Although the chemical form of nutrients varies widely, all organisms require six bioelements: carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur to survive, grow, and reproduce.

Nutrients are categorized by the amount required (macronutrients or micronutrients), by chemical structure (organic or inorganic), and by their importance to the organism's survival (essential or nonessential).

Microorganisms are classified both by the chemical form of their nutrients and the energy sources they utilize.

Nutrient requirements of microorganisms determine their respective niches in the food webs of major ecosystems.

Nutrients are transported into microorganisms by two kinds of processes: those requiring expenditure of energy and those which occur passively.

The molecular size and concentration of a nutrient determines which method of transport is used.

Environmental Factors That Influence Microbes
The environmental factors that control microbial growth are temperature, pH, moisture, radiation, gases, and other microorganisms.

Environmental factors control microbial growth by their influence on microbial enzymes.

Microorganisms are classified by their temperature requirements as psychrophiles, mesophiles, or thermophiles.

Three cardinal temperatures for a microorganism describe its temperature range and the temperature at which it grows best. These are the minimum temperature, the maximum temperature, and the optimum temperature.

Most eucaryote microorganisms are aerobic, but bacteria vary widely in their oxygen requirements from faculative to anaerobic.

Microorganisms live in symbiosis with other species that range from mutually beneficial to parasitic.

The Study of Microbial Growth
Microbial growth refers both to increase in cell size and increase in numbers of cells in a population.

The generation time is a measure of the growth rate of a microbial species. It varies in length according to environmental conditions.