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Ancient Symbiosis Found Between Animals and Bacteria
A marine flatworm discovered in the early 1970s, lacking both mouth and gut, led to a research project to understand how these little creatures acquire food. The unexpected results are detailed in Proceedings of the National Academy of Sciences (PNAS) on June 27.
The nematode worms belong to the genus Paracantenula, and inhabit the interstitial spaces between sand grains from tropical oceans to the Mediterranean. They are only several millimeters long.
So how do these tiny beings survive? They contain "chemoautotrophic sulfur-oxidizing bacteria," according to the research article, which are also found in six animal phyla and one archaeon—micro-organisms that resemble bacteria. Paracatenula belong to the phylum Nematoda.
In simple language, this means that several animal phyla share an interdependent relationship with certain bacteria inside their cells. The bacteria oxidize reduced sulfur compounds within the hosts’ cells to fix carbon into organic matter (food), rather like plants use solar energy to make sugars.
All the animals derive their energy completely from these intracellular bacteria. Despite the great diversity of these host animals, the known bacteria partners seem to belong to only two classes—Gamma and Epsilon Proteobacteria.
A major surprise in the study was that Paracatenula worms’ intracellular bacteria, known as “Riegeria” symbionts, are actually Alpha-Proteobacteria. Other important intracellular symbionts in this class include mitochondria, which power cells in higher organisms; nitrogen-fixing bacteria in legume root nodules; and pathogens such as Rickettsiae, the parasite that causes typhus.
{etRelated 58317}Evidence from several recent studies suggests symbiotic and pathogenic relationships may operate on similar or even identical mechanisms. Further research on Paracatenula and its Riegeria symbionts may uncover the underlying genetic factor that has enabled Alpha-Proteobacteria to successfully colonize eukaryotic cells.
The bacteria comprise between 33 and 50 percent of the body volume of the various worm species, which is “the highest proportion among all known endosymbiotic associations between bacteria and metazoans [all multi-celled animals except sponges]," according to the article.
Extrapolated data imply that the Paracatenula-Riegeria relationship originated more than 500 million years ago, making this the oldest known animal-bacteria symbiosis.
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