Millions of individual cells form green patches on ponds and lakes. Microalgae only require a few mineral salts, carbon dioxide from the air and sunlight to grow and prosper. It would be possible to harvest the patches of microalgae to extract multiple unsaturated fatty acids and vitamins for use in food supplements. Another idea would be to isolate pigments and produce food dyes, or thickening agents for the cosmetics industry. And of course they could be energy carriers for the fuel cells of the future: hydrogen gas, biodiesel or bioethanol. But none of this is easy. Industry cannot sit around waiting for natural ponds and lakes to sprout growth, which is why for many years companies have grown microalgae in photobioreactors. Sunlight is able to enter these photobioreactors where the organisms are kept in an optimal culture medium, and where they carry out photosynthesis and proliferate. They are thus able to produce up to five times more biomass per area than traditional energy crops such as rape and maize.

“However, currently used photobioreactors do not yet lead to the desired product yields,” said Dr. Rosa Rosello, process engineer at the Institute for Bio- and Food Technology at the Karlsruhe Institute of Technology (KIT). “The major problem we face is that not all algal cells contained in a tank are exposed to the same amount of light.” The reason for this is that the light that shines through the glass of the reactor has to penetrate all the layers of algal cells. On its way to the centre of the tank, the light is scattered and absorbed. That is why a large number of cells live in the shade and use up energy, rather than producing energy through photosynthesis. So the question arises as to how the availability of light can be optimised.

One possibility of trying to expose all cells to the same amount of light is to constantly stir the contents of the tank. However, in order to do this, it is important to find out how quickly the culture can be stirred without damaging the cells. The question can also be asked in a different way: How long can the algal cells remain in the shade before switching from photosynthesis to the degradation of energy?

In order to find answers to these questions, Rosello and her team have developed a bioreactor in which they are able to investigate the light conditions independently from all other factors, including for example the composition of the medium or the application of carbon dioxide. This bioreactor is a glass tank ten centimetres in diameter. This glass tank represents a volume element of a production reactor that is exposed to ideal light intensities and gas quantities. It contains a solution of essential minerals such as nitrogen, phosphate and sulphate, as well as trace elements such as cobalt, iron or manganese. Carbon dioxide can be introduced into the tank from the outside. The researchers use a lamp equipped with light emitting diodes to generate the intensity of natural sunlight and variable light and darkness phases. "This set-up enables us to test which temporal changes of light irradiation favour the optimal growth rate of the microalgae in the tank," said Rosello. The bioengineers from Karlsruhe use this set-up to test the growth of different microalgae species, and to measure the growth of the algae and the production of different metabolic products such as hydrogen, lipids or carbohydrates.

In her doctoral thesis, Rosello investigated the growth of Phorphyridium cruentum from which interesting products can be derived, including polysaccharides with so-called sulphate ester groups. The algae release these substances into the culture medium, which are used in the cosmetics and food industries as thickening agents. The substances also have antiviral and anticarcinogenic properties and are therefore of great interest for the pharmaceutical industry. Rosello tested the maximum length of shade phases (i.e. the time during which the algae are not exposed to light) that would still support the optimal growth of the algae. The maximum growth rate serves as point of reference. Normally, algae grow faster when the intensity of sunlight increases. From a certain degree of intensity, the algae are nevertheless saturated with light, which means that the enzymes that convert light energy into carbon compounds have reached their limit. The researchers’ goal is to establish photobioreactor conditions that enable the algae to grow at the maximum possible speed.

The investigations provided Rosello with information on the existence of light-darkness frequencies that cause the maximum growth rate to decrease by up to 50%. This is the case of changes in the second range such as those that were obtained during the mixing of the culture in so-called open photobioreactors. In contrast, a maximum growth rate can be achieved with millisecond intervals, for example twenty milliseconds of light and twenty milliseconds of shade. The growth rate can even be increased by reducing the phases to three milliseconds of light and three milliseconds of shade. This has direct consequences for the mixing speed that needs to be applied in a photobioreactor on the industrial scale. “It is still unclear whether these high frequencies can realistically be achieved,” said Rosello. “It might potentially damage the cells and further investigations need to be carried out to find out more.” Such investigations provide the KIT researchers with important information about the “well-being and ill-being” of the microalgae under different light pattern conditions in outdoor facilities. These findings form the basis for optimising the design of photobioreactors in order to achieve maximum light use and hence high yields.

Rosello and her team at the KIT are currently elucidating the potential of microalgae in numerous projects. They are part of an international consortium (Solar Biofuels), in which biologists and engineers from Germany, Australia and England work together. The KIT researchers focus on elucidating the question as to which light conditions different genetically modified Chlamydomonas reinhardii strains need in order to be able to produce the highest amount of hydrogen. The researchers are also working on determining the optimal conditions for the production of different lipids for the production of biodiesel, a project that involves the use of the algae Nannochloropsis salina and Phaeodactylum tricornutum. Which types of oils do the algae produce? Which light parameters are required for doing so? And how can this be implemented in the large scales required by industry (i.e. for example in a 250-l tank)? “The major problem faced by industry is that photobioreactors are not yet as lucrative as it would like them to be,” said Rosello. “Therefore, our major challenge is to minimise the investment and energy costs required for the mixing of the culture media and to maximise the yield of interesting biological products,” said Rosello. A major goal of the KIT researchers is to develop a photobioreactor for the economic production of microalgae that can be upscaled to the production scale. Therefore, the researchers are focusing on optimising incident light input and the energy required for gas transport and mixing. More than 25,000 microalgae species are known. Intelligent approaches, as well as technically optimised methods are required in order to industrially exploit the broad range of interesting metabolic products.

Further information:
Dr.-Ing. Rosa Maria Rosello Sastre
Karlsruhe Institute of Technology (KIT)
Institute for Bio- and Food Technology
Division III: Bioprocess Engineering
Fritz-Haber-Weg 2 
Building 30.44, 1st floor
76131 Karlsruhe
Tel.: +49 (0)721/608-5203
Fax: +49 (0)721/608-5202
Email: rosa.rosello(at)kit.edu