Sponges (Porifera) are generally considered to be the oldest extant animal group, with a fossil record that extends considerably into the Precambrian; phylogenomic analysis suggests they are the sister group to all other animals, which also suggests an early origin for the group.
Calcified tissues appear suddenly in Sponge-like fossils in the late Ediacaran Period then in a variety of metazoan groups (multi-cellular Animals) at the start of the Cambrian, an event widely known as the Cambrian Explosion. This is generally considered a major step forward in animal evolution, as mineralized body parts are extremely useful both in defending against predation and penetrating the defences of potential prey, and serve as excellent support for muscles, although many modern sea creatures are still able to survive and thrive without any mineralized tissues.
Many, perhaps all, sponges contain colonies of symbiotic bacteria, and it has been suggested that this relationship may also date back to the Precambrian. This makes sponges good subjects for scientists studying the origins of symbiotic relationships between Animals and Bacteria.
Sponges can secrete skeletal elements (spicules) of either calcium carbonate (Class Calcarea) or silica (Classes Hexactinellida and Dermospongeiae),with one group in Class Dermospongeiae, the Sclerosponges, capable of producing both. It has previously been suggested that Coraline (calcium carbonate secreting) Dermosponges may have gained the ability to biomineralizecalcium carbonate by horizontal gene transfer from a Bacterial donor.
In a paper published in the journal Evolution in October 2012, Maria Uriz and Gemma Agell of the Centre d’Estudis Avançats de Blanes, Andrea Blanquer, also of the Agell of the Centre d’Estudis Avançats de Blanes, as well as the Observatoire Océanologique at Université Paris-Sorbonne, and Xavier Turon and Emilio Casamayor, again of the Centre d’Estudis Avançats de Blanes, discuss the discovery of endosymbiotic calcifying Bacteria in non-calcifying Dermosponges of the genus Hemimycale, and the implications of this for the origins of calcification in metazoans.
Uriz et al.collected specimens of Hemimycale from northeast Spain, the Mediterranean, the Red Sea and East Africa. These were dried in a stove at 80°C for 48 hours then weighed, then placed in a second oven at 600°C to remove the organic portion, leaving silica and calcium carbonate. Finally the samples were boiled in nitric acid to remove the calcium carbonate, before being washed and redried to give the weight of the silica spicules. Samples were also subjected to Scanning Electron and Transmission Electron Microscopy, Energy-Dispersive X-ray Analysis and in situ DNA hybridization with bacterial and archaeal probes.
The Calcibacteria harbouring Sponge, Hemimycale columella, from the Atlantic and Mediterranean. Uriz et al. (2014).
The sponges were found to contain 30-60% calcium carbonate (dry weight). They contained numerous Coccoid Calcibacteria within vacuoles in specialist cells (Calcibacteriocytes). These Calcibacteria secreted calcareous coatings as they matured, which built up within the calcibacteriocytes. As this happened the calcibacteriocytes migrated to the surface of the sponge, eventually lysing at the surface to contribute to a Calcibacterial coating.
During the sponges reproductive cycle maternal calcibacteriocytes surrounded the embryo, and were then phagocytised, releasing the Bacteria into the embryo’s mesohyl, where they were absorbed by embryonic proto-calcibacteriocytes.
The Calcibacteria harbouring Sponge, Hemimycale sp., from the Indo-Pacific region. Uriz et al. (2014).
Hemimycale hosts a low diversity of bacteria compared to many sponges, but these bacteria make up about 60% of the dry bodyweight, compared to about 38% by volume for most higher diversity sponges. This is not a direct comparison and highlights a need for further study in the field.
Work is underway to identify the Calcibacteria found; 16S rRNA gene tag-pyrosequencing suggests that 65% of the bacteria present within Hemimycale are Alphaproteobacteria, though it does not confirm that these are the Calcibacteria. Mitochondria are modified Alphaproteobacteria, and many symbiotic bacteria found in other invertebrates belong to this group, making them good candidates.
Dividing Calcibacteria entrapped within the calcareous coat (SEM); scale bar is
600 nm. Uriz et al. (2014).
Calcarious nodules have been reported in Sponges before, but this is the first time that endosymbiotic Calcifying Bacteria have been reported; Uriz et al. suggest that this trait might be quite widespread, but that it may be widely overlooked by the practice of treating glass sponges with nitric acid to release the silica spicules.
Uriz et al. consider that the Calcibacteria appear to be fully adapted to a symbiotic lifestyle; they lack normal bacterial cell walls, live within specialized cells within the host, and do not contain much cytoplasmic material of their own. The relationship appears to be a reasonably old one, since the bacteria are found in all members of the genus studied, in widely varying ecological conditions, and there is a well-developed mechanism for propagating the bacteria from one generation to the next, and the relationship appears to be beneficial to the sponge; very little eats Hemimycale, despite a poorly developed silica skeleton and no obvious chemical defences, and the sponges appear to grow faster than related species in similar environments.
High density of Calcibacteria released from broken calcibacteriocytes after squeezing the sponge (SEM); scale bar is 1 μm. Uriz et al. (2014).
Studies of the Coralline Demosponge Astrosclera willeyana suggest that the genes responsible for calcification in this sponge are fully integrated into the sponge genome, but that these genes appeared to be of prokaryotic (Bacterial) origin. Furthermore, since the same genetic pathway is used in the related Astrosclera queenslandica, which is thought to have shared a last common ancestor with Astrosclera willeyana in the Triassic, this horizontal gene transfer must have happened at least 265-220 million years ago.
Lynn Margulis’sendosymbiotic hypothesis suggests that eukaryotic cells evolved at least in part by the integration of symbiotic bacteria into the cells as organelles. This is widely accepted, though scientists vary in the degree to which they agree with the theory; most accept that mitochondria and chloroplasts have bacterial origins, but not all accept such an origin for other organelles. Uriz et al.’s findings suggest that the Calcibacteria within the cells of Hemimycale are currently evolving towards full integration as organelles.
It has previously been suggested that the development of mineralized tissues in animals was triggered not by a need to develop a skeleton to support the body, but by a need to remove (toxic) calcium ions from the cells. Under this scenario, biomineralization occurred initially as an excretory process, probably provoked by an abrupt rise in environmental calcium, then was co-opted for skeleton building later.
The presence of endosymbiotic calcifying bacteria in Hemimycale, combined with the probable prokaryotic origin of the calcifying genes in Astrosclera, suggest a possible scenario in which Calcifying Bacteria were co-opted by early metazoans, then eventually integrated completely into the animal genomes. Calcifying Bacteria are known to have developed long before the origin of multi-cellular life-forms, making it plausible that a variety of different animals may have co-opted Bacteria for this purpose, particularly if they all suffered a simultaneous rise in environmental calcium which needed to be overcome.
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