Improving Solar Energy Efficiency by Extension of the Range of Available Light that Drives Photosynthesis

Expanding the solar harvesting efficiency of oxygenic photosynthesis has the potential to improve crop growth rate, yield, and season length and is applicable to both food crops and biofuels crops. One way to improve solar harvesting efficiency is to expand the range of solar radiation that can be absorbed and utilized by plants or algae. Currently, Photosynthetically Active Radiation (PAR) is defined as the range of solar incident light (from 400 nm to 700 nm) that can be absorbed and converted into chemical energy for biomass production. This range of PAR represents only roughly 35% of the total incident solar radiation. Expanding the range of useable light into the longer-wavelength near-infrared spectral region could result in substantial improvements in energy availability. At wavelengths longer than 700 nm there is a significant amount of usable solar energy. Certain cyanobacteria, and all anoxygenic phototrophs, derive their energy from these wavelengths of light. Therefore, it might be possible to incorporate whole photosystems, or more likely components of these photosystems, into the electron-transport system of higher plants and algae. Fig. 1 illustrates the difference intransmission of incandescent light through two cyanobacterial samples, highlighting the extended range of PAR for A. marina due to utilization of chlorophyll d. in the unique cyanobacterium Acaryochloris marina. All higher plants, algae and most cyanobacteria utilize chlorophyll a as their major photopigment, while A. marina uses almostexclusively chlorophyll d (2). Chlorophyll d with a peak absorbance in-vivo of ~717 nm can absorb significant amounts of light out to 750 nm and perform oxygenic photosynthesis.

Fig. 1. Total light transmitted through cultures of Synechocystis PCC6803 and Acaryochloris marina as a function of wavelength (nm)

The extended range of PAR for A. marina permits its growth in environments in nature where the incident light is enriched with far red light such as under a layer of algae or other typical cyanobacteria, although recent evidence suggests that chlorophyll d containing organisms may be much more widely distributed than previously thought (3). The extension of PAR to 750 nm represents an additional 9.5% increase in useable solar energy. In an environment such as an algal pond that effectively absorbs all the incident PAR, this is expected to translate into a significant improvement in biomass production. The genes that code for the enzymes that result in the formation of chlorophyll d have recently been identified in our lab and mechanistic characterization of these enzymes is underway, as well as insertion of these genes into other photosynthetic organisms so that the ability to make chlorophyll d can be extended to a wider range of systems.