Producers in an ecosystem are organisms that manufacture organic matter from inorganic substances using an external energy source, forming the primary biological entry point for energy into most ecosystems. They are commonly called primary producers or autotrophs. Most producers use photosynthesis, capturing light energy and using carbon dioxide and water to synthesise organic molecules. Plants, algae and cyanobacteria are major photosynthetic producers.
Some microorganisms are chemolithoautotrophs that obtain energy by oxidising inorganic substances such as hydrogen sulfide, ammonia, hydrogen or ferrous iron and use that energy to fix inorganic carbon. Producers are therefore defined by their mode of acquiring energy and carbon rather than simply by being green plants. They form the foundation of most food webs because other organisms depend directly or indirectly on the organic matter they create.
Photosynthetic producers convert radiant energy into chemical energy stored in organic compounds. In oxygenic photosynthesis, light energy drives reactions that ultimately use water as an electron source and release oxygen, while carbon dioxide is incorporated into organic molecules. The overall simplified equation is carbon dioxide plus water plus light producing organic matter and oxygen, although the actual process consists of numerous reactions occurring in specialised molecular systems.
In plants and algae, photosynthesis occurs primarily in chloroplasts, while cyanobacteria perform photosynthesis within their cells without chloroplasts. Light intensity, wavelength, carbon dioxide availability, temperature, water availability and nutrient supply can influence photosynthetic productivity. Photosynthesis therefore links atmospheric chemistry, climate, biological energy capture and ecosystem productivity.
Primary production describes the rate at which producers create organic matter. Gross primary production is the total amount of chemical energy or carbon fixed by producers, while net primary production is what remains after producers use some of the captured energy in respiration. Net primary production is particularly important ecologically because it represents organic material available for growth and for consumption by herbivores and other organisms. Producers allocate assimilated carbon among leaves, roots, stems, storage compounds, reproduction and respiration.
In aquatic systems, phytoplankton can have rapid turnover and high productivity despite maintaining relatively low standing biomass. In forests, large amounts of carbon may be stored in long lived trunks and roots. Productivity therefore concerns rates of material and energy fixation rather than simply the visible amount of plant biomass present at one moment.
Producers require more than an energy source and carbon dioxide. They also require mineral nutrients and suitable physical conditions. Nitrogen is required for proteins, nucleic acids and chlorophyll, phosphorus is important in nucleic acids and energy metabolism, and elements such as potassium, magnesium, sulfur and iron have important cellular functions. Nutrient availability can limit primary production in both terrestrial and aquatic environments. In oceans, nitrogen, phosphorus or trace elements such as iron can limit phytoplankton growth in particular regions. On land, soil nutrient availability, water and temperature often strongly influence plant productivity. Producers interact with microorganisms that can improve nutrient acquisition, including mycorrhizal fungi associated with plant roots and nitrogen fixing bacteria associated with particular plants.
Producers can be classified according to how they obtain energy and carbon. Photoautotrophs use light energy and inorganic carbon, while chemoautotrophs or chemolithoautotrophs obtain energy from oxidation of inorganic compounds and use inorganic carbon. Some organisms have mixed nutritional strategies and can combine autotrophic and heterotrophic processes depending on environmental conditions. Certain algae and microorganisms can therefore switch or supplement their energy acquisition methods. This diversity is especially important in environments where sunlight is limited, such as deep ocean hydrothermal vents, subsurface environments and sediments. Chemosynthetic microbial communities demonstrate that primary production does not fundamentally require sunlight, although sunlight powered photosynthesis dominates primary production across the global biosphere.
Producers are central to global biogeochemical cycles. Through photosynthesis, producers remove carbon dioxide from the atmosphere or water and incorporate carbon into biomass. Some of this carbon returns through respiration, while some enters soils, sediments, food webs or long term carbon stores. Plants influence water cycling through transpiration and affect soil development, erosion and nutrient retention. Marine phytoplankton contribute substantially to global carbon cycling and oxygen production. Producers also influence atmospheric composition over geological and ecological timescales. However, gross statements about plants simply producing oxygen should be qualified because producers also respire and consume oxygen, and net ecosystem oxygen balance depends on production, respiration, decomposition and physical exchange.
Producers support consumers by supplying energy rich organic compounds. Herbivores obtain much of their carbon and energy by feeding directly on producers, while carnivores and omnivores obtain producer derived energy indirectly. Dead plant material also enters detrital food webs, where fungi, bacteria and detritivores break it down. Thus producers support both grazing and decomposition pathways. The efficiency with which consumers obtain energy from producers depends on plant chemistry, digestibility, nutrient content, herbivore physiology and defence mechanisms. Plants may produce structural compounds such as cellulose and lignin that are difficult for many animals to digest, while some herbivores possess specialised microorganisms that help break down these materials. Producer chemistry can therefore strongly influence the structure of consumer communities.
Producers are important in agriculture, forestry, fisheries, conservation and climate science. Crop plants convert solar energy into food and feed, while forests and grasslands provide biomass, habitat, soil protection and carbon storage. Aquatic primary producers support fisheries through the production of organic matter consumed by zooplankton and other organisms. Changes in nutrient availability can increase productivity but excessive nutrient inputs can also cause eutrophication, algal blooms, oxygen depletion and ecological disruption.
Climate change can alter productivity through temperature, carbon dioxide, precipitation and extreme events, but responses vary among ecosystems and species. Producers therefore cannot be understood merely as food sources. They are biochemical energy converters that influence ecosystem structure, carbon cycling, nutrient dynamics, habitat formation and the availability of energy to almost every other trophic component of an ecosystem.
