Biogeochemical Cycles and Ecosystem Energetics — Study Notes

The Carbon Cycle

• Definition: movement of carbon atoms and carbon-containing molecules between sources and sinks.
• Carbon sink: natural reservoir that absorbs more carbon than it releases; examples include forests and the ocean; duration of storage depends on the reservoir (short-term vs long-term).
• Carbon source: releases significant carbon into the atmosphere; examples include deforestation and raising cattle.
• Core processes: cycles between photosynthesis and cellular respiration in living organisms; decomposition leads to carbon storage over long timescales (millions of years) under certain conditions; burning fossil fuels releases stored carbon as carbon dioxide (CO₂) into the atmosphere.
• Anthropogenic impact: term anthropogenic refers to environmental change caused or influenced by people, directly or indirectly.

Photosynthesis and Cellular Respiration

• Plants perform both photosynthesis and respiration; animals perform cellular respiration only.
• Photosynthesis is the process that fixes carbon from CO₂ into a six-carbon sugar (glucose) and releases oxygen as a byproduct.
• Cellular respiration uses glucose and O₂ to generate ATP energy and releases CO₂ and H₂O as byproducts; this process is essential for life.
• Chemical equations:
  • Photosynthesis: $6\ CO<em>2 + 6\ H</em>2O \rightarrow C<em>6H</em>{12}O<em>6 + 6\ O</em>2$
  • Cellular respiration: $C<em>6H</em>{12}O<em>6 + 6\ O</em>2 \rightarrow 6\ CO<em>2 + 6\ H</em>2O + \text{ATP}$
• The products of photosynthesis are reactants for respiration, and vice versa; this cycle sustains energy flow in ecosystems.
• Plants’ carbon turnover supports growth and metabolism; respiration by plants and animals consumes part of the fixed carbon.

The Nitrogen Cycle

• The nitrogen cycle moves nitrogen atoms and nitrogen-containing molecules between sources and sinks.
• The atmosphere is a major reservoir of nitrogen (N₂), but most organisms cannot use N₂ directly.
• Most nitrogen reservoirs hold nitrogen for short periods; transformations move nitrogen between forms usable by organisms and forms unavailable to them.
• Key processes:
  • Nitrogen fixation
  • Nitrification
  • Assimilation
  • Ammonification
  • Denitrification

Nitrogen fixation

• Abiotic fixation: lightning and cosmic radiation convert atmospheric N₂ to reactive nitrogen species (e.g., NO, NO₂) that can lead to formation of nitrates via rain (HNO₃ formation).
• Biotic fixation: soil microorganisms (including rhizobia in symbiosis with legumes) convert N₂ to ammonia (NH₃) or ammonium (NH₄⁺).
• Nodules on legume roots (e.g., soy, chickpeas) house nitrogen-fixing bacteria to make nitrogen biologically available to plants.

Nitrification

• Ammonia (NH₃) is oxidized to nitrite (NO₂⁻) and then to nitrate (NO₃⁻) by nitrifying bacteria.
• Nitrates (NO₃⁻) are the forms most readily taken up by plants; note: the transcript mentions nitrates as the most unusable form, which is incorrect in standard ecology; plants primarily utilize NO₃⁻ and NH₄⁺.

Assimilation

• Plants absorb nitrates (NO₃⁻) and ammonium (NH₄⁺) through their roots.
• Animals obtain nitrogen by consuming plant or animal tissue.
• Nitrogen is essential for DNA, RNA, and proteins.

Ammonification

• Decomposers (bacteria and fungi) break down organic nitrogen into ammonium (NH₄⁺), making nitrogen available again to plants.

Denitrification

• Denitrifying bacteria reduce nitrates (NO₃⁻) to gaseous nitrogen (N₂), returning it to the atmosphere.

The Phosphorus Cycle

• The phosphorus cycle moves phosphorus atoms and phosphorus-containing molecules between sources and sinks.
• Major reservoirs: rocks and sediments containing phosphorus minerals; no atmospheric component is involved.
• Because there is no atmospheric phase, phosphorus is difficult to return to land after reaching marine environments.
• Phosphorus is a limiting nutrient in many ecosystems, constraining primary productivity where scarce.

The Water Cycle

• Driven by solar energy; water exists in solid, liquid, and gaseous forms.
• Major reservoirs: oceans are the primary reservoir; glaciers and groundwater are smaller reservoirs.
• Processes include evaporation, condensation, precipitation, and run-off, maintaining global hydrological balance.

