Browsing by Subject "cyanobacteria"

Now showing 1 - 6 of 6
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    Abiotic Mechanisms for Cyanobacteria Physiology and Distribution in Lakes: A Multi-Scale Approach
    (2018-05) Wilkinson, Anne
    Harmful Algal Blooms (HABs) are a ubiquitous ecological and public health hazard because they are comprised of potentially toxic freshwater microorganisms, called cyanobacteria. Cyanobacteria are capable of accumulating in large concentrations in fresh-water ecosystems during summer and producing a toxin (microcystin) that in high concentration can be harmful to humans and animals. The occurrences of toxic HABs are highly spatially and temporarily variable in freshwater ecosystems and are difficult to predict. These HABs can be governed by abiotic environmental conditions including water temperature structure, light, nutrient abundance, and mixing. This dissertation increases the understanding of abiotic environmental conditions, i.e. different mixing scales, on the physiology and distribution of cyanobacteria in nutrient invariant eutrophic systems using field and laboratory studies. In the laboratory, we investigated the effect of small-scale turbulence on the growth and metabolism of Microcystis aeruginosa. The laboratory bioreactor setup included two underwater speakers, generating a quasi-homogeneous turbulent flow, comparable to field values in the lacustrine photic zone (Reλ =0, Reλ =33 and Reλ =15). The results suggest that turbulence mediates the metabolism of Microcystis aeruginosa, quantified by the net oxygen production, oxygen uptake, and inorganic carbon uptake, which is not manifested in changes in growth rate. In the field, we investigate the abiotic drivers for cyanobacteria and microcystin vertical distribution using a research station to quantify a wide range of local meteorological conditions, water temperature, and water chemistry, including phycocyanin, in two different eutrophic stratified Minnesota lakes. The monitoring effort was coupled with discrete weekly sampling measuring nutrients, cyanobacteria composition, and microcystin concentrations. Our objective was to describe the distributions of cyanobacteria biovolume (BV) and microcystin concentrations (MC) using easily measurable physical lake parameters. The analysis of vertical heterogeneity of cyanobacteria in the entire water column revealed high positive correlations among BV stratification, surface water temperature, stratification stability, quantified by the Schmidt stability. During strong stratification, the MC and BV accumulated above the thermocline and were highly correlated. Although, the cyanobacteria BV is significant only above the thermocline during stratification where cyanobacteria are exposed to high phosphate, temperature and light, there is still further vertical variability to explain within this region. Two types of BV distributions were observed above the thermocline. The first distribution depicted BV uniformly distributed over the diurnal surface mixed layer (SL). The second BV distribution displayed local BV maxima near and under the surface in the SL. A quantitative relationship was developed to determine the probability of observing a uniform distribution as a function of the surface Reynolds number (ReSL), the dimensionless ratio of inertial to viscous forces, over the SL. The uniform distribution was observed for ReSL>50,000. The outcome of this analysis is the first step towards the quantification and prediction vertical stratification of cyanobacteria biovolume and microcystins as a function of local meteorological and physical conditions in a stratified lake.
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    Characterization and cellular roles of a bacterial O-GlcNAc transferase in Synechococcus elongatus PCC 7942
    (2015-05) Sokol, Kerry
    The post-translational addition of a single O-linked 2-N-acetylglucosamine (O-GlcNAc) to serine or threonine residues is an important element in an ever-increasing array of metazoan cellular and regulatory processes. The enzyme responsible for this modification, O-GlcNAc transferase (OGT), is conserved among a wide variety of organisms and is critical for viability in many eukaryotes, including humans, mice, and Drosophila. Although OGTs with domain structure similar to eukaryotic OGTs are predicted for numerous bacteria species, their cellular roles remain unknown. I have identified an OGT in the cyanobacterium Synechococcus elongatus PCC 7942 that has active site homology and similar domain structure to eukaryotic OGTs. An OGT deletion mutant was created ([delta]ogt), which is viable and has no defect in growth rate, but has several phenotypes of interest. Without agitation, [delta]ogt cells aggregate, and settle out of media. Compared to wild type cells, the [delta]ogt cells also have higher free cellular phosphate levels, wider thylakoid lumen, and differential organization of electron dense inclusion bodies. These phenotypes are rescued by re-introduction of the wild type OGT, but are not fully rescued by OGTs with single amino acid substitutions corresponding to mutations that reduce eukaryotic OGT activity. S. elongatus OGT purified from E. coli hydrolyzes the donor sugar, UDP-GlcNAc, while mutant OGTs that do not fully rescue the deletion mutant have reduced or no activity. These results suggest that the eukaryotic-like OGTs of bacteria affect multiple processes. Although the substrates for the SeOGT remain elusive, I have uncovered a relationship between the SeOGT and pili through a mutation that suppresses the [delta]ogt settling phenotype. A single amino acid substitution (alanine 107 to aspartic acid) in the protein PilA suppresses two mutant phenotypes, settling and loss of pili. PilA is a subunit of type four pili (fimbriae), which are frequently found on the surfaces of gram-negative bacteria. Fimbriae are involved in virulence, DNA uptake, twitching motility, and adhesion. Although pilus assembly is a complex process, pili are primarily homopolymers of PilA, which is added or subtracted via ATPases to either extend or retract the pilus. No pili are detectable in [delta]ogt cells, while pili are restored to [delta]ogt cells by the pilA(A107D) mutation. When present together, mutant and wild type pilA genes prevent the assembly of pili, suggesting that there is a toxic interaction between mutant and wild type proteins which results in inability to assemble pili. This occurs regardless of the presence of OGT. Glycosylation of extra-cellular appendages have been widely reported. Pili are known substrates of glycosylation and glycosylation states have been reported to be important in the function of pili. The suppression of [delta]ogt phenotypes by mutant PilA suggests that PilA is either a substrate of the OGT or that another protein involved in pilus synthesis is modified by OGT.
