500 Million-Year-Old Symbiosis
June 29, 2011
Marine shallow water sandy bottoms on the surface appear desert-like and empty, but in the interstitial space between the sand grains, a diverse fauna flourishes. In addition to bacteria and protozoa, numerous animal phyla have been found here - some found only here. One of the strangest members of this interstitial fauna is Paracatenula, a several millimeters long, mouth- and gut-less flatworm, which is found in areas from tropical oceans to the Mediterranean. These worms are the focus of a research project led by Jörg Ott at the Department of Marine Biology of the Univ. of Vienna. The surprising results of this research have now been published in the Proceedings of the National Academy of Sciences (PNAS).
In the early 1970s, at the time of the discovery of Paracatenula, it was already a mystery how the worms acquire their food without a mouth and gut. The solution to this question came unexpectedly: At deep ocean hot vents, giant mouth-less tubeworms were found. These – like Paracatenula – live in symbiosis with intracellular bacteria that oxidize reduced sulfur compounds. The energy obtained in this chemical process is used by the symbiotes to fix inorganic carbon into biomass – just like plants do using sunlight. Because of the high productivity of the symbiotes, their hosts can derive all their nutrition from them.
Many animals of different phyla from several habitats have been found to live in such a symbiotic association. Compared to the great diversity of these hosts, the diversity of the microbial symbiotes was strictly limited to members of only two classes, the Gamma and Epsilon Proteobacteria.
One of the biggest surprises in the current study was that the symbiotes of Paracatenula are indeed sulfur-oxidizing bacteria, but they are Alpha-Proteobacteria. Several other important intracellular symbiotes come from this class, most notably the mitochondria, which are the crucial power plants in the cells of all higher organisms. The nitrogen-fixing root nodule bacteria of leguminous plants, as well as dangerous pathogens such as the causative agent of epidemic typhus, also belong to this class. In recent years, several studies have presented evidence that the mechanisms in symbiotic and pathogenic relationships are similar, or even identical. Future projects with Paracatenula and its symbiotes could give fundamental insights into the mechanisms that have allowed Alpha-Proteobacteria several times to establish an intracellular lifestyle independently.
Another captivating detail of the Paracatenula-Riegeria symbiosis is that the symbiotes that live in specialized cells known as Bacteriocytes account for up to 50 percent of the total tissue. That is significantly more than in all other known symbioses between animals and bacteria.
Based on genetic sequences of the symbiotes, the scientists have roughly extrapolated the age of the symbiosis – the estimated age of 500 million years makes this symbiosis the oldest known animal bacteria association.
Comparing the phylogenies of hosts and symbiotes, another subtle but nontrivial detail was uncovered – the worms have been passing on their symbiotes to their offspring in every generation, without any symbiote switches for the last 500 million years. How this secured symbiote transmission is accomplished is the focus of the current studies in Ott's lab.
Source: Univ. of Vienna