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Module 1: Ecological Interactions and Energetics

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Primary Production

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Today, we move forward with our discussion on ecological energetics, and look at primary
production. We look at some definitions. Primary production is the synthesis of organic
compounds from atmospheric or aqueous carbon dioxide, through the process of
photosynthesis or chemosynthesis. Here we are looking at the autotrophs or the organisms
that are responsible for primary production, as we have seen in one of our earlier lectures.
And these are of two kinds they are photo autotrophs or chemo autotrophs.
Photo autotrophs are those organisms that use light; photo is light, auto is self, troph is
nutrition. With the help of light, they are doing self-nutrition which means that they are
fixing up carbon dioxide into organic molecules. These organic molecules such as
carbohydrates or fats or proteins, and so on, then make up the bodies of these organisms.
And these organic molecules are not only required to make up the bodies of these
organisms. When these organisms are eaten up, by other organisms which we call as
consumers, so in that process these organic molecules move up in the food chain.

And with those the energy that was fixed up by the primary producers is also moved up in
the food chain. Primary production which is the synthesis of organic compounds from
atmospheric or aqueous carbon dioxide. Atmospheric carbon dioxide in situations, where
we have the photo autotrophs; that are exposed to the air. And in the case of those photo
autotrophs that are or in the case of those chemo autotrophs that are not exposed to the air,
but are residing in an aqueous environment or water environment in that case, they also
use the aqueous carbon dioxide.
Examples would be organisms that are living in the oceans, in the rivers, in different water
bodies, ponds, lakes, and so on. So, synthesis of organic compounds from atmospheric or
aqueous carbon dioxide through the process of photosynthesis or chemosynthesis; and the
organisms that are doing primary production are autotrophs which are of two kinds, we
have photoautotrophs, and the chemoautotrophs. The examples include trees, plants, algae,
and so on.
(Refer Slide Time: 02:29)

Why that important to learn about the primary production? It is important for three main
reasons. The first reason is that the plants form 99.9 percent of the earth’s living mantle.
There is this report from Whittaker, which says that 99.9 percent of the earth’s living
mantle or all the organisms that are living, so, 99.9 percent of those is made up of the
autotrophs or the plants, so because they form a very major portion of the ecosystem. So,
it becomes extremely important to know about those.

This is also evident if we go to any of our forested areas, so if you visit any forest, you
will see so many trees around, but so less number of animals that are there which makes
this ratio of 99.9 percent. The second reason is that primary production is responsible for
the conversion of the ultimate source of energy, which is the sun to biological energy
which fuels the complete ecosystem.
(Refer Slide Time: 03:34)

In the case of any food chain, if we talk about sun and the energy goes to the plants, from
there it goes to the herbivores, from there it goes to the carnivores. Now, in such a food
chain, the primary source of energy is the sun. And if this portion is not there, if plants are
not there, so the rest of the food chain will also collapse.
Now, even in the case of the detritus food chains. So, when we talk about detritus, which
is then fed by detritivores, which is then fed upon by carnivores, and then the next higher
level carnivore and so on. So, in this case when we talk about this detritus, this detritus is
coming from the plants or the herbivores or the carnivores or other parts of this food chain.
In this case also, we can trace the ultimate source of energy to the sun. Except in those
very few instances in which the source of energy is chemical reactions, when we are
starting a food chain through the process of chemo synthesis, through chemo autotrophs.
Even in that case, the conversion of the energy into the biological molecules in the very
first place occurs through the action of autotrophs, which is the same as talking about the
primary production.

