Phytoplankton are amazing little microscopic plants that the entire world depends on for oxygen, food and carbon removal. They live in water (oceans, lakes and rivers), near the surface. There are approximately 5000 different kinds (which is surprising considering they are autotropic). They are fascinating to look at because they form all kinds of geometric shapes, looking like something off a UFO (http://www.cedareden.com/). They produce about half of the world's oxygen, and absorb thousands of tonnes of carbon dioxide a year. When they die, they sink to the bottom of the ocean or lake, taking the carbon with them. Lots of things eat them, like krill. They are the bottom of the food chain, but they are highly inspiring! Buddha or Christ would be proud of their meekness.
All they need to grow is sunlight, and minerals. Ocean currents bring minerals up from the bottom of the ocean (or the mouths of rivers) which the phytoplankton use to grow, using sunlight for energy. They absorb carbon from the atmosphere and organic compounds such as carbohydrates, utilising minerals dissolved in the water and produce oxygen. They particularly need iron as well as other minerals.
There is a modern problem for phytoplankton though. Because of all the unstable acids, salts from fertilisers, carbon overload in the air etc, and other chemical rubbish we pump into our waterways and oceans, the sea is becoming more acidic. This is causing difficulties for all lifeforms there, such as weakening crustacean's protective shells, damaging coral, and reducing the productivity of phytoplankton (and reducing the food supply of the animals who rely on them). This is a problem.
There are saving graces in the case of phytoplankton though. Phytoplankton reproduce rapidly, living for only a couple of days. There are, of course, mutations that occur regularly. If our oceans get irrepairably acidic we may lose most of our food sources, but at least phytoplankton will evolve fast enough to survive. Lets just hope that whatever evolves next is still producing oxygen, not some other gas, considering it is utilising very different chemicals, and levels of chemicals, to the ones it grew used to over millennia.