Metabolism Beyond the Cell: Ecosystems, the Carbon Cycle and Technologies

Your cells and the whole planet

The air you breathe is about 21% oxygen because of billions of years of photosynthesis. The CO₂ that is warming the climate comes mostly from burning coal, oil and natural gas, which are the remains of ancient photosynthesizers. Even a summer "dead zone" in Lake Erie comes down to photosynthesis and respiration getting out of balance. Expectation C1.1 asks you to analyse how metabolic processes work in, and connect, biotic (living) and abiotic (non-living) systems. This final lesson zooms out from the cell to the planet, and then looks at technologies that put metabolism to work.

Metabolism shaped Earth's atmosphere

Early Earth's atmosphere had almost no free O₂. When cyanobacteria evolved oxygen-producing photosynthesis, O₂ slowly accumulated; geologists date a major rise in atmospheric oxygen (the Great Oxidation Event) to roughly 2.4 billion years ago. That change made aerobic respiration, with its much larger ATP yield, possible, and eventually allowed large, active, multicellular life. It also created the ozone layer (O₃ formed from O₂), which shields land organisms from ultraviolet radiation.

Today, photosynthesis by land plants and ocean phytoplankton removes huge amounts of CO₂ each year, and respiration by all organisms (especially decomposers) returns a similar amount. This balance links the biosphere (biotic) with the atmosphere, hydrosphere and lithosphere (abiotic).

The carbon cycle

• Photosynthesis moves carbon from atmospheric CO₂ into organic molecules.
• Feeding moves it from producers to consumers.
• Cellular respiration (by producers, consumers and decomposers) returns it to the atmosphere as CO₂.
• Decomposers (bacteria and fungi) respire the carbon in dead matter and wastes; in waterlogged, oxygen-poor places (bogs, wetlands, deep sediments) decomposition is slow and anaerobic, so carbon builds up as peat. Canada's northern peatlands store enormous amounts of carbon this way.
• Over millions of years, buried organic matter can become fossil fuels. Combustion releases that ancient carbon rapidly.
• The oceans exchange CO₂ with the air; dissolved CO₂ forms carbonic acid (as in your BTB tests), and rising CO₂ is making seawater slightly more acidic (ocean acidification).

Because burning fossil fuels adds carbon faster than photosynthesis and the oceans can remove it, atmospheric CO₂ has risen from about 280 ppm before the Industrial Revolution to about 430 ppm in 2026, the main cause of current climate change (you studied climate change in Grade 10).

When the balance tips: eutrophication and dead zones

Phosphorus and nitrogen from farm fertilizer, manure and sewage can wash into lakes. These nutrients act like fertilizer for algae and cyanobacteria, which bloom at the surface (lots of photosynthesis). When the bloom dies, it sinks, and decomposers respire the dead cells, using up dissolved O₂ in the deeper water. The result can be hypoxia (very low O₂), which kills fish and other organisms, creating a "dead zone".

Lake Erie, the shallowest and warmest of the Great Lakes, is a well-known example: it has had large algal blooms in summer and low-oxygen zones in its central basin. In 2016, Canada and the United States set a target to cut the phosphorus entering Lake Erie by 40% (compared with 2008 levels), and Ontario farmers, municipalities and conservation authorities are working on ways to reduce runoff. Some cyanobacteria in blooms can also release toxins, which is why public health units sometimes issue beach and drinking-water advisories.

Technologies that use metabolism

Fermentation in food and drink. Bread, yogurt, cheese, sauerkraut, kimchi, soy sauce and vinegar all depend on microbial metabolism. Food scientists control temperature, pH and the microbe used to get the product they want.

Fuel ethanol. Yeast fermentation turns sugars from corn (and, increasingly, from other plant material) into ethanol, which is blended into gasoline. Benefits: renewable; the CO₂ released was recently taken from the air by the crop. Drawbacks: growing, fertilizing, harvesting and processing the crop uses energy and land that could grow food, so the net climate benefit depends on how it is produced.

Anaerobic digestion and biogas. In sealed tanks with no O₂, anaerobic bacteria and methane-producing archaea break down manure, food waste and sewage sludge into biogas (mostly methane), which can be burned for heat and electricity. Some Ontario farms and municipalities run digesters. Capturing the methane is important, because methane is a much more powerful greenhouse gas than CO₂ if it escapes.

Wastewater treatment. In the activated sludge process, blowers bubble air through sewage so that aerobic microbes can respire dissolved organic matter into CO₂ and water. This removes the material that would otherwise cause oxygen depletion in rivers and lakes.

Composting uses aerobic decomposers (turning the pile adds O₂). Green-bin programs in many Ontario cities send food waste to composting or anaerobic-digestion facilities.

Research frontiers: scientists are working on "artificial leaves" that split water using sunlight, and on engineering crops for more efficient photosynthesis (for example, research aimed at giving rice C₄-like features, or at reducing photorespiration). These are still in development.