Human consumption of fossil fuels for energy production emits approximately 31 billion tons of carbon dioxide (CO2), the most abundant greenhouse gas, into the atmosphere each year (Boden et al. 2011). Since the start of the Industrial Revolution, atmospheric CO2 concentrations have increased by about 40%, and there have been similar or greater increases in other greenhouse gases (see Chapter 25). Increasing concentrations of these greenhouse gases are causing global temperatures to rise at a rate greater than has occurred on Earth since the end of the last ice age (IPCC 2013). Climate change will not only cause significant ecological changes, many of which are described in the textbook and other Online Climate Change Connections, but will also lead to upheaval in economic and social systems.
To minimize the impact of future climate change, the use of “clean” or renewable energy sources, which emit few or no greenhouse gases, has increased. These energy sources include hydroelectric dams, wind turbines, biofuels, and solar energy. These sources, however, will not be able to provide enough energy to meet global energy demand for at least several decades. As of 2014, renewable energy accounted for only about 19.2% of global energy demand, and traditional biomass burning (e. g., fuelwood, peat, animal dung) provided almost half of that energy (REN21 2016).
Solar energy holds substantial promise for providing sustainable energy, and it is one of the fastest-growing sources of clean energy. It still constitutes a relatively small share of clean energy production, however, in part due to the historical high cost of photovoltaic (PV) panels, which are made up of solar cells that convert solar energy directly into electricity (Figure 1). Lowering the cost of PV panels and improving their efficiency remain major technological challenges. One approach has been to use energy-absorbing dyes that mimic pigments used by autotrophs in photosynthesis. Shuguang Zhang of the Massachusetts Institute of Technology constructed a solar cell using proteins from spinach that contained light-harvesting complexes (McAlpine 2010). Zhang and colleague Michael Grätzel are developing solar cells using inexpensive light-sensitive dyes similar to the light-harvesting complexes of plants.
Similarly, some researchers and policymakers have advocated using biofuels—fuels made from biomass, such as crops or cultured algae—as a renewable energy resource. However, significant challenges must be met before biofuels can provide renewable energy on a large scale, including lowering the environmental costs of their production.
The discovery of a new form of chlorophyll may lead to greater efficiency of energy production from both PV panels and biofuels. Min Chen and colleagues found a pigment they called chlorophyll f in samples from stromatolites in Australia’s Shark Bay (Chen et al. 2010). Stromatolites are layered sedimentary deposits composed of cyanobacterial communities found in shallow marine waters (Figure 2). Cyanobacteria are found on the surface of the sediments, where they capture energy from sunlight that supports the stromatolite community. The turbidity of the water and the density of the cyanobacteria themselves can limit the availability of sunlight to the cyanobacteria (Figure 3). The evolution of chlorophyll f has allowed some cyanobacterial species to take advantage of solar radiation not used by other species. Most chlorophyll absorbs light in blue and red wavelengths and reflects green light (see textbook Figure 5.6), but chlorophyll f absorbs light in the far-red and infrared range (about 706 nm). Cyanobacteria with chlorophyll f take advantage of this underutilized red-shifted portion of the sunlight resource to support photosynthesis. The specific identity of the cyanobacteria within the stromatolites that produce chlorophyll f, however, is unknown.
Despite the uncertainties about its origin, the discovery of chlorophyll f has provided hope that a wider range of solar radiation can be used to bolster energy yield from both PV panels and biofuels. Zhang likens the potential use of chlorophyll f to widening a net to collect more light energy and believes it may serve as a model for technologies that will enhance the spectrum of light used by PV panels (McAlpine 2010). Robert Blankenship and colleagues (2011) suggested that bioengineering chlorophyll f and other red-shifted photosynthetic pigments into algae and plants used for biofuels would enhance energy capture by as much as 10%. The discovery of chlorophyll f may therefore augment multiple efforts to decrease human emissions of greenhouse gases and help lower the amount of global warming expected in the future.
Blankenship, R. E. and 17 others. 2011. Comparing photosynthetic and photovoltaic efficiencies and recognizing the potential for improvement. Science 332: 805–809.
Boden, T. A., G. Marland and R. J. Andres. 2011. Global, Regional, and National Fossil-Fuel CO2 Emissions. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.
Chen, M., M. Schliep, R. D. Willows, Z. Cai, B. A. Neilan and H. Scheer. 2010. A red-shifted chlorophyll. Science 329: 1318–1319.
IPCC. 2013. Climate Change 2013. Cambridge University Press, Cambridge; also available on the IPCC web site: www.ipcc.ch/report/ar5/wg1/.
McAlpine, K. 2010. Infrared chlorophyll could boost solar cells. New Scientist; doi:10.1016/S0262-4079(10)62078-7.
REN21. 2016. Renewables 2016 Global Status Report. (Paris: REN21 Secretariat).