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    Cyanobacteria phenology and toxicity across six Minnesota temperate lakes
    (2022-10) Egan, Leah
    Cyanobacteria harmful algal blooms (cHABs) represent both chronic and emerging water quality threats in lakes globally and are the consequence of complex, interacting stressors. While we know that water temperature, nutrient loading and availability, and water column mixing conditions are important drivers of cHABs, the combination of abiotic conditions leading to bloom development, maintenance, and toxicity remain poorly understood across different lake types. To better understand relationships among cyanobacteria abundances and composition, toxin concentrations, and nutrient conditions, we monitored six temperate Minnesota lakes with differing watershed land uses and lake morphometric characteristics across a latitudinal gradient. This project combined limnological approaches, comprehensive phytoplankton community analyses using taxonomic approaches, and advanced analytical characterization of toxin molecules to determine mechanisms leading to bloom formation and toxicity. Findings show that our study lakes had differing bloom phenologies influenced by different community assemblages and nutrient limitation states. The southern lakes had contrastingly different watershed land uses, such that Peltier Lake was predominantly urbanized, and Carrie Like was highly agricultural, which led differing nutrient growth conditions. Peltier (low N:P) experienced chronic surface blooms, whereas Carrie (extremely high N:P) did not. In Peltier Lake, the dominant cyanobacteria taxa present switched midsummer from nitrogen fixers (Dolichospermum spp.) to non-nitrogen fixers (Microcystis spp.) which tracked with decreasing nitrogen to phosphorus ratios. This community shift was counterintuitive based on changes in nutrient deficient growth conditions, suggesting that other drivers were likely impacting the shift in dominant cyanobacteria genera. Random Forest Models predicted major drivers of cyanotoxins in the bloom dominated lake, Peltier, to be in-lake growth conditions including dissolved organic carbon, soluble reactive phosphorus, and total phosphorus concentrations. Cyanotoxins were surprisingly detected in all lakes, including our least productive systems such as a northernly located lake, White Iron, and we observed a gradient of microcystin congeners present among our sample lakes. Cyanotoxin production can vary at the species (strain) level, therefore, it is essential to determine abiotic drivers of cHABs for various strains in differing lake types to properly inform management and mitigation of future system specific HABs.
