by Matthew Norton
The seas of the world are full of chemical mixtures, not just the sodium chloride that makes up table salt, and many living things are able to extract these chemicals directly from the water for their own purposes. In some cases, they have been quite inventive, using chemicals that most other creatures would not look twice at (assuming they have eyes).
Diatoms, a group of microscopic algae (i.e. seaweeds), are one such group who have devised a way of extracting silica (also called silicon dioxide) from the water and using it to make protective cases out of glass. These cases are called frustules and they come in a variety of shapes (e.g. square, circular, long and thin) and the diatoms that make them may live alone, or joined together in a chain.
We don’t really know the exact process that diatoms use to make their glass cases and given the wide variety made by different species it is possible that there is no single process used by all diatoms, but there are some things we do know. We know that the amount of silica available can control diatom growth, although the ‘pools’ of silica they can store in their bodies may give them an edge when supplies from the water are running low. We have also made some progress in identifying the genes that ultimately make the proteins that grab the silica molecules from the water and transport them to where they are needed in the diatom cell. In recent years, we have also discovered some strange molecules in diatom cells which may yield further clues, but only time will tell.
The glass cases made by diatoms demonstrates how resourceful these little seaweeds can be, but it comes with the problem of glass being very heavy and this extra weight could drag them into deeper water, out of reach of the sunlight they need to make their food. Diatoms rise to the challenge by using their vacuoles, a closed off space inside their single-celled bodies, as a float. They can also make their whole body (except the glass case) heavier during the day by using sunlight to accumulate food reserves and then burn through those reserves to make themselves lighter when they have sunk too far away from the sunlight.
Despite the difficulties that come with living in a glass case, diatoms are a very successful group of organisms who are found floating around in coastal seas, living under sea ice in polar waters and various other habitats in the water and even on land to some extent. They also fulfil a very important role in the world’s oceans, supporting many ocean food chains through sheer numbers at both the surface and as their dead bodies sink into the ocean depths as particles of ‘marine snow’.
The influence of diatoms on the natural world is particularly dramatic when there are rapid explosions in diatom growth called ‘blooms’, but this is not always good for other creatures in the area. For example, anything living under the bloom may suffocate as the dead diatoms suck out all the oxygen as they rot. Some diatom species also release toxic chemicals into the water which can cause serious health problems to animals that get themselves caught in their blooms.
Diatom evolution has produced some pretty impressive techniques for turning a readily available resource into a valuable tool for survival. This seems to have made them a very important part of the ocean ecosystem, which is not bad for a bunch of microscopic plants that we can’t even see with the naked eye.
From a human perspective
There is little doubt that diatoms find their glass cases useful, otherwise they wouldn’t go to the trouble of making them, but we have also found quite a few uses for them because they are durable, long lasting and come in a variety of microstructures.
For example, the glass cases from long dead diatoms can give us vital clues on what life was like millions of years ago through fossilised samples. The patterns of the preserved diatom cases can be compared to the patterns from modern day diatoms and use to the closest matches to work out what the environment was like when the fossilised diatom was alive. Diatom mats on the sea floor can also help to keep the remains of dead animals intact, eventually producing some very well preserved fossils.
Diatoms are also useful in modern detective work by providing forensic investigators with evidence to determine the exact cause of death in victims who have ‘appear’ to have drowned. This is based on the idea that if the victim really did die from drowning in the water body (lake, river, canal etc) that their body is found than they would have inhaled the diatoms from that water body and those diatoms would have been circulated around the body. This can be confirmed by comparing taking samples from the water body and the victim’s major organs and bone marrow and seeing if there is a match. If there is a large difference between these samples then it may suggest that the victim’s body was moved after they had died, which would be very suspicious indeed.
It is worth noting that diatom sampling is not a perfect forensic tool for determining a cause of death and there are several circumstances that can produce misleading results. For example, if the victim drowned quickly then any diatoms they inhaled might not have had the chance to get round the body before their circulation shut down. On the other hand, if the victim’s body isn’t discovered for a long time after death then decomposition can directly expose the major organs to the water and the diatoms it contains. Still, diatoms can provide a useful source of evidence in potential homicide cases, even though other pieces of evidence are needed for a conclusive result.
Finally, there has been some interest in using diatom cases in nanotechnology to design micro-electronic devices, chemical sensors and drug delivery systems inside the human body. The advantage to using diatoms is that they we don’t need extreme conditions (e.g. extreme heat to melt metal) to modify the microstructure of their cases, although some clever genetic engineering is still required. There have also been some pretty bold claims that diatom-based nanotechnology can be used to extract tiny particles of silver and gold and make new materials by using the glass case a mould from which the silica can be replaced at the atomic level. This might actually be a step towards creating a machine very similar to the replicators seen in Star Trek.
It is amazing to think that, through the production of their detailed glass cases, diatoms are so useful to us and in some cases they really can change a person’s life. It seems that human innovation and the result of millions of years of evolution is a winning combination that can be applied to a number of problems in our modern world.
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