C4.2 Transfers of energy and matter

An ecosystem is a community of organisms together with its abiotic environment. Ecosystems are open systems: both energy and matter cross their boundaries.

• Energy enters mainly as light and leaves mainly as heat.
• Matter enters and leaves too: CO₂ and O₂ diffuse in and out, water flows through, seeds and pollen blow in, and animals migrate.

In a closed system only energy can pass in or out; matter stays inside. A sealed bottle garden is close to a closed system: light enters and heat leaves, but the same atoms must be recycled inside. The Earth as a whole is also nearly closed for matter but open for energy.

Sunlight as the main energy source

In almost every ecosystem, sunlight is the source of energy. Photosynthesis by plants, algae and cyanobacteria converts light energy into chemical energy in carbon compounds, and every other organism depends on these compounds directly or indirectly. There are exceptions:

Organic matter that drifts into a cave or sinks into the deep sea was made by photosynthesis elsewhere, so only the chemoautotroph-based communities are truly independent of sunlight.

A food chain is a sequence of organisms in which each one feeds on the one before it. When a consumer eats an organism at the previous stage, the chemical energy in that organism’s carbon compounds passes to the consumer.

oak leaves → winter moth caterpillar → blue tit → sparrowhawk

Arrows show the direction of transfer

The arrows mean “is eaten by” and point from food to feeder: the direction in which energy and biomass move. Most consumers eat more than one type of food and are eaten by more than one predator, so the chains in a community link up into a food web.

Trophic levels

Many organisms have a varied diet, so they occupy different trophic levels in different food chains. A red fox is a secondary consumer when it eats a rabbit but a primary consumer when it eats blackberries. A wood mouse eating acorns is a primary consumer, but when it eats caterpillars it is a secondary consumer, so a tawny owl that eats that mouse is acting as a tertiary consumer.

All organisms need carbon compounds. Autotrophs make their own from simple inorganic substances: CO₂ (or hydrogen carbonate ions in water) plus sources of other elements such as nitrate and phosphate ions. They need an external energy source for two things:

1. Carbon fixation: converting CO₂ into an organic compound (a reduction reaction).
2. Anabolic reactions that build the fixed carbon into larger molecules: polysaccharides, proteins, lipids and nucleic acids.

Autotrophs differ in where that energy comes from.

Photoautotrophs

Photoautotrophs use light as their energy source, through photosynthesis (see C1.3 Photosynthesis). Plants, algae and cyanobacteria are photoautotrophs, and they are the producers in almost all ecosystems.

Chemoautotrophs

Chemoautotrophs use energy released by oxidation reactions of inorganic substances. Oxidation reactions are exothermic, so they release energy that cells can use. Only some bacteria and archaea are chemoautotrophs.

Iron-oxidizing bacteria, such as Acidithiobacillus ferrooxidans, live in water that contains dissolved iron(II) ions, for example acidic drainage from mines. They oxidize Fe²⁺ to Fe³⁺:

Fe²⁺ → Fe³⁺ + e⁻

The electrons removed pass along an electron transport chain in the bacterium’s plasma membrane. The energy released is used to make ATP (by chemiosmosis) and reduced NAD, which are then used to fix CO₂ into carbon compounds. The Fe³⁺ often forms orange-brown deposits of iron hydroxide on stream beds. Each Fe²⁺ yields only a small amount of energy, so these bacteria must oxidize huge quantities of iron.

Heterotrophs obtain carbon compounds from other organisms. Large molecules such as proteins, polysaccharides and nucleic acids cannot be absorbed into cells, so they are first digested into small, soluble molecules (amino acids, sugars, nucleotides). These are absorbed and then assimilated: used to build the heterotroph’s own carbon compounds. For example, a rabbit digests grass proteins to amino acids and then makes rabbit proteins from them.

• Internal digestion: animals take food into the gut and digest it with enzymes there; unicellular consumers such as Paramecium digest food inside vacuoles.
• External digestion: saprotrophs (many fungi and bacteria) secrete enzymes onto their food and absorb the products.

