Giant bacteria are intriguing, underexplored biological enigmas. Many well-studied giants use abundant internal small-molecule stores and/or light to satisfy their oversized energy demands. However, the ways giant heterotrophs (organisms that cannot produce its own food, instead taking nutrition from other sources of organic carbon) fulfill their expanded needs remain elusive. Bacteria of the genus Epulopiscium are intestinal symbionts of tropical marine surgeonfish that are exceptional and unique in the bacterial world. They are the largest known heterotrophic bacteria — a large cigar-shaped individual is a million times the volume of Escherichia coli. To determine how Epulopiscium bacteria fuel their robust metabolism, biologists generated a high-quality draft genome of Epulopiscium viviparus and reconstructed its metabolic potential using a comprehensive approach.
“This incredible giant bacterium is unique and interesting in so many ways: its enormous size, its mode of reproduction, the methods by which it meets its metabolic needs and more,” said Cornell University Professor Esther Angert, senior authors of the study.
“Revealing the genomic potential of this organism just kind of blew our minds.”
First discovered in 1985, Epulopiscium bacteria live symbiotically within the intestinal tracts of surgeonfish in the family Acanthuridae in tropical marine coral reef environments, such as the Great Barrier Reef and in the Red Sea.
Because of its gargantuan size, scientists initially believed it was some distinct type of protozoan.
“Studying these giant bacteria requires capturing the fish in which they live and preserving the cells or extracting DNA and RNA as quickly and carefully as possible,” Professor Angert said.
Professor Angert and her colleagues were especially interested to learn how Epulopiscium viviparus fuels its extreme metabolic needs.
Bacteria that feed off nutrients in their environment, rather than creating their own energy from…
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