Primary Productivity

• Primary productivity is the rate at which autotrophs synthesize new biomass.
• Energy source: sunlight drives photosynthesis; some ecosystems also rely on chemical energy (chemosynthesis) in the presence of volcanic activity or hydrothermal systems.
• Expressions: productivity can be measured as energy per unit area per unit time or as biomass added per unit area per unit time.

Gross Primary Productivity (GPP) and Net Primary Productivity (NPP)

• GPP: total amount of organic material (glucose) synthesized by autotrophs during photosynthesis.
• NPP: portion of GPP available to consumers; accounts for autotrophic respiration.
  • Formulas:
  • $NPP = GPP - R_{auto}$
  • $ext{Biomass growth per unit area per unit time} = \frac{\Delta\text{Biomass}}{Area\times Time}$
• Not all GPP becomes biomass; some is used for metabolic respiration and maintenance.

Factors affecting primary productivity

• Light: both quantity and quality affect photosynthetic rate.
• Temperature: warm temps generally increase productivity up to a point; excessively high temperatures can denature enzymes and slow processes.
• Availability of CO₂ and water.
• Nutrients: nitrogen and phosphorus availability can limit productivity; excess nutrients can cause algal blooms when limiting nutrients become abundant.
• Herbivory: grazing can reduce net productivity.
• Water availability.
• Ecosystem-specific notes:
  • Most productive ecosystems have warm temperatures, ample water, strong light, and ample nutrients.
  • Not limited to terrestrial ecosystems; aquatic ecosystems can be highly productive, and some aquatic systems may exhibit inverted biomass pyramids due to rapid turnover at the base.
  • Algae blooms are common when limiting nutrients become abundant (e.g., NO₃⁻ or PO₄³⁻ enrichment).

Trophic Levels, Energy Flow, and Pyramids

• All ecosystems rely on a continuous influx of energy to maintain structure and function.
• Energy flow in terrestrial and near-surface aquatic ecosystems moves from the sun to producers, then to herbivores and higher trophic levels.
• In deep-sea ecosystems, energy can come from hydrothermal vents via chemosynthesis because sunlight does not reach these depths.

The 10% Rule

• When energy transfers from one trophic level to the next, only about 10% is transferred to the next level; the rest is lost as heat or used by the organism.
• This rule is consistent with the First and Second Laws of Thermodynamics:
  • First Law: energy cannot be created or destroyed; it can only change forms.
  • Second Law: energy quality decreases over time; some energy is lost as heat and becomes less usable.
• Example: If a producer receives 1000 kcal of energy from the sun, about 10% (i.e., 100 kcal) is passed to the primary consumer, and about 10% of that (i.e., 10 kcal) is passed to the secondary consumer.

Pyramids

• Biomass pyramid (terrestrial) and energy pyramid generally share the same shape and are closely related.
• Biomass in aquatic ecosystems can be inverted due to rapid turnover; energy pyramids remain upright because energy flow is constrained by the 10% rule.

Food Chains and Food Webs

• Food chain: a simple sequence of organisms where each one is eaten by the next; food web: a network of interlinked food chains showing multiple feeding relationships.
• Arrows in diagrams point in the direction of energy flow.

Feedback Loops in Ecosystems

• Positive feedback loops amplify changes in the same direction, reinforcing trends.
  • Examples discussed: melting Arctic sea ice reduces albedo, leading to more solar absorption and further melting; permafrost thaw increases greenhouse gas release, enhancing warming.
• Negative feedback loops dampen changes and stabilize systems (corrective effects).
  • Examples discussed: recycling of mined materials (e.g., aluminum) and predator-prey dynamics can stabilize populations; changes in the water and carbon cycles can act to buffer extremes (in many contexts).

Connections to Foundational Principles and Real-World Relevance

• The carbon, nitrogen, phosphorus, and water cycles are interconnected and respond to human activities (deforestation, fossil fuel combustion, agriculture, nutrient runoff).
• Understanding energy flow and the 10% rule helps explain why energy pyramids are narrower at higher trophic levels and why ecosystems support fewer high-level consumers.
• Knowledge of primary productivity and limiting nutrients informs ecosystem management, conservation, and responses to climate change.
• Ethical and practical implications include sustainable land use, emissions reductions, nutrient management, and protection of ecosystems to maintain nutrient cycling and energy flow.
• Conceptual links to thermodynamics (energy conservation and quality) underpin explanations of why energy transfer between trophic levels is inefficient.

Notable clarifications

• The transcript’s statement that nitrates are the most unusable form for plants is inaccurate; plants readily use nitrate (NO₃⁻) and ammonium (NH₄⁺). This note is included for exam accuracy.