Biogeochemical Cycles

Algae and the Nitrogen Cycle The growth of all organisms depends on the availability of mineral nutrients, and none is more important then nitrogen, which is required in large amounts as an essential component of peptides, proteins, enzymes, chlorophylls, energy-transfer molecules (ATP, ADP), genetic materials (RNA, DNA), and other cellular constituents.

Algae and the Nitrogen Cycle Nitrogen is present in all the four different spheres of the Earth: the lithosphere contains about 98% of the global N, distributed among its different compartments (soil and sediments of the crust and core). This N is not readily available to be cycled in the near surface Earth environment. Some periodically enters the atmosphere and hydrosphere through volcano eruptions, primarily as ammonia (NH3) and nitrogen (N2) gas.

Algae and the Nitrogen Cycle In atmosphere gas (N2) comprises more than 78% of the volume. The hydrosphere and the biosphere together contain relatively little N compared with the other spheres.

Algae and the Nitrogen Cycle Nitrogen has many chemical forms, both organic and inorganic, in the atmosphere, biosphere, hydrosphere, and lithosphere. It occurs in the gas, liquid (dissolve in water), and solid phases. N can be associated with carbon and with other elements other than carbon (inorganic species).

Algae and the Nitrogen Cycle

Algae and the Nitrogen Cycle Important inorganic species include nitrate

(NO3), nitrite (NO2), nitric acid (HNO3) ammonium (NH4), ammonia (NH3), the gas N2, nitrous oxide (N2O), nitric oxide (NO), and nitrogen dioxide (NO2).

Most organic N species in the four spheres are biomolecules, such as proteins, peptides, enzymes and genetic materials.

Algae and the Nitrogen CycleThe key processes linking the major pathways of

the nitrogen cycle are the following: N-fixation, that is, reduction of atmospheric N2

into ammonia NH3

Assimilation that is, conversion of (nitrate) NO3 and (ammonium) NH4 to organic nitrogen

Ammonification, that is, conversion of organic nitrogen to NH4

Denitrification, that is, conversion of NO3 to gaseous forms of nitrogen (NO, N2O, N2)

Algae and the Nitrogen Cycle

Algae and the Nitrogen Cycle Though complex microbial relationships

regulate these processes We can assume that fixation,

ammonification, nitrification, and denitrification are carried out almost exclusively by bacteria

Whereas algae play a main active role only in nitrogen fixation and assimilation.

Algae and the Nitrogen Cycle From an algal point of view: Atmospheric molecular nitrogen is converted by

prokaryotic algae (Cyanophyta) to compounds such as ammonia (fixation, which are in part directly converted into amino acid, proteins and other nitrogen-containing cell constituents of the fixators, and in part excreted into the open environment.

Eukaryotic algae, unable to perform fixation, incorporate fixed nitrogen, either ammonium or nitrate, into organic N compounds by assimilation.

Algae and the Nitrogen Cycle When organic matter is degraded, organic

compounds are broken down into inorganic compounds such as NH3 or NH4 and CO2 through the ammonification process.

The resultant ammonium can be nitrified by aerobic chemoautotrophic bacteria that use it as electron donor in the respiration process.

Cycle completed by denitrification carried out usually by facultative anaerobic bacteria that reduce nitrate used as electron acceptor in respiration to nitrogen gas.

Algae and the Nitrogen Cycle All life forms require nitrogen compounds the

most abundant protein of it (98%) is buried in the rocks, therefore deep and unavailable, and the rest of nitrogen, the gas (2%) can be utilized only by very few organisms.

This gas cannot be used by most organisms because the triple bond between the two nitrogen atoms makes the molecule almost inert.

Algae and the Nitrogen Cycle

In order for N2 to be used for growth this gas must be “fixed” in the forms directly accessible to most organisms, that is, ammonia and nitrate ions.

Algae and the Nitrogen Cycle A semitemporal separation of nitrogen fixation and

oxygenic photosynthesis combined with spatial heterogeneity was the first oxygen-protective mechanism developed by marine cyanobacteria such as Trichodesmiun sp and Katagnymene sp.

A full temporal separation, in some non-heterocystous filamentous diazotrophs e.g. Oscillatoria limosa and Plectonema boryanum.

Other filamentous organisms, complete segregation of N2 fixation and photosynthesis was achieved with the cellular evolution of heterocystous cyanobacteria e.g. Nostoc and Anabaena.

