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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
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