Nutrient Cycles in Ecosystems

Matter cycles between the biotic environment and inthe abiotic environment.

• Simple inorganic molecules (such as CO2, N2and H2O) are assimilated (or fixed) from the abiotic environment by producers and microbes, and built into complex organic molecules (such as carbohydrates, proteins and lipids).
• (In science organic compounds contain carbon–carbon bonds, while inorganic compounds don’t.)
• These organic molecules are passed through food chains and eventually returned to the abiotic environment again as simple inorganic molecules by decomposers.
•  Major nutrients are taken from the environment  in the form of carbon dioxide (C and O) and water (H).
• All the other nutrients are usually required as soluble mineral ions, so are often referred to as“minerals”.
• While the major nutrients are obviously needed in the largest amounts, the growth of
  producers is often limited by the availability of macro nutrients such as nitrogen and phosphorus.

Photosynthesis is the only route by which carbon dioxide is “fixed” into organic carbon compounds.

Photosynthesis is balanced by respiration, decay and combustion, which all return carbon dioxide to the atmosphere.

Different ecosystems have a different balance:

•  A carbon source is an ecosystem that releases more carbon as carbon dioxide than it accumulates in biomass over the long term. Carbon sources include  farmland (since crops are eaten and respired quickly and decay is encouraged by tilling) and areas of deforestation (since the tree biomass is burned or decayed).
•  A carbon neutral ecosystem is one where carbon fixation and carbon  release are balanced over the long term. Carbon neutral ecosystems include mature forests, where new growth is balanced by death and decay.

• Carbon sink is an ecosystem that accumulates more carbon in biomass than it releases as carbon dioxide over the long term. This accumulation happens when the conditions are not suitable for decomposers (too cold, too dry, too acidic, etc). Carbon sinks include peat bogs (since the soil is too acidic for decay), the ocean floor (since it is toocold and anaerobic for detritus to decay); and growing forests (since carbon is being incorporated into growing biomass). In a carbon sink the carbon remains fixed in organic form and can even form a fossil fuel given enough time.

Decay (also known as decomposition, putrefaction or rotting) is the breakdown of detritus by organisms collectively called decomposers.

There are two groups of decomposers: saprobionts and detritivores.

Saprobionts

• Saprobionts (or saprotrophs) are microbes (fungi and bacteria) that live on detritus. Saprobionts use saprobiotic nutrition, which means they do not ingest their food, but instead use extracellular digestion, secreting digestive enzymes into the detritus that surrounds them and absorbing the soluble products.
• The absorbed products are then further broken down in aerobic respiration to inorganic molecules such as carbon dioxide, water and mineral ions.

• Only a few bacteria posses the cellulase enzymes required to break down the plant fibres that comprise much of the detritus biomass.

• Herbivorous animals such as cows and termites depend on these bacteria in their guts.

• In aquatic ecosystems the main saprobionts are bacteria, while in terrestrial ecosystems the main saprobionts are fungi.
• Fungi are usually composed of long thin threads called hyphae.
• These hyphae grow quickly throughout soil giving fungi a large surface area to volume ratio.

Detritivores

• Detritivores are small invertebrate animals (such as earthworms and woodlice) that eat detritus.
•  Like  all animals, they use holozoic nutrition, i.e. they ingest food, digest it in a gut, absorb the soluble products and egest the insoluble waste.
• This egesta consists largely of plant fibres (cellulose and lignin), which  animals can’t digest.

Detritivores speed up decomposition by helping saprobionts

•  Detritivores physically break up large plant tissue (like leaves or twigs) into much smaller pieces, which they egest as faeces. The faeces has a larger surface area making it more accessible to the saprobionts.
•  Detritivores aerate the soil, which helps the saprobionts to respire aerobically.
•  Detritivores excrete useful minerals such as urea, which saprobionts can metabolise.

Neither saprobionts nor detritivores can control their body temperature, so their activity (metabolism and reproduction) depends on the environmental temperature.

Decay therefore happens much more rapidly in summer than in winter.

Nitrogen is needed by all living organisms to make proteins and nucleic acids.

There are several different forms of inorganic nitrogen that occur in the nitrogen cycle: N2, NO2 and NH3.

I Nitrogen Fixation.

• The triple bond linking the two nitrogen atoms makes it a very stable molecule, which doesn't readily take part in chemical reactions.
• N2 therefore can’t be used by plants or animals as a source of nitrogen.
• The nitrogen in N2is "fixed" into useful compounds by nitrogen fixing bacteria.
• They reduce nitrogen gas to ammonia (N2+ 6H →2NH3), which dissolves to form ammonium ions (NH4+).
• This reaction is catalysed by the enzyme nitrogenase and it requires a great deal of energy: 15 ATP molecules need to be hydrolysed to fix each molecule of N2.

• Some nitrogen-fixing bacteria are free-living in soil, but most live in colonies inside the cells of root nodules of leguminous plants such as clover or peas.
• This is a classic example of mutualism, where both species benefit.
• The pea plants gain a source of useful nitrogen from the bacteria, while the bacteria gaincarbohydrates from the plant, which they respire tomake the large amounts of ATP they need to fix nitrogen.
• Nitrogen gas can also be fixed to ammonia by humans using the Haber process (N2+ 3H2 →2NH3) to make nitrogenous fertilisers, which are spread on to soil.
• Nitrogen can also be fixed by oxidising it to nitrate (N2+ 2O2 →2NO2).
• This reaction happens naturally by lightning.

II Nitrification

• Nitrifying bacteria can oxidise ammonia to nitrate in two stages: first forming nitrite ions (NH4 +→NO-2) then forming nitrate ions (NO− 2→NO-3).
• This oxidation reaction is exothermic, releasing energy, which these bacteria use to make ATP, instead of using respiration.  

III Assimilation

• Plants are extremely self-sufficient: they can make carbohydrates and lipids from CO2
  and H2O, but to make proteins and nucleotides they need a source of nitrogen.
• Plants require nitrogen in the form of dissolved nitrates, and the supply of nitrates is often so poor that it limits growth (which is why farmers add nitrate fertilisers to crops).
• Plants use active transport to accumulate nitrate ions in their root hair cells against a concentration gradient.
• Most plant species have symbiotic fungi associated with their roots called mycorrhizae.
• These mycorrhizae aid mineral absorption since the hyphae are thinner than roots and so have a larger surface area : volume ratio.
•  Some plants living in very poor soils have developed an unusual strategy to acquire nitrogen: they trap and digest insects.
• These so-called carnivorous plants don't use the insects as a main  source of nutrition as a consumer would do, but just as a source of nitrogen-containing compounds.

IV Ammonification

• Microbial saprobionts break down proteins in detritus to form ammonia in two
  stages: first they digest proteins to amino acids using extracellular protease enzymes, and then they remove the amino groups from amino acids using deaminase enzymes to from ammonia (NH4+ ).
• The deaminated amino acids, now containing just the elements CHO, are respired by the saprobionts to CO2and H2O.

V Denitrification

• The anaerobic denitrifying bacteria convert nitrate to N2 and NOxgases, which are
  then lost to the air.