The phototrophic way of life

Photosynthesis is the utilization of radiant energy for the synthesis of complex organic molecules. The phototrophic way of life implies the capture of electromagnetic energy (see section Light Absorption and Excitation Transfer in Prokaryotes in this chapter), its conversion into chemical energy (see section Conversion of Light into Chemical Energy in this chapter), and its use for cellular maintenance and growth (see section Efficiency of Growth and Maintenance Energy Requirements in this chapter). The mode of growth in which photosynthesis is coupled to the reduction of carbon dioxide into organic molecules is defined as photoautotrophy. The solar electromagnetic energy reaching the Earth's surface (163 W·m^-2; see section Light Energy and the Spectral Distribution of Radiation) represents 48 % of the electromagnetic energy reaching the top of the atmosphere (solar constants = 1.361 kW·m^-2, corresponding to 340 W·m^-2 per surface of the Earth, equivalent to a total incoming energy of 173 PW; Kopp and Lean 2011). Despite its losses in the atmosphere due to reflection, absorption/reflection, or evaporation, the electromagnetic energy available still surpasses the energy contributed by all other sources by at least 4-5 orders of magnitude. Thus, present-day geothermal energy derived from stored heat and heat produced by radioactive decay is ≤0.08 W·m^-2 or 47 TW total (Davies and Davies 2010; for primordial Earth the estimate is ∼0.0062 W·m^-2, and was delivered by electric discharge, radioactivity, volcanism, or meteoritic impacts; Mauzerall 1992).