Metabolic Versatility

Purple nonsulfur bacteria normally inhabit muddy lake bottoms and sewage lagoons.They have a remarkable ability to adapt metabolically in response to changes in oxygen tension，light intensity，and available nutrients in their environment.Such changes are likely to occur in the mud of ponds and ditches；in the littoral zones of lakes，rivers，and seas；and in all kinds of sewage Iagoons（Imhoff et al.1984）.

The multiple modes of growth seem to parallel the evolution of the earth's atmosphere from an initial reducing environment of hydrogen，ammonia，and methane through the synthesis of organic acids and depletion of ferrous iron in the oceans，to the development of an oxidizing environment through the action of the cyanobacteria.Typical purple nonsulfur bacteria，R.capsulatus and R.sphaeroides，are capable of six distinct modes of growth；anaerobic photoautotrophy growing on hydrogen and carbon dioxide under light；anaerobic photoheterotrophy in the light using organic compounds and carbon dioxide；anaerobic chemoheterotrophy in the dark，utilizing a variety of organic carbon sources plus a terminal electron acceptor such as DMSO；dark aerobic chemoautotrophy，using hydrogen as the sole source of energy and reducing power；aerobic chemoheterotrophy growing on a diverse group of carbon compounds and generating ATP through oxidative phosphorylation；and anaerobic dark fermentation in the absence of oxygen，utilizing substrate-level phosphorylation.Among the various environmental variables，oxygen tension and light intensity，which change frequently in the vertical ecological zone，are the most important in regulating these metabolic shifts（Drews 1978；Kaplan 1978；Drews and Oelze 1981；Kaplan and Arntzen 1982；Chory et al.1984；Drews et al.1986；Youvan and Marrs 1984 and 1985）.(https://www.daowen.com)

When growing chemoheterotrophically，purple nonsulfur bacteria contain a typical Gram-negative outer membrane and a cytoplasmic membrane.Under these conditions cell growth is supported by an aerobic respiratory chain with components structurally and functionally similar to those found in mitochondria（Zannoni and Baccarini-Melandri 1980）.Upon the removal of oxygen，the cell develops an extensive intracytoplasmic membrane（ICM）that comprises the photosynthetic apparatus.The amount of ICM per cell and the composition of the ICM are functions of the light intensity（Drews 1978 and 1986；Kaplan 1978）.Increase in the light intensity depresses the formation of the photosynthetic apparatus.The quick response of facultative phototrophic bacteria to environmental changes provides an attractive system for studying the regulation of the expression of photosynthetic genes by oxygen and light.