BIO 20 Chapter 2: Cycles of Matter and the Hydrologic System

Water in the Biosphere

• Earth is considered a closed system, meaning matter must cycle within it rather than being imported or exported.
• Surface water sources vary in origin; they may arise from melting snow and ice, ocean bodies, or as metabolic products of cellular respiration.
• Atmospheric water acts as a potent greenhouse gas, responsible for trapping heat and maintaining the temperature of the Earth.
• Water is the primary agent for heat transfer throughout the biosphere due to its unique ability to absorb and store massive amounts of thermal energy.

The Hydrologic Cycle

• The hydrologic cycle involves the continuous movement of water through various reservoirs and processes:     * Precipitation: Water falling from rain clouds to the Earth's surface.     * Infiltration and Percolation: Water moving into the soil and through the bedrock, eventually reaching groundwater.     * Deep Percolation: Movement of water into deep underground layers.     * Surface Runoff: Water flowing over the land surface into lakes and streams.     * Storage: Water held in lakes, the ocean, and groundwater.     * Evaporation: Conversion of liquid water from the ocean and soil into vapor.     * Transpiration: The release of water vapor into the atmosphere from vegetation.     * Cloud Formation: The condensation of water vapor into clouds.

Physical and Chemical Properties of Water

• The Universal Solvent:     * Water is a polar molecule, characterized by a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom.     * This polarity allows water to pull apart the ions in ionic compounds, effectively dissolving them.     * Hydrogen bonds form between nearby water molecules, allowing them to surround and isolate compound particles during the dissolution process.
• Hydrogen Bonding and Phase Changes:     * The relatively strong hydrogen bonds between water molecules require significant energy to break, leading to high heats of fusion and vaporization.     * Compared to similar hydrogen-based compounds, water exhibits significantly higher melting and boiling points.     * Density of Ice: Forces of repulsion between oxygen atoms and the orientation of hydrogen bonds cause an open crystal structure in ice. Consequently, ice is less dense than liquid water, causing lakes to freeze from the top down. This prevents bodies of water from freezing solid and facilitates the cycling of oxygen and nutrients during seasonal turnovers (spring and fall).
• Cohesion and Adhesion:     * Cohesion: The attraction between water molecules created by hydrogen bonds, resulting in surface tension.     * Adhesion: The attraction between water molecules and other substances, such as glass. This force drives capillary action, which is essential for transporting water within the xylem of plants.
• Temperature Moderation:     * Due to hydrogen bonding, water has a high specific heat capacity, allowing it to store large quantities of heat.     * Large bodies of water moderate regional temperatures. During the day, land warms faster than the sea, creating a cool sea breeze; at night, the sea remains warmer than the land, creating a land breeze.     * In individual organisms, this high specific heat capacity protects body temperatures from rapid, dangerous fluctuations.

Biogeochemical Cycles: Carbon and Oxygen

• Matter must be recycled constantly because the quantity of matter in the ecosystem is finite. Key cycles include oxygen, carbon, nitrogen, sulfur, and phosphorus.
• The Carbon-Oxygen Cycle: These two cycles are deeply intertwined and often illustrated together.     * Photosynthesis: Conducted by plants, algae, and cyanobacteria to convert $CO_2$ into organic matter.     * Cellular Respiration: Performed by high-level consumers, primary consumers, and plants to release energy and $CO_2$.     * Decomposition: Detritivores (soil microbes) break down detritus, releasing carbon back into the cycle.     * Combustion: The burning of wood and fossil fuels releases stored carbon into the atmosphere as $CO_2$.
• Rapid vs. Slow Cycling:     * Rapid Cycling: The immediate exchange of carbon through photosynthesis and cellular respiration in living organisms.     * Slow Cycling: Carbon stored for long periods, such as in the tissues of large trees that live for centuries. This carbon is only recycled once the tree dies and decomposes.
• Carbon Sinks:     * A carbon sink is a reservoir that stores carbon.     * The largest stores are the Earth's oceans, containing billions of tonnes of dissolved $CO_2$.     * Other significant sinks include forests, petroleum deposits, and limestone rock ($CaCO_3$).