And the third importance is that it releases oxygen as a by-product. Oxygen is required by
most of the other organisms to convert these biological molecules into energy, and because
this is a by-product of primary production. There also it becomes extremely important to
learn about primary production.
(Refer Slide Time: 05:28)

When we are talking about these reactions, there are two processes that are happening in
tandem or at the same time. In the case of a plant; a plant is doing photosynthesis, and it
is also doing respiration. In the process of photosynthesis, you have carbon dioxide and
water which are being acted upon through the action of enzymes, which are present in the
chloroplast. Chlorophyll is also important here, and they are fixing up solar energy into
these sugars. Here we are talking about glucose, so it is converting carbon dioxide and
water into glucose, and is releasing oxygen as a by-product.
Most of the cells of the plants, and most of the cells that are present in animal bodies are
also doing respiration. Respiration is a reverse process. In the process of respiration, these
molecules that were made by the plants are then burnt to generate energy. When we talk
about respiration, you just can invert this arrow. So, you will have this glucose plus oxygen
in the presence of metabolic enzymes, it is giving you carbon dioxide and water. And the
solar energy that was fixed in the process of photosynthesis is then released in the form of
energy molecules such as ATP.

(Refer Slide Time: 06:53)

When both of these reactions are act are acting at the same point, we can define three
terms. First is the gross primary production. Gross primary production is the energy or
carbon that is fixed by photosynthesis per unit time.
(Refer Slide Time: 07:09)

What we are asking here in the case of gross primary production is when we are talking
about this reaction. When we have this reaction, when we are talking about gross primary
production, what we are asking is how much of this carbon dioxide is getting fixed? or
how much of this energy is getting fixed? or how much of these biological molecules are

getting formed? or how much amount of oxygen is getting released? When we ask this
question that this reaction is happening, but what is the rate at which this reaction is
happening, then we are talking about the gross primary production. The energy or carbon
that is getting fixed via photosynthesis per unit time is the gross primary production.
(Refer Slide Time: 08:14)

Now, as we saw before, there are two reactions happening in tandem. Now, in any plant
cell or in those cells that have chloroplast, we are having this process of photosynthesis.
And at the same time we are also observing respiration in the whole of the plant. So, when
we talk about photosynthesis some amount of carbon is getting fixed, but then when we
talk about respiration some of that carbon is again getting released back.
When we are talking about the gross primary production, we are asking about the rate of
photosynthesis. But, when we subtract respiration from this, so photosynthesis minus
respiration. In that case, we are talking about the net primary production. Net primary
production is the gross primary production or the energy or carbon that was fixed by
photosynthesis minus the energy or carbon that is lost via respiration. When we express it
per unit time, we are talking about the net primary production.
In some of the books we say that, when we are talking about gross primary production, it
is the energy or carbon that is fixed. When we talk about gross primary productivity, in
that case it is fixation per unit time. This is a semantic difference that we see in some
literature.

(Refer Slide Time: 09:43)

When we say production that is amount of CO2 or energy fixed and when we say
productivity, that means, production per unit time, but then there are some books that use
these terms interchangeably. When we talk about gross primary production, we can also
say that we are referring to the energy that is being fixed per unit time. There is this third
term called compensation point. Compensation point is the equilibrium point for plants,
where photosynthesis equals respiration.
(Refer Slide Time: 10:35)

What is compensation point? Suppose you have this plant, and when you have the sun,
then you have two processes that are happening. One is photosynthesis, and the second
process is respiration, now this is during the daytime. Now, during the night time, we do
not have the sun, and so the photosynthesis stops, and we only have a respiration that is
going on.
In this case, we can say that respiration occurs at all times, whereas photosynthesis happens
only in the daytime, only when light is available. If we look at the amount of carbon or the
amount of carbon dioxide that is getting fixed, we will find that in the case of respiration,
you will have a constant amount. So, this is fixed or released.
Here we have respiration, because that is happening at all times whereas, in the case of
photosynthesis, so on the x axis here we have the time. Let us say here you have from 0
hours 6, 12, 18, and 24 hours. Now, 24 hours is midnight. Now, suppose the sun rises at
around 6’o clock in the morning, so in that case the photosynthesis reaction would start at
this time. And then, let us say that the sun sets at around 6’o clock in the evening , and
then this would peak at some time. So, this is the amount of carbon dioxide or oxygen that
is getting absorbed. And in the case of respiration that would be somewhere below this or
something like this. So, here we have respiration.
Now, if we look at this curve, we can divide it into these three regions. Now, region 1, 2,
and 3; now, in the first region from 0000 hours, till say around 0615 hours. So, 00:00hrs
to 06:15hrs. Here we have a time where the where the carbon dioxide that is released
because of respiration is greater than the carbon dioxide that is getting fixed by the process
of photosynthesis. So, here we have a net release of carbon or net release of carbon dioxide.
In this stage, from 06:15 hrs to say around 17:45 hrs here we see that the amount of carbon
that is released in the process of respiration is less than the amount of carbon dioxide that
is getting fixed because of photosynthesis.
In section-2 we will have a net absorption of CO2. And in this third stage that is from your
17 45hours to 2400 hours, here again we have a net release of carbon dioxide. In this curve
we can delineate two points; one is this, and the second one is this. At both of these points,
we have the amount of carbon dioxide that is getting released because of respiration is
equal to the amount of carbon dioxide that is getting fixed because of photosynthesis. And
both of these points go by the name of compensation points.

