Producers are organisms capable of creating simple carbohydrates such as glucose, from gaseous carbon dioxide. This process of producing organic molecules from inorganic carbon sources is called primary production. The energy for this process can come from solar radiation, chemical reactions or from the heat in deep ocean geothermal vents. On land, most producers are plants. Marine production is dominated by algae and plankton.
Types of Producers
There are two major types of primary producers – phototrophs and chemotrophs.
Phototrophs use the energy from the sun to convert carbon dioxide into carbohydrates. The process by which this occurs is called photosynthesis. Later, the chemical bond energy in carbohydrates is released through respiration and used to fuel metabolic pathways. A similar process occurs in chemotrophs, except that the energy source is inorganic oxidation and reduction reactions. Chemotrophs are nearly always microscopic and are found in regions where water and light are scarce.
Occasionally, terms such as ‘secondary producers’ and ‘tertiary producers’ are used. Animals that consume plants are considered secondary producers since they ‘produce’ the biomass for their predators. Similarly, carnivores eaten by other species are considered ‘tertiary producers’. At each trophic level the consumer can only obtain 10% of the calories available to its producer. Therefore, it is rare to see energy pyramids containing more than four levels, or ‘quaternary producers’.
Examples of Producers
Photosynthetic producers can be broadly grouped under three categories: plants, cyanobacteria and phytoplankton.
From microscopic species to redwoods that tower over the earth, there is astonishing diversity among plants. Remarkably, these varied species share exactly the same mechanism for photosynthesis. Photosynthesis occurs in specialized organelles called chloroplasts that contain pigments such as chlorophyll.
These pigments are usually located in a scaffold of membrane-bound proteins called light-harvesting complexes. Here, light-driven oxidation induces the pigment to lose an electron. The high-energy electron then enters an electron transport chain, where it travels from one protein to another, losing energy at each step, coupled with a series of oxidation and reduction reactions. The movement of a charged particle through the electron transport chain also fuels a proton gradient across the membrane. The proton gradient and the electron transport chain together, power the formation of the energy currency of the cell, Adenosine Triphosphate (ATP). The photo-oxidized chlorophyll pigment is then returned to its native state through the splitting of a water molecule, which releases molecular oxygen.
Cyanobacteria are photosynthesizing prokaryotes. They are among the earliest life forms to have appeared on earth with a fossil record that stretches back to over three billion years. They also contributed towards creating an oxygen-rich atmosphere over the course of two billion years, paving the way for the kind of life forms we see today. Due to their photosynthetic activity, they were initially classified as algae and the term ‘blue-green algae’ continues to be used informally to refer to these prokaryotes.
Cyanobacteria are considered to be the endosymbionts that evolved into modern day chloroplasts. These prokaryotes also have membrane protein complexes in their cell membrane. Some of these membranes form cylindrical thylakoid sheets, that resemble the internal structure of chloroplasts. These similarities make them useful as model organisms for the study of photosynthesis. However, there are some differences between the pathways used in modern cyanobacteria and plants. One of these arises from the marine nature of these prokaryotes, that requires them to ‘concentrate’ carbon dioxide in small vesicle-bound compartments to improve the efficiency of photosynthetic enzymes such as RuBisCO.
They are crucial for the health and survival of marine ecosystems because they play an important role in creating bioavailable carbon and nitrogen. Nitrogen is fixed as ammonia and used to create nitrogen-containing compounds such as proteins and nucleic acids. Since cyanobacteria are consumed by organisms in the ocean bed, in shallow waters as well as in open seas, they are among the most important marine primary producers.
Phytoplankton are microscopic free floating plants that perform most of the photosynthetic activity of the ocean. They are at the base of marine ecosystems and maintain the oxygen levels of the ocean as well as the atmosphere. They are consumed by microscopic herbivorous animals called zooplankton which are then eaten by organisms higher up in the food pyramid.
The appearance of phytoplankton is said to have contributed to a major evolutionary explosion 250 million years ago. After a mass extinction at the end of the Paleozoic era, an increase in nutrients and a reduction in predation allowed these plants to proliferate in the oceans. Their abundance and enhanced nutritional content also allowed primary consumers like zooplankton, to proliferate. As these groups of organisms grew and colonized larger ocean tracts, some populations diversified, adapted to new environments which finally led to an enormous increase in species diversity in the oceans.
Functions of Producers
Producers are the primary source of biomass on earth. They form the bottom of all energy pyramids and are the first trophic level in every ecosystem. Primary producers harness the energy from the sun or from chemical reactions and fix inorganic carbon in the form of carbohydrates. Their role in sequestering carbon dioxide makes them crucial for weather patterns across the globe, maintaining optimal temperature and annual rainfall. Photosynthesis also releases oxygen as a byproduct and this is consumed by all organisms to release the chemical energy stored in carbohydrates.
Producers such as lichens, are important as pioneer species, altering the abiotic environment to make it more habitable. They accelerate weathering and enhance the deposition of organic matter, leading to the formation of soil.
Along with abiotic factors, producers play a crucial role in determining the species diversity in a region. For example, when plankton are the primary producers, filter feeding herbivores will proliferate, followed by carnivores that can consume these organisms. On the other hand, regions that contain tall trees will end up favoring herbivores like giraffes that can reach the higher branches and thereafter select for predators that can hunt these quick animals. Thus, the primary producer underpins the entire ecosystem.