Converting Solar Energy to Biomass

Ecosystems are made up of a mixture of biotic (living) and abiotic (non-living) components. Biotic components include producers (plants), consumers (e.g. herbivores, carnivores, omnivores), and detritus feeders/decomposers (e.g. fungi, bacteria, worms). Abiotic components involve all non-living components such as chemical nutrients, minerals, and their associated cycles and processes. However, probably the most important abiotic component of ecosystems is solar energy.

Energy Flows: Photosynthesis and Respiration

Solar energy is energy from the sun, and is what primarily powers the biosphere. The process through which solar energy becomes part of the biosphere is photosynthesis, the process of turning solar energy into plant matter. By 'fixing' solar energy into carbohydrate energy within plant tissues, solar energy is effectively the foundation of Earth's biosphere. (A limited number of ecosystems use chemical reactions rather than photosynthesis for energy; these ecosystems are in very dark locations without any light such as the deep ocean floor or caves.)

Photosynthesis is done by primary producers - plants (terrestrial ecosystems) and algae (aquatic ecosystems). Producers are also known as autotrophs (self-feeders) because they produce their own food from solar energy through photosynthesis. Producers are the critical biotic link between solar energy and the biosphere - they are what make solar energy available to use by the rest of the ecosystem.

Through photosynthesis, producers 'manufacture' starches and sugars through chemical reactions within plant leaves. This process creates energy-rich food for the plant, releases oxygen, and removes carbon dioxide (CO₂) from the atmosphere. (In fact, primary producers remove an estimated 100 billion tons of CO₂ from the atmosphere every year!)

Carbohydrates are the organic result of photosynthesis, and are what makes up plant tissue. As the name indicates, carbohydrates are made up of Carbon (C), Oxygen (O), and Hydrogen (H). Plants begin by making the simple sugar glucose (C₆H₁₂O₂). They use these sugars to build starches, or complex carbohydrates. Starches are the principle food stored within plant tissue - plants store energy in the bonds within carbohydrates to use later. The energy stored within plant tissues is also available for other organisms (consumers) to eat, facilitating the flow of energy throughout the ecosystem.

To access and consume this energy, plants convert the stored carbohydrates within their tissues back into energy, a process known as respiration. Respiration is essentially the reverse of photosynthesis: while solar energy is the input for photosynthesis, respiration releases heat energy as an output.

In the process of photosynthesis, plants consume light, carbon dioxide (CO2), nutrients, and water (H2O) and produce outputs of oxygen (O2) and carbohydrates (sugars) as stored chemical energy. Plant respiration, illustrated on the right during the night, approximately reverses this process. The balance between photosynthesis and respiration determines net photosynthesis and plant growth.

Net Primary Productivity and Biomass

The net photosynthesis for an ecosystem (meaning the amount of photosynthesis left over after respiration is accounted for) is that ecosystem's net primary productivity. It can also be described as the amount of stored chemical energy that the ecosystem generates, in the form of biomass.

Biomass is the total amount of organic matter in an ecosystem, including living and recently living, animal and plant. Net primary productivity determines the amount of biomass available for consumption by other organisms, known as consumers (heterotrophs). Consumers are organisms that feed on organic matter (biomass) for energy.

Productivity levels (i.e. the amount of plant growth, and thus the amount of total biomass) are tied to sunlight and precipitation. More sunlight and precipitation results in higher productivity, meaning more biomass. As a result, net primary productivity is highest in Earth's equatorial regions, between the Tropics of Cancer and Capricorn (located about 23.5 degrees north and south of the equator, respectively). The zone between these latitudes are characterized by a hot, humid climate; for example, tropical rainforests in this region are highly productive in terms of total biomass.

"This map shows the land surface resized according to its annual gross primary production (GPP)...GPP is the net amount of energy that is produced by the main energy producers of an ecosystem" (Terrestrial Ecosystem Productivity Links to an external site., World Mapper). The map is adjusted to proportionately show the terrestrial ecosystem productivity of Earth's different regions, with larger areas in green/blue having higher primary productivity.

On the other hand, net primary productivity decreases at higher latitudes and elevations. Less sunlight and precipitation in these regions results in lower productivity, and thus lower total biomass. For example, desert (very little precipitation) or tundra (very high latitude) ecosystems have far less biomass than a tropical rainforest. As an illustration, a hectare (2.5 acres) of sugarcane grown in the tropics can fix 45 metric tons of carbon per year. However, a hectare of desert plants can only fix 1% of that amount.

Net primary productivity can also vary seasonally in temperate ecosystems and higher latitudes, with higher productivity during spring and summer and lower during fall and winter.

"Terrestrial ecosystems rely almost exclusively on the sun's energy to support the growth and metabolism of their resident organisms. Plants are quite literally biomass factories powered by sunlight, supplying organisms higher up the food chain with energy and the structural building blocks of life." (Gough, 2011 Links to an external site.: p. 28)