Nature Blog Network

Saturday, July 21, 2007

Porifera Double Whammy! Huge Silicate Spicules (or are you just happy to see me?) and the Evolution of Calcification!

Craig at Deep Sea News posted on new research about a deep sea sponge, Monorhaphis chuni (Hexactinellida), with the world's largest known biosilica structure! This is a silcate spicule that can grow up to 3 meters long. Thats at least a meter longer than you!

Picture is copyright Emily S. Damstra and used by permission.

I don't really know how to construe to enormity of that structure. I could make it so you have to scroll down this post 3 meters, but that would just be annoying. The study Craig is referring to on DSN is by Müller et al., published in the most recent issue of Cell and Tissue Research. The authors carefully studied the formation of these giganto-spicules and helped along the way with silicatein-related proteins. Silica is not a common element in the ocean, though rare at the surface it increases in concentration as you go deeper by about 10-fold. The ability scavenge this rare element and incorporate into a biostructure in itself is a feat, and an expensive one at that! Now multiply that over time to about 3 meters...

This study is an excellent exercise in integrative biology. It merges biochemistry, histology, genetics, morphology and systematics. They determined there were different chemical layers to the spicules, including collagen and the silicateins (potentially a first for the Hexactinellida). The conclusion:
"Based on the data gathered here, we suggest that, in the Hexactinellida, the growth of the spicules is mediated by silicatein or by a silicatein-related protein, with the orientation of biosilica deposition being controlled by lectin and collagen."-Müller et al.
Keeping with the theme of sponge skeletons, but moving away from silica-based to carbonate-based, a study by Jackson et al. in June 29 issue of Science used an approach called Paleogenomics to determine the role of precursor alpha-Carbonic Anhydrases (a-CA's) in calcareous skeleton formation. Paleogenomics uses modern techniques, such as gene and protein expression and phylogenetics, on extant organisms in combination with knowledge of their evolutionary history. a-CA's have evolved through several gene duplication events in the Metazoa for a variety of physiological purposes:
"The chemical reaction [CO2 + H2O ⇆ HCO3− + H+] functions in processing metabolic wastes, regulating pH, fixing carbon, and transporting ions across organic membranes. The metalloenzyme carbonic anhydrase is pivotal to these processes by catalyzing this reaction approximately 1 million fold."-Jackson et al.
"(D-F) Spherulite calcification begins with an ovoid structure, with subsequent stages increasing in size. Scale bars, 5 mm." Reproduced from Jackson et al.

A very important enzyme with a diverse set of functions cascading down throughout the Metazoa. The aim of this paper, in my opinion was to see what a-CA's looked like in the last common ancestor to the Metazoa (LCAM). Sponge genomes are great to look at for these types of questions because of their basal position on the animal tree of life. They determined the a-CA enzyme is used in biocalcification of the Demosponges. The sponges (and presumably biocalcification) radiated in the Cambrian Explosion, 520-540- million years ago. The LCAM most likely used the a-CA enzyme for a similar purpose. And in fact we see this feature, biocalcification, presevered in several protostome and deuterostome taxa. As with any well written Science paper, there is a succinct final paragraph concluding their results:
"From our data we infer that a core molecular toolkit capable of catalyzing the production of HCO3− (and ultimately CaCO3) was present in the first metazoans and included an a-CA. Subsequently, various metazoan lineages inherited this toolkit and have added to and elaborated upon its key elements to guide, enhance, and inhibit the deposition of CaCO3 in the spectacular variety of ways we see today."-Jackson et al.

Two well-written sponge papers with important evolutionary conclusions. What more could you ask for?

See also a perspective written by Taylor et al. on how sponges are providing insights into animal evolution.

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