Compensation point is the equilibrium point for plants, where photosynthesis equals
respiration or the amount of carbon dioxide that is getting fixed by photosynthesis is the
amount of carbon dioxide that is getting released because of respiration or in terms of
energy the amount of energy that is getting fixed because of photosynthesis is equal to the
amount of energy that is being released through the process of respiration.
At these two points, the plant is neither absorbing carbon dioxide nor is giving out oxygen.
Let us say, it is neither absorbing carbon dioxide nor is it releasing carbon dioxide, and it
is neither absorbing oxygen or it is release of oxygen. These two points as they normally
occur during early mornings and late evenings, these two points are known as the
compensation points.
(Refer Slide Time: 15:46)

Now, how do we measure the gross primary production or the net primary production? So,
here we have three different methods through which we can measure the amount of energy
that is being fixed or the amount of carbon that is being fixed. When we write this reaction,
6 CO2 + 6 H2O in the presence of chlorophyll enzymes, and solar energy is giving you
glucose + 6 O2.
So, in terms of energetics we can ask this question, how much amount of solar energy is
required in this process. So, if we compute the amount of solar energy that is required, it
comes to around 2966 kilo joules, when you have one mole of glucose that is being
produced. So, for each mole of glucose that is being produced, you have 2966 kilojoules

of energy that is getting absorbed. 6 moles of carbon dioxide that is getting utilized and 6
moles of oxygen that is getting released.
Now, when we say 6 moles of carbon dioxide, it means 134.4 liters at the standard
temperature and pressure, which is defined as 0 degrees Celsius, and a pressure of 1 bar.
Now, when we have these values, we can measure the amount of carbon dioxide that is
getting fixed by either measuring the rate at which this carbon dioxide is getting utilized.
So, for instance you have a plant, you cover it with a glass jar, and you measure the amount
of carbon dioxide that is present in the air there. And then throughout the day, you try
measuring the amount of carbon dioxide at different points of time. And when you come
to this conclusion that this ‘x’ amount of carbon dioxide has been utilized, so we can say
that x moles divided by 6 moles is the amount of glucose in moles that has been produced
or in place of measuring carbon dioxide, we can even measure oxygen.
So, we can measure the amount of oxygen that has been released by the plant to make an
estimate of the amount of carbon dioxide that is getting fixed or the amount of these
biological molecules that are getting synthesized. So, this is a way of measuring the gross
primary production or productivity. Now, in this case, if we also include the amount of
carbon that is getting released because of the process of respiration, we are measuring the
net primary production or productivity.
(Refer Slide Time: 18:04)

Now, another method is this. So, here we see that in place of carbon dioxide, if we replace
it with radioactive carbon dioxide. So, if we replace carbon-12 with carbon-14, then this
carbon 14 will also get incorporated in these sugar molecules that are being produced. So,
we can put this plant into a chamber in which it is not having our normal CO2, but all the
carbons in the CO2 have been labeled. So, they are all carbon-14.
In that case we can measure the amount of carbon-14 that is getting incorporated in the
plants. And then we can use it to make an estimate of the amount of carbon dioxide that
has been absorbed by this plant in the process of photosynthesis. Now, even in this case,
because the plant is also doing some amount of respiration, so some amount of carbon-14
will also be lost. And so in that case, we are measuring the net primary production or the
productivity.
(Refer Slide Time: 19:17)

