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Facultative autotrophs are abundant components of communities inhabiting geothermal springs. However, the influence of uptake kinetics and energetics on preference for substrates is not well understood in this group of organisms. Here, we report the isolation of a facultatively autotrophic crenarchaeote, strain CP80, from Cinder Pool (CP, 88.7°C, pH 4.0), Yellowstone National Park. The 16S rRNA gene sequence from CP80 is 98.8% identical to that from Thermoproteus uzonensis and is identical to the most abundant sequence identified in CP sediments. Strain CP80 reduces elemental sulfur (S8°) and demonstrates hydrogen (H2)-dependent autotrophic growth. H2-dependent autotrophic activity is suppressed by amendment with formate at a concentration in the range of 20-40 μM, similar to the affinity constant determined for formate utilization. Synthesis of a cell during growth with low concentrations of formate required 0.5 μJ compared to 2.5 μJ during autotrophic growth with H2 These results, coupled to data indicating greater C assimilation efficiency when grown with formate as compared to carbon dioxide, are consistent with preferential use of formate for energetic reasons. Collectively, these results provide new insights into the kinetic and energetic factors that influence the physiology and ecology of facultative autotrophs in high-temperature acidic environments., (© FEMS 2016. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Rates of dissolved inorganic carbon (DIC), formate, and acetate mineralization and/or assimilation were determined in 13 high-temperature (>73 °C) hot springs in Yellowstone National Park (YNP), Wyoming, in order to evaluate the relative importance of these substrates in supporting microbial metabolism. While 9 of the hot spring communities exhibited rates of DIC assimilation that were greater than those of formate and acetate assimilation, 2 exhibited rates of formate and/or acetate assimilation that exceeded those of DIC assimilation. Overall rates of DIC, formate, and acetate mineralization and assimilation were positively correlated with spring pH but showed little correlation with temperature. Communities sampled from hot springs with similar geochemistries generally exhibited similar rates of substrate transformation, as well as similar community compositions, as revealed by 16S rRNA gene-tagged sequencing. Amendment of microcosms with small (micromolar) amounts of formate suppressed DIC assimilation in short-term (<45-min) incubations, despite the presence of native DIC concentrations that exceeded those of added formate by 2 to 3 orders of magnitude. The concentration of added formate required to suppress DIC assimilation was similar to the affinity constant (K(m)) for formate transformation, as determined by community kinetic assays. These results suggest that dominant chemoautotrophs in high-temperature communities are facultatively autotrophic or mixotrophic, are adapted to fluctuating nutrient availabilities, and are capable of taking advantage of energy-rich organic substrates when they become available., (Copyright © 2015, American Society for Microbiology. All Rights Reserved.)

We investigated the molecular mechanisms regulating differences in mitochondrial volume density between heart ventricles of Antarctic notothenioids that vary in the expression of hemoglobin (Hb) and myoglobin (Mb). In mammals, peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) and nuclear respiratory factor 1 (NRF-1) stimulate mitochondrial biogenesis and maintain mitochondrial density in muscle tissues. We hypothesized that these factors would also maintain mitochondrial density in the hearts of Antarctic notothenioids. The percent cell volume occupied by mitochondria is significantly lower in hearts of the red-blooded notothenioid Notothenia coriiceps (18.18+/-0.69%) in comparison with those of the icefish Chaenocephalus aceratus (36.53+/-2.07%), which lacks both Hb and cardiac Mb. Mitochondrial densities are not different between hearts of N. coriiceps and Chionodraco rastrospinosus, which lacks Hb, but whose heart expresses Mb. Despite differences in mitochondrial volume density between hearts of N. coriiceps and C. aceratus, the levels of transcripts of the genes encoding PGC-1alpha, NRF-1 and citrate synthase, and the copy number of mitochondrial DNA do not differ. Our results indicate that the high mitochondrial densities in hearts of C. aceratus may result from an increase in organelle size. The surface-to-volume ratio of mitochondria from N. coriiceps is 1.9-fold greater than that of mitochondria from C. aceratus. In addition, the levels of PGC-1alpha correlate with mitochondrial density in muscle tissues of notothenioids possessing mitochondria of similar size and morphology. Finally, the levels of PGC-1alpha are 4.6-fold higher in the aerobic pectoral adductor muscle in comparison with the glycolytic skeletal muscle of N. coriiceps. The potential physiological significance of an increase in mitochondrial size in hearts of Antarctic icefishes is discussed.