Occurrence and distribution of viruses and picoplankton in tropical freshwater bodies determined by flow cytometry

Potential influence of overwintering benthic algae on water quality

Overwintering benthic algae not only directly impact drinking water safety, but also affect the algae recruitment in warm spring seasons. Thus, understanding the characteristics of overwintering benthic algae can provide scientific references for formulating preventative strategies of reasonable water resource. However, they have received less attention. In this study, the spatiotemporal variation of benthic algae and their harmful secondary metabolites were studied from autumn to summer in Qingcaosha Reservoir. Benthic algae (picophytobenthos accounting for 55.42%) had a high biomass during overwintering, and the groups of overwintering benthic algae included pico-Cyanobacteria, pico-Cryptomonas, pico-Chlorophyta, pico-Diatoms, Cyanobacteria, Chlorophyta, Cryptomonas and Diatoms, which were consistent with the planktonic algae species in warm seasons. In oligotrophic or mesotrophic water bodies, micronutrients of iron and manganese were key nutrient factors influencing the biomass of benthic algae. Furthermore, picophytobenthos were important potential contributors of harmful secondary metabolites. The content of microcystins, anatoxin-a, geosmin and 2-methylisoborneol in sediment were 15.75 µg/kg·FW, 48.16 µg/kg·FW, 3.91 ng/kg·FW, and 11.76 ng/kg·FW during winter, which had potential to be released into water bodies to impact water quality. These findings indicate that water quality monitoring programs need to consider sediment in winter as a potential source of toxins and preventative measures to prevent excessive proliferation of algae should be implemented in winter.

A snapshot of the global drinking water virome: Diversity and metabolic potential vary with residual disinfectant use

Viruses are important drivers of microbial community ecology and evolution, influencing microbial mortality, metabolism, and horizontal gene transfer. However, the effects of viruses remain largely unknown in many environments, including in drinking water systems. Drinking water metagenomic studies have offered a whole community perspective of bacterial impacts on water quality, but have not yet considered the influences of viruses. In this study, we address this gap by mining viral DNA sequences from publicly available drinking water metagenomes from distribution systems in six countries around the world. These datasets provide a snapshot of the taxonomic diversity and metabolic potential of the global drinking water virome; and provide an opportunity to investigate the effects of geography, climate, and drinking water treatment practices on viral diversity. Both environmental conditions and differences in sample processing were found to influence the viral composition. Using free chlorine as the residual disinfectant was associated with clear differences in viral taxonomic diversity and metabolic potential, with significantly fewer viral populations and less even viral community structures than observed in distribution systems without residual disinfectant. Additionally, drinking water viruses carry antibiotic resistance genes (ARGs), as well as genes to survive oxidative stress and nitrogen limitation. Through this study, we have demonstrated that viral communities are diverse across drinking water systems and vary with the use of residual disinfectant. Our findings offer directions for future research to develop a more robust understanding of how virus-bacteria interactions in drinking water distribution systems affect water quality.

Metabarcoding Reveals Lacustrine Picocyanobacteria Respond to Environmental Change Through Adaptive Community Structuring

Picocyanobacteria (Pcy) are important yet understudied components of lake foodwebs. While phylogenetic studies of isolated strains reveal a high diversity of freshwater genotypes, little is known about abiotic drivers associated with Pcy in different lakes. Due to methodological limitations, most previous studies assess potential drivers using total cell abundances as a response, with often conflicting and inconsistent results. In the present study, we explored how picocyanobacterial communities respond to environmental change using a combination of epifluorescence microscopy and community data determined using 16S rRNA gene metabarcoding. Temporal shifts in picocyanobacterial abundance, diversity and community dynamics were assessed in relation to potential environmental drivers in five contrasting lakes over 1year. Cell abundances alone were not consistently related to environmental variables across lakes. However, the addition of metabarcoding data revealed diverse picocyanobacterial communities that differed significantly between lakes, driven by environmental variables related to trophic state. Within each lake, communities were temporally dynamic and certain amplicon sequence variants (ASVs) were strongly associated with specific environmental drivers. Rapid shifts in community structure and composition were often related to environmental changes, indicating that lacustrine Pcy can persist at high abundances through collective community adaptation. These results demonstrate that a combination of microscopy and metabarcoding enables an in-depth characterisation of picocyanobacterial communities and reveals strain-specific drivers. We recommend that future studies cease referring to picocyanobacterial as one functional group and take strain specific variability into consideration.

