Photosynthesis is the process by which plants, some bacteria, and some protistans use the energy from sunlight to produce sugar, which cellular respiration converts into ATP, the "fuel" used by all living things. The conversion of unusable sunlight energy into usable chemical energy, is associated with the actions of the green pigment chlorophyll. Most of the time, the photosynthetic process uses water and releases the oxygen that we absolutely must have to stay alive. Oh yes, we need the food as well! 

We can write the overall reaction of this process as: 
6H2O + 6CO2 ----------> C6H12O6+ 6O2 

Most of us don't speak chemicalese, so the above chemical equation translates as: 

six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen. 

What is Chemosynthesis? 

Chemosynthesis is a process certain organisms use to produce energy, akin to photosynthesis, but without the utilization of sunlight. The energy comes from the oxidization (burning) of chemicals which seep up from the Earth's crust. The organisms that use chemosynthesis, all bacteria, manufacture carbohydrates and other organic molecules from the oxidization of sulfates or ammonia. The hydrogen they use comes from hydrogen sulfite, whereas the nitrogen comes from ammonia or nitrates. The organisms that use chemosynthesis are found around hydrothermal vents on the ocean floor. They are adapted to circumstances which would have been commonplace billion of years ago, leading some to call them descendants of the earliest life on Earth. 

Organisms that use chemosynthesis are extremophiles, living in harsh conditions such as the absence of sunlight and a wide range of water temperatures, some approaching the boiling point. These organisms are known for living inside one another, engaging in symbiotic and parasitic relationships to maximize their chances of survival. Chemosynthetic microbes provide the foundation for larger communities of organisms which consume the microbes to survive. One interesting example is the tubeworm, filled with billions of chemosynthetic bacteria. The tubeworm starts life with a mouth and gut, which it uses to intake many bacteria. Its mouth then closes and it continues to survive by consuming food produced by its internal bacteria. 

Chemosynthetic species are autotrophs, organisms capable of manufacturing organic matter directly from inorganic feedstock. Autotrophs of different types can produce energy either through photosynthesis or chemosynthesis. The gases that autotrophs use to create energy would be poisonous to most organisms. They use unusual enzymes capable of resisting high temperatures and pressures. Since these organisms live on the bottom of the ocean floor, they are subject to much pressure from the water above. Ecologies surrounding deep sea vents are extremely prosperous relative to those located further away from such chemical sources, which must survive solely on dead organic matter slowly descending from the waters above. 

Chemosynthetic organisms have been viewed by the biotech industry as a means of converting toxic chemicals into harmless organic variants. If life exists on other planets or moons such as Mars or Titan, it has been postulated that they may use chemosynthesis.