Close to Critical planet's air is a little less than the density of liquid water will the raindrops descend slowly; otherwise, they would come down like meteors and make life on Tenebra impossible. But life is possible on Tenebra. Although Clement doesn't say so explicitly, Tenebra's plants do not use photosynthesis. The optical depth of the atmosphere is too great for enough of Altair's light to reach the ground. Instead, Tenebra's plants use chemosynthesis based on the transformation of sulphur oxides. The process is much like that used by plants growing around the hydrothermal vents known as black smokers on the bottom of
Photosynthesis allowing carbon fixation to 3-phosphoglycerate by RuBisCO. Sixteen thousand species of plants use CAM. Cyanobacteria possess carboxysomes, which increase the concentration of around RuBisCO to increase the rate of photosynthesis. An enzyme, carbonic anhydrase, located within the carboxysome releases CO from the dissolved hydrocarbonate ions (HCO). Before the CO diffuses out it is quickly sponged up by RuBisCO, which is concentrated within the carboxysomes. HCO ions are made from CO outside the cell by another carbonic anhydrase and are actively pumped into the cell by a membrane protein. They cannot cross the membrane as they are charged, and within the