Biogeochemical cycle P1

Definition and scope

  • Biogeochemical cycles (cycles of matter) are the movement and transformation of chemical elements and compounds between living organisms (biotic) and nonliving reservoirs (abiotic) such as the atmosphere, lithosphere, and hydrosphere.

  • Major biogeochemical cycles include:

    • Carbon cycle

    • Nitrogen cycle

    • Phosphorus cycle

    • Water cycle

  • These cycles involve transformation and cycling of elements through biological, geological, and chemical processes across various reservoirs (e.g., atmosphere, soil, oceans).

  • Conceptual framework: the pathway by which a chemical substance cycles through the biotic compartment (biosphere) and the abiotic compartments (atmosphere, lithosphere, hydrosphere).

  • Biotic compartment = biosphere; Abiotic compartments = atmosphere, lithosphere, hydrosphere.

  • In many cycles, substances are stored for long periods in geological reservoirs (sequestration), and can be released later.

Major cycles and representative processes

  • Carbon cycle (example):

    • Atmospheric CO₂ is absorbed by plants via photosynthesis, converting CO₂ into organic compounds used for energy and growth.

    • Carbon is released back to the atmosphere via respiration and decomposition.

    • Carbon can be stored in fossil fuels and released to the atmosphere through burning of fossil fuels by humans.

  • Nitrogen cycle (example):

    • Atmospheric N₂ is fixed by plants (and microbes) into usable forms such as ammonia (NH₃) and nitrates (NO₃⁻) via nitrogen fixation.

    • These forms are used by other organisms for growth and metabolism.

    • N is returned to the atmosphere through denitrification and other processes.

  • Water cycle (example):

    • Evaporation of water from land and oceans forms clouds in the atmosphere.

    • Precipitation returns water to surfaces; it can infiltrate soils to become groundwater or runoff to lakes/rivers.

    • Subterranean water can flow to the ocean along with river discharges, carrying dissolved/particulate organic matter and nutrients.

  • Additional elements and cycles include:

    • Oxygen, hydrogen, phosphorus, calcium, iron, sulfur, mercury, selenium, etc.

    • Cycles for molecules (e.g., water, silica).

    • Macroscopic cycles (e.g., rock cycle).

    • Human-induced cycles for synthetic compounds (e.g., PCBs).

  • Geological reservoirs can sequester substances for long times; cycles involve interactions of biological, geological, and chemical processes.

Microorganisms and biogeochemical cycling

  • Microorganisms are critical drivers of biogeochemical cycling across macronutrients and micronutrients.

  • Many processes would be greatly diminished or halted without microorganisms, impacting land and ocean ecosystems and planetary cycles.

  • Changes to cycles can impact human health and well-being; cycles regulate climate, support plant/phytoplankton growth, and maintain ecosystem health.

Interconnectedness, drivers, and human impact

  • The cycles are interconnected; energy enters ecosystems as sunlight (or inorganic molecules for chemoautotrophs) and leaves as heat across trophic transfers.

  • Matter in living organisms is conserved and recycled rather than endlessly replenished; global cycles are closed or effectively closed for the elements involved.

  • The six most common elements in organic molecules are C, N, H, O, P, and S, and they occur in multiple chemical forms across reservoirs.

  • Geologic processes (weathering, erosion, drainage and subduction) contribute to material recycling.

  • Because geology and chemistry are central to these processes, biogeochemical cycles describe inorganic matter transfer between living and nonliving components.

  • The interplay of biota and abiotic factors connects cycles across the biosphere, lithosphere, atmosphere, and hydrosphere.

  • .In ecosystems, C, N, O, P, S, and other elements cycle in interconnected ways; for example, water movement facilitates the leaching of S and P into rivers and oceans.

Essential elements and their roles

  • Hydrogen and Oxygen: found in water and organic molecules; essential to life.

  • Carbon: found in all organic molecules.

  • Nitrogen: essential in nucleic acids and proteins.

  • Phosphorus: used in nucleic acids and phospholipids in cell membranes.

  • Sulfur: crucial to the three‑dimensional structure of proteins.

