Researchers at the University of California, Berkeley are developing an approach to improving the solar-to-biofuels energy conversion efficiency of algae in mass culture by genetically truncating the size of the light-harvesting chlorophyll arrays that serve to absorb sunlight in the photosynthetic apparatus. A paper on their work appears in a special energy issue of the open-access journal Optics Express.
Researchers have calculated, based on a quantum yield of 0.103 O2 per photon absorbed, that the productivity of microalgae under bright sunlight could be up to 75 g dry weight m-2 d-1. However, small-scale cultures of microalgae grown under full sunlight show maximal photosynthetic productivity of about 20-30 g dw m-2 d-1. The reason for this discrepancy, the Berkeley team of Mautusi Mitra and Anastasios Melis notes, is that green algae assemble large arrays of light absorbing chlorophyll (Chl) antenna molecules in their photosystems.
Up to 600 Chl a and Chl b molecules can be found in association with PSII and PSI. At high solar intensities, the rate of photon absorption by the large Chl antenna of the first few layers of cells in the mass culture far exceeds the rate at which photosynthesis can utilize them, resulting in dissipation and loss of excess photons as heat or fluorescence. Up to 80% of the absorbed photons could thus be wasted, minimizing solar-to-product conversion efficiencies and photosynthetic productivity to unacceptably low levels. In addition to the wasteful dissipation of excitation, and also due to the high rate of photon absorption by the photosynthetic apparatus, cells at the surface of the mass culture are subject to photoinhibition of photosynthesis, a phenomenon that compounds losses in productivity. Meanwhile cells deeper in the culture are deprived of much needed solar energy, as this is strongly attenuated due to the filtering.—Mitra and Melis 2008
Mitra and Melis suggest that algae can get along with many fewer Chl antenna molecules.
The main objective of the research in the field of bioengineering of the optical properties of microalgae is to minimize the Chl antenna size of the two photosystems to a combined low of 132 Chl molecules (37 for PSII and 95 for PSI). This is the smallest Chl antenna size that will permit assembly of the photosystems in chloroplasts. Such Chl antenna size configuration of the photosystems would compromise the competitive ability and survival of the cells in the wild. However, it would enable efficient solar-to-product conversion by the cells in mass culture, leading to high rates of biomass accumulation, and hydrogen (H2) or hydrocarbon (HC) production by these microorganisms.—Mitra and Melis 2008
Truncating the Chl antenna size in green microalgae through bioengineering allows grater transmittance of sunlight deeper into the culture, thereby allowing more cells to contribute. Other research has also shown that a truncated Chl antenna size enables an approximate 3-fold greater solar energy conversion efficiency and photosynthetic productivity.
The Berkeley researchers had earlier identified the Tla1 gene as responsible for defining the Chl antenna size in green microalgae.
To identify currently unknown genes that determine the Chl antenna size in photosynthetic organisms, and to demonstrate that a truncated Chl antenna size would minimize absorption and wasteful dissipation of sunlight by individual cells, resulting in better light utilization efficiency and greater photosynthetic productivity under mass culture conditions, the team is using an approach comprising DNA insertional mutagenesis, screening, biochemical and molecular analyses in a study of the green algae C. reinhardtii.
Current progress suggests that a partially truncated chlorophyll antenna size of the microalgae alleviates the over-absorption of incident sunlight by individual cells in a high density culture, and minimizes the wasteful dissipation of irradiance. A truncated light harvesting chlorophyll antenna size in such mutants diminishes the severe cell shading that occurs with normally pigmented wild type, permitting a more uniform illumination of the cells in a mass culture, and resulting in a greater solar-to-product conversion efficiency and photosynthetic productivity of the algae under high cell density and bright sunlight conditions. Accordingly, the truncated light-harvesting chlorophyll antenna size (tla) property may find application in the commercial exploitation of microalgae for the generation of biomass, biofuel, chemical feedstock, as well as nutraceutical and pharmaceutical products.—Mitra and Melis 2008
Mautusi Mitra and Anastasios Melis (2008) Optical properties of microalgae for enhanced biofuels production. Optics Express, Vol. 16, Issue 6