Primary production of organic matter
Primary production of organic matter is the process through which certain organisms, primarily plants, algae, and cyanobacteria, convert sunlight into chemical energy through photosynthesis. This complex mechanism involves the absorption of carbon dioxide and water to create organic compounds, such as carbohydrates, which serve as the foundation for more complex molecules like proteins and lipids. The vast majority of global primary production, estimated at about 105 gigatons of carbon per year, is generated by terrestrial plants and oceanic algae.
Environmental conditions significantly influence primary production, with factors such as light availability, temperature, and nutrient levels playing crucial roles. For instance, tropical forests exhibit the highest rates of terrestrial primary productivity, while oceanic productivity peaks in nutrient-rich shallow seas and estuaries.
In the ecological hierarchy, primary production supports all heterotrophic life forms, including herbivores and detritivores, which rely on living and decomposed organic matter for energy. Human activities, including agriculture and urbanization, have a profound impact on primary production, as they alter ecosystems and can lead to the depletion of natural resources. The interplay between primary production, climate, and human influence highlights its critical importance in maintaining ecological balance and supporting human livelihoods.
Primary production of organic matter
Primary production is the amount of organic matter produced per unit of time by those organisms capable of photosynthesis, notably green plants on land and algae in the oceans. A proportion of this productivity provides food for animals, fish, and humans and is thus a vital resource. Primary productivity is an important component of the global carbon cycle and contributes to climate regulation.
Background
Almost all life on Earth is fueled by radiant energy from the Sun. Capturing this energy is restricted to plants and algae plus thecyanobacteria that can photosynthesize. This is a complex process in which these organisms, which contain chlorophyll, a green pigment capable of absorbing light, extract carbon dioxide and water from their surroundings. All three are combined in a series of intricate chemical steps to produce complex organic molecules such as carbohydrates (sugars and starches). These are used by the organisms to produce more complex molecules such as amino acids, lipids, and proteins, which, in turn, generate all their component parts (lignin, cellulose, DNA).
In terrestrial plants photosynthesis occurs mainly in the leaves. In a few organisms, known as chemoautotrophs, the energy source is not the Sun; the energy is obtained as they break down inorganic chemicals in rocks. Most chemotrophs are bacteria that live in hostile environments, such as deep-ocean vents or the hot environments created where geysers or thermal waters occur. All organisms that can harness an energy source and store energy in their components are known as autotrophs, a term which literally means “self-feeding.” More than 99 percent of global primary production is generated by green plants on land and algae in the oceans, even though less than 1 percent of the Sun’s energy reaching the Earth is captured. The measurement of primary production is difficult, and estimates can be presented in various ways, including in terms of carbon fixation. One estimate, published in the journal Science in 1998, considers that the total global annual primary production of the biosphere, 104.9 gigatons of carbon per year (Gt C yr1) of 56.4 Gt C yr1, is generated by terrestrial photosynthesis, and 48.5 Gt C yr1 is generated by oceanic photosynthesis. This energy capture is the first stage in all food chains and webs.
The rate at which plants and algae photosynthesize is determined by environmental conditions that may limit primary production. Constraints in the terrestrial include climate-related factors such as water availability and annual temperature regimes. For example, periods of drought or low temperatures will reduce rates of photosynthesis, as will periods of high temperatures, when high evaporation rates limit water availability. Soil conditions may also constrain growth. Factors such as degree of acidity or alkalinity; nutrient availability, especially in relation to nitrogen, phosphorus, and iron; and a range of essential micronutrients are important. For example, iron is essential for the formation of chlorophyll. Other limiting factors include shading, pests, and diseases. In aquatic environments constraints on productivity include the availability of light and nutrients such as nitrogen and iron. Variations in these factors help to explain patterns of terrestrial primary production which, at the global scale, is highest in forest ecosystems in tropical regions and lowest in high-latitude tundra ecosystems. In the oceans productivity is greatest in shallow seas, estuaries, and coral reefs, where light and nutrients are abundant.
