Cycles within Ecosystems

The carbon cycle moves carbon atoms between the atmosphere, oceans, living organisms and fossil fuels. The atmosphere contains ~0.04 % CO₂ — small but vital.

Processes that REMOVE CO₂ from the atmosphere:

1. Photosynthesis (the ONLY major sink). $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$ Plants, algae and some bacteria absorb CO₂ → fix it into glucose using light energy. The carbon now enters the food chain.

2. Ocean dissolution. CO₂ dissolves in seawater forming carbonic acid (H₂CO₃) and bicarbonate ions (HCO₃⁻). Marine producers (phytoplankton) use this dissolved CO₂. Some carbon is locked into carbonate shells (forams, corals) → eventually limestone.

Processes that RETURN CO₂ to the atmosphere:

3. Respiration. All living organisms (plants, animals, decomposers) respire continuously: glucose + O₂ → CO₂ + H₂O + energy. This is the main natural source of atmospheric CO₂.

4. Decomposition. When organisms die, saprobiotic bacteria and fungi break down the dead matter. As they respire the absorbed nutrients, they release CO₂.

5. Combustion (burning). Burning wood OR fossil fuels (coal, oil, natural gas) releases CO₂. Fossil fuels are the compressed remains of plants and animals that died millions of years ago in anaerobic conditions — burning them releases carbon that has been locked away geologically.

Carbon flow up the food chain (feeding):

• Plants fix CO₂ → glucose, then convert glucose to starch, cellulose, proteins, lipids, DNA — all organic carbon compounds.
• Herbivores eat plants → digest carbon compounds → use for their own tissues + respiration.
• Carnivores eat herbivores → same process.
• All organisms eventually die → decomposers break them down → CO₂ returned to atmosphere.

Long-term lock-up:

• Fossil-fuel formation: dead organisms buried in anaerobic conditions (peat bogs, ocean sediments) over millions of years → coal, oil, natural gas. Carbon locked out of the active cycle.
• Limestone: carbonate shells of marine organisms accumulate on the seabed → become sedimentary rock. Released only by weathering or volcanism over geological timescales.

Human disruption. In the natural cycle, photosynthesis and respiration roughly balance. Burning fossil fuels and deforestation add CO₂ faster than photosynthesis can remove it → atmospheric CO₂ has risen from 280 ppm (pre-industrial) to >420 ppm today → ENHANCED greenhouse effect → global warming.

Atmospheric nitrogen (N₂) makes up 78 % of air — but it is INERT. Plants and animals cannot use N₂ directly. It must be CONVERTED (fixed) into ammonia, then nitrate, before plants can absorb it.

Step 1 — Nitrogen fixation (N₂ → NH₃ / NH₄⁺).

Three routes:

• Rhizobium (a bacterium) lives in MUTUALISTIC root nodules of LEGUMINOUS plants (peas, beans, clover, lentils). The plant supplies sugars; Rhizobium fixes N₂ → ammonia → used by the plant. After harvest, the nitrogen-rich roots are ploughed back into the soil — natural fertiliser.

• Azotobacter (a free-living soil bacterium) fixes atmospheric N₂ → ammonia in the soil itself, without a plant host. Some species live in water.

• Lightning. The huge energy in a lightning strike combines atmospheric N₂ with O₂ → nitrogen oxides → dissolved in rain → reach the soil as nitric acid → nitrate.

• (Industrial: the Haber process N₂ + 3H₂ → 2NH₃ makes ammonia fertiliser — not biological, but a major source of soil nitrogen on farmland.)

Step 2 — Ammonification (proteins / urea → ammonia).

Saprobiotic decomposers (bacteria and fungi) break down:

• DEAD ORGANISMS (proteins, nucleic acids).
• ANIMAL WASTE (urea in urine).

They release AMMONIA (NH₃) / AMMONIUM ions (NH₄⁺) into the soil.

Step 3 — Nitrification (NH₃ → NO₂⁻ → NO₃⁻).

Nitrifying bacteria in well-AERATED (aerobic) soil carry out a TWO-STEP oxidation:

• Nitrosomonas: NH₃ / NH₄⁺ → NO₂⁻ (nitrite). Requires O₂.
• Nitrobacter: NO₂⁻ → NO₃⁻ (nitrate). Requires O₂.

Both reactions release energy that the bacteria use for their own metabolism.

Step 4 — Plant uptake of nitrate (spec 4.16P).

Plants absorb NITRATE (NO₃⁻) from the soil through their ROOT HAIR CELLS by ACTIVE TRANSPORT (nitrate concentration is higher inside the cell than outside). The nitrate is transported in the xylem to leaves.

In the leaves, plants use nitrate to make AMINO ACIDS, which they then link together into PROTEINS. Plants also need nitrogen for:

• DNA and RNA (nitrogenous bases).
• Chlorophyll (contains N).
• ATP.

Step 5 — Consumers — nitrogen passes UP the food chain.

Animals eat plants (or other animals). They DIGEST plant proteins → amino acids → use these to build their own proteins.

Step 6 — Excretion.

When animals break down excess amino acids in the LIVER (deamination), they form UREA, which they excrete in urine. Decomposers then convert this back to ammonia (Step 2 again).

Step 7 — Denitrification (NO₃⁻ → N₂).

Denitrifying bacteria (e.g. Pseudomonas) are anaerobic — they thrive in WATERLOGGED soils where there is no O₂.

