Conclusions H. modesticaldum is one of the only two cultured anoxygenic phototrophs that can fix nitrogen at temperatures above 50°C. Only acetate, lactate and pyruvate have been reported previously to support the photoheterotrophic growth of H. modesticaldum, and it is necessary to further explore carbon sources in order

to understand energy metabolism in-depth. In this paper, we developed the growth medium close to a minimal growth medium, and report the first studies, with comprehensive experimental evidence supported, that D-ribose, D-glucose and D-fructose can be photoassimilated as sole carbon sources to generate cell material. Additionally, in the absence of autotrophic CO2 fixation, H. modesticaldum uses two CO2-anaplerotic pathways Epacadostat during MAPK inhibitor phototrophic growth: pyruvate:ferredoxin oxidoreductase (PFOR) and phosphoenol-pyruvate carboxykinase (PEPCK). The CO2-anaplerotic pathways by PFOR and PEPCK are essential for acetate assimilation, pyruvate metabolism and introducing carbon flow into the rTCA cycle for generating cell materials, buy MI-503 including photosynthetic pigments (Figure 5). Our studies suggest that PFOR and ferredoxin-NADP+ oxidoreductase (FNR)

are required for generating reducing power (Fdred and NAD(P)H) during chemotrophic growth. A similar ratio of acetate excretion/pyruvate consumption is observed in pyruvate-grown cultures during phototrophic versus chemotrophic growth, and conversion of acetyl-CoA to acetate can generate ATP for the energy required for H. modesticaldum in darkness. Also, since energy and reducing power produced by H. modesticaldum during chemotrophic growth are rather limited

compared to phototrophic growth, cellular functions demanding high-energy input, such as nitrogen fixation and hydrogen production, are down-regulated. Nevertheless, our studies indicate that H. modesticaldum produces sufficient energy and reducing power for both carbon metabolism and nitrogen fixation during chemotrophic growth, albeit at a relatively low growth rate. An overview of energy metabolism pathways of H. modesticaldum is shown in Figure 8. Resveratrol In summary, our reported studies not only significantly broaden our current knowledge, but also provide new and essential insights on the energy metabolism of H. modesticaldum. Methods Materials Chemicals and enzymes for enzymatic activity assays were purchased from Sigma-Aldrich. The 13C-labeled glucose and pyruvate were from Cambridge Isotope Laboratories (CIL), Inc. The DNA oligomers were from Integrated DNA Technology (IDT) without further purification. The source culture of Heliobacterium modesticaldum Ice1T was a gift from the laboratory of Dr. Michael T. Madigan at Southern Illinois University, Carbondale.