Individual marine phytoplankton are too tiny to see without a microscope, but their impact on the world’s environment is huge. Found in every ocean, phytoplankton are the basis of the marine food chain. They produce much of the oxygen that air-breathing animals everywhere need to live. They affect global temperatures by removing vast quantities of the greenhouse gas carbon dioxide from the air. They also sometimes create poisons that kill fish and sicken people.
Phytoplankton are single-cell plant-like organisms that float in uncountable numbers throughout the world's oceans and in fresh water lakes. They are 63-153 microns long--the smallest objects people see unaided are 1,000 microns, or 1 mm. Like land plants, phytoplankton grow by using a process called photosynthesis to create carbohydrates out of sunlight, water and chlorophyll.
Observations of phytoplankton can warn of changes in the environment because phytoplankton populations respond rapidly to differences in the water, sunlight and various nutrients they need to live. Alterations in the size, density or rate of growth of phytoplankton populations can be measured from space by satellites. Scientists compare these observations with other global trends information to better understand interactions between phytoplankton, the environment and global climate.
Under certain conditions, some species of phytoplankton suddenly produce toxic compounds harmful to marine life and people. These poisonous phytoplankton outbreaks are often called “red tides” because they sometimes, though not always, appear red or red-orange in color. Since some red-colored outbreaks are harmless, Woods Hole Oceanographic Institution says the preferred term for a poisonous outbreak is harmful algae bloom, or HAB. HABs are estimated to cost more than $80 million per year in health care costs and damage to fisheries and tourism in the United States. People can get sick by eating shellfish from HAB-contaminated water.
Increasingly acidic oceans are making it harder for phytoplankton to absorb iron, a key nutrient for keeping phytoplankton populations healthy, according to a January 2010 Princeton University study published in "Science." Acidity levels are thought have risen due to increased amounts of carbon dioxide in the atmosphere. All ocean fish and mammals eat phytoplankton either directly or by eating fish that eat them. If acidity continues to rise, phytoplankton populations could shrink, reducing food available for marine life, the study states.
The Iron Hypothesis--the idea that global warming can be reversed by dosing the oceans with iron supplements--has been controversial since it was proposed by an oceanographer named John Martin in the 1980s. Martin suggested that the resulting larger phytoplankton populations could absorb enough carbon to cool global temperatures. It’s also possible the opposite could happen because phytoplankton have been shown to warm water by absorbing sunlight that otherwise would be reflected into space, according to a Scripps Institution of Oceanography study published in the Journal of Geophysical Research in 2002.