Decomposers

Decomposers are saprotrophs that obtain energy and carbon from dead organic matter:

• faeces (egested, undigested food);
• dead parts of organisms, such as fallen leaves, shed skin, hair and moulted exoskeletons;
• whole dead organisms.

This material still contains chemical energy in its carbon compounds. Detritivores such as earthworms and woodlice also feed on it, but they ingest it and digest it internally. Without decomposers, dead matter would pile up and the elements in it would be locked away.

Both autotrophs and heterotrophs respire

All cells need a continuous supply of ATP (for anabolic reactions, active transport, movement and, in birds and mammals, keeping warm). In both autotrophs and heterotrophs, this ATP comes from cell respiration (see C1.2 Cell respiration): carbon compounds such as glucose and fatty acids are oxidized, releasing energy that is used to phosphorylate ADP. Plants respire all the time, in the light and the dark; photosynthesis does not replace respiration.

Only a small fraction of the energy in one trophic level reaches the next, typically around 10%, though values between about 5% and 20% are common. There are three main reasons:

1. Not all of the organisms are eaten. Some die of other causes, and some parts (roots, bark, bones, fur) are left uneaten. Their energy passes to decomposers and detritivores instead.
2. Not all of what is eaten is digested and absorbed. Indigestible material such as cellulose, lignin or bone is egested in faeces, and its energy also goes to decomposers.
3. Cell respiration. Much of what is absorbed is oxidized to CO₂ and water to release energy. That energy is used by the organism and ends up as heat; the carbon compounds respired can never pass to the next level.

Heat is produced at every stage

Energy transfers in cells are never 100% efficient. Heat is produced:

• when ATP is made in cell respiration, because only part of the energy released by oxidizing glucose is captured in ATP;
• when ATP is used to do work in cells (muscle contraction, active transport, synthesis), because these processes are also inefficient.

This happens in autotrophs and heterotrophs alike, and in decomposers. Heat radiates away into the environment and cannot be converted back into chemical energy by organisms, so energy cannot be recycled: it flows through the ecosystem once. Ecosystems therefore need a continuous input of energy, from sunlight in most cases.

Decomposers and detritivores are not usually drawn as part of food chains, but energy that “leaves” a chain in uneaten matter and faeces passes to them, and they release it as heat in their own respiration.

An energy pyramid shows how much energy each trophic level of an ecosystem gains per year, so it represents both the energy transferred along the food chain and the energy lost between levels.

Constructing an energy pyramid from data

H. T. Odum measured energy flow in Silver Springs, Florida. Converted to kJ m⁻² yr⁻¹:

The transfer efficiency from primary to secondary consumers is 1,600 ÷ 14,100 × 100 = 11.3%, so 88.7% of the energy was lost. The other transfers were about 16% and 5.5%: the “10% rule” is only a rough average.

Why food chains are short

Because most energy is lost at each step, the energy available falls geometrically: with 10% efficiency, the fourth level receives only 0.1% of what producers gained. After four or five levels there is not enough energy to support a viable population of another level, so food chains rarely have more than four or five trophic levels.

At each successive level there are fewer organisms, or smaller ones, so there is less biomass. However, the energy content per unit mass is not lower: a gram of hawk contains about as much energy as a gram of blue tit. The top levels simply have less biomass. This is why top predators are rare and need large territories.

Production is the accumulation of carbon compounds in biomass, which happens when organisms grow or reproduce. It is a rate, measured as mass of carbon per unit area per unit time, usually g m⁻² yr⁻¹.

Primary production

Primary production is the accumulation of carbon compounds in biomass by autotrophs.

• Gross primary production (GPP): the total carbon compounds made by photosynthesis.
• Net primary production (NPP) = GPP − carbon compounds used by the producers in their own respiration. NPP is the biomass available to consumers and decomposers.

Biomes vary in their capacity to accumulate biomass. Production is highest where conditions for photosynthesis are good all year: warm temperatures, plenty of water and light, a long growing season and enough mineral nutrients. It is lowest where one factor is severely limiting: water in deserts, temperature and season length in tundra, mineral nutrients in the open ocean.

