A group of researchers at the University of Konstanz led by Prof. Dr. Peter Kroth is working on an organism that is an extraordinarily successful survivor. Its chemical, biological and biochemical properties can be put to many different uses and it has the potential to be used in the healthcare market and industry to an even greater and more effective extent in the future. We are referring to diatoms.
Scientists and artists alike have been interested in diatoms with their fascinating geometric shapes for a long time and their structures and shapes appear in modern design. Prof. Dr. Peter Kroth from the University of Konstanz has been working on diatoms for quite some time. His group was also involved in sequencing the Phaeodactylum tricornutum genome, which was published in 2009.
Diatoms were present in freshwater and oceans long before the appearance of humans. They are one of the most common types of phytoplankton. It is likely that the endosymbiotic combination of several algae types, which gave rise to diatoms, contributes to the diatoms’ capacity to survive extreme conditions. They are found in almost all aquatic habitats, including Arctic crevasses. One fifth of all global photosynthetic activity and hence CO2 fixation can be attributed to diatoms; so diatoms therefore contribute to reducing atmospheric CO2 concentration.
In the same way as plants, diatoms convert light energy into chemical energy (carbohydrates, lipids) by way of photosynthesis. Carbohydrates and lipids normally serve as energy stores that can be tapped during periods of darkness. However, energy does not seem to be of great importance for diatoms: some of them excrete huge quantities of carbohydrates, which they use to move forward. “Diatoms excrete polysaccharides, and we assume that these drive the cells forward by adhering to substrate where they expand,” explains Professor Kroth.
Non-planktonic diatoms are found on the surface of lakes and oceans where they form colonies. These biofilms consist mainly of carbohydrates that are secreted by the diatoms and are used as food source by the bacteria in the same biofilms. It would appear that these diatoms and bacteria form specific communities.
Professor Kroth goes on to explain: “Diatoms have a large proportion of lipids. The cultivation of diatoms for the production of biodiesel has been a major focus of scientific research since the oil crisis in the 1970s. But the cultivation of diatoms is not that easy. The algae need light for growth, and the lamps that provide this light need a lot of energy. In addition, harvesting, drying and extraction uses a lot of energy. This means that the cultivation of algae requires more energy than it produces in the form of biodiesel. Therefore, the use of crops such as maize for energy generation is still more efficient, simpler and hence cheaper than producing energy from algae. However, the use of maize for energy production has become highly controversial since maize is a staple food for many people around the world. In addition, huge areas of agricultural land would be needed to cover global biofuel requirements with the production of crops; this idea is really just an illusion. Therefore, algal biomass has huge potential to be used for the production of car fuel, bioethanol for example. However, at present, the process steps need to be further optimised and cautious estimates assume that the economically viable production of bioethanol from algal biomass will only be possible in around 10 to 20 years’ time. The production of fuel from algae is also highly interesting considering the fact that algal biomass is a byproduct in the algal production of substances such as carotinoids or unsaturated fatty acids.
The potential of different algal species is already being exploited in the production of animal feed supplements: for example, carotinoids are industrially produced from algae and added to animal feedstuff. Green algae, which contain large quantities of astaxanthin, a red carotinoid, are used to feed farmed salmon which do not usually have the typical red colour of wild salmon. Wild salmon absorbs astaxanthin as part of their natural diet (small crustaceans). Other types of algae are also being successfully used in fish and mussel breeding where they are used as suppliers of carbohydrates, fatty acids, steroids and vitamins.
Diatoms are encased in a shell made of silica (hydrated silion dioxide), which is called a frustule. These frustules display fine structures and holes, which are genetic, i.e. species-specific. When the cells die, they are either eaten by other organisms or they sink to the river or ocean bed where they accumulate. As silica only degrades slowly, over time the diatoms have created thick sediments. Diatomite is one particular material that consists mainly of diatom shells. As the material has well-formed pores and is chemically highly stable, diatom shells have long been used as filter material in the production of wine and beer: efficient filter layers consisting of organic diatom material help to make wine and beer ’clear’, i.e. remove turbid substances. Diatomite is a very mild abrasive and, for this purpose, has been used for many years in toothpaste.
“Docosahexaenoic acid (DHA, 6-fold unsaturated omega-3 fatty acid) is produced from heterotrophic dinoflagellates and is used in baby food,” said Professor Kroth citing an example from the food sector where algae already play a successful role.
Algae are also produced commercially and processed into food supplements. Blue algae such as Spirulina and green algae are dried and pressed into tablets and used to reverse or prevent mineral deficiencies. However, the effect has not yet been scientifically proven. As the algae used are not usually cultivated in closed systems, but in well-fertilised ponds and lakes, their cultivation is associated with the risk of contamination with toxic cyanobacteria that can produce liver and nerve toxins. Prof. Dietrich from the University of Konstanz recently tested a number of commercial blue alga preparations made of Aphanizomenon flos-aquae and found that some of them exceeded the legally permitted quantities of toxic microcystin.
Diatom genomes have been sequenced and scientists around the world are now working hard to put this knowledge to good use. “Around 8% of the diatom DNA originates from bacteria that were taken up at some point during the phylogenesis of diatoms; the diatoms then incorporated fragments of bacterial DNA into their DNA,” said Prof. Kroth explaining that this might be the reason why diatoms are so successful. Based on this knowledge, researchers attach specific DNA sequences to wolfram particles and inject them into the cells where they integrate into the genome. This method is used to make diatoms express specific proteins that might potentially be excreted in a similar way to polysaccharides.If this were possible, human antibodies could be produced in the algal cytosol or algal chloroplasts. Cytokine antibodies could for example be used for the treatment of humans. However, the purification of algal proteins is very time-consuming and expensive as these processes have to be carried out according to strict regulations, in the same way as all other biologically produced substances that are used for pharmaceutical applications.
Diatoms can be used in “bioreactors” for the production of genetically modified molecules: they can easily be transformed using genetic engineering methods, and are easy to cultivate under sterile conditions, in relatively simple fermentation tanks and in high quantities. It will need to be assessed on a case-to-case basis whether the production and isolation of substances from algae is worth the effort. A large number of researchers and biotechnology companies around the world are currently working on this possibility.
Prof. Dr. Peter KrothUniversity of KonstanzDepartment of Plant EcophysiologyPhone: +49 7531 88-4816Fax: +49 7531 88-3042E-mail: peter.kroth(at)uni-konstanz.de