Photon management for augmented photosynthesis

P hotosynthesis is the model process for storing solar energy in complex chemical bonds. Annually it results in the fixation of upwards of 120 billion tons of carbon through terrestrial plants alone1, and nearly as much again inside the world’s oceans2. Humankind has sought over decades to mimic this process synthetically; however, to date, photosynthesis remains the only option for the sustainable production of many complex chemicals. In particular photosynthesis provides a sustainable path for the synthesis of high-energy-density liquid biofuels—an important priority in a world increasingly stressed by anthropogenic CO2. For these reasons, cultivation of photosynthetic plants and microalgae for biofuel production has attracted great interest. Biofuel production from microalgae can follow several routes. Biodiesel can be produced by reacting triacylglycerols (a type of cellular energy storage lipid) with an alcohol, such as methanol, to produce fatty acid methyl esters (biodiesel), a process known as transesterification. For biodiesel production green algae and diatoms show particular promise as feedstocks owing to their high lipid concentration which can exceed 50% of the cell’s dry-weight3. In addition, microalgal carbohydrates can be converted to biomethane or biohydrogen, through anaerobic digestion or to bioethanol through fermentation4. Alternatively, raw biomass in its entirety can be converted into biocrude oil using thermochemical conversion processes such as pyrolysis or hydrothermal liquifaction4. To avoid harvesting and processing of raw biomass for biofuel extraction, direct photobiological production of hydrogen is possible with certain species, such as Chlamydomonas reinhardtii5. With genetic modifications, particularly of cyanobacteria, other biofuels and biofuel precursors can similarly be evolved including isobutyraldehyde6, isobutanol6, 1-butanol7 and isoprene8. Bio-electricity production in microalgal bio-photovoltaic cells has also been demonstrated9,10. This suite of microalgal energy generation options is a distinct advantage of photosynthesis over other solar energy conversion techniques which are DOI: 10.1038/ncomms12699 OPEN