Bacteria symbionts

Associations between endosymbiotic bacteria and the Homoptera, Blattaria, and Coleoptera are common. One of the best known is that between Biicli-nera and the aphids (149,150). Both partners are obligate and mutualistic symbionts, and the aphids cannot survive without the bacteria (150). Buclmera, in fact. 

An inventory of known biomacromolecules is provided in Table 22.3. Many of these play essential metabolic roles in enabling growth and reproduction, such as the carbohydrates, lipids, proteins, and polynucleotides. Others are components of cell walls and exoskeletons. Some organisms, such as bacteria, plankton, plants, and lower invertebrates, synthesize biomolecules, called secondary metabolites, that are used to control ecological relationships, including predator/prey, host/symbiont, mating/spawning, and competition for food or space. 

Diazotrophs Nitrogen-fixing bacteria. In the marine environment, some nitrogen fixers live as symbionts in phytoplankton, particularly diatoms. 

In symbiosis with Fabales, bacteria live as bacteroids in root nodules inside the plant cells. The plant supplies the bacteroids with nutrients, but it also benefits from the fixed nitrogen that the symbionts make available. 

It is now known that the initial interaction between plants and bacteria of the Rhizobiaceae is a chemical detection by the microbe of a susceptible host, i.e., the host produces compounds which act as signals for the microbial pathogen or symbiont. The microbe responds to these signals by expression of genes necessary in subsequent stages of the interaction. For a few of the Rhizobiaceae some signal compounds involved have been identified (1-7). 

Only certain prokaryotes can fix atmospheric nitrogen. These include the cyanobacteria of soils and fresh and salt waters, other kinds of free-living soil bacteria such as Azotobacter species, and the nitrogen-fixing bacteria that live as symbionts in the root nodules of leguminous plants. The first important product of nitrogen fixation is ammonia, which can be used by all organisms either directly or after its conversion to other soluble compounds such as nitrites, nitrates, or amino acids. 

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