Primary producers are a basic part of an ecosystem. They can be thought of as the first and most important step in the food chain. Along with decomposers, they make up the base of a food web and together their populations number more than any other part of the web. Primary producers are consumed by primary consumers (generally herbivores), which are then consumed by secondary consumers and so on. Organisms at the top of the chain eventually die and are then consumed by decomposers, which fix the nitrogen levels and provide the organic material necessary for the next generation of primary producers.

Primary producers are vital to the survival of an ecosystem. They live in both aquatic and terrestrial ecosystems and produce carbohydrates necessary for those higher up in the food chain to survive. Since they are small in size and can be susceptible to changing environmental conditions, ecosystems with more diverse populations of primary producers tend to thrive more than those with homogeneous populations. Primary producers reproduce rapidly. This is necessary to sustain life as the species' populations get smaller as you go further up the food chain. For example, up to 100,000 pounds of phytoplankton may be necessary to feed the equivalent of only one pound of a predator species at the top end of the chain.

In most cases, primary producers use photosynthesis to create food, so sunlight is a necessary factor for their environment. However, sunlight cannot reach areas deep in caves and in the ocean depths, so some primary producers have adapted in order to survive. Primary producers in those environments use chemosynthesis instead.

Aquatic primary producers include plants, algae and bacteria. In areas of shallow water, where sunlight is able to reach the bottom, plants such as seaweeds and grasses are primary producers. Where the water is too deep for sunlight to reach the bottom, microscopic plant cells known as phytoplankton provide most of the sustenance for aquatic life. Phytoplankton are affected by environmental factors such as temperature and sunlight as well as the availability of nutrients and the presence of herbivorous predators.

About half of all photosynthesis happens in the oceans. There, phytoplankton take carbon dioxide and water from their surroundings, and they can use energy from the sun to create carbohydrates through the process known as photosynthesis. As the primary source of food for zooplankton, these organisms form the base of the food chain for the entire ocean population. In turn, zooplankton, which include copepods, jellyfish and fish at the larval stage, provide food for filter-feeding organisms such as bivalves and sponges as well as amphipods, other fish larvae and small fish. Those that are not consumed right away eventually die and drift to the lower levels as detritus where they may be consumed by deep-sea organisms that filter their food, such as coral.

In freshwater areas and shallow saltwater areas, producers include not only phytoplankton such as green algae, but also aquatic plants such as sea grasses and seaweed or larger rooted plants that grow on the surface of water such as cattails and provide not only food but also shelter for larger aquatic life. These plants provide food for insects, fish and amphibians.

Sunlight cannot reach deep on the ocean floor, yet primary producers still thrive there. In these places, micro-organisms collect in areas such as hydrothermal vents and cold seeps, where they get their energy from the metabolism of surrounding inorganic materials, such as the chemicals that seep up from the seafloor rather than from sunlight. They also may settle on whale carcasses and even shipwrecks, which act as a source of organic material. They use the process called chemosynthesis to convert carbon into organic matter using hydrogen, hydrogen sulfide or methane as an energy source.

Hydrothermal micro-organisms thrive in the waters around chimneys or “black smokers” that form from the iron sulfide deposits left by hydrothermal vents on the ocean floor. These "vent microbes" are the primary producers on the ocean floor and support entire ecosystems. They use the chemical energy found in the minerals of the hot spring to create hydrogen sulfide. Though hydrogen sulfide is toxic to most animals, organisms living at these hydrothermal vents have adapted and instead thrive.

Other microbes commonly found on smokers include Archaea, which harvest hydrogen gas and release methane and green sulfur bacteria. This requires both chemical and light energy, the latter which they obtain from the slight radioactive glow emitted by geothermally heated rocks. Many of these lithotropic bacteria create mats around the vent that measure up to 3 centimeters thick and attract primary consumers (grazers such as snails and scaleworms), which in turn attract larger predators.

The terrestrial or soil food chain is made up of a large number of diverse organisms, ranging from microscopic single-celled producers to visible worms, insects and plants. The primary producers include plants, lichens, moss, bacteria and algae. Primary producers in a terrestrial ecosystem live in and around organic matter. Since they are not mobile, they live and grow where there are nutrients to sustain them. They take nutrients from organic matter left in the soil by decomposers and transform them into food for themselves and other organisms. Like their aquatic counterparts, they use photosynthesis to convert nutrients and organic materials from the soil into food sources to nourish other plants and animals. Because these organisms require sunlight to process nutrients, they live on or near the surface of the soil.

