A Shell Game Exposed 
by Andrew Kerr
May 2008

If I were to drop a heaping spoonful of diatoms into the palm of your hand your first reaction would likely be disgust. From our relatively lofty perspective, diatoms are literally scum. But put that scum under a microscope and, like the teen movie bookworm who magically becomes a beauty when she removes her glasses, their gorgeousness becomes readily apparent. Diatoms are housed in ornate and intricate silica shells which, like snowflakes, come in a dazzling array of shapes (there are about 100,000 species and each has its own shell shape). Diatoms have already made friends in engineering; the sheer variety of shell shapes means one can fish around for one that might address a particular nanotechnological need. For example, if you need a microscopic tube and one species of diatom builds just such a structure, then you're in luck. We wrote about this a while back.

Turns out diatoms have more tricks up their sleeves. Ellery Ingall, of Georgia Tech's School of Earth and Atmospheric Science, along with PhD candidate Julia Diaz, have found that diatoms in the ocean are chowing down on an element (and nutrient) called phosphorous in order to make an under-recognized, phosphorus-rich molecule called polyphosphate. Scientists have long scratched their heads over the discrepency between the amount of phosphorous pouring from rivers into the ocean and the amount of phosphorous actually detected in the ocean. Now the (ahem) shell game is over. Diatoms seem to play a critical role in the Case of the Missing Phosphorous.

Ingall and Diaz found that polyphosphates derived from diatoms are transported to marine sediments. Once there, polyphosphate transforms into a common phosphorus mineral called apatite. Until now, scientists were not certain where most of the world's apatite came from. This discovery seems to go a long ways towards explaining the origins of those minerals.

One might fairly ask, "So what?" Since half the photosynthesis conducted on the planet is performed by diatoms, and diatoms comprise 1% of the earth's biomass, anything we discover these organisms doing likely has a signficant impact on, well, everything.

Phosphorous is an essential building-block of life. This is why phosphates are a key component in fertilizers. But if diatoms are storing phosphorus as polyphosphate, and then polyphosphate is transforming into the mineral apatite (which is not so easily tapped as a nutritional source by other life), then that lowers the amount of available phosphorous for other marine life to consume. That could significantly lower populations of other lifeforms and thus have an impact on levels of atmospheric gases (since organisms produce those). To put it more succinctly, Ingall's and Diaz’s discovery may increase our understanding of global climate.

Our world actually faces a phosphorous shortage. Some scientists fear that the phosphate supply is going to dip to worryingly low levels in just the next few decades. Since we need the stuff in fertilizers, that would be very bad. As Dana Cordell of the Institute of Sustainable Futures tersely notes in this article from The Austalian: "Quite simply, without phosphorus we cannot produce food."

Ingall told your correspondent that diatoms will not likely be the source of our phosphate salvation. "The downside of diatoms as a source of phosphate for fertilizer is that in addition to the polyphosphate granules there is a whole lot of other solid material in a diatom which essentially dilute the phosphorus," he says.

However, another area Ingall and his team plan to explore is the role types of bacteria play in the polyphosphate formation process. Since bacteria don't grow the pretty, hard shells that diatoms do, the polyphosphates there may be more readily accessible. "This may indeed represent a way of concentrating phosphorus for use as fertilizer," he says.