Encased in Glass

At the beginning of the Triassic period, a single celled eukaryotic predator ate a red algae. Instead of digesting this snack, the predator kept it’s passenger alive. In exchange, the red algae photosynthesized and fed the predator. This endosybiosis lead to the rise of the diatoms. This wasn’t the first endosymbiosis: the red algae itself was a eukaryotic predator a billion years before and consumed a Cyanobacterium to become the phototroph we know today. The secondary endosymbiosis of red algae by the diatom ancestor lead to the incorporation of many genes from the red algae it’s plastid into the diatoms genome, leading to the eventual dissapearence of the red algae nucleus and a reduction of the plastid genome. Eventually the Triassic predator with it’s red algal endosymbiont became a photosynthetic autotroph.

            Diatoms adapted to their new role as primary producers extremely well. Somewhere along the line, the diatom ancestor began taking the abundant silica dissolved in the oceans and encased itself in shells of glass, with numerous pores to let nutrients thru of course. . These glass walls are very diverse among the different diatoms and have a wide variety of intricate shapes and patterns. Even so, they all have the same basic structure. Two halves, called valves, one slightly smaller than the other fit snuggle together bound by girdle bands to form the cell wall called a frustule.

The silica cell walls are not only extremely strong, but very energetically inexpensive to make. Compared to organic cell walls the other phytoplankton make, a silica cell wall costs less than 1/10^th the energy to make. What happens when you are a phytoplankton with a cell wall that’s stronger and cheaper than your competitors? The diatoms have the answer: take over. By and large, they have. Diatoms fix about 20% of the earths carbon. When nutrients are not limiting, diatoms make up up to 70% microbial communities. When there are blooms of microbes on the ocean, chances are diatoms are the most numerous.

            How the diatoms make these cell walls is not easily explained or well understood. We do know that diatoms regulate and control the formation of their walls with extreme precision because each generation reproduces the same pattern as the previous generation with high fidelity. Scientists working with nanotechnology can only dream of being as proficient at making such precise silica structure, so they are exploiting the diatoms unique ability to do it for them. What is known about their cell walls is that they contain three main classes of proteins in their structure (silafins, long chain polyamines and silacidins) and are synthesized in a specialized vesicle. Different diatom species express these proteins in different amounts, leading to the hypothesis that the varying amounts of the proteins contribute strongly to cell wall structure.

            Because of the advantage of their cell walls and photosynthetic plastids, diatoms have spread over the world and now dominate many phytoplankton communities. This widespread prevalence has lead many higher marine organisms such as fish relying on diatoms for food. Diatoms have been shown to be a better food in terms of fisheries health than other phytoplankton communities. In many areas, silica or iron are limiting and diatoms only make up a small segment of the population. These areas include the dead zones in the ocean where only a small number of stingier bacteria can live. Seeding dead zones in the ocean has lead to huge diatom blooms, but unfortunately most of the carbon is eaten and leads to higher predator populations instead of fixed carbon.

            Floating through the ocean, glass armored phototrophs go where the water will take them. The plastid provides energy, the wall protection and the diatom grows. Understanding the unique chemistry and biology of the silica cell wall can have applications in nanotechnology to create specific tiny structures at will. Their importance as primary producers is profound, so the unique biology that allows this organism to be ahead of its competitors becomes very important in understanding ecosystems that rely on this eukaryotic microbe.

Submitted by Matthew Dargis

Bibliography

E. Virginia Armbrust. (2009). The Life of Diatoms in the World’s Oceans. Nature 459:185-193.

E. Virginia Armbrust, et al. The Genome of the Diatom Thalassiosira psuedonana: Ecology, Evolution and Metabolism. (2004). Science 306: 79-86. 

Townley, H., Parker, A. and White-Cooper, H. (2008), Exploitation of Diatom Frustules for Nanotechnology: Tethering Active Biomolecules. Advanced Functional Materials, 18: 369–374.