Community Composition and Diversity of Coastal Bacterioplankton Assemblages in Lakes Michigan, Erie, and Huron

Microbiome processing of organic nitrogen input supports growth and cyanotoxin production of Microcystis aeruginosa cultures

Nutrient-induced blooms of the globally abundant freshwater toxic cyanobacterium Microcystis cause worldwide public and ecosystem health concerns. The response of Microcystis growth and toxin production to new and recycled nitrogen (N) inputs and the impact of heterotrophic bacteria in the Microcystis phycosphere on these processes are not well understood. Here, using microbiome transplant experiments, cyanotoxin analysis, and nanometer-scale stable isotope probing to measure N incorporation and exchange at single cell resolution, we monitored the growth, cyanotoxin production, and microbiome community structure of several Microcystis strains grown on amino acids or proteins as the sole N source. We demonstrate that the type of organic N available shaped the microbial community associated with Microcystis, and external organic N input led to decreased bacterial colonization of Microcystis colonies. Our data also suggest that certain Microcystis strains could directly uptake amino acids, but with lower rates than heterotrophic bacteria. Toxin analysis showed that biomass-specific microcystin production was not impacted by N source (i.e. nitrate, amino acids, or protein) but rather by total N availability. Single-cell isotope incorporation revealed that some bacterial communities competed with Microcystis for organic N, but other communities promoted increased N uptake by Microcystis, likely through ammonification or organic N modification. Our laboratory culture data suggest that organic N input could support Microcystis blooms and toxin production in nature, and Microcystis-associated microbial communities likely play critical roles in this process by influencing cyanobacterial succession through either decreasing (via competition) or increasing (via biotransformation) N availability, especially under inorganic N scarcity.

Impacts of an invasive filter-feeder on bacterial biodiversity are context dependent

Bacteria represent most of the biodiversity and play key roles in virtually every ecosystem. In doing so, bacteria act as part of complex communities shaped by interactions across all domains of life. Here, we report on direct interactions between bacteria and dreissenid mussels, a group of invasive filter-feeders threatening global aquatic systems due to high filtration rates. Previous studies showed that dreissenids can impact bacterial community structure by changing trait distributions and abundances of specific taxa. However, studies on bacterial community effects were conducted using water from Lake Michigan (an oligotrophic lake) only, and it is unknown whether similar patterns are observed in systems with differing nutrient regimes. We conducted ten short-term dreissenid grazing experiments in 2019 using water from two eutrophic lake regions-the western basin of Lake Erie and Saginaw Bay in Lake Huron. Predation by dreissenids led to decline in overall bacterial abundance and diversity in both lakes. However, feeding on bacteria was not observed during every experiment. We also found that traits related to feeding resistance are less phylogenetically conserved than previously thought. Our results highlight the role of temporal, spatial, and genomic heterogeneity in bacterial response dynamics to a globally important invasive filter feeder.

Bacterial eutrophic index for potential water quality evaluation of a freshwater ecosystem

Water quality evaluation of freshwater ecosystems has been widely reported based on the physical and chemical parameters of water (e.g., Carlson's trophic state index (TSI)), while the aquatic microorganisms are actually a more intuitive way to reflect the eutrophic levels. This article was based on 27 global freshwater ecosystems including freshwater rivers, lakes, and reservoirs. Bacterial eutrophic index (BEI) was determined as the function of temperature and abundances of Cyanobacteria and Actinobacteria. BEI and TSI values of the freshwater ecosystems were determined and the correlation analysis of TSI and BEI indicated their positive correlation (ρ = 0.452, p < 0.01). Furthermore, an eutrophication classification based on BEI was proposed. It turned out that BEI was a possible feasible method for water quality evaluation. The aquatic microorganism-based method such as BEI should be considered for water quality evaluation of a freshwater ecosystem. Complicated models combined with physicochemical (e.g., TSI) and microbial (e.g., BEI) method are recommended for water quality evaluation of a freshwater ecosystem in future.

Microbial communities of the Laurentian Great Lakes reflect connectivity and local biogeochemistry

The Laurentian Great Lakes are a vast, interconnected freshwater system spanning strong physicochemical gradients, thus constituting a powerful natural laboratory for addressing fundamental questions about microbial ecology and evolution. We present a comparative analysis of pelagic microbial communities across all five Laurentian Great Lakes, focusing on Bacterial and Archaeal picoplankton characterized via 16S rRNA amplicon sequencing. We collected samples throughout the water column from the major basins of each lake in spring and summer over 2 years. Two oligotypes, classified as LD12 (Alphaproteobacteria) and acI‐B1 (Actinobacteria), were among the most abundant in every sample. At the same time, microbial communities showed distinct patterns with depth during summer stratification. Deep hypolimnion samples were frequently dominated by a Chloroflexi oligotype that reached up to 19% relative abundance. Stratified surface communities differed between the colder, less productive upper lakes (Superior, Michigan, Huron) and warmer, more productive lower lakes (Erie, Ontario), in part due to an Actinobacteria oligotype (acI‐C2) that averaged 7.7% of sequences in the lower lakes but <0.2% in the upper lakes. Together, our findings suggest that both hydrologic connectivity and local selective pressures shape microbial communities in the Great Lakes and establish a framework for future investigations.

