Growth on yeast extract As previously reported [2], we also found that yeast extract (0.4%) alone can support growth of H.

modesticaldum (Figure 2A). It is known that many undefined carbon sources, vitamin Wortmannin mixtures and amino acids included, are included in yeast extract. We successfully replaced yeast extract with vitamin B12 for supporting the growth of a different photoheterotrophic bacterium MS-275 cell line [9]. In all of the growth media of H. modesticaldum, vitamin B12 has always been included, and it is not yet known what carbon sources in the yeast extract support the photoheterotrophic growth of H. modesticaldum. With approaches listed in Materials and Methods, we have estimated that the amount of pyruvate, acetate and lactate in yeast extract is negligible. However, the inclusion of pyruvate or acetate as a defined organic carbon source, along with yeast extract, can significantly enhance growth (Additional file 2: Figure S2). Alternatively, JSH-23 molecular weight it is possible that some amino acids in yeast extract may support the growth of H. modesticaldum, and the oxidation of amino acids transported into the cell can generate reducing power and chemical energy. To test this hypothesis, we grew H. modesticaldum on casamino acids

that contain predominately a mixture of free amino acids, and observed comparable cell growth with 1.0% casamino acids versus with 0.4% yeast extract after 48 hours of growth (OD625 is ~0.7-0.8). Also, we didn’t observe significant growth enhancement with vitamin mixtures included in casamino acids-grown cultures. Together, our studies support the idea that amino acids contribute to the growth of H. modesticaldum. Further, we have probed the contribution

of glutamate and glutamine for cell growth of H. modesticaldum. Glutamate can serve as a nitrogen source for H. modesticaldum [6], while our current studies indicate that either glutamate or glutamine (up to 100 mM each) cannot support the growth of H. modesticaldum as a sole carbon source during phototrophic and chemotrophic growth. To investigate the impact of yeast extract on metabolic GNAT2 pathways, we compared transcriptomic data from cultures containing PYE (pyruvate and yeast extract are carbon sources) and PMS (pyruvate as the sole organic carbon) growth media (all of the growth media are described in Materials and Methods section and Table 1). It is generally assumed that proteomic and transcriptomic data are related [11], and that higher mRNA levels normally lead to more protein production, particularly in prokaryotes with no mechanism of post-transcriptional modification. Our data show that the addition of yeast extract to the culture media has little effect on the transcriptional levels of most genes involved in carbon metabolism and other cellular functions (Additional file 3: Table S1). Table 1 Organic carbon sources used in growth media reported in this paper.