Biological Compounds, Metabolism, and Biogeochemical Cycles

Energy and Carbon Acquisition

Phototrophs: Organisms that obtain their energy directly from sunlight.
Chemotrophs: Organisms that obtain their energy from chemical sources.
Autotrophs: Organisms that obtain their carbon from inorganic sources, primarily carbon dioxide (CO_2) from the atmosphere.
Heterotrophs: Organisms that obtain their carbon from organic compounds produced by other organisms.
Combinations of Energy and Carbon Sources:
- Photoautotrophs: Use light for energy and CO_2 for carbon (e.g., plants, algae, some bacteria).
- Photoheterotrophs: Use light for energy and organic compounds for carbon (e.g., some bacteria).
- Chemoautotrophs: Use chemical compounds for energy and CO_2 for carbon (e.g., some bacteria and archaea).
- Chemoheterotrophs: Use chemical compounds for energy and organic compounds for carbon (e.g., animals, fungi, many bacteria).
Organism Classifications:
- Animals: Primarily chemoheterotrophs.
- Plants: Primarily phototrophs (photoautotrophs), but can also be photoheterotrophs or chemoautotrophs in certain contexts.
- Bacteria and Archaea: Exhibit a much broader range of metabolic strategies, capable of using all four combinations of energy and carbon sources.

Photosynthesis:
- Uses light energy to synthesize organic compounds (sugars) from CO_2. Carbon atoms are linked together.
- Generates oxygen (O_2) as a byproduct.
- Photosynthetic organisms remove approximately 25\% of the CO_2 in the atmosphere.
Respiration:
- Breaks down organic compounds (sugars) to release energy (for ATP) and CO_2.
- Reverse process of photosynthesis.
- The amount of CO_2 released through respiration generally balances the amount removed by photosynthesis.
Anaerobic Respiration and Photosynthesis:
- Bacteria and Archaea are capable of carrying out respiration and photosynthesis in the absence of oxygen.
- This expands their viable habitats, notably extreme environments.

Archaea: More so than bacteria, thrive in extreme environments.
- Example: Deep-sea hydrothermal vents, characterized by no oxygen and very high temperatures.
- These microorganisms are primary producers and consumers in such harsh conditions.
Thermophiles: Heat-loving organisms (thermo = heat, phile = loving).
- Their enzymes are adapted to high temperatures, unlike most organisms whose enzymes would unravel.
Halophiles: Salt-loving organisms (implied by the example of a highly saline pink lake in Australia—though not explicitly named in the transcript).

Definition: The movement of chemical elements through organisms (bio), the environment (geo), and chemical processes.
Key Elements Discussed: Nitrogen and Carbon.
Storage (Reservoirs):
- Short-term storage: Elements held for a short duration in the atmosphere, water, or soil before being used by living things and recycled (e.g., CO_2 in the atmosphere, nitrogen in the soil).
- Long-term storage: Elements tied up for extended periods, not immediately usable by organisms (e.g., nitrogen in rocks, carbon in fossil fuels like peat and coal).

Atmospheric Nitrogen (N2): Most organisms cannot directly utilize atmospheric N2.
Nitrogen Fixation:
- Atmospheric N2 is converted to ammonia (NH3) by specific bacteria.
- This process makes nitrogen available in a form that plants can absorb.
- Less common methods include lightning and industrial processes.
Nitrification:
- Another type of bacteria converts ammonia (NH_3) into nitrates.
- Plants can take up both ammonia and nitrates.
Denitrification:
- If excessive nitrification occurs, other bacteria can convert nitrates back into nitrogen gas (N_2), releasing it into the atmosphere.

Role of Autotrophs (Producers):
- Photosynthetic organisms (plants, algae, some bacteria) remove CO_2 from the atmosphere and water.
- They convert CO_2 into organic compounds (sugars) through photosynthesis.
- In aquatic environments, bacteria are significant producers, carrying out photosynthesis to convert CO_2 into organic compounds.
Role of Respiration:
- Heterotrophs (animals, most bacteria) and autotrophs (plants also respire) break down organic compounds.
- This process releases CO_2 back into the atmosphere or water.
Role of Decomposition:
- Bacteria play a crucial role in decomposing dead organic matter (plants, animals).
- This decomposition releases carbon and other elements back into the soil and atmosphere, making them available for reuse.
- This is a primary way bacteria contribute to the carbon cycle, beyond just respiration.
Role of Bacteria and Archaea:
- They expand the range of carbon cycling possibilities, particularly in extreme habitats where eukaryotic life might not exist.
- They are significant in the short-term carbon cycle, especially in environments like the deep ocean lacking oxygen.

Trophic Level: An organism's typical place within a food web.
Energy Flow:
- Origin: Solar energy is the ultimate source of energy in most ecosystems.
- Primary Producers: (e.g., plants) incorporate solar energy through photosynthesis.
  - Plants utilize approximately 1\% of available sunlight energy for photosynthesis, which is sufficient to sustain entire ecosystems.
- Primary Consumers (Herbivores): Consume primary producers.
- Secondary Consumers: Consume primary consumers.
- Carbon (and energy) flows up through the food web as organisms are consumed.
Energy Loss:
- Not all producer biomass is consumed by herbivores.
- A significant portion of assimilated energy is lost through metabolic processes (respiration) and waste products (feces) at each trophic level.
- Some energy is directed towards growth and reproduction.
Biomass (Dry Weight):
- Typically, there is a decrease in biomass at successively higher trophic levels (e.g., 1,000 kg of primary producers might support less biomass of primary consumers).
- Exception (Aquatic Systems): In some aquatic environments, the biomass of primary producers (e.g., phytoplankton) can be smaller than that of primary consumers (e.g., zooplankton). This is possible because phytoplankton have very rapid regeneration rates, allowing a smaller standing crop to support a larger population of consumers over time.