Lec 18 | MIT 7.014 Introductory Biology, Spring 2005

All righty. So we’ve been talking about,
the last time we talked about the
different ways that this extraordinary process called
evolution has resulted in different ways that organisms
exploit the carbon and energy sources that
are available on earth. And as we talked about that we
find that whatever is thermodynamically
possible in terms of redox gradients in the environment you
will find an organism that has evolved to
exploit it. And one thing I haven’t
mentioned to you guys yet — Maybe I’ve mentioned it but I
haven’t driven it home is that in the microbial world right now
99.9% of the microbes that live on
earth have never been cultivated. So it’s
a vastly unknown universe, the details. So the metabolic pathways
that I’m describing here are just the tip of the iceberg in
terms of what’s possible. So last time we were talking at
the biochemical scale. This time we’re now going to
scale up and think of these
biochemical processes on a global level. And
we’re going to talk about primary productivity which is
really just another word for the collective
photosynthesis of all of the photosynthesizers on earth. So who are the dominant primary
producers? Well, we’ve already talked
about this. And just to get
ourselves oriented let’s think of in water the primary
photosynthetic organisms, the primary
producers are phytoplankton. And let’s just look
at one phytoplankton cell, OK? This is a single cell so this
would be maybe ten microns in size. And inside that cell we have a
chloroplast, and you’ve learned all this in
other lectures and including my lectures,
which takes up CO2 and evolves oxygen and makes sugars
which go to, what’s the organelle where
respiration takes place? Mitochondria. Which goes to the mitochondrion
and is respired. CO2
is evolved and oxygen is taken up. So this little single cell gets
through life through photosynthesis primarily,
but it also takes the products of photosynthesis and
burns them inside the cell. So this is
what we call respiration. And it’s important to remember
that even photosynthetic organisms
respire, OK? But then there’s this
whole other group of organisms that last time we called
heterotrophs that can only respire. They rely on the photosynthetic
product from photosynthetic organisms. So that’s in water. And then on land the main
primary producers, productivity are plants. And we’ll just symbolize these
as a tree. And we all know that the tree
takes up CO2, evolves oxygen. If you blow this up,
you just take a leaf, OK? That’s a blowup. And then you blowup again and
take a cell from the leaf. This cell is identical in
function to the single microscopic phytoplankton cell,
OK? So the process in both of these
ecosystems, this could be grass, it could be moss,
it could be anything else, the process is the same
in terms of what’s involved in productivity. OK. Now, you have a handout for
this lecture that has a lot of the definitions that I’m
going to now very briefly put on the board. Oh, I should tell you also that
in the last ten minutes of the
lecture I’m going to show you another
incredible clip from the Blue Planet series that I showed you
last time so you have something to look
forward to. So let’s
define some terms. Biomass we’re going to call B. And as I said this is all
in the handout. So I’m just going to give you
the short version here. And the units of this can be of
course anything, but normally it’s something
like grams carbon per meter squared. And
what does that mean? Well, it means if you have,
in a terrestrial ecosystem, OK,
where you have trees, it would mean that you would
calculate the biomass. You’d take a square meter and
you’d integrate up and collect all of
the biomass in that surface area, OK? So that’s what that means. So grams carbon in that square
meter. For an
aquatic ecosystem it’s the opposite. You take a square meter of the
surface water, let’s put some waves on here,
and you integrate all the way down. How far? When will you not have
anymore primary, oh, no, biomass? When will you no longer have
any more photosynthetic biomass in
the oceans or in a lake? Pycnocline. Oh, that’s a good
answer. Not necessarily,
though. But it often corresponds to
that. The pycnocline is a density
gradient in aquatic ecosystems. But what is
absolutely essential for photosynthesis? Light. Light. So
you go down until there’s no light. Absolutely. There won’t be any
photosynthesis where there’s no light. This is the one thing we
know for sure, OK? And that’s usually in the
oceans around 200 meters. And in lakes it depends on how
rich they are. In the Charles
River it’s about one meter because it’s such a mess. In fact, well,
I shouldn’t say a mess, but it’s very productive. In
fact, legend has it, and I don’t know if this is
true, that it’s actually safe to swim in the
Charles in terms of the water quality, but
the reason you’re not supposed to swim is because the
