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.
See
also…
Chalinid Dermosponges are among the hardest Sponges to classify
taxonomically due to their simple anatomies and variable morphologies.
They are encrusting Sponges with skeletons made up of...
Theonellid Sponges are predominantly deepwater Sponges found across the
globe, with a rigid skeleton made up of interlocking silica spicules.
They are noted for the production of an array of unusual chemicals, the
majority of which are thought to be produced by...
Unlike most Sponges (Porifera), which feed by filter feeding water
pumped through their bodies, Carnivorous Sponges (Cladorhizidae) feed by
capturing Crustaceans and other small animals on hooked spicules on
filaments, then digesting them externally. The group are predominantly
found in deep water, where carnivory is presumed to be a better feeding
strategy than filter...
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