New paper in Science Advances sheds light on biosilicification in sponges

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The new paper led by our team member Manuel Maldonado (and collaborators the Bedford Institute of Oceanography and the London Museum of Natural History) describes how the cooperation between passive and active silicon transporters clarifies the ecophysiology and evolution of biosilicification in sponges

You can access the paper Cooperation between passive and active silicon transporters clarifies the ecophysiology and evolution of biosilicification in sponges here.

Marine sponges are the first animals that existed on our planet and all other animals derived from their ancestors. Sponges are organisms with an extremely simple anatomy, which makes them unique, lacking the nervous system, sensory organs, muscles, digestive tract and other organs typical of the rest of animals. Within their bodies, sponges contain huge numbers of skeletal pieces, which are the anatomical equivalent of the bones of other animals. But again, sponges are also unique when it comes to their bones. Most animals build bones from calcareous material. Typical examples are the calcium carbonate skeletons of corals (which end up forming the reefs), the spines of the sea urchins and the clam shells; also the calcium phosphate bones of humans and other vertebrate animals. Most of the sponges, on the contrary, building their bones with silicon, elaborating with this chemical element a compound called silica and whose chemical formula is exactly the same as the glass of our windows. In other words, sponges literally make “glass bones”. This silica production happens through a process that is still poorly known but suspected to be of enormous biotechnological interest. A better understanding of this biological process will allow, for example, to imitate industrially the mechanisms developed by sponges to process isotopically purified silicon, which is a highly appreciated semiconductor material in microelectronics.

The authors of the study used the sophisticated underwater robot ROPOS to experiment with living glass sponges at the ocean floor. Alive sponges were also successfully taken to the laboratory on shore to characterize and model, for the first time, the kinetics of silicon consumption and skeletal growth in these deep-sea glass sponges. Importantly, through molecular techniques, it has been uncovered how silicon – which is naturally dissolved in seawater – is transported into the sponges to form the silica bones. A mechanism has been identified in which both passive channels for silicon in the cell membranes cooperate with transmembrane proteins for active silicon transport, a much more complex transport scheme than was anticipated for such simple animals. The features discovered in the silicon transport system have numerous implications for interpreting sponge evolution. The discoveries also explain why modern sponges are outcompeted by diatoms when using dissolved silicon, and why the sponge mechanisms to incorporate silicon have been unable to evolve and get better adapted in response to the intense competition with diatoms that started over 65 million years ago.

Even more relevant is the discovery that the passive silicon channels used by sponges seem to have been preserved throughout the evolution of animals, despite the fact that animals other than sponges are essentially calcifying but not silicifying. In fact, homologous genes to those of the sponge silicon transport channels have been found to remain operational even in humans, where they appear to supply the bone-forming cells with silicon during certain phases of bone growth. The discovery made in the sponges now allows to understand why the supply of silicon through diets and other experimental treatments helps to repair broken bones in vertebrates. This study, which was not initially applied research, provides a biological basis that will allow redesigning experimental therapies for bone regeneration in humans and other vertebrates, and even inspire treatments to alleviate bone growth deficiencies. From an evolutionary point of view, the study also raises the novel hypothesis that silicification and calcification are not, in fact, alternative skeletal processes, but rather biomineralization mechanisms that have been closely intertwined from the very origin of animals.