BIO(Bio-geo-chemical cycles)

                 Bio-geo-chemical cycles

            Bio-geo-chemical cycles

A constant interaction, between the

biotic and abiotic components of the

biosphere, makes it a dynamic, but stable

system. These interactions consist of

transfer of matter and energy between the

different components of the biosphere.

Bio-geo-chemical pathways determine the

path of transfer of matter on earth. Let us

look at some of the major biogeochemical

cycles.

Bio-geo-chemical cycles as we may

see from the name itself includes both

biological, geological and chemical or

physicochemical pathways. This means the

reservoir or pool of nutrients on earth may

contain some chemicals of biological

origin while others may be purely inorganic

in nature also may be geochemical

(obtained from rocks and soil) in origin.

Water though not considered as a biogeo-chemical cycle by most ecologists

actually is the precursor of the major

elements Hydrogen and Oxygen as some

living organisms use them for making the

basic food molecules for several

organisms in nature.

Water is also a universal solvent and

essential for various reactions to take place

within a living cell. Thus we shall also take

up water cycle briefly in this chapter.

Though the nutrient pool involves several

elements of nature but, we shall study just

the cycling of some major elements like

Oxygen, Nitrogen and Carbon.

The water cycle

All the water that is on the earth has

always been here? Earth never gets water

added to it nor does water disappear from

the earth. Water is constantly recycled in a

process known as the Hydrological or

Water cycle.

Fresh water is more scarce than you

might think.Nearly 97% of all the water on

the earth is in the oceans, and so only about

3% is fresh water. About 2% of this fresh

water is permanently frozen in Glaciers and

at the Polar Ice caps.

Thus only about 1% is available fresh

water. Again about 1/4 of this 1% is present

as groundwater. Only about 0.009% of

water on earth is in the rivers and lakes.

Rest is present in the bodies of living

organisms, as soil moisture, as humidity of

atmosphere etc. Water is the most essential,

abundant substance in living things.

The human body for example, is

composed of about 70% water (remember

all living organisms together constitute

only 0.005% of water on earth). Water

participates in many biochemical

mechanisms, including photosynthesis,

digestion and cellular respiration. It is also

the habitat for many species of plants,

animals and microorganisms, and it

participates in the cycling of the materials

used by living things. So, it is important

that we protect our water

resources.

The Nitrogen cycle

Nitrogen is both the most abundant

element in the atmosphere and, a building

block of proteins and nucleic acids. The

Nitrogen cycle is a complex

biogeochemical cycle in which Nitrogen

is converted from its inert atmospheric

molecular form (N2

) into a form that is

useful in biological processes.

The element Nitrogen is constantly

moving in a giant circle from the air,

through the soil, into the bodies of plants

and animals, and eventually back to the air

by the process of Nitrogen cycle. All living

things need nitrogen mainly for growth,

repair and development (Nitrogen being

essential for protein formation). Even

though the Earth’s atmosphere is made up

of 78% nitrogen, plants and animals cannot

use it in this form.

The atmospheric nitrogen is thus

converted into certain compounds which

plants may take up from the soil by some

biochemical process (caused by certain

bacteria like Rhizobium, Nitrosomonas etc)

and physicochemical processes (caused by

lightning). Animals get the required amount

of nitrogen from plants either directly

(herbivores) or indirectly (carnivores).

The nitrogen cycle contains several

stages:

1. Nitrogen fixation:

Atmospheric nitrogen occurs primarily

in inert form (N2) or non reactive form that

few organisms can use; therefore it must

be converted into a compound - or fixed -

 form in a process called nitrogen fixation.

Most atmospheric nitrogen is ‘fixed’

through biological processes. A number of

bacteria and blue green algae are known to

be able to fix atmospheric nitrogen into

compounds in their own body. These may

be symbiotic (Rhizobium) or freeliving

(Nitrosomonas) respectively. These

organisms convert atmospheric nitrogen

into the organic nitrogen for their own

cells. As they die rapidly( they grow rapidly

as well), this nitrogen, now present in the

soil as compounds become available to

plants. In leguminous plants like pea, beans

etc there is a symbiotic relationship of the

nitrogen fixing bacteria with the plant.

2. Nitrification:

Nitrates can also be converted to

Ammonia by the de Nitrifying Bacteria in

the soil (especially in waterlogged soils).

The nitrifying bacteria may then use this

ammonia to synthesize compounds for

their own cell and eventually convert to

Proteins, Nucleic acids, Nitrites and

Nitrates. Nitrites are produced mainly by

Nitrosomonas, while nitrates by

Nitrobacters that are also capable of

utilizing nitrites and converting them to

nitrates. Death of these microorganisms

add the nitrogenous compounds to the soil.

Plants take up nitrate as well as ammonium

ions from the soil to convert them to

proteins and nucleic acids.

3. Assimilation:

Nitrogen compounds mainly as nitrates

or ammonium ions(NH4+) are taken up

from soil by plants which are then used in

the formation of plant proteins and as

animals eat these plants, animal proteins

are synthesised.

4.Ammonification:

Production of Ammonia (NH3

) from

Nitrates and other Nitrogenous compounds

in called Ammonification.

Describe a path of ammonification

discussed in the above section.

Ammonification also occurs when

plants and animals die, or when animals

emit wastes, the nitrogen in the organic

matter reenters the soil and water

bodies where it is broken down by

other microorganisms, known as

decomposers. This decomposition

produces ammonia which is then

available for other biological

processes.