Now, one other variant could be that we are not replacing all of CO2 with carbon 14, but
what we are doing is that we have a mixture of carbon-12 and carbon-14. And then, if we
know the ratio that was there in the beginning, we can use this ratio as well to figure out
the amount of sugars that are getting produced by looking at an amount of carbon-14 that
has been fixed in this process.

(Refer Slide Time: 19:39)

Now, the third method which is the easier method and is the most widely utilized method,
it says that the amount of plant material that is being produced can be measured as delta
B, where B is the biomass. So, delta B is the change in the biomass between two time
periods t2 and t1. And B2 is the biomass at time t2, and B1 is the biomass at time t1 .
(Refer Slide Time: 20:11)

So, if we take this difference, so what we are doing in this case is that, suppose you have
a forest, now in this forest we go there at say time t1, and we measure the total amount of
biomass that is there in the system. Now, how do we measure the biomass, well we can

make an estimate of the total amount of wood that is present, the total amount of leaves
that are present. And we can also make an estimate of the total amount of biomass in the
form of roots that is present in this forest. So, you add the leaves from this plant, this plant,
this plant, and so on.
And also you can make an estimate of the amount of litter that has gone down. So, litter
consists of the dead wood or the decaying wood or the dead leaves that have come down.
Now, at time t, when you make this measurement of the total amount of biomass that is
present in this forest. And you measure that it is B1. Now, you go back to this forest, after
say one year, and at time t2, you measure the amount of biomass and that is B2.
Now, in this period of t2 minus t1, you have a total change of biomass of B2 minus B1, so
that is the amount of biomass that has been produced or destroyed depending on whether
it is a positive or negative, in time t2 minus t1. So, this is the amount of biomass that was
produce were destroyed, and divided by that the time period of measurement. So, from
this as well we can make an estimate of the net primary productivity of this particular
forest.
(Refer Slide Time: 22:11)

Now, we can also define the efficiency of production. The efficiency of gross primary
production is defined as the energy that is fixed by gross primary production divided by
the energy in this incident sunlight.

(Refer Slide Time: 22:33)

What we are asking here is, suppose you have a plant, and this plant intercepted say 1000
calories from the sun. Now, when it intercepted this amount of calories, how much was
the amount of energy that it was able to fix in the form of the biological molecules, because
in this process as well, this will not be a 100 percent efficient process. You will also be
losing out some amount of energy. Suppose this tree got 100 calories of energy, and it was
say able to fix 40 calories through the process of photosynthesis.
In this case we will define the efficiency of gross production as 40 calories divided by
1000 calories into 100 percent. Here we will have a 4 percent efficiency, which is the gross
efficiency. Now, in place of using this term energy fixed by gross primary production, if
we remove the amount that was released back because of respiration. We are using the net
primary production, so in that case we can define the net efficiency. So, suppose out of
this 40 calories, we have a situation in which 30 calories are lost due to respiration.
The net amount of energy that gets fixed is 40 calories minus 30 calories is 10 calories.
And in that case, we define the net efficiency as 10 calories divided by these 1000 calories
in to 100 percent, which is a 1 percent efficiency of net primary production.

(Refer Slide Time: 24:34)

We can also define another term which is productivity. And productivity is defined as
production per unit time. If we say that net primary production for a particular forest was
say 1 ton of biomass that was produced, and that amount of biomass was produced in a
period of say 2 years. So, we will say that productivity is 1 ton divided by 2 years, which
is 0.5 tons per year, so that is productivity; production divided by time.
We can define or we can try to compute net primary productivity using this equation. The
net primary productivity is given by APAR multiplied by LUE, where APAR is the
absorbed photosynthetically active radiation multiplied by the light use efficiency.

