They are invisible to the naked eye. Only in massive quantities do they color water green. Aquatic microalgae are tiny organisms, which usually consist of a single cell and sometimes a few connected cells. The biologists Elly Spijkerman and Sabrina Lachmann are interested in the carbon uptake of these organisms because algae, as green terrestrial plants, contain chlorophyll and carry out photosynthesis. These inconspicuous little organisms play a large role in the global carbon cycle. They fix a considerable amount of the greenhouse gas carbon dioxide (CO2). The different species of algae seem to use different strategies for CO2-fixation. Spijkerman and Lachmann, both researchers at the chair Ecology and Ecosystem Modelling, want to find out which metabolic pathways algae can use and how these are influenced by environmental conditions.
A culture cabinet provides the algae with everything they need. The small culture flasks stand on racks at a constant temperature of 20°C and receive 16 hours of light a day. Chlamydomonas acidophila, Chlorella emersonii, and Chlamydomonas pitschmannii are written on the glass flasks. These are the names of the algae floating in a solution containing all the nutrients they need, mainly phosphorus and nitrogen as well as different trace elements and, of course, the inorganic carbon that algae need to perform photosynthesis and create biomass.
In her dissertation Lachmann examines how these three types of algae from different aquatic ecosystems assimilate carbon. It appears that each has developed its own strategy. “Carbon assimilation depends on the pH,” Lachmann explains. This is because not all carbon is the same. Understanding this involves submerging into water chemistry. Water has basically two inorganic carbon sources that algae can use for photosynthesis: CO2 and bicarbonate. The higher the pH, the less CO2 and the more bicarbonate is available, and vice versa.
Spijkerman is holding up a tube containing a greenish-brown liquid. “When the pH is above 6.3, the inorganic carbon is primarily bicarbonate, which is useless to this alga,” says the biologist who is supervising Lachmann’s doctoral thesis. The glass tube contains a culture of Chlamydomonas acidophila, whose name says that it “loves acid”, and can be found in the extremely acidic mining lakes in Lusatia. It would “starve” if it had only bicarbonate. It lacks the transport mechanism to subsist on this type of carbon.
Most rivers, lakes, and oceans have very little carbon dioxide and much bicarbonate. The algae, however, need a special transporter to be able to use bicarbonate as a carbon source - a disadvantage because the uptake requires additional energy and nutrients. Aquatic ecologists therefore want to unravel which environmental conditions promise an efficient strategy of carbon assimilation.
“We want to understand the mechanisms of carbon assimilation and how they affect the ecosystem,” Spijkerman describes their research goal. For this the biologists have performed growth tests with different algae and under different conditions. They let the algae grow with either a sufficient supply of nutrients or a nutrient deficiency and regularly checked the cultures’ pH. “The algae change the pH of the medium by photosynthesis“, Lachmann explains. The pH increases until all usable carbon has been consumed. The researchers have been able to establish which type of carbon the different algae species can use and if the nutrient supply influenced it.
The results of these and previous experiments show the complexity of carbon assimilation system. “It is fascinating,” Spijkerman explains, “that the algae can use two different carbon sources and three to four different mechanisms for each source to channel carbon into the cell”. Each strategy requires different physiological adaptations.
As already mentioned, Chlamydomonas acidophila prefers an acidic environment. The alga occurs globally in acidic waters with a pH between 1.5 and 5 and uses CO2 exclusively for photosynthesis. It lacks an assimilation system for bicarbonate. Its strategy exhausts little energy and few nutrients, but only works with enough CO2. Chlorella emersonii, on the other hand, is a “normal alga that occurs in neutral waters,” Spijkerman says. It uses both CO2 and bicarbonate. This, however, increases its nutrient requirements. Nutrient shortage inhibits the alga’s ingestion. Unlike some other species it can use two different carbon sources if there are enough nutrients. The third alga is a real all-rounder compared to the others. Chlamydomonas pitschmannii grows both in acidic and alkaline waters and even likes it a bit hotter. In addition to CO2 it also uses bicarbonate. The two researchers are currently examining how this species of alga reacts to nutrient shortage.
Biologists are not the only ones interested in which conditions allow algae to assimilate carbon most efficiently and build up biomass. Determining how they physiologically adapt to optimally utilize the carbon source could prove important for industrial biomass production. This is all the more important because algae cultures do not compete with food production like biomass agriculture, such as corn and rapeseed. They do not use up any arable land. Research into the use of microalgae as a future source of raw material has been underway for years now. The organisms’ physiology is significant because the less energy a cell needs for transport and conversion, the more energy it has for growth, leading to higher yields.
Spijkerman will soon be able to apply her knowledge. She will be leaving the university to continue research at a Berlin company that produces ethanol from marine blue-green algae, a base substance for biofuels. “I will be directly engaging in the commercial use of microalgae,” the researcher says.
The Researchers
Dr. Elly Spijkerman studied biology at University of Amsterdam. Since 2002 she worked at the University of Potsdam. She examines how stress factors influence algae physiology. In 2015 she will start working at a company dealing with the commercial use of microalgae.
Contact
Universität Potsdam
Institut für Biochemie und Biologie
Maulbeerallee 2
14469 Potsdam
E-Mail: spijkeruuni-potsdampde
Sabrina Lachmann studied biosciences and ecology at the University of Potsdam. Since 2013 she has been working on her doctoral thesis, which analyzes the carbon assimilation of different types of algae.
Contact
E-Mail: salachma@uni-potsdam.de
Text:Heike Kampe
Online-Editing: Agnes Bressa, Translation: Susanne Voigt
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