By Marilyn Wiebe
Algae, including here microscopic, single cell organisms, macroscopic “seaweeds” and cyanobacteria or “blue-green algae”, are responsible for at least half of the world’s photosynthesis. During photosynthesis, CO2 is taken into a cell, either as CO2 or as bicarbonate, the carbon is converted into biomass or other products and oxygen is released to the atmosphere. The potential for algal capture of CO2 is generally agreed to be higher than that of land plants and many algae are able to exploit environments which are not suitable for traditional agricultural activities, such as areas with brackish or saline water. Algal biomass can be used as short- (e.g. recycling carbon in biofuels) to medium- (e.g. slow release of carbon from fertiliser) term storage of CO2.
Various test and pilot facilities around the world have been investigating the use of different flue gases for algal cultivation. Earlier work at VTT highlighted the methods of transfer of CO2 from production facilities to algal ponds or culture systems. Flue gas may be directly injected into an algal culture system to provide mixing as well as CO2 to the system or it may be captured as carbonate or bicarbonate using an aqueous or chemical scrubber and provided in liquid. Direct injection lowers the pH of aqueous systems, for which low pH tolerant organisms could be beneficial. Alternatively high pH values are needed to ensure that bicarbonate remains in solution and is not released to the atmosphere. Alkaline tolerant algae are desirable when CO2 is fed as bicarbonate/carbonate. These considerations led to questions of how well CO2 is captured by algae in conditions of extreme pH. VTT together with the Finnish Environment Institute SYKE have been investigating some of the species with potential at both extremes.
Three acid tolerant and three alkali tolerant algae were identified for this work, including brown diatoms, green, motile protists, a non-motile green alga and one cold-tolerant, motile, green alga. Each alga has its own growth characteristics, and they did not all grow equally well. However, it was striking that acid tolerant and alkali tolerant strains were equally able to capture CO2 at low (acid tolerant) or high pH (alkali tolerant) as at neutral pH – i.e. uptake of CO2 will not suffer by using acidic or alkali conditions with appropriate strains.
Low pH cultures require addition of CO2 but high pH cultures do not
There are, however, differences in operation of the cultures at low or high pH. At low pH the cultures may become CO2 limited and require addition of CO2 in the gas feed (i.e. direct injection of flue gas would be desirable). The low pH strains captured up to about 40% of CO2 from air and up to about 15% of CO2 when it was fed at 2-3%. Although the proportion of CO2 captured is lower when the air is supplemented with CO2 than when it is not, the total amount of CO2 captured by the algae is higher. These acid tolerant strains primarily take up CO2 and one strain was not able to grow at pH values above 7 at which CO2 becomes available primarily as bicarbonate. The acid tolerant strains generally produced more biomass than the alkaline strains.
The alkali tolerant strains did not require CO2 supplementation to grow well (i.e. no flue gas is needed). The diatoms were able to capture up to 60% of the CO2 from air at both pH 7 and pH 9. Provision of additional CO2 in the gas stream resulted in the formation of alkali salts, rather than additional algal growth, so less CO2 was captured when more was provided in alkaline conditions (i.e. direct injection of flue gas would be deleterious). Even at neutral pH, uptake of CO2 from CO2-enriched air was not as efficient as with the acid-tolerant strains. In terms of capturing CO2 from flue gas, providing the CO2 as bicarbonate and recycling some of the salts to the CO2 scrubber should enable the provision of higher amounts of CO2 to such strains with less precipitate formation. A number of recent publications have been addressing the question of how bicarbonate solutions could be better exploited in the cultivation of alkali-tolerant algae.
Algae may also be used to capture CO2 in cool climates
The cold-tolerant alga captures up to about 25% of the CO2 from air, growing slower than the other algae. Nonetheless, this strain also grows similarly at pH 9 as at pH 7 and demonstrates that cold-tolerant algae can provide an option for CO2 capture in areas with cool climates.
As this study draws to an end we are still considering whether bicarbonate solutions could be used with algae growing at near neutral pH values to increase the proportion of CO2 captured.
Dr. Marilyn Wiebe is a principal research scientist at Bioprocess engineering team, with a background in physiological studies and cultivation of yeast and filamentous fungi. During the past 12 years her research has focused on the use of microorganisms, including algae, for biofuels and the development of biorefinery concepts. She is interested in the use of various cultivation methods (photo-, mixo- and heterotrophic, batch, fed-batch and continuous) to understand algal growth and carbon metabolism. This interest has led to several co-authored publications related to algal growth and productivity. firstname.lastname@example.org