Decomposers and natural Recycling

How elements are re-used

Aim: (k) describe the role of decomposers in the decomposition of organic material;(l) describe how microorganisms recycle nitrogen within ecosystems. (Only Nitrosomonas, Nitrobacter and Rhizobium need to be identified by name).

The syllabus says:

Ass

imila

tio

n

Loss to erosion or leaching into groundwater

Soil nutrient pool

Decomposerfood web

Detritus

Death

Herbivore

Uptake

Plants

Feces or urine

Forest: nutrients are put back into the soil slowly, so organic material builds up

Tropical rain forest: decomposition is rapid so there is very little organic build up

Result: if living material is removed from tropical rain forests, the soil is nutrient poor to support

new growth

MushroomMushroom

EarthwormEarthworm

Primary Primary decomposers:decomposers:

Bacteria and archaeaBacteria and archaeaMillipedeMillipede

NematodesNematodes

Puffball

Breaking down organic material

Organisms that feed on dead organic matter, releasing molecules and minerals which then become available to other organisms in the ecosystem

http://www.youtube.com/watch?v=5kG18FYISaY

http://www.youtube.com/watch?v=jrSHku6-LFo&feature=related

Decomposers• Consumers feed on organisms or

particulate organic matter.• Decomposers utilize complex

compounds in dead organic material.• Bacteria and fungi are the main groups

of decomposers.• Bacteria are the main feeders on

animal material.• Fungi feed primarily on plants, although

bacteria also are important in some plant decomposition processes.

The Nitrogen Cycle

• All living things need nitrogen-containing compounds, which include proteins and nucleic acids.

• 78% of the atmosphere is nitrogen gas, but most organisms cannot use nitrogen in this form.

• The nitrogen cycle is an important nutrient cycle describing how nitrogen is stored and transferred.

• There are five main processes in the nitrogen cycle:

1) Nitrogen fixation

2) Assimilation

3) Ammonification

4) Nitrification

5) Denitrification http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp29/animations/ch29/1_nitrogen_cycle.swf

http://www.pdf-tube.com/download/swf/Nitrogen%20cycle%20talk/aHR0cDovL3d3dy5tYXJpZXR0YS5lZHUvfnNwaWxhdHJzL2Jpb2wyMDIvYW5pbWF0aW9ucy9uaXRyb2dlbl9jeWNsZS5zd2Y

nitrogen in the atmosphere (N2)

animal protein

denitrifyingbacteria

ammonia (NH3)

nitrites (NO2-)

plant protein

nitrates (NO3-)

nitrifying bacteria nitrifying bacteria

assimilation

nitrogen fixing bacteria

nitrogen fixation

ammonification

nitrification (1)

decomposers

nitrification (2)

denitrification

The Nitrogen Cycle

http://www.biology.ualberta.ca/facilities/multimedia/uploads/ecology/ncycle.swf

Nitrogen Fixation • This is the process whereby nitrogen in the atmosphere is converted into

a form plants can use.

• 78% of the atmosphere is nitrogen gas (N2):

The triple bond linking the two nitrogen atoms makes it a very stable molecule, so it doesn't readily take part in chemical reactions and can’t be used by plants or animals as a source of nitrogen.

• There are three main ways in which N2 is "fixed" into useful compounds:

1) Biological - through the action of bacteria (Enzymes)

2) Atmospheric - through chemical reactions caused naturally by lightning

3) Industrial - through human-induced chemical reactions (Haber process)

N N

Nitrogen Fixation1) Biological

• Nitrogen fixing bacteria reduce nitrogen gas (N2) to ammonia (NH3) which then dissolves to form ammonium ions (NH4

+).

• This reaction is catalysed by the enzyme nitrogenase and requires lots of energy: 15 ATP molecules to fix each molecule of N2.

• Some nitrogen-fixing bacteria live freely in soil (Azotobacter), but most live in colonies inside root nodules of leguminous plants such as clover or peas (Rhizobium)

• This is a classic example of symbiosis, where both organisms benefit. The leguminous plants gain a source of useful nitrogen from the bacteria and the bacteria gain carbohydrates from the plant, which they respire to make the ATP they need to fix nitrogen.

Rhizobium bacteria live in nodules in the roots of legumes,

here they fix nitrogen.

Nitrogen Fixation

2) Atmospheric

• Atmospheric nitrogen (N2) can be oxidised to form nitrate ions (NO2

-) and other oxides of nitrogen (N2O)

• This reaction happens naturally through the action of lightning.

• This is thought to have been a significant process in the earth’s early atmosphere.

• Volcanic activity both in ancient times and now also leads to increase in oxides of nitrogen in the atmosphere

Nitrogen Fixation

3) Industrial • Nitrogen can be converted to ammonia by humans

using the Haber process: N2 + 3H2 2NH3

• This reaction is used to make nitrate fertilisers. • Today, almost a third of all nitrogen fixed is fixed

in this way.

Nitrification

• Nitrification is the process by which ammonium is converted into nitrates by bacteria in a series of oxidative reactions.

• The bacteria oxidise ammonia in two main stages, first bacteria of the genus Nitrosomonas converts ammonia to nitrite ions (N02

−), then the bacteria of the genus Nitrobacter convert nitrite ions to nitrate ions (NO3

−)

• These oxidative reactions are exothermic. Energy is released, which the nitrifying bacteria use to make ATP, instead of using respiration.

• These bacteria are called chemoautotrophic bacteria.

Assimilation• Assimilation is the building up of organic molecules

in organisms.

• Plants can make carbohydrates and lipids from CO2 and H2O, but to make proteins and nucleotides they need a source of nitrogen.

• Plants acquire nitrogen in the form of dissolved nitrates. Plants use active transport to accumulate nitrate ions in their root hair cells against a concentration gradient.

• The supply of nitrates is a limiting factor in plant growth. That’s why farmers add nitrate fertilisers to crops.

• Some plants living in extremely poor soils have developed an unusual strategy to acquire nitrogen: they trap and digest insects.

• Animals get nitrogen by consuming plants and excrete excess nitrogen as urea, ammonia or uric acid.

Ammonification

• Ammonification is the process in which nitrogen compounds in the soil from dead animals and plants are converted into ammonia.

• Microbial saprophytes (decomposers) break down proteins in animal and plant matter to form ammonia in two stages1) They digest proteins to amino acids using

extracellular protease enzymes.2) They remove the amino groups from amino acids

using deaminase enzymes. • The deaminated amino acids, containing Carbon,

Hydrogen and Oxygen, are respired by the saprophytes to CO2 and H2O (this links in with the carbon cycle.)

Denitrification

• Denitrification releases nitrogen from nitrites and nitrates into the atmosphere.

• Anaerobic denitrifying bacteria, which thrive in waterlogged soil, convert nitrates to N2 (nitrogen)and NOx (nitrous oxides) gases, which are then lost to the air.

• Nitrous oxides can increase global warming, they can cause acid rain (nitric acid)

• Nitrous oxides also reduce the ozone layer which leads to increases in UV rays which can cause skin cancer

• Another problem is that “useful” nitrogen is constantly being lost from the soil.

http://www.mhhe.com/biosci/genbio/casestudies/nitrogen.mhtml