Marine microalgae: an address plaque for calcium

A biochemical mechanism controls which nanostructures are formed in calcite-forming microorganisms

Microalgae, mussels, sea urchins and starfish are the master builders of the nanoworld: They create elaborate structures using only calcium, proteins and sugars as building blocks. Scientists at the Potsdam-based Max Planck Institute of Molecular Plant Physiology and the Max Planck Institute of Colloids and Interfaces have now discovered a key mechanism by which marine microalgae produce their filigree structures. The findings could also be relevant to other products of biomineralization, for example bones and teeth, and could even lead to technological applications.

Microalgae known as coccolithophores create designs for mass production. Although even the heaviest specimen of these marine creatures weighs less than a billionth of a gram, single-celled microalgae produce up to 500 million tonnes of calcium carbonate, or chalk, globally. That corresponds to around one third of the world’s annual steel production. They build the crystalline chalk, known as calcite, into filigree structures that look as if they were designed on a drawing board. Nevertheless, these architectural wonders are not the outcome of deliberation but the product of biochemical processes within the microorganisms. “We’ve discovered a biochemical mechanism that results in the crystals being formed precisely where they’re needed,” says André Scheffel, researcher at the Max Planck Institute of Molecular Plant Physiology and head of the recent study.

The research team, which also included scientists from the Max Planck Institute of Colloids and Interfaces and the German Research Centre for Geosciences in Potsdam, studied the marine microalga Pleurochrysis carterae. P. carterae is a member of the coccolithophorids, a group of single-celled marine algae whose name is derived from the calcite scales, called coccoliths, on their surface. The tiny calcite scales have a base plate consisting of organic material, mainly cellulose fibres, which is surrounded by a raised edge, like a pastry case. Two different forms of calcite crystals are arranged in an alternating order along the edge and only along the edge. The coccoliths are formed inside the cell in a special vesicle, a membrane-enclosed space. The finished coccoliths are then transported out of the cell and are integrated in the coccolith armour that surrounds each algal cell.

Polysaccharides transport calcium but no calcium carbonate

“Until now it was unclear how the regular structure is formed within the special membrane-enclosed space,” says André Scheffel. “It was previously thought that the chemical structure of the base plate determined that the calcite crystals would form only along the edge of the coccoliths,” says Scheffel. The Potsdam researchers have now refuted this assumption with experiments in a test tube. First they dissolved the crystals of isolated coccoliths and separated the released organic material in the base plate as well as the soluble polysaccharides and proteins. They then offered various ions – calcium, carbonate and other metal ions – to the base plate with and without the soluble organic components.

The experiments showed that no crystals formed on the base plate in the absence of the organic components. “Interestingly, the soluble negatively charged polysaccharides are essential for the calcite crystals to form at the right location,” André Scheffel explains. However, they only transport positively charged calcium – but no calcium carbonate – to the end of the base plate, where they settle along with the calcium in the form of small clumps. Thus, the proteins are not involved in the navigation, despite the fact that earlier investigations had shown that the soluble constituents are integrated during the crystallization of calcium carbonate. However, it was not known what role polysaccharides and proteins play in the process.

The researchers also found that precise localization works only with calcium. The polysaccharides distribute metal ions such as magnesium and sodium on the base plate indiscriminately, if at all. It’s as if a logistics company were only able to deliver a cargo reliably to the right address because the cargo itself helps with the navigation.

Ideas for nanotechnology

“Our findings are probably relevant not only to P. carterae but to all organisms that process calcium: other microalgae, mussels and sea urchins,” André Scheffel says. “The mechanism may even play a role in biomineralization in general, for example in the formation of teeth and bones.” In these cases, too, it is still unclear why mineral crystals form exactly where they belong.

The researchers now want to investigate whether soluble biomolecules such as polysaccharides also play a role in ensuring that crystals are placed at the right locations in other organisms that form biominerals. In addition, they would like to find out which components of the base plate and the soluble polysaccharides interact with calcium to ensure that the calcium is precisely localized. It also remains to be explained how P. carterae forms calcite crystals with two different forms from the clumps of calcium and sugars.

One reason that André Scheffel and his colleagues want to answer these questions is that marine microalgae bind large quantities of carbon dioxide along with the calcite and are therefore of enormous ecological importance. Moreover, knowledge of how marine animals build calcite structures could also have technological applications: Nature’s tricks could provide materials scientists with ideas for producing tiny structures for nanotechnology.

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