Effects of dietary Gelsemium elegans alkaloids on intestinal morphology, antioxidant status, immune responses and microbiota of Megalobrama amblycephala

Numerous plant extracts used as feed additives in aquaculture have been shown to stimulate appetite, promote growth and enhance immunostimulatory and disease resistance in cultured fish. However, there are few studies on the famous Chinese herbal medicine Gelsemium elegans, which attracts our attention. In this study, we used the Megalobrama amblycephala to investigate the effects of G. elegans alkaloids on fish intestinal health after diet supplementation with 0, 5, 10, 20 and 40 mg/kg G. elegans alkaloids for 12 weeks. We found that dietary G. elegans alkaloids at 40 mg/kg improved intestinal morphology by increasing villus length, muscle thickness and villus number in the foregut and midgut and muscle thickness in the hindgut (P < 0.05). These alkaloids also significantly improved intestinal antioxidant capabilities by increasing superoxide dismutase, catalase, total antioxidant capacity and malondialdehyde levels and up-regulated intestinal Cu/Zn-SOD and Mn-SOD (P < 0.05) at 20 and 40 mg/kg. Dietary G. elegans alkaloids improved intestinal immunity via up-regulating the pro-inflammatory cytokines IL-1β, IL-8, TNF-α and IFN-α and down-regulating expression of the anti-inflammatory cytokines IL-10 and TGF-β (P < 0.05) at 20 and 40 mg/kg. The expression of Toll-like receptors TRL1, 3, 4 and 7 were also up-regulated in intestine of M. amblycephala (P < 0.05). In intestinal microbiota, the abundance of Proteobacteria was increased while the Firmicutes abundance was decreased at phylum level after feeding the alkaloids (P < 0.05). The alkaloids also increased the abundance of the probiotic Rhodobacter and decreased the abundance of the pathogenic Staphylococcus at genus level (P < 0.05). In conclusion, dietary G. elegans alkaloid supplementation promoted intestine health by improving intestine morphology, immunity, antioxidant abilities and intestinal microbiota in M. amblycephala.

Dynamics of Bacterial and Fungal Communities during the Outbreak and Decline of an Algal Bloom in a Drinking Water Reservoir

The microbial communities associated with algal blooms play a pivotal role in organic carbon, nitrogen and phosphorus cycling in freshwater ecosystems. However, there have been few studies focused on unveiling the dynamics of bacterial and fungal communities during the outbreak and decline of algal blooms in drinking water reservoirs. To address this issue, the compositions of bacterial and fungal communities were assessed in the Zhoucun drinking water reservoir using 16S rRNA and internal transcribed spacer (ITS) gene Illumina MiSeq sequencing techniques. The results showed the algal bloom was dominated by Synechococcus, Microcystis, and Prochlorothrix. The bloom was characterized by a steady decrease of total phosphorus (TP) from the outbreak to the decline period (p < 0.05) while Fe concentration increased sharply during the decline period (p < 0.05). The highest algal biomass and cell concentrations observed during the bloom were 51.7 mg/L and 1.9×10⁸ cell/L, respectively. The cell concentration was positively correlated with COD_Mn (r = 0.89, p = 0.02). Illumina Miseq sequencing showed that algal bloom altered the water bacterial and fungal community structure. During the bloom, the dominant bacterial genus were Acinetobacter sp., Limnobacter sp., Synechococcus sp., and Roseomonas sp. The relative size of the fungal community also changed with algal bloom and its composition mainly contained Ascomycota, Basidiomycota and Chytridiomycota. Heat map profiling indicated that algal bloom had a more consistent effect upon fungal communities at genus level. Redundancy analysis (RDA) also demonstrated that the structure of water bacterial communities was significantly correlated to conductivity and ammonia nitrogen. Meanwhile, water temperature, Fe and ammonia nitrogen drive the dynamics of water fungal communities. The results from this work suggested that water bacterial and fungal communities changed significantly during the outbreak and decline of algal bloom in Zhoucun drinking water reservoir. Our study highlights the potential role of microbial diversity as a driving force for the algal bloom and biogeochemical cycling of reservoir ecology.