Population growth, dynamics, and blooms of bacterial, unicellular eukaryotes, and toxic algae are described in this book. Microbes are used to illustrate both exponential and logistic population growth. Microbes are also used to illustrate dynamics in other aspects of ecological systems, including nutrient cycling. The movement of nitrogen in ecological systems is largely affected by microbes, some of which have symbiotic relationships with legumes. The effects of the environment on the growth of microbes and the effects of the microbes on ecological systems are described in reference to nutrient cycles and harmful algal blooms. Populations of harmful algal can quickly grow and exceed carrying capacity, with resulting negative effects on other species, including humans.