Acid Deposition

• The combustion of sulfur-rich fossil fuels releases sulfur oxides ($SO_x$) into the atmosphere.
• Sulfur dioxide reacts with atmospheric oxygen and water vapor to produce sulfurous acid ($H_2SO_3$) and sulfuric acid ($H_2SO_4$).
• These acids fall as acid precipitation, which can drastically alter the pH of soil and water, threatening the survival of organisms.
• In Alberta, the soil is naturally alkaline (basic). When acid rain falls, a neutralization reaction occurs between the acid and the basic soil, which helps reduce the negative impact of acid deposition.

The Nitrogen Cycle

• Nitrogen is essential for building proteins (amino acids) and DNA structure, yet atmospheric nitrogen ($N_2$) is unusable by most organisms.
• Processes in the Nitrogen Cycle:     * Nitrogen Fixation: Converting atmospheric $N_2$ into ammonium ($NH_4^+$). This is done by nitrogen-fixing bacteria in the root nodules of legumes or via lightning.     * Ammonification: Decomposers break down organic matter to produce ammonium ($NH_4^+$).     * Nitrification: A multi-step process converting ammonium into usable nitrites ($NO_2^-$) and then nitrates ($NO_3^-$). The overall chemical pathway involves:         1. $NH_4^+$ converted by Nitrosomonas species (using ammonium monooxygenase) to $NO_2^-$.         2. $NO_2^-$ converted by Nitrobacter species (using nitrite oxidoreductase) to $NO_3^-$.     * Assimilation: Plants absorb nitrates to build tissues.     * Denitrification: Occurs in anaerobic (low oxygen) environments where bacteria break down nitrogen compounds and release $N_2$ gas back into the atmosphere.
• Agricultural Applications: Legumes are used in methods like the "Three Sisters" to naturally restore soil nitrogen levels.

The Phosphorus Cycle

• Phosphorus is required for DNA, phospholipids, and ATP. Unlike other cycles, it has no atmospheric phase; it is found primarily in soil, water, and rock.
• Long-Term Cycle: Phosphorus is stored in rocks. Weathering and erosion by water carry dissolved phosphates from land to streams and eventually to oceans. Geological activity (uplifting) eventually pushes these sediments back up to form new land.
• Short-Term Cycle: Phosphates dissolved in water enter the food chain through producers (plants). Animals eat the plants, and decomposers return the soluble phosphates back to the soil upon the death of the organism.
• Environmental Impact: Since phosphorus is typically a limiting factor for growth, an overabundance (excess) can lead to uncontrolled algal blooms. This reduces the available oxygen in aquatic ecosystems, harming other life forms.

Homeostasis and the Gaia Hypothesis

• The Gaia Hypothesis: Proposed by James Lovelock in 1979, this hypothesis suggests that the Earth acts as a self-regulating system that maintains global homeostasis.
• Living Components: The atmosphere's composition is a result of biological activity (respiration and photosynthesis), and many geological features originate from biological sources.
• Stromatolites:     * These are sedimentary rock formations composed partly of cellular debris from photosynthetic cyanobacteria (blue-green algae).     * Originating over $3 \times 10^9$ years ago, they are the oldest known fossils.     * Early layers show oxygen trapped in iron oxides, while later layers show oxygen moving into the atmosphere.     * Fossilized specimens from the East Arm of Great Slave Lake are approximately $1.8 \times 10^9$ years old.     * Today, they are nearly extinct but exist in rare locations like Shark Bay, Western Australia.

Human Impact and Future Research

• Biosphere 2: An experiment in the 1990s in Arizona where "biospherians" were sealed inside a dome to replicate Earth's systems. The experiment failed as the participants suffered from starvation and oxygen depletion, highlighting the immense complexity of our biosphere.
• NASA Projects:     * Advanced Life Support (ALS): Studying how to grow plants in space for food and oxygen recovery.     * Haughton-Mars Project: A research station in the Canadian Arctic simulating Mars' surface conditions to study requirements for sustainable manned missions.
• Management: Humans can significantly disrupt the flow of matter and energy. Current efforts focus on alternative energy and better resource/land management to preserve natural stability and reduce environmental impact.