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    Growth, Motility and Metabolism of Harmful Cyanobacteria and Lipid-Producing Microalgae in Fluid Environments: From Laboratory to Field Study
    (2020-08) You, Jiaqi
    Microorganisms have been playing important roles in aquatic environments, including the beneficial roles in ecosystem functioning and metabolites production as potential nutrient and energy sources, and the harmful roles in water quality such as harmful algal blooms (HABs). Cyanobacteria blooms have been a worldwide threat to the ecological integrity and environmental health of the freshwater bodies due to the progressive anthropogenic activities and climate change. The complex and combined interactions of environmental variables on the growth, buoyancy and metabolism (e.g., toxin production) make the prediction and management of cyanobacteria blooms and their toxicity difficult. On the other hand, microalgae have been shown as a potential bioresource for food, biofuel, and pharmaceutical products. During the growth phases with corresponding environmental conditions, microalgae accumulate different amounts of various metabolites. The neutral lipid content accumulated in the lipid-producing microalgae cells, which can be transferred to biodiesel, varies with growth conditions. This dissertation improves the understanding of growth, motility (swimming or buoyancy regulations) and metabolism of cyanobacteria and lipid-producing algae in fluids with influences of various environmental variables, in order to maximize the efficieney of microalgal biofuel production and to minimize the harmful effects of cyanobacteria HABs. In the laboratory study of cyanobacteria, batch cultures of Microcystis aeruginosa (M. aeruginosa) were cultivated at seven different temperatures to measure the specific growth rate at each temperature. A relationship between temperature and specific growth rate was established. We propose a cardinal temperature model for M. aeruginosa with the inflection point (optimal temperature) located at 27.5˚C. The model describes 98% of the variability of experimental data from 5˚C to 35˚C. A digital inline holographic microscope was employed to visualize and analyze the buoyancy of the M. aeruginosa colonies at two different temperatures. The results demonstrated a five times difference in buoyant velocities of M. aeruginosa colonies at 17.5˚C and 28˚C. A model was derived to calculate the density of a colony using the buoyant velocity and colony size. The findings provide a better understanding of temperature effects on the growth and buoyancy of M. aeruginosa. The results could facilitate the prediction of cyanobacteria blooms and the development of water quality models for freshwater ecosystems. In the laboratory study of lipid-producing microalgae, the neutral lipid accumulation was quantified and the swimming signatures (speed and trajectories) were analyzed for the motile green alga, Dunaliella primolecta, during the lag-exponential-stationary growth cycle at different nutrient concentrations. We discovered significant changes in the neutral lipid content and swimming signatures of microalgae across growth phases. The timing of the maximum swimming speed coincided with the maximum lipid content and both maxima occurred under nutrient stress at the stationary growth phase. Furthermore, the swimming trajectories suggested statistically significant changes in swimming modes at the stationary growth phase when the maximum intracellular neutral lipid content was observed. The results provide the potential exploitation of microalgal swimming signatures as possible indicators of the cultivation conditions and the timing of microalgal harvest to maximize the lipid yield for biofuel production. The findings can also be implemented to explore the production of food and antibiotics from other microalgal metabolites with low energy costs. In the field study of cyanobacteria blooms, we investigated the concentrations of cyanobacteria and microcystins in a small stratified lake and examined the influence of the abiotic environmental factors on the vertical and temporal heterogeneities. The results demonstrated the similarities in the vertical heterogeneities of cyanobacteria biovolume and total microcystin concentration. Similar patterns were discovered in vertical variations of macronutrient ratio of nitrogen over phosphorus (N:P) and biovolume ratio of non-N-fixing over N-fixing cyanobacteria. Moreover, temporal lags were revealed between the maxima of cyanobacteria biovolume, total microcystin level and Microcystis colony size. The stability of water column significantly affected the maximum Microcystis colony size, the surface cyanobacteria biovolume and the surface microcystin concentration. Correlations were established between the temporal heterogeneities of cyanobacteria community composition and the macronutrient dynamics. The findings and their implications on the environmental health will facilitate the development of prediction models and management strategies in the effort to control the impacts of cyanobacteria and cyanotoxins in small to medium size stratified lakes.
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    The Land Of Blue Green Waters? Describing The Algal Community Dynamics Of Six Minnesota Lakes By Examining Cyanobacterial Dominance And Toxicity
    (2020-02) Bambach, Matthew
    Cyanobacteria are a diverse and ancient group of phytoplankton that are a normal component of aquatic primary producer communities. They can become a problem when they reach high cell densities and form blooms capable of producing toxins that can threaten human and wildlife populations. These occurrences are referred to as cyanobacterial harmful algal blooms (cHABs). Increasing global cHAB frequency and potency have been attributed to warming temperatures and nutrient over-enrichment, but drivers of local and regional occurrence remain poorly understood. In this thesis, I examined six inland Minnesota lakes with different physical, chemical, and biological attributes to highlight patterns in cyanobacterial dominance of the phytoplankton community and understand which lake attributes were closely related to cHAB activity, with a focus on the cyanotoxin Microcystin, from June to September in 2016 and 2017. Cyanobacteria were found to dominate most study lakes, and visible blooms were observed at southern, central, and northern latitude lakes. Microcystin-producing taxa were observed in all study lakes. July and August were the months most likely to experience cHABs, and 2017 showed increased cHAB activity associated with elevated algal biovolume across systems. Specific drivers of cHABs differed among study lakes, but aggregated data for all lakes suggests that increased cyanobacterial dominance of the phytoplankton, total kjeldahl nitrogen, and chlorophyll-a pigment were the attributes most closely associated with harmful conditions. Finally, different harmfulness metrics to safeguard the public from cHABs are compared to discuss their respective protectiveness.
