cross section (transmission electron micrograph) of a Mycoplasma cell. of the 530 genes of Mycoplasma genitalium, 43 code for transfer, ribosomal, and other non-messenger RnAs. Functions are known, or can be guessed, for 339 of the genes coding for protein: of these, 154 are involved in replication, transcription, translation, and related processes involving dnA, RnA, and protein; 98 in the membrane and surface structures of the cell; 46 in the transport of nutrients and other molecules across the membrane;  71 in energy conversion and the synthesis and degradation of small molecules; and 12 in the regulation of cell division and other processes. note that these categories are partly overlapping, so that some genes feature twice. (A, from s.
Razin et al., Infect. Immun. 30:538–546, 1980. With permission from the American society for microbiology; B, courtesy of Roger cole, in medical microbiology, 4th ed. [s. Baron ed.]. Galveston: university of Texas medical Branch, 1996.) are called organotrophic (from the Greek word trophe, meaning “food”). Others derive their energy directly from the nonliving world. These primary energy converters fall into two classes: those that harvest the energy of sunlight, and those that capture their energy from energy-rich systems of inorganic chemicals in the environment (chemical systems that are far from chemical equilibrium). Organisms of the former class are called phototrophic (feeding on sunlight); those of the latter are called lithotrophic (feeding on rock). Organotrophic organisms could not exist without these primary energy converters, which are the most plentiful form of life.
Phototrophic organisms include many types of bacteria, as well as algae and plants, on which we—and virtually all the living things that we ordinarily see around us—depend. Phototrophic organisms have changed the whole chemistry of our environment: the oxygen in the Earth’s atmosphere is a by-product of their biosynthetic activities.
Lithotrophic organisms are not such an obvious feature of our world, because they are microscopic and mostly live in habitats that humans do not frequent— deep in the ocean, buried in the Earth’s crust, or in various other inhospitable environments. But they are a major part of the living world, and they are especially important in any consideration of the history of life on Earth.
Some lithotrophs get energy from aerobic reactions, which use molecular oxygen from the environment; since atmospheric O2 is ultimately the product of living organisms, these aerobic lithotrophs are, in a sense, feeding on the products of past life. There are, however, other lithotrophs that live anaerobically, in places where little or no molecular oxygen is present. These are circumstances similar to those that existed in the early days of life on Earth, before oxygen had accumulated.
The most dramatic of these sites are the hot hydrothermal vents on the floor of the Pacific and Atlantic Oceans. They are located where the ocean floor is spreading as new portions of the Earth’s crust form by a gradual upwelling of material from the Earth’s interior (Figure 1–11). Downward-percolating seawater is heated and driven back upward as a submarine geyser, carrying with it a current of chemicals from the hot rocks below. A typical cocktail might include H2S, H2, CO,
Mn2+, Fe2+, Ni2+, CH2, NH4 , and phosphorus-containing compounds. A dense
SEA dark cloud of hot, mineral-rich water 350°C percolation contour of seawater