Algae-based biofuels hold the promise of a green, sustainable energy supply and scientists at the University of Pennsylvania are looking at an odd ally to help make this a reality – giant clams. A coral reef resident may seem like a strange source to make biofuels practical, but the characteristic iridescent blue muscle tissues that the giant clam shows when it's open is giving researchers clues on how to produce algae more efficiently.

These massive bivalve molluscs of the family Tridacnidae that weigh over 200 kg (440 lb) and can reach 47 in (120 cm) across like living in the coral reefs of Australia and the islands of the western tropical Pacific. There they sit with their shells gaping open. In such shallow, clear water the sun isn't filtered very much and the clams should be bleached and killed by all that light – not to mention that there isn't enough food to feed such large animals.

The reason they aren't dead is that the clams are home to a brown, single-cell algae of the genus Symbiodinium with which they have a photosymbiotic relationship. In other words, the algae help protect the clams by absorbing light, then providing their host with supplemental nutrition.

The clam's contribution to this partnership is also what gives its mantle its distinctive blue iridescent appearance. The mantle is made up of iridocytes, which are specialized cells where the algae grow in microscopic pillars about 2 cm (0.8 in) deep made up of around 300 algae cells. The iridocytes channel yellow and green light deep into the tissues, spreading it out and to each individual algae cell, with the leftover blue light giving the mantle its sparkly color. For clam and algae, this is a win-win situation.

How this fits into biofuels is through the problem of scalability that has caused more than one biofuel project to falter as it comes up against a common barrier. On the lab bench, algae seems like the perfect way to turn sunlight into fuel. Single organisms are very efficient converters of light into chemicals and they reproduce at an incredible rate. The snag is that when this idea goes beyond the lab to full-scale production, the obstacles mount – not the least of which is that unless enough sunlight can get to each cell, the whole system breaks down.

Using the clams as a model, Alison Sweeney and Shu Yang of the University of Pennsylvania are looking for ways to create a material that does what the iridocytes do. They did this by synthesizing nanoparticles and mixing them with an emulsion of water, oil, and soapy molecules that act as surfactants to lower the surface tension between the two normally unmixable liquids. By shaking the mixture, the nanoparticles help produce microbeads that mimic the iridocytes. The tricky bit is shaking it enough at the right speed to make the microbeads the right size.

"It's very efficient, and it's very difficult to achieve," says Yang. "People are trying to do this by designing nanoparticles, but you need to do a lot of synthesis and find ways to precisely control their size, shape and optical properties, which becomes complicated and expensive. Our method is both simple and inexpensive and at the same time achieves better results than all these other systems."

The next step will be to find a way to get the algae to grow in gel pillars and from there to get them to convert light to fuel with the same efficiency as the giant clam. If that can be achieved, the researchers see the technology as being used not only in biofuel production, but also in solar panels to heat and cool buildings while storing energy for later use.

The results of the research was published in Advanced Materials.

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