6 kJ/kg), resulting in a photosynthetic conversion efficiency of about 29.8%. This value for algal open ponds is considered to be very conservative, with the actual value likely a few percent lower. Finally, for the theoretical maximum, we use the value computed in Zhu et al. (2008) for a maximum photosynthetic efficiency of 29.1% (obtained by combining the loss for photochemical inefficiency and carbohydrate synthesis). Cellular maintenance Maintenance energy is a variable that may affect photoefficiency by drawing away energetic currencies of

ATP and NADPH for cell division, repair, and other functions not directly associated with product formation. The maintenance energy in any given process situation depends on rates of metabolism, cell division, etc., as shown in differences in measured values in dividing versus resting cells (Pirt 1965; Pirt 1975). A batch bioprocess, therefore, wherein cell Vorinostat manufacturer division and product formation are proceeding simultaneously Small molecule library manufacturer versus a continuous process where growth is minimized and carbon is partitioned to a secreted product may differ considerably in maintenance energy. However, because the concept and measurement are controversial, we have attributed a 5% loss to the analyses of

all three scenarios. Mitochondrial respiration Under illumination, eukaryotic photosynthetic organisms, e.g., plants and algae, lose efficiency because of respiratory metabolism in the mitochondria. Because cyanobacteria have no subcellular organelles and the engineered organisms Janus kinase (JAK) are partitioning nearly all fixed carbon Ruboxistaurin in vivo to product, we have assumed negligible respiration loss in the direct process and have also zeroed out this loss in the theoretical practical maximum scenario. The algal open-pond analysis includes a 30%

loss for mitochondrial respiration. This value is based on the plant value used by Zhu et al. (2008). Photorespiration According to Zhu et al. (2008), processes at atmospheric CO2 concentrations, such as an open algal pond, will have a substantial loss (≈49%) due to photorespiration. This loss is minimized at high-CO2 levels (>1%) maintained in the enclosed direct process (see text for explanation). Biomass versus fuel production In the direct process, most fixed-carbon output is in the form of a chemical product from a cloned heterologous pathway. For the algal process, we assume a generous value for oil yield of 50% by weight and thus apply a 50% loss to productivity. The losses discussed above are summarized in Table 3. We define conversion factor as (1 – loss factor) for each of the above losses. For instance, the conversion factor for cellular maintenance (loss = 5%) is 95%. Total conversion efficiency, as shown in Fig. 2, is computed by taking the product of each of the conversion factors computed from the values in Table 3. Acknowledgments The authors declare a competing interest via their association with Joule Unlimited.