(Refer Slide Time: 25:28)

In this case, what we are saying is that we are talking about the net primary productivity
is given by APAR into LUE. Now, APAR is the absorbed photosynthetically active
radiation. So, essentially how many joules of energy or how many mega joules of energy
was absorbed per square meter of area, and per unit time, say into x hours.
If we have say y mega joules of energy that was absorbed by the plants, now this energy
is coming from the sun. So, out of the incident radiation, there was y mega joules of energy
that got absorbed by the plant, and this is the photosynthetically active radiation. What do
we mean by photosynthetically active radiation? When we talk about the whole of the
spectrum the VIBGYOR, then the wavelengths that are mostly responsible for
photosynthesis come in the blue region, and in the red region. And the other radiations say
green, yellow, orange, they are mostly reflected by the plants, they are not used for
photosynthesis.
Now, because green is mostly reflected, so this is why the leaves look green in color. So,
green is not being used for photosynthesis, whereas the red and blue are being used for
photosynthesis. So, we are only considering that portion of the spectrum that is being used,
because in the process of photosynthesis, so that is photosynthetically active radiation.
Out of that photosynthetically active radiation, the total amount that gets absorbed is the
APAR. Now, that photosynthetically active radiation is given in terms of how many mega

joules of energy was there per unit time, per unit area, so which is why we have mega
joules per unit time, per unit area.
Light use efficiency is the efficiency of the plants to use this light. In this case what we are
asking is the plant was able to absorb, these many mega joules of energy and when these
many mega joules of energy were converted into carbon that was fixed. Here we have the
grams of carbon per mega joule of energy. In this case, you will have mega joule and mega
joule that will get canceled out, and we will have an estimate of the grams of carbon that
are being sequestered or that have been converted in the form of biomass per unit area,
and per unit time, which is an estimate of the net primary productivity.
So, net primary productivity, we had defined it as x amount of carbon or x grams of carbon
that was getting generated per unit time. So, per unit time is say in 1 hour. And in the case
of net primary productivity, we can define it for a forest or for any area or we can define
it per unit area. So, in this equation, this was there in an area of say square meters. So, in
that case we have the net primary productivity that is given by the multiplication of
absorbed photosynthetically active radiation multiplied by the light use efficiency. Now,
this gives us a way of estimating the net primary productivity for any area on earth, because
the absorbed photosynthetically active radiation will depend on how much amount of
radiation is actually made available at that particular area.
(Refer Slide Time: 29:47)

For instance if we consider the earth, and in this case this is the equator. If we have the sun
here, so in this case, if you consider a point here, so this point is getting much more amount
of sunlight as compared to a point here, because this point is receiving a light that is
incident at a very flat angle. So, the amount of photosynthetically active radiation that gets
absorbed can be figured out by looking at the latitude of the place that can also be looked
at by looking at the aspect of that place.
(Refer Slide Time: 30:37)

So, for instance in the case of India, if we have a hill and because India is in the northern
hemisphere, this is north and this is south. So, in this case the southern aspect gets more
amount of sunlight as compared to the northern aspect. So, if more amount of sunlight is
getting incident on the southern aspect, so more amount of light is made available to the
plants and so more amount of light will be absorbed by the plants.
APAR can be discerned by looking at the location of that place, it will also depend on the
amount of cloudiness in that area, because clouds are able to block the sunlight. So, if there
is an area that has more amount of clouds in a year, so in that case the APAR will be less.
Similarly we can compute the light use efficiency, light use efficiency will depend on
different species for instance, it will also depend on the fertility of that area or the amount
of water that the area has or the amount of nutrients, mineral salts that are there in the soil
in that particular area, so, that makes it possible to model the APAR, and the light use
efficiency to make an estimate of the net primary productivity.
Using that we can compute the net primary productivity. Net primary productivity can also
be computed using satellite data. In the case of satellite data, what we are trying to measure
is the amount of chlorophyll that is there per unit area. The amount of chlorophyll that is
present here if you have more amount of chlorophyll and you know what kind of species
are there, so you can figure out, what is the amount of productivity that we can expect from
that area. We can make an estimate of the net primary productivity of different areas of
the earth. And we can also compute the net primary productivity and the gross primary
productivity using modeling.
(Refer Slide Time: 32:40)