5.Denitrification:

Nitrogen makes its way back into

the atmosphere through a process

called denitrification, in which

solid nitrate (NO3) is converted back to

gaseous nitrogen (N2). Denitrification

occurs primarily in wet soils where water

makes it difficult for microorganisms to

get oxygen. Under these conditions, certain

organisms - known as denitrifiying bacteria

- will process nitrate to gain oxygen,

leaving free nitrogen gas as a byproduct.

Thus, the nitrogen content of the earth

and its atmosphere remains in a perfect

balance.

Human intervention and nitrogen

cycle

Unfortunately, humans are interfering

with the natural balance when they overuse

artificially produced nitrates as agricultural

fertilizers that are often washed into water

bodies by rain as well as by releasing

exponential amounts of untreated domestic

sewage into water bodies. Before these

nitrates can be converted into atmospheric

nitrogen, they are often carried off by rain

or irrigation to streams and rivers and even

seep down to groundwater.

In some parts of the world, water for

humans and animals contains such high

concentrations of nitrates that it is unsafe

for consumption. These excessive amount

of nitrates and other nitrogenous

compounds, when they reach rivers and

lakes, cause too much algal growth. This

over-abundance of algae uses up too much

of the oxygen in the water. When oxygen

level falls, other forms of life in the water

bodies die off.

The Carbon cycle

Carbon is found in various forms on the

Earth. It occurs in the elemental form as

say Soot, Diamond and Graphite. In the

combined state, it is found as gases, Carbon

dioxide and Carbon monoxide in the

atmosphere, as carbonate and hydrogen

carbonate salts in various minerals, while

all life-forms are composed of carbon

containing molecules like Proteins,

Carbohydrates, Fats, Nucleic acids and

Vitamins. The endoskeletons and

exoskeletons of various animals are also

formed from carbonate salts.

Carbon dioxide is also responsible for

maintaining the Earth as a Green house with

temperature conditions suitable for life.

Thus, Carbon exists in the biosphere as the

central element of life. Carbon Dioxide or

CO2

, now makes up about 0.04% by volume

of air.

Have you ever thought how this level

of Carbon is being maintained in the

nature?

Carbon is incorporated into life

through various processes. The main

reservoirs of carbon are sedimentary rocks,

fossilized organic carbon including the

fossil fuels, the oceans, and the biosphere.

Photosynthesis:

The first step in the biological

carbon cycle is the conversion of

inorganic atmospheric carbon into a

biological form. This ‘fixing’ of

carbon in biological form takes place

within plants and other organisms -

 known as producers - in a process

called photosynthesis, by which

energy from sunlight is converted into

chemical form.

In photosynthesis, light energy

helps to combine carbon dioxide and

water to create the simplest of sugars,

the carbohydrate molecules known as

Glucose (C6H12O6). In oceans,

photosynthesis is carried out by

microscopic aquatic plants called

phytoplankton. The carbohydrates

then become the source of chemical

energy to cells in all plants and

animals. In plants, some carbon

remains as simple glucose for short-term 

energy use, while some are converted

to large complex molecules such as starch

for long term energy storage.

The Green house effect:

A greenhouse is a small house made of

glass that is used to grow plants. It traps

the sun’s rays and keeps the heat from

escaping. It is warm inside. In the same way

that the glass traps heat in a greenhouse,

some gases present in the atmosphere such

as Carbon di Oxide, Carbon monoxide,

Methane and Water vapour trap heat from

radiating back to the space. The natural

greenhouse gases act like a big blanket

around the earth, keeping it warm and

making life possible without which

temperatures would have fallen to sub zero

values. This phenomenon of naturally

warming up is called ‘‘Greenhouse effect”.

But the extent of this natural warming

up process have been grossly affected now.

Due to various human activities like burning

of fossil fuels, deforestation and

industrialization, an excessive amount of

carbon dioxide and other green house gases

has been emitted to the environment. As a

result more heat gets trapped. This causes

the temperature of the earth to rise, which

results in Global Warming. 

Oxygen cycle:

Oxygen is an abundant element,

next to Nitrogen, on our Earth. It is

found in the elemental form in the

Atmosphere to the extent of nearly 21%.

It also occurs extensively in the combined

form in the Earth’s Crust as well as in the

air in the form of carbon dioxide. In the

crust, it is found as the oxides of most

metals. It is also present as carbonate,

sulphate, nitrate and other compounds. It

is also an essential component of most

biological molecules like carbohydrates,

proteins, nucleic acids and fats (or lipids).

The cycle and storage:

 Oxygen from the atmosphere is used

up mainly by the processes, combustion,

respiration and in the formation of oxides

of elements like Nitrogen, Iron etc. Oxygen

is returned to the atmosphere in only one

major process, that is, Photosynthesis.

Ozone layer:

The Earth’s atmosphere is divided into

several layers. The lowest region, the

Troposphere, extends from the Earth’s

surface up to about 10 kilometers (km) in

altitude. Virtually all human activities occur

in the Troposphere. Mount, Everest, the

tallest mountain on the planet, is only about

9 km high. The next layer, the Stratosphere,

continues from 10 km to about 50 km.

Most commercial airline traffic occurs in

the lower part of the Stratosphere. most

atmospheric ozone is concentrated in a

layer in the stratosphere, about 15-30

kilometers above the Earth’s surface.

Ozone is a molecule containing three

Oxygen atoms. It is blue in colour and has

a strong odour.

Normal oxygen, which we breathe, has

two oxygen atoms and is colourless and

odourless. Ozone is much less common

than normal oxygen. Out of each 10 million

air molecules, about 2 million are normal

Oxygen, but only 3 out of 10 millions are

Ozone.