Diatoms: algae that live in glass houses

Figure 1 diatom species captured in a plankton net  |  photos and post by Michael Konrad

We don’t notice much of the life in the water around our floating homes because the individual organisms are too small. Diatoms fit that description; they range in size between 2 and 200 microns (0.002 to 0.2 mm). Anything 2 microns long, about 4 wavelengths of light, is well below the size we can see. The thinnest pencil lead you can buy is 0.3 mm in diameter, so you can just see the largest diatoms. Each diatom may be small, but the huge population of diatoms represents almost half of the total biomass in the ocean. They are a major reason our bay looks green in the summer and are the major food for larger organisms that in turn are eaten by the fish we put on the table.

The diatom shell

They really do live in glass houses, or shells. Glass is mostly silicon oxide, or silica, mixed with small amounts of other elements. The molten silica below the earth’s cool hard crust, mixed with other minerals, when cooled quickly becomes granite, the major rock at the surface. When cooled more slowly it forms small crystals: the sand on our beaches, a major component of concrete. Cooled even more slowly it can form large, beautiful quartz crystals of pure silica.

Silicon is a chemical element just below carbon in the periodic table and so, like carbon, can make 4 chemical bonds to other atoms. It has some properties of a metal, but is a poor, or semi-conductor of heat and electricity. It can form two bonds to each of two oxygen atoms, so silicon oxide can be written O=Si=O, which certainly looks like carbon dioxide, O=C=O. Writers of science fiction have proposed alien life forms based on silicon compounds instead of the large collection of carbon molecules that make up life on our earth, and silicon is indeed part of a few metabolic processes in plants.

A commercially important silicon compound is di-methyl silicon acetate, a thick gel, sold in tubes as a caulking and sealing material. When it reacts with the moisture in the atmosphere it polymerizes to form long molecules that are strong but flexible, like rubber. The acetate combines with water to form acetic acid vapor (think vinegar), which you smell as the silicone cures. Various silicon based rubbers are used in many industrial and consumer products. The transistors that power our cell phones and laptops are fabricated from single crystals of pure silicon, made by removing the oxygen from silica through reaction with carbon at high temperature to form carbon dioxide, which escapes. It’s easy to get silicon (the metalloid), confused with silica (its oxide) or silicone (a rubber polymer based on silicon).

Granite on mountains is slowly dissolved by years of rain and flows down into the oceans. It is this dissolved silica that the diatoms use to make their shells, which often reflect light as beautiful irradiant colors. Diatom shells are a silica-protein matrix which contains many small pores allowing water, and dissolved minerals and carbon dioxide, to enter, and oxygen to escape.

Diatoms contain chlorophyl, which adsorbs sunlight to convert the carbon dioxide dissolved in the water to all the organic molecules needed to make diatom tissue. The oxygen released into the water and then atmosphere by this process, is used by animals like us to generate energy by converting some of our food back to carbon dioxide: cycles within cycles within cycles.

Diatoms come in a large variety of shapes, but the basic one is a cylinder made of two cups that fit closely into each other, thus the name “cut in two” from the Greek [Fig 1]. When conditions are favorable diatoms multiply rapidly, doubling every 24 hours. A two fold increase in number every 24 hours means a 1000 fold increase in just ten days. A barely noticeable green tinge becomes a dense green soup and people wonder “where has all this stuff come from”? The diatoms were always there, just in much smaller numbers. This type of rapid growth is often called a “bloom”. As the diatom grows the two halves move apart to accommodate the larger volume. Then the two cups separate and each forms a new complementary shell. After several cycles of growth, the algae must form a completely new double shell.

Life and death
Most diatoms can’t swim or move, they just drift with the currents, thus they are called plankton (from the Greek wandering). However, some pennate shaped diatoms can glide on a surface. Reproduction is mostly by cell division; however occasionally male and female gametes are produced and combine to create progeny. The silica shells of dead diatoms don’t dissolve or decay rapidly but sink to the bottom to form thick layers in regions with large populations. Dried diatom shells, diatomaceous earth, is used as filter material in water purification. Dried diatoms from the bottoms of lakes that covered the Dodele Depression in the Sahara Desert about 10,000 years ago, are blown by easterly winds over the Atlantic Ocean to fall in the Amazon forests and provide much of the nutrient minerals for vegetation in that region.

Figure 1. A large variety of diatom species captured in a plankton net — a nylon cone net about 40 cm in diameter and 300 cm long. The net fabric is a simple weave with openings 50 µ wide. It was towed just under the water surface at Schoonmaker Marina for 70 m and the plankton were captured in a PVC cylinder at the end. The field of view of this photograph is about 600 x 600 µ.

Figure 2. Four diatom species.

The top left panel is a side view of a diatom with the basic shape of two interlocking cylinders. The end view of this type of diatom can be seen in Figure 1 at the lower left corner. The other three panels represent only a sample of many other species present in the bay, with panels 3 and 4 showing colonies of individuals. The colors are mostly due to iridescence, color dependent interference between different wavelengths of light, also seen with butterfly and hummingbird wings. They are also dependent on the settings of Photoshop™, the program used to format these images.