Cell size, cell integrity and DNA content of S. solfataricus transferred from 80 °C to 45 °C . Samples of cells growing exponentially at 80°C (0 h), or after being transferred to 45°C (time points: 7d, 14d and 21d), or transferred back to 80°C (24 h) were collected. The cell density was monitored by measuring the OD 600nm(A) and by flow cytometry (D). The time point (0 h) indicates when 80°C exponentially growing cells (closed gray diamond) were transferred from 80°C to 45°C (open black diamond). The arrow (↑) indicates when cells were transferred back to 80°C (closed gray diamond) after 21 days at 45°C. (B) Cell size was measured by phase-contrast microscopy (a-e). Bar, 2 μm. (C) Cell membrane integrity was analyzed by fluorescence microscopy. Merged images of Syto 9 (green) and propidium iodide (red) are shown (a-e). The same cells were studied by phase-contrast (B) and by fluorescence microscopy (C). DNA content distribution and cell size of S. solfataricus cells transferred from 80°C to 45°C for three weeks and back to 80°C (D). DNA content distribution was analyzed as the fluoresence of propidium iodide and the cell size determined by forward light scattering in flow cytometry (a-e).