Similarly to the ocean floor, sunlight does not reach deep into caves. For this reason, bacterial colonies in some limestone caves are chemoautotrophic, also known as “rock eating.” These bacteria, like those in the ocean depths, get their necessary nourishment from the nitrogen, sulfur or iron compounds found in or on the surface of rocks that have been carried there by water seeping through the porous surface.

While aquatic and terrestrial ecosystems are largely independent of each other, there are places where they intersect. At these points, the ecosystems are interdependent. The banks of streams and rivers, for example, provide some of the food sources to support the stream’s food chain; land organisms also consume water organisms. There tends to be a greater diversity of organisms where the two meet. Higher levels of phytoplankton, likely due to greater availability of nutrients and longer “residence” time have been found in marsh systems than in nearby coastal estuaries. Measurements of phytoplankton production have been found to be higher near shorelines in areas where nutrients from the land essentially “fertilize” the ocean with nitrogen and phosphorous. Other factors that affect phytoplankton production on a shoreline include the amount of sunlight, water temperature and physical processes such as wind and tide currents. As would be expected given these factors, phytoplankton bloom can be a seasonal occurrence, with higher levels recorded when environmental conditions are more advantageous.

An arid desert ecosystem does not have a consistent water supply, so its primary producers, such as algae and lichen, spend some periods of time in an inactive state. Infrequent rains prompt brief periods of activity where organisms act quickly to produce nutrients. In some cases these nutrients are then stored and only released slowly in anticipation of the next rain event. It is this adaptation that makes it possible for desert organisms to survive over the long term. Found on soil and stones as well as some ferns and other plants, these poikilohydric plants are able to transition between active and resting phases depending on whether they are wet or dry. Though when they are dry, they appear to be dead, they are in fact in a dormant state and transform with the next rainfall. After a rain, algae and lichens become photosynthetically active and (due to their ability to reproduce rapidly) provide a food source for higher-level organisms before the desert heat causes the water to evaporate.

Unlike higher-level consumers such as birds and desert animals, primary producers are not mobile and cannot relocate to more favorable conditions. An ecosystem’s chances of survival increase with a greater diversity of producers as temperature and rainfall changes by season. Conditions that are right for one organism may not be for another, so it benefits the ecosystem when one can be dormant while another thrives. Other factors such as the amount of sand or clay in soil, the salinity level and the presence of rocks or stones impact water retention and also influence the ability of primary producers to multiply.

At the other extreme, areas that are cold much of the time, such as the Arctic, are unable to support much plant life. Life on the tundra is much the same as that in an arid desert. Varying conditions mean that organisms can only thrive in certain seasons and many, including primary producers, exist in a dormant stage for part of the year. Lichens and mosses are the most common primary producers of the tundra.

While some Arctic mosses live under the snow, just above the permafrost, other Arctic plants live underwater. The melting of sea ice in the spring along with the increased availability of sunlight triggers algae production in the Arctic region. Areas with higher nitrate concentrations demonstrate higher productivity. This phytoplankton blooms under the ice, and as the ice level thins and reaches its yearly minimum, the ice algae production slows. This tends to coincide with the movement of the algae into the ocean as the bottom ice level melts. Production increases correspond to periods of ice thickening increases in the fall, while there is still significant sunlight. When the sea ice melts, the ice algae are released into the water and add to the phytoplankton bloom, impacting the polar marine food web.

This changing pattern of sea ice growth and melt, along with a sufficient nutrient supply, appears to be necessary to the production of ice algae. Changing conditions such as an earlier or faster ice melt may reduce the levels of ice algae, and a change in the timing of the algae release could impact the survival of consumers.

Algal blooms can occur in almost any body of water. Some may discolor the water, have a foul odor or make the water or fish taste bad, but not be toxic. However, it is impossible to tell the safety of an algal bloom from looking at it. Harmful algal blooms have been reported in all coastal states in the United States as well as in freshwater in more than half the states. They also occur in brackish waters. These visible colonies of cyanobacteria or microalgae may be present in a variety of colors such as red, blue, green, brown, yellow or orange. A harmful algal bloom is fast-growing and affects animal, human and environmental health. It may produce toxins that can poison any living thing that comes in contact with it, or it may contaminate aquatic life and cause illness when a person or animal eats the infected organism. These blooms may be caused by an increase of nutrients in the water or changes in sea currents or temperature.

Although few species of phytoplankton produce these toxins, even beneficial phytoplankton can be damaging. When these micro-organisms multiply too quickly, creating a dense mat on the water's surface, the resulting overpopulation can cause hypoxia or low levels of oxygen in the water, which disrupts the ecosystem. So-called “brown tides,” while not toxic, can cover large areas of the water’s surface, preventing sunlight from reaching below and subsequently killing off those plants and the organisms that depend on them for life.