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    Modeling the vertical distributions of Microcystis aeruginosa
    (2022-06) Taylor, Jackie
    Harmful algal blooms (HABs) occur when algae populations accumulate in large concentrations near the air-water interface in aquatic environments. HABs---which, due to climate change, are increasing in severity, frequency, and global distribution---are hazardous to both environmental and public health. A particularly concerning algal species is the ubiquitous cyanobacterium Microcystis, which produces toxic microcystins and is especially competitive in thermally stratified lakes. Despite the growing body of evidence that suggests vertical heterogeneity of Microcystis can be a precursor to HAB formation, the abiotic drivers of vertical distribution of Microcystis are poorly understood in the field environment. The prediction of subsurface peaks in cyanobacteria concentrations is also pertinent because subsurface concentrations are not easily recognizable to the public or lake system managers, creating a risk of exposure to harmful algal toxins. To understand how vertical distributions of Microcystis are impacted by lake temperature profiles and hydrodynamics, we conducted a field study with novel monitoring technology. High-frequency temporal and vertical data were collected from a research station anchored in a stratified and eutrophic lake for five months, which is detailed in Chapter 2. Using a combination of dimensional analysis and machine learning approaches, data show that the magnitude of the subsurface Microcystis concentration peak and the center of gravity of the deep cyanobacteria layer are statistically significantly mediated by the thermal structure of the lake. The peak subsurface cyanobacteria biovolume is related to the thermocline depth and temperature, whereas the center of gravity of the subsurface cyanobacteria biovolume is related to the mixed layer depth and temperature. Furthermore, our data suggest there is a seasonal evolution of the subsurface cyanobacteria center of gravity that could potentially indicate timing of HAB onset. Based on easily measured parameters associated with the vertical lake temperature profile and meteorological conditions, we provide evidence of predictable trends in subsurface cyanobacteria variables. The field investigation revealed observations connecting lake hydrodynamics and Microcystis vertical distributions that make it possible to construct a mathematical model to understand the underlying physical phenomena behind the field observations (i.e, the lake thermal profile mediation of subsurface peaks in cyanobacteria concentration). For many harmful algae species in eutrophic lakes, the formation of such blooms is controlled by three factors: the lake hydrodynamics, the vertical motility of the algae organisms, and the ability of the organisms to form colonies. Here, using the common cyanobacterium Microcystis aeruginosa as an example, we develop a model that accounts for both vertical transport and colony dynamics. At the core of this treatment is a model for algal aggregation, described using Smoluchowski dynamics containing parameters related to Brownian motion, turbulent shear, differential setting, and cell-to-cell adhesion. To arrive at a complete description of bloom formation, we place the Smoluchowski treatment as a reaction term in a set of one-dimensional advection diffusion equations which account for the vertical motion of the algal cells through molecular and turbulent diffusion and self-regulating buoyant motion. This model is rich with interesting mathematics, both analytical and numerical. We have depth-dependent dispersion, spatiotemporal oscillatory advective velocities, a mixture of discrete and continuous variables, and all of this in a complex system of hundreds of partial differential equations. Before interpreting the implications of model results on Microcystis vertical distributions in the field, we must understand how the model behaves numerically. To this end, we investigate the accuracy and stability of various numerical schemes in Chapter 3. We offer estimations of numerical dispersion for an upwind scheme with temporally oscillating velocity fields. The numerical dispersion of the first-order upwind is then compared to a quadratic upwinding scheme with a flux-limiter, switching between first-order and quadratic upwind to improve accuracy while retaining stability. Finally, we draw conclusions on the impacts of various model features on numerical accuracy and stability. Namely, while first-order upwind schemes underestimate peak concentrations and overestimate concentration pulse widths in advection-dominated flow regimes, upwinding still predicts peak location and time to large colony appearance accurately in all flow regimes. Thus, due to the robustness and simplicity of the scheme, first-order upwind is an appropriate numerical scheme for the transport and aggregation of harmful algae. Once the numerics are clearly understood, Chapter 4 investigates model performance for predicting Microcystis vertical distributions in a field environment. Results indicate Smoluchowski aggregation qualitatively describes the colony dynamics of M. aeruginosa. Further, the model demonstrates wind-induced mixing is the dominant aggregation process and the rate of aggregation is inversely proportional to algal concentration. Essentially, a large wind event and high algal concentrations are necessary and sufficient conditions for the rapid aggregation of M. aeruginosa. Because blooms of Microcystis typically consist of large colonies, both of these findings have direct consequences to harmful algal bloom formation. While the theoretical framework outlined in this chapter was derived for M. aeruginosa, both motility and colony formation are common among bloom-forming algae. As such, this coupling of vertical transport and colony dynamics is a useful step for improving forecasts of surface harmful algal blooms, which is the guiding motivation of this thesis. Using a combination of data-driven and mechanistic models, the work described herein can help stakeholders assess the risk of toxic HAB formation, thereby mitigating the adverse environmental and public health impacts.