Question

A classic experiment involving a phototrophic organism demonstrates how rapidly microbial evolution can occur within a population of cells. What attributes of the metabolism of this model organism were important for the study and what did the experiment show?

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Answer 1

The growth rate of microbes depend upon the composition of the medium. The growth rate will be maximum if the medium is provided with a better energy source and more of the biosynthetic intermediates that the cell have to make for itself. Bacteria need nutrients to provide energy for their metabolic processes and they use the carbon, nitrogen, phosphorus or sulfur from their nutrient source to produce energy. During cellular respiration the elements are broken down and a coenzyme called adenosine triphosphate. The function of this coenzyme is to transport the chemical energy within the cell. Other bacteria may get their energy from sunlight by metabolic methods. An experiment was performed using concentrated growth media which contains readily available carbon, nitrogen, sulfur and phosphorus. Many vitamins and minerals are also added in the medium. The experiments shows the following:

Phototrophic cell growth requires a source of electrons to replace the electrons that are consumed during biosynthetic reactions. Photosynthetic organisms like eukaryotic algae and the cyanobacteria have two photoreaction centres where it contains the pigment chlorophyll a. Water can be the electron source in reactions that generate oxygen. Their photosynthetic apparatus have light-absorbing pigments that can capture light energy and transfer it to the reaction centres. Some photosynthetic bacteria have a single type of reaction centre with a pigment called bacteriochlorophyll. These organisms do not release oxygen and need an electron donor. The electron is transported through an electron transport chain, giving out energy by formation of a proton gradient and ATP synthesis. The electron is returned to the chlorophyll and this cyclic reaction is the source of energy needs of the cell. The Calvin cycle of carbon dioxide are activated, and electrons are transferred to the cofactor NADP to form NADPH in order to grow the cell. During the process of evolution, some bacteria might have lost genes that encode certain biosynthetic reactions and are hence likely to require nutritional supplements.