Growth, Motility and Metabolism of Harmful Cyanobacteria and Lipid-Producing Microalgae in Fluid Environments: From Laboratory to Field Study

Abstract

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.