This is the gross primary productivity of different areas. And here we can observe that it
starts from 0 and goes to 2400. And so these areas are the most productive areas. So, like
this area is the Amazonian rainforest. These rainforests have a very high amount of gross
primary productivity. In comparison these areas so like this is the Sahara desert. Sahara
desert will be having a very less amount of gross primary productivity, because you have
less number of plants, and you have a dearth of water in that area. Now, most of the areas
of Europe will come in a moderate amount of productivity. In comparison India has a
much higher level of gross primary productivity and so is the Southeast Asian nations.
(Refer Slide Time: 33:37)

We can also compute the net primary productivity or we can also compute the things such
as the light use efficiency for gross primary productivity and the net primary productivity.

(Refer Slide Time: 33:39)

(Refer Slide Time: 33:41)

(Refer Slide Time: 33:46)

Now to recap, what does productivity depend on? Productivity depends on the solar
constant; the rate at which energy reaches the earth’s surface from the sun. And this is
usually taken to be 1388 watts per square meter. This is the amount of energy that the sun
is giving. We know how much amount of that energy is photosynthetically active radiation.
We can figure out a proportion of photosynthetically active radiation using the solar
constant. This is the energy that is being received by the sun on average, but different areas
would be receiving different amounts of energy, depending upon the latitude of that place,
the cloudiness of that place.
Also the dust and water that are there in the atmosphere, because dust and water will also
occlude or block the photosynthetically active radiation that is reaching the plants. The
amount of sunlight that is received by the plants will also depend on the area of leaves that
the plant has, and the arrangement of leaves.
The plant needs to have an arrangement that maximizes the amount of radiation that is
being intercepted by the leaves. It will also depend on the amount of carbon dioxide that
is there in the atmosphere as will be the amount of water that is being available in that
area. Using all of these different factors, we can model different factors of productivity.

(Refer Slide Time: 35:10)

This is once such simulation exercise that we had done, and this was to understand the
impact of global warming on the carbon sequestration potential and stand dynamics of
Chir Pine forests.
(Refer Slide Time: 35:30)

What we did here was that we considered in area and this area was Almora district of
Uttarakhand. And in this Almora district we were considering the pine trees. If we have
pine trees, and they are at the current ambient conditions. The current ambient conditions
means the amount of carbon dioxide that we have in the air at present, and also the location

of this place, what is the latitude of this place, how much is the amount of cloudiness that
is there in this place, how much amount of water is there that this area is receiving, what
is the level of fertility that the soil have, so those are all the ambient conditions.
We wanted to understand if in the process of global warming, now when we have global
warming, there are two things that are happening. One is that we have an increase in the
carbon dioxide levels in the atmosphere. And because of that and because of the
greenhouse effect, we will observe an increase in temperature. Now, for most of the plants
if you increase carbon dioxide, so because in the process of photosynthesis you have
carbon dioxide plus water is giving you the sugars and oxygen.
If you increase the amount of carbon dioxide, then the amount of production of your
glucose or the sugars will increase. This will have an impact of fertilization on the plants.
Whereas, an increase in temperature might be useful for the plants or it might be harmful
for the plants that would depend on the existing conditions. For instance, if you have a
banyan tree that is there in Uttarakhand, now banyan tree is normally found in the tropical
area. It is a tree that wants to have a higher temperature, but then you have planted it
somewhere in Uttarakhand, where it is very cold.
In that case if you increase the temperature, so this plant will be more comfortable and it
will be much more efficient in absorbing carbon dioxide or sequestering carbon dioxide.
On the other hand, if you consider say a pine tree that is planted in Madhya Pradesh. So,
in that case, your pine tree which is a tree of cold areas has already been put in an area that
is extremely warm.
If you increase temperature further, its efficiency will go down even further.