Secondary production

Secondary production is the accumulation of carbon compounds in biomass by heterotrophs (consumers and decomposers). It is always lower than primary production in an ecosystem because, at every trophic level, a large part of the carbon compounds is oxidized to CO₂ and water in cell respiration and so is lost from biomass. Uneaten and undigested material also limits what consumers can turn into their own tissue.

Unlike energy, carbon is recycled. A carbon cycle diagram shows pools (boxes: where carbon is stored) and fluxes (arrows: transfers between pools). The three fluxes caused directly by organisms are:

• photosynthesis: CO₂ from air or water is fixed into carbon compounds by producers;
• feeding: carbon compounds pass to consumers, and to decomposers via dead organic matter;
• cell respiration: all organisms oxidize carbon compounds, returning CO₂ to the air or water.

Carbon sinks and carbon sources

For a whole ecosystem, the rates of photosynthesis and respiration are not always equal:

Combustion releases stored carbon

Carbon can stay locked in some pools for very long periods. These stores formed at very different times:

• Biomass (wood): carbon fixed over years to centuries.
• Peat: partly decomposed plant matter in waterlogged bogs, mostly accumulated over the last 10,000 years or so, and still forming.
• Coal: from plant matter buried in swamps, much of it about 300–360 million years ago (the Carboniferous).
• Oil and natural gas: mostly from the remains of marine plankton buried in sediments over tens to hundreds of millions of years.

Combustion oxidizes these carbon compounds and releases CO₂ into the atmosphere. Fires can start naturally, when lightning strikes dry vegetation or peat, but human activities — burning coal, oil and gas for energy, and burning forests to clear land — have greatly increased the rate of combustion since the Industrial Revolution.

The Keeling curve

In 1958 Charles David Keeling began measuring atmospheric CO₂ at Mauna Loa Observatory, Hawaii, far from local sources of pollution. The record, the Keeling curve, shows two patterns.

1. Annual fluctuation (the zig-zag). CO₂ falls from about May to October and rises from about October to May, by roughly 6 ppm at Mauna Loa. Most land, and so most vegetation, is in the Northern Hemisphere. In the northern summer, photosynthesis exceeds respiration and removes CO₂; in the northern autumn and winter, many plants lose their leaves or stop growing while respiration (especially by decomposers) continues, so respiration exceeds photosynthesis.
2. Long-term trend. Each year’s rise is not fully reversed, so concentration climbs: from about 316 ppm in 1959 to over 420 ppm in the 2020s (before the Industrial Revolution it was about 280 ppm). This is due mainly to combustion of fossil fuels, with deforestation adding to it. The rise has accelerated, from under 1 ppm per year in the 1960s to over 2 ppm per year recently.

The consequences of rising CO₂, including climate change and ocean acidification, are covered in D4.3 Climate change.

Photosynthesis and respiration depend on each other

Aerobic respiration depends on oxygen in the atmosphere (or dissolved in water), and almost all of that oxygen was released by photosynthesis, when water is split. In turn, photosynthesis depends on carbon dioxide that is released by cell respiration (and combustion). Autotrophs also respire, but the overall exchange links autotrophs and heterotrophs.

The fluxes are huge: on land alone, photosynthesis fixes more than 100 gigatonnes (10¹⁷ g) of carbon each year, and a similar amount is returned by respiration. This makes it one of the largest interactions between autotrophs and heterotrophs on Earth.

All elements are recycled

Carbon is not special. Every element that organisms use is recycled: C, H, O, N, P and S in organic molecules, and ions such as K⁺, Ca²⁺, Mg²⁺ and Fe²⁺/Fe³⁺. The amount of each on Earth is finite, yet none has run out in over three billion years of life, because:

• autotrophs absorb elements from the abiotic environment as inorganic ions and molecules (e.g. CO₂, NO₃⁻, PO₄³⁻);
• the elements pass along food chains in carbon compounds, or are absorbed by heterotrophs as ions;
• decomposers digest dead organic matter and release the elements back into soil, water and air as inorganic substances, which producers can absorb again.

Atoms are not changed by any of these steps, so they can be reused indefinitely. Details of the nitrogen cycle and other nutrient cycles are not required. For how populations and communities interact, see C4.1 Populations and communities.