Gross and Net Primary Productivity
Gross primary productivity (GPP) is the total amount of organic matter produced per unit of area per unit of time. All plants and photosynthesizing organisms need to use some of the energy they have captured for metabolic processes, transpiration, reproduction, and especially respiration. As much as 70 percent of GPP may be used up in these processes. The remaining energy, which is stored as organic matter, is net primary productivity (NPP) and is stored as living tissue (the standing-crop biomass in, above, and below ground plant parts or algae). However, plants lose their leaves and algae die; this dead organic material enters the soil or water body and is described as detritus. This, plus the standing-crop biomass, constitute the total biomass of an or area, and is a measure of carbon storage. The living and dead organic matter provide the energy for organisms that cannot capture the Sun’s energy and so cannot make their own food. These are known as heterotrophs (consumers). Living plant material and algae are grazed by herbivores, mainly animals and insects, as the second stage in a grazing food chain or web. Further stages involve omnivores and carnivores. Dead organic matter provides energy for another group of organisms known as detrivores (also known as saprotrophs), which includes fungi, bacteria, many types of insects and worms, and a range of marine species. They are organisms that obtain energy by decomposing organic matter and constitute the detrital pathway of energy transfer. Although detrital pathways are less understood than grazing pathways, they predominate in terms of energy transfers in most ecosystems and may account for as much as 90 percent.
Primary Production and Climate
Primary production is closely linked with climate because of the major role played by carbon dioxide in energy capture and global temperatures. From the early days of Earth’s history, climate and life have been intimately linked (this is an essential tenet of the Gaia hypothesis) through biogeochemical cycles such as those of oxygen, nitrogen, and carbon. This mutual evolution has given rise to an oxygen-rich that facilitated the evolution of mammals and, eventually, humans, while huge volumes of carbon have been removed from the atmosphere in primary production and stored as limestone, coal, oil, and natural gas. All constitute vital resources for building materials and sources of fossil-fuel energy.
This relationship has also influenced global temperatures, maintaining them with a range suitable for the survival and evolution of life-forms. The removal of carbon dioxide from the atmosphere led to global cooling, but the persistence of a small amount of the gas (about 0.04 percent) is essential to create the “greenhouse effect,” which traps heat radiated back into space from the Earth’s surface. The extent and function of the Earth’s standing-crop biomass on land and in the oceans plays a role in maintaining this balance. However, two processes are jeopardizing the stability of this life-atmosphere relationship, specifically the destruction of the Earth’s natural terrestrial ecosystems, especially deforestation, and the vast scale of fossil fuel use. Both extant and stored primary production are thus increasing the volume of atmospheric carbon dioxide to such an extent that global climatic change is now occurring, the chief signature of which is global warming.
Primary Production and Society
Primary production underpins all aspects of human endeavor. By 2023, the world's population had exceeded 8 billion. The United Nations warned that there is a carrying capacity for human life on Earth, and the population cannot continue to grow indefinitely. However, it is difficult for experts to calcuate Earth's carrying capacity because resources, such as the direct consumption of crops or the consumption of meat and animal products, are not distributed equally. According to the United Nations, the world's population was expected to exceed 9.5 billion by 2050 and experts were concerned that the Earth may not have the resources to sustain so many people. Of Earth's cropland, about 34 percent has had its natural vegetation cover removed or substantially altered. The resources on this land are not only used for primary production. The land is also the source of many other “goods” used by humans, not least of which is wood. Used for furniture, fuel, house construction, fencing and paper, the world’s woodland and forest NPP has been, and continues to be, a vital resource. An inadvertent human influence on primary production also exists. For example, urbanization and road, rail, and dam construction take land out of primary production. Deliberate and accidental fires destroy NPP in forests, woodlands, heathland, and grassland, while pollution impairs the production capacity of all types of ecosystems at the micro and macroscale.
Estimates have been made as to just how much of global NPP humans control and/or influence. Given the wide scope of human appropriation of NPP, for everything from perfumes to pharmaceuticals, such estimates vary widely, ranging between 50 and 80 percent. This appropriation of NPP will increase as human population increases and as standards of living improve, but it will be to the detriment of the integrity of biogeochemical cycles.
Bibliography
Fahey, Timothy J., and Alan K. Knapp. Principles and Standards for Measuring Primary Production. New York: Oxford University Press, 2007.
Haberl, Helmut, et al. “Quantifying and Mapping the Human Appropriation of Net Primary Production in Earth’s Terrestrial Ecosystems.” Proceedings of the National Academy of Sciences of the USA 104, no. 31 (July 31, 2007): 12,942-12,947.
Ramankutty, N., et al. “Farming the Planet: One. Geographic Distribution of Global Agricultural Lands in the Year 2000.” Global Biogeochemical Cycles 22, no. 1 (March 1, 2008).
Townsend, Colin R., Michael Begon, and John L. Harper. Essentials of Ecology. 3d ed. Malden, Mass.: WileyBlackwell, 2008.
Wilmoth, John, et al. "As the World's Population Surpasses 8 Billion, What Are the Implications for Planetary Health and Sustainability?" United Nations, 11 July 2023, www.un.org/en/un-chronicle/world-population-surpasses-8-billion-what-are-implications-planetary-health-and. Accessed 6 Jan. 2024.