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The Role of Algae in Biogeochemistry
  • Biogeochemisty: study of interactions between atmosphere, biosphere (living organisms), hydrosphere (water systems), and lithosphere (crustal minerals)
  • Algae are important in cycles of C, N, O, S, P
  • Nitrogen cycle: N is major nutrient for plants and algae, change in nitrogen load of waters alters algal composition and biomass
  • Phosphorus cycle: P is another major nutrient for plants and algae; municipal sewage input largely increases P concentrations
  • Sulfur cycle: Some algae produce volatile sulfur compounds that are active in cloud formation and consequently have a cooling effect on our climate
  • Carbon cycle: Algae are the base of the aquatic food chain by photosynthesis; they can also draw down atmospheric CO2 (in discussion), counterbalancing man-made greenhouse effects
Limitation of Algal Growth
  • Essential nutrients: nitrogen, phosphorus, CO2, iron, silicate
  • Nutrient limitation: Algal growth is dependent on the concentration of essential nutrients up to their maximum nutrient uptake capacity. 

  • Nutrient requirements are different for each element: 
    Redfield Ratio: C:N:P = 106 : 16 : 1
  • Liebieg‘s Law of the Minimum: Algal growth is limited by the single nutrient whose concentration is closest to the minimal level required.
  • Oligotrophic water: low nutrient concentration, often limiting
  • Eutrophic water: high nutrient concentrations above limitation levels; the process of nutrient enhancement = eutrophication
  • Freshwater: mostly P limited; Marine systems: mostly N limited
  • Antarctic oceans: high concentrations of N and P, probably iron limited
Algae and the Nitrogen Cycle
  • Nitrogen is required to build amino acids, proteins, chlorophyll, nucleic acids, etc. 
  • Ammonium (NH4+) and nitrate (NO3-) are the most common and used nitrogen sources; some algae can take up urea or amino acids; NH4+ is preferred because it can be utilized directly (energy saving)
  • NO3- assimilation by nitrate reductase requires iron (phytoplankton cannot exhaust high NO3- pools in Fe-limited Antarctic oceans)

  • Cyanobactera are unique in that they can fix atmospheric nitrogen; N2 is converted to NH4+, process known as diazotrophy; 25-60% of fixed nitrogen can be released into the water and serves as nitrogen source for other algae
The Biological Pump
  • Part of the phytoplankton production is not consumed by animals but sinks to the sea floor; burried in anoxic deep-sea sediments, degradation of this organic carbon is extremely slow
  • Zooplankton fecal material, which ultimately originated from algal production, also sinks to the deep-sea floor (marine snow)
  • Both processes draw carbon from the surface layers to the deep-sea; because surface water and atmospheric CO2 concentrations are in balance, sedimentation of organic carbon can reduce the atmospheric CO2 concentration: the biological pump
  • Algae are believed to have lowered Earth‘s atmospheric CO2 concentration by a factor of 16 over the last 500 million years, thereby lowering Earth‘s average surface temperature
  • Organic carbon burried in anoxic deep-sea sediments can eventually become hydrocarbon source (e.g. North Sea oil fields)
Carbon Utilization by Algae
  • The primary source of carbon is CO2
  • Whereas CO2 can become limiting in freshwater, the huge pool of carbonate and bicarbonate in seawater prevents CO2 becoming limiting: upon utilization of CO2, carbonate and bicarbonate „re-fill“ the CO2 pool: 
    CO32- + 2H+ = HCO3- + H+ = CO2 + H2O
  • Rubisco (Ribulose biphosphate carboxylase/oxygenase) catalyzes the conversion of CO2 to organic carbon (PGA, phosphoglyceric acid; or phosphoglycolate):
    RuBP + CO2 + H2O = 2 PGA
    RuBP + O2 = PGA + phosphoglycolate
  • Phosphoglycolate can be lost from the cell, leading to a pathway called photorespiration, which is an energy loss
Carbon Concentration Mechanisms in Algae
  • Storage within the cell is mostly as HCO3-, because it diffuses less easily out of the cell than CO2
  • Carbonic anhydrase converts HCO3-
    HCO3- +H+ = CO2 + H2O
  • Cyanobacteria: CA located in carboxysomes, together with Rubisco
  • Eukaryotes: CA in pyrenoid (substructure of plastids) or in thylacoid membranes; cytoplasmid and plastidic forms of CA discussed
  • Periplasmic space CA: between cell membrane and cell wall, forms CO2 from HCO3- at cell surface for faster uptake (HCO3- charged); referred to as „external CA“, regulated by environmental [CO2]
  • Some algae release acids, which convert HCO3- to CO2 for carbon uptake; especially important in calcification
    CO2 balance of calcification: Calcification produces CO2 !!
    Ca2+ + 2 HCO3-  =  CaCO3 + H2O + CO2
    Oceanic blooms of coccolithophorids and production of coral reefs DO NOT help decreasing the atmospheric increase in CO2