They convert NO₃⁻ → N₂ gas, which escapes to the atmosphere. This step is UNDESIRABLE for farmers because it REMOVES nitrate from the soil → plants are short of nitrogen → poor growth.

Why farmers care.

• Drain and plough fields → keep soil aerated → favours nitrifying bacteria over denitrifying.
• Plant legumes (clover, peas) in rotation → Rhizobium adds nitrogen.
• Apply nitrate fertiliser → direct route to plant nitrogen.
• Avoid over-applying (causes EUTROPHICATION in streams).

The water cycle (hydrological cycle) moves water between oceans, atmosphere, land and living organisms — continuously, powered by solar energy.

Stages:

1. Evaporation. Heat from the sun causes liquid water in oceans, rivers, lakes, soil, etc. to evaporate → water vapour rises into the atmosphere. The largest source is tropical oceans.

2. Transpiration. Water inside plant leaves evaporates from the mesophyll cell surfaces and diffuses out through the STOMATA. In rainforests, transpiration can account for >50 % of atmospheric moisture.

3. Condensation. As water vapour rises, it cools (temperature decreases with altitude). At a low enough temperature, the vapour condenses around tiny dust / aerosol particles → forms droplets → forms clouds.

4. Precipitation. Droplets merge until heavy enough to fall as RAIN (or snow, hail, sleet). Returns water to land or ocean.

5. Surface runoff. On land, water flows over the surface → into streams → rivers → back to the sea.

6. Infiltration / percolation. Water seeps through soil into groundwater (aquifers) → eventually feeds rivers and springs.

7. Plant uptake. Roots absorb soil water by osmosis through ROOT HAIR CELLS. The water rises through the xylem to leaves — the transpiration stream — driven by the suction of evaporation from leaves. Delivers dissolved mineral ions to all tissues.

Why plants matter to the water cycle:

• Transpiration cools the local climate — like sweat cools the body, transpired water carries latent heat away.
• Tree cover slows runoff → roots hold soil; canopy intercepts rain → more water enters groundwater (recharges aquifers) rather than washing away.
• Deforestation disrupts this — without trees, less transpiration → less local rainfall (drier climate); more runoff → flash floods; soil erosion downstream.

Drinking-water resources. Most accessible fresh water is in groundwater (aquifers); a small fraction is in rivers and lakes. Over-extraction faster than recharge (e.g. for irrigation) depletes aquifers — a major sustainability concern in agriculture.

Decomposers are saprobiotic bacteria and fungi that break down DEAD organic matter and waste. They are essential for recycling nutrients.

How they work (saprobiotic nutrition):

1. Secrete digestive ENZYMES onto the dead material.
2. Enzymes break large molecules down into smaller, soluble nutrients (proteins → amino acids; carbohydrates → sugars; lipids → fatty acids + glycerol).
3. Absorb the soluble nutrients across their cell wall.
4. Use the nutrients for respiration (release CO₂) and growth.

Role in the carbon cycle:

• They RESPIRE the carbon they absorb → release CO₂ back to the atmosphere.
• Without decomposers, carbon would stay locked in dead bodies → atmosphere starved of CO₂ → plants couldn't photosynthesise → ecosystem collapses.

Role in the nitrogen cycle:

• They break down PROTEINS (in dead organisms) and UREA (in urine) → release AMMONIA (ammonification).
• This ammonia is the starting material for nitrifying bacteria (Nitrosomonas → Nitrobacter) → eventually plant nitrate.
• Without decomposers, plant nitrogen would have no way back into the soil → crops would fail.

Factors affecting rate of decomposition:

• Temperature — higher = faster (enzymes work better). Compost heaps heat themselves up.
• Moisture / water — decomposers need water for enzyme activity. Dry conditions slow decay.
• Oxygen — most decomposers are aerobic. Waterlogged anaerobic conditions slow decomposition and produce methane.
• pH — most decomposers prefer near-neutral pH; acidic soils slow decomposition.
• Material — soft tissues decay faster than woody/lignin-rich material; bones and shells last longest.

Practical relevance:

• Composting: mix garden waste with food scraps; keep moist + warm + aerated → fast decomposition → nutrient-rich compost for plants.
• Sewage treatment: uses aerobic bacteria (decomposers) to break down sewage in trickling filters.
• Bioremediation: specific bacteria are used to clean up oil spills, heavy metals, etc.

Without decomposers: within a few years, dead bodies would pile up; soil nutrients would deplete; plant growth would crash; the entire ecosystem would fail.

Common features of biogeochemical cycles:

Key contrasts with energy flow:

• Atoms (C, N, water molecules) are RECYCLED through the cycle — they pass between organisms and environment endlessly.
• Energy is NOT recycled — it flows in ONE direction (sun → producers → consumers → heat lost to space). The Sun must continuously resupply energy via photosynthesis.

Why this matters. When humans interfere with a cycle:

• Carbon cycle disruption (excess combustion of fossil fuels + deforestation) → atmospheric CO₂ rises → enhanced greenhouse effect → climate change.
• Nitrogen cycle disruption (excess fertiliser / sewage) → eutrophication of waterways.
• Water cycle disruption (deforestation, over-extraction of groundwater) → drought, flooding, water shortages.

Sustainable management seeks to keep cycles in balance — reforest, reduce fossil-fuel use, treat sewage, manage fertiliser carefully.