Algae and the Nitrogen CycleNostoc sp.

Nostoc colony

Trichodesmium sp.

Algae and the Nitrogen Cycle The non-heterocystous filamentous

cyanobacteria Trichodesmiun sp and Katagnymene sp., unlike all other non-heterocystous species fix nitrogen only during the day.

Algae and the Silicon Cycle The Biogeochemical cycle of silicon does

not facilitate a high biospheric abundance of the element, in fact silicon cycle differs from the cycles of carbon, nitrogen, sulfur and it is similar to phosphorous in that there is no atmospheric reservoir.

Algae and the Silicon Cycle Silicification occurs in three clades of

photosynthetic heterokonts: Chrysophyceae Bacillariophyceae and Dinophyceae

Algae and the Silicon Cycle Diatoms being the world’s largest

contributors to biosilicification Because amorphous silica is an essential

component of the diatom cell wall, silicon availability is a key factor in the regulation of diatom growth in nature; in turn, the use of silicon by diatoms dominates the biogeochemical cycling of silicon in the sea.

Algae and the Silicon Cycle Several thousand million years ago little if any of

the life on Earth was involved in the processing of silicic acid to amorphous silica (SiO2 * H2O) The concentration of silicic acid in the aqueous environment was high

Environment rich in silicic acid is indicated in the fossil record by evidence of blue-green algae

Algae and the Silicon Cycle

Algae and the Silicon Cycle Once the silica frustules have settled to the

bottom their silica enters the sedimentary cycle whereupon it is unlike to reappear in the biosphere for tens of millions of years

Diatoms in sedimentary deposits of marine and continental, origin belong to different geologic ranges and physiographic environments

Algae and the Silicon Cycle Aside from their role in the silicone cycle,

the diatoms have also attracted attention because of their importance to export of primary production to the ocean’s interior

Algae and the Silicon Cycle

Algae and the Sulfur Cycle Sulfur is an essential element for

autotrophs and heterotrophs In reduced oxidation state: Nutrient sulfur

plays an important part in the structure and function of proteins

In fully oxidized state: sulfur exists as sulfate and is the major cause of acidity in both natural and polluted rainwater

Algae and the Sulfur Cycle Sulfur cycle can be thought of as beginning with

the gas sulfur dioxide (SO2) or the particles of sulfate (SO4) compounds in the air

These compounds either fall out or are rained out of the atmosphere

Algae and plants take up some forms of these compounds and incorporate them into their tissues

Algae and the Sulfur Cycle As nitrogen, these organic sulfur

compounds are returned to the land or water after the algae and plants die or are consumed by heterotrophs.

Bacteria are important here as well because they can transform the organic sulfur to hydrogen sulfide gas (H2S).

Algae and the Sulfur Cycle In oceans, certain phytoplankton can

produce a chemical that transforms organic sulfur to sulfur dioxide (SO2) that resides in the atmosphere

These gases can re-enter the atmosphere, water, and soil, and continue the cycle

Algae and the Sulfur Cycle All living organisms require S as a minor

nutrient Roughly the same atom proportion as

phosphorus Sulfur is present in freshwater algae at a

ratio of about 1 S atom to 100 C atom and the S content varies with species, environmental conditions, and season

Algae and the Sulfur Cycle Vascular plants, algae and bacteria have the

ability to take up, reduce, and assimilate sulfate (SO4) into amino and convert SO4 into ester sulfate compounds

Sulfate is assimilated from the environment, reduction inside the cell, and fixed into sulfur-containing amino acid and other organic compounds

Algae and the Oxygen/ Carbon Cycle

They are directly associated with photosynthesis and respiration processes

Oxygen constitutes about 21% of the atmosphere, 85.8% of the ocean and 46.7% by volume of the Earth’s crust

When CO2 released by the respiration of algae, plants, bacteria and animals, more bicarbonate and carbonate ion produce

CO2+H2O H2CO3 HCO3+H CO3+H HCO3 CO2+OH

Algae and the Oxygen/ Carbon Cycle Some marine organisms combine calcium

with carbonate ions in the process of calcification on manufacture calcareous skeletal material

After death, this skeletal material sinks and is either dissolved, in which case CO2 is again released into the water or it becomes buried in sediments, in which case the bound CO2 is removed from the carbon cycle

Algae and the Oxygen/ Carbon Cycle