Streamlining theory
[3] Two of the free-living bacterial taxa with the smallest genomes are Prochlorococcus and Pelagibacter ubique,[4][5] both highly abundant marine bacteria commonly found in oligotrophic regions.
[8] This selective pressure is stronger in large marine prokaryotic populations, because intra-species competition favours fast, efficient and inexpensive replication.
[2] This may explain why genome streamlining seems to be particularly prevalent in prokaryotic organisms, as they tend to have larger population sizes than eukaryotes.
[9] It has also been proposed that having a smaller genome can help minimize overall cell size, which increases a prokaryotes surface-area to volume ratio.
[4] P. ubique has complete biosynthetic pathways and all necessary enzymes for the synthesis of 20 amino acids and only lack a few cofactors despite the genome's small size.
The genome size for this microorganism is achieved by lack of, "pseudogenes, introns, transposons, extrachromosomal elements, or inteins".
The genome also contains fewer paralogs compared to other members of the same clade and the shortest intergenic spacers for any living cell.
[14] As a cyanobacteria, they have an incredible ability to adapt to environments with very poor nutrient availability, as they maintain their energy from light.
[15] Nitrogen-fixing marine cyanobacteria are known to support oxygen production in oceans by fixing inorganic nitrogen using the enzyme nitrogenase.
[17] Furthermore, anabolic enzymes needed for creating amino acids such as valine, leucine and isoleucine are missing, as well as some which lead to phenylalanine, tyrosine and tryptophan biosynthesis.
[17] Bacterial symbionts, commensals, parasites, and pathogens often have even smaller genomes and fewer genes than free-living organisms, and non-pathogenic bacteria.
[18] Pathogenic organisms are thought to undergo genome reduction due to genetic drift, rather than purifying selection.
[1] In this case, DNA mutations happen by chance, and thus often lead to maladaptive genome degradation and lower overall fitness.
