembled and binned to generate 1,894 non-redundant metagenome-assembled genomes (MAGs) with ≥50% completeness and ≤10% contamination. Phylogenomic analysis revealed that the MAGs were nearly exclusively bacterial, dominated by Pseudomonadota (Proteobacteria, N = 623) and Bacteroidota (N = 321). Nine eukaryotic MAGs were identified by eukCC with six assigned to the phylum Chlorophyta. Additionally, 6,350 high-quality viral sequences were identified by geNomad with the majority classified in the phylum Uroviricota. This expansive coassembled metagenomic dataset provides an unprecedented foundation to advance understanding of microbial communities in freshwater ecosystems and explore temporal ecosystem dynamics.