  • The cycling of these elements is tightly interconnected across the biosphere and abiotic spheres.

Energy flow vs. matter cycling

  • Energy flow in ecosystems is directional and open: input from the Sun (or inorganic sources for chemoautotrophs), transformed through trophic levels, and lost as heat.

  • Matter, however, is conserved within biogeochemical cycles: atoms are reused and transformed rather than created or destroyed.

Interconnections and examples of processes

  • The movement of water drives leaching of sulfur and phosphorus into rivers, which flow to oceans, illustrating coupling between water cycle and other elemental cycles.

  • Minerals cycle through the biosphere between biotic and abiotic components and from one organism to another.

  • Ecological systems (ecosystems) host multiple biogeochemical cycles (e.g., water cycle, carbon cycle, nitrogen cycle).

Biogeochemical vs geochemical cycles

  • Biogeochemical cycles involve both living (biotic) and nonliving (abiotic) components; they span the biosphere and the lithosphere, atmosphere, and hydrosphere.

  • Geochemical cycles focus more on crustal and subcrustal reservoirs; there is overlap with biogeochemical cycles.

  • Exchanges between rocks, soils, and oceans are generally slower than exchanges with the biosphere and atmosphere.

Global ocean context and marine biogeochemistry

  • The global ocean covers >70% of Earth's surface and is highly heterogeneous.

  • Marine productive areas and coastal ecosystems, though a small fraction of surface area, have a large impact on global biogeochemical cycles due to microbial activity.

  • Microbial communities represent about 90% of the ocean’s biomass and drive major cycling processes.

  • Research has historically focused on carbon and macronutrients (N, P, silicate); other elements like sulfur and trace elements are less studied due to technical/logistical challenges.

  • Anthropogenic pressures threaten marine life and nutrient recycling, including cultural eutrophication from agricultural runoff (increased N and P), algal blooms, deoxygenation, and heightened greenhouse gas emissions.

  • Ocean changes linked to climate: cryosphere changes (glacier and permafrost melt) lead to intensified marine stratification and redox-state shifts, reshaping microbial communities rapidly.

  • Global change affects key processes such as primary productivity, CO₂ and N₂ fixation, organic matter remineralization, and burial of fixed CO₂.

  • Ocean acidification: a pH decrease of about
    \Delta \mathrm{pH} \approx -0.1

Reservoirs & Exchange Pools

  • Reservoir = A place where chemicals (like carbon) are stored for a long time.

    • Example: Coal deposits store carbon for millions of years.

  • Exchange pools = Places where chemicals stay for a short time.

    • Examples: Plants and animals.

  • Plants and animals use carbon to make carbohydrates, fats, and proteins,
    then release it back into the air or surroundings.

  • Reservoirs are usually abiotic (non-living) like rocks or soil,
    while exchange pools are biotic (living) like plants and animals.

  • The time a chemical stays in one place is called:

    • Residence time

    • Turnover time

    • Renewal time

    • Exit age

  • Main point: Carbon stays much longer in reservoirs (e.g., coal)
    than in living organisms.

    Fast and Slow Cycles

    Fast cycle (biological):

    • Moves carbon between land, air, and oceans.

    • Happens quickly (years).

    • Example: Carbon moves through plants, animals, and back to the atmosphere.

  • Slow cycle (geological):

    • Involves rocks, volcanoes, and tectonic activity.

    • Takes millions of years.

    • Moves carbon through Earth’s crust, soil, ocean, and atmosphere.

Deep Cycles (Subsurface Carbon)

  • Terrestrial subsurface = largest carbon reservoir (14–135 Pg, 2–19% of biomass).

  • Microorganisms control biogeochemical cycles underground.

  • Knowledge is limited:

    • Less than 8% of known 16S rRNA sequences come from subsurface organisms.

    • Few complete genomes or isolates available.

  • Microbes may be connected by metabolic handoffs (one organism’s product used by another).

  • Lack of detailed data makes it hard to create accurate carbon cycle models.

  • New techniques (genome-resolved metagenomics) can help understand these deep ecosystems.