The spatial distribution of the photosynthetic picoeukaryotes community structure in Lake Hongze

The spatial dynamics of picophytoplankton were investigated by flow cytometry and high-throughput sequencing in Lake Hongze, a large river-connecting lake. Picophytoplankton were mainly composed of phycocyanin-rich picocyanobacteria (PCY) and photosynthetic picoeukaryotes (PPEs). Picocyanobacteria was the dominant picophytoplankton group since the contribution of PPEs to total picophytoplankton was only 11.78%. However, PPEs were highly diverse and were composed of Chlorophyta, Bacillariophyta and Chrysophyceae. Environmental factors showed spatial differences, particularly in total phosphorus (TP), suspended solids (SS) and chemical oxygen demand (CODMn), which showed relatively high concentrations around the river channel. The abundances of PPEs and PCY showed similar spatial patterns, which were relatively low in the river course since they were negatively related to SS and CODMn. The top 10 OTUs contributed 79.18% of the total PPEs reads and affiliated with main species in PPEs. CCA results showed that, PPEs community composition was significantly impacted by temperature and DO at sites away from river channel, while was only influenced by nitrite at sites around the river channel.

Novel cyanotoxin-producing Synechococcus in tropical lakes

Picocyanobacteria are small cyanobacteria, being about 0.8-1.5 µm in size. They are present in freshwater environments all over the world and are known to cause harmful algal blooms, although their effects are not well understood. Algal blooms are important to manage because they threaten freshwater resources, with potentially severe effects on ecological and human health. There is also increased urgency due to urbanization and climate change trends which are expected to exacerbate these bloom dynamics. These changes are expected to especially favour picocyanobacteria groups, emphasizing the need for better characterization of their effects in the environment. In this study, we report the discovery that Synechococcus sp. could produce cylindrospermopsin (CYN) and anatoxin-a (ATX). This ability had never been previously reported for this species. Their toxin genes were also partial compared to other major producers such as Raphidiopsis sp. and Anabaena sp., demonstrating potentially unique synthesis pathways that provides insight into the various mechanisms of genetic variation that drives toxin synthesis. The Synechococcus sp. strains were found to produce about 9.0 × 10^-5-6.8 × 10^-4 fg CYN cell^-1 and 4.7 × 10^-4-1.5 × 10^-2 fg ATX cell^-1. The potential for Synechococcus sp. to be toxic highlights a global concern due to its widespread distribution, and through environmental trends that increasingly favour its productivity within freshwater systems around the world.

Bacterial viability and diversity in a landscape lake replenished with reclaimed water: a case study in Xi'an, China

To understand the characteristics of bacterial viability and diversity in landscape waters replenished with reclaimed water, the typical landscape lake using reclaimed water was investigated in this study. Samples were collected from a reclaimed water inlet (P1), a reclaimed water distribution outlet (P2), and a landscape lake replenished by reclaimed water (P3). By means of measuring adenosine triphosphate (ATP), flow cytometry (FCM), and 16S rRNA gene high-throughput sequencing, the bacterial viability and diversity in reclaimed water distribution system and landscape lake were illustrated. The bacterial ATP contents at P1, P2, and P3 were 3.55 ± 1.79 ng/L, 3.31 ± 1.43 ng/L, and 18.97 ± 6.39 μg/L, and the intact bacterial cell concentrations were 5.91 ± 0.52 × 10⁴ cells/mL, 7.95 ± 2.58 × 10⁴ cells/mL, and 5.65 ± 2.10 × 10⁶ cells/mL, respectively. These results indicated a significant increase of bacterial viability in the landscape lake. The Shannon diversity index of 6.535, 7.05, and 6.886 at P1, P2, and P3, respectively, demonstrated no notable change of bacterial diversity from reclaimed water distribution system to landscape lake. However, the relative abundance of Pseudomonas sp. at P3 was significantly higher than that at P1. These findings indicated that viable but non-culturable (VBNC) bacteria could be revived in the landscape lake. The bacterial viability during reclaimed water reuse should deserve special attention.