Microbial Ecology

Organisms are combined in a population. Populations are combined in a microbial community. The microbial communities of the flocs of activated sludge, the fixed biofilm of a biofilter, or soil microbial communities are used often in environmental biotechnology. Microorganisms live in natural or engineered habitats in which their growth is affected by interactions with populations of other microbes. There may be positive interactions (cooperation, commensalism, mutualism), negative interactions (competition, amensalism, predation, parasitism) or no interactions (neutralism) between the cells of a microbial population, between different microbial populations, or between microorganisms and macroorganisms. Bacteria called R-tactics are fast growing in a rich medium but can quickly die under a shortage of nutrients. Bacteria called L-tactics grow fast in a rich medium but form dormant forms like spores under a shortage of nutrients. Bacteria called K-tactics are adapted to grow slowly in media with low concentrations of nutrient. Aerobes are microorganisms that grow at the atmospheric pressure of oxygen. Microaerophiles prefer low concentrations of oxygen because they have oxygen-sensitive molecules. Obligate anaerobes are very sensitive to oxygen because they have no protection against highly reactive radicals of oxygen reduction. The maximum temperature for growth depends on the sensitivity of protein structure to temperature. The minimum temperature depends on the temperature of the lipid membrane melting. Psychrophiles have optimal temperatures below 15^oC. Mesophiles have optimal growth temperatures in the range between 20^oC and 40^oC. Thermophiles grow best above 50^oC. Microbes grow within the pH range of 6–8 (neutrophiles), at pH lower than 5 (acidophiles), and at pH higher than 9 (alkaliphiles). The intracellular pH is an approximately neutral pH. The extracellular pH influences the dissociation of the carboxylic, phosphate, and amino groups of the cell surface, thus changing its charge and adhesive properties. It is important for environmental biotechnology in the sedimentation of activated sludge and in the fixed biofilm process. A tutorial with solutions and a quiz bank are added to this chapter.