Cyanobacterial harmful algal bloom (CHABs) has been a great concern due to the detrimental effects on ecosystem, animal and human health. Overgrowth of CHABs causes hypoxia and some species produce unpleasant odor compounds. More importantly, multiple toxins (cyanotoxins) produced by CHABs can be harmful to animal and human health by damaging internal organs, including liver, kidney, reproductive and neurological systems. Water-mediated activities are believed to be the common exposure routes for human to these toxins. Many studies reported that these toxins can be accumulated into food or dietary supplements via contaminated water, since the guideline for agriculture and aquaculture water use is not established yet. Therefore, strategies for controlling both CHABs and their toxins are important and imperative for protecting environmental and food safety. Chapter 1 summarized the literature review about CHABs and their toxins, and identified knowledge gaps. Furthermore, previous research regarding to cyanophages and their ecological roles are also reviewed because understanding of cyanophage, natural predators of cyanobacteria, is critical to understanding the bloom formation and evolution.
Among the cyanotoxins, the most abundant cyanotoxin is microcystin (MC), which is a seven-amino-acid peptide, consisting of more than 150 congeners. MC shows toxicity via inhibiting protein phosphatase 1 and 2A, and generate reactive oxygen species (ROS), leading to cell apoptosis in the involved organs. MCs are chemically stable against sunlight, extreme pH and boiling, therefore, it is difficult to treat the contaminated water once MCs are released. Physical, chemical and biological methods have been applied to treat MCs in water, but few of them can be successfully applied for MC- contaminated food. New technologies for MC treatment are proposed in Chapter 2. The objective of Chapter 2 was to develop sustainable and non-chemical-based methods for controlling MCs: 1) ultraviolet with TiO2, which is a common food additive as well as a photocatalyst; and 2) cold plasma, which is a non-thermal treatment using ionized gas. Natural MCs were extracted from Microcystis aeruginosa and treated with several combinations: 1) UV at intensity of 1470 µW/cm2 [high] or 180 µW/cm2 [low]; 2) cold plasma; and 3) no treatment in a dark room (control). To determine synergistic effects, nanoparticles (TiO2) were coated on the outside of the UV treatment chamber prior to irradiation. The MC degradation efficiency was enhanced by the reusable TiO2 coating at lower UV intensity by 10 percent, but no significant difference was observed at higher intensity of UV. Cold plasma removed MCs rapidly (80% and 92% in 1 and 2 hours, respectively) under experimental conditions, indicating that it can be easily and practically used in household and industrial setting.
In environmental settings, controlling CHABs formation is a better and effective way to reduce toxin production. Many previous studies have focused on environmental factors on blooms, but the role of cyanophage (viruses whose hosts are cyanobacteria) in the ecology of toxin production is much less understood, especially in freshwater. Cyanophage are known to contribute to biological controlling of CHABs by influencing the metabolism and evolution of cyanobacteria. Therefore, the information on the characteristics of cyanophage infection is crucial in potential bloom control. In Chapter 3, findings about cyanophage isolated from Lake Erie during bloom season were reported. The objectives of this study are to 1) isolate and characterize cyanophages from Lake Erie; and 2) examine the host-cyanophage interactions using multiple tools, especially atomic force microscopy (AFM). Cyanophage isolates were inoculated into toxin-producing Microcystis aeruginosa (host) originated from Lake Erie. The dynamic interactions between phages and hosts were monitored using spectrophotometer, fluorimeter, quantitative PCR and AFM. The structural and genetic characters of the cyanophages were identified by using transmission electron microscope (TEM) and PCR. The Podoviridae ~300 nm in size were infectious against the toxic strain of M. aeruginosa. The growth and photosynthesis capacity of hosts was significantly inhibited after cyanophage infection and cellular damages on membranes over time were clearly observed. We also found that UV irradiation with proper dose can induce the cyanophage shifting from their lysogenic stage to lytic stage. The psbA (photosynthesis core protein A) and the gp58 (special capsid protein) genes were identified, which showed high similarities of the same genes identified in marine cyanophages, indicating a close evolutionary root.
In summary, this thesis reports comprehensive knowledge about CHABs and their toxins. In addition, emerging techniques were applied in this study to provide informative and essential basis to develop controlling strategies in various settings when toxins are present.