Lecture 10: The biosphere

Lecture 10: The biosphere

1. Primary production/photosynthesis

   • Photosynthesis originates in cyanobacteria (blue-green algae).
   • Oxygenation of the atmosphere was triggered by the evolution of photosynthesis.

2. Ecosystems and trophic structure

   • Decomposers are one of the most important aspects of ecosystems as they are responsible for nutrient cycling.

3. Primary production in aquatic ecosystems

   • Phytoplankton are the main primary producers in aquatic ecosystems.
   • Marine primary producers
     • Coccolithophores (calcareous)
       • Have external calcium carbonate shells to protect themselves.
       • Formed the White Cliffs of Dover.
     • Diatoms (siliceous)
       • Organisms are found within a glass-like silica shell.
   • Factors controlling primary production
     • Light intensity
       • Inhibited by depth and turbidity (sediment).
     • Nutrient availability
       • The main limiting factor.
       • Typically N or P is the limiting nutrient.
         • Geoengineering considers the possibility of artificially increasing productivity by increasing nutrient availability

4. Nutrient cycling

   • The top layer of the sea bed, the 'mixed layer', is rich in bacteria, like soil.
     • DOM is remineralised by decomposers.
   • Trees and vegetation extract key nutrients from rock and soil.
     • So they can be used to increase agri-productivity.

5. Limiting nutrients in marine systems

   • Dominant nutrients
     • Nitrate
     • Phosphate
     • Iron
     • Silica
   • Upwelling provides the upper layers of the ocean with nutrients from bottom water.

6. Ecosystems, energy and nutrients

   • Ecosystems involve the transfer of both energy and mass (nutrients), making them open systems.

7. The long-term carbon cycle

   • Interlinked with other cycles.
   • Essential for life on Earth.
   • Controlled by abiotic and biotic processes.
   • Volcanic outgassing is the largest natural source of atmospheric carbon dioxide.
   • Due to plate tectonics, carbon stored in the lithosphere will eventually be outgassed from magma (over millions of years).
   • Silicate weathering naturally removes carbon dioxide from the atmosphere.
     • This is important for carbon sequestration.
       • Can be accelerated by distributing crushed rock, such as basalt, on beaches, farmlands, or forestry areas.
     • It does so by producing alkalinity, which drives carbonate precipitation in oceans.

8. Chemosynthesis

   • Definition
     • The conversion of carbon-containing molecules (typically carbon dioxide or methane), and nutrients into organic matter via the the oxidation of inorganic compounds.
   • Uses energy derived from reactions involving inorganic chemicals, typically in the absence of sunlight.
     • E.g. Documented to occur in hydrothermal vents, where chemosynthesis harnesses hydrothermal energy rather than solar energy.
   • Is another, newly-discovered type of primary production.
   • Whereas autotrophs are primary producers that photosynthesize, chemoautotrophs are primary producers that chemosynthesize.