visibility is so bad that if anything happened
they’d never find you. But I don’t know if
that is true. That could be an urban legend. OK, so we’re going to define
gross primary productivity. GPP, gross
primary productivity is the rate of photosynthesis —
— and grams carbon per meter squared per year. And of course the units here
are not, the absolute units,
this could be per day and this could be per
square millimeter if you wanted, but the units are amount
per unit area per unit time,
OK? And then we’re going to define
the respiration rate, the respiration of the
autotrophs. So this would be the
respiration rate of the photosynthetic organisms,
which is why we call them R sub A. And that’s going to have the
same units, OK? So it’s grams carbon per meter
squared per year respired. And net primary productivity is
then gross primary productivity minus R
sub A so net primary productivity of an ecosystem is
the amount of carbon CO2 that’s drawn into the
system through photosynthesis minus the
amount that was respired by the plants, by the organisms that
did the photosynthesis. In a sense, you can think of it
as the amount of carbon that
actually goes into a plant growing, OK, that
goes into the biomass of a tree or that goes into the division
of a single celled phytoplankton
into two phytoplankton. And then a lot of
that is lost through respiration. OK. We can also define mean
residance time — — as MRT, which is the biomass
divided by the net primary productivity. So what are the units of the
mean residance time? It should be obvious but —
Years, right, or time. Mean residence time is time. And that’s really,
if you want to think about it, if you think of yourself as
a carbon atom that’s drawn into a tree
through photosynthesis, it’s the average amount of time
you will spend, your one atom in that
tree. OK? It’s the average residence,
the mean residence time. Well, actually,
that’s wrong, right? It’s
not the amount of time that you, that one atom will spend,
but it’s on average the amount of time
that atoms will spend in the tree. OK. And then the fractional
turnover — — is equal to one over the
mean residence time. And it has the
units of obviously years to the minus one. And it’s the fraction,
if you think of a tree again, it’s the fraction of the carbon
in that tree that is renewed by new
carbon every year. Now, these two concepts
will become very important again when we start to talk
about global biogeochemical cycles. And we’ll talk about the
residence time, the various elements in
various components in the earth system. OK. So now let’s go on and look at,
first of all, I should have
shown this slide last time. I’m trying to not walk around
too much because this fellow is filming
me or filming these classes. But we should briefly take a
look at the absorption spectrum of all of
the plants in the biosphere. Could we turn the lights down a
little? Or I guess I’m in charge of
that. There. A little bit better maybe. This is the absorption spectrum
of the pigment chlorophyll,
chlorophyll A that is the pigment that all green
plants have and they use to absorb sunlight. And you’ll notice that
over the course of evolution all of these white bands are
accessory pigments. That different organisms have
evolved to also capture light. And they pass on that energy to
the chlorophyll molecule. And the point
is that if you look across all of the visible light and even
beyond there are pigments that have
evolved to capture that solar energy
collectively in the ecosystem. OK, so let’s look at,
now we’re going to look at world net primary
productivity. So essentially
photosynthesis on a global scale. And I’m going to tell
you right up front. These numbers are extremely
approximated. And I’ve taken these numbers
from various textbooks. Your textbook doesn’t even have
a table like this in it. In fact,
your textbook is very weak in this section of biology. But there are
always tradeoffs in choosing textbooks. So I’ve taken this from
textbooks and I’ve rounded off these numbers. And so I just want you to
go through and understand the structure of the table. The idea is not to memorize
particular numbers but understand
and have a feeling for the relative amounts of productivity
in different ecosystems. So, first of all,
here’s our units of grams per year,
world net primary productivity. These are grams of carbon,
OK? And
if we first look at the total amount on land,
in this particular table is 177 versus the marine total is
54. But the new estimates,
I’ve taken this out of sort of more
primarily literature than textbooks, really
show these numbers to be more like this. That shows you how variable
this is. It changes every decade. Showing that the amount of
photosynthesis in the oceans is roughly on par with the amount
on land. And to remind you of our units
here, weight-wise this is 50 to 70 billion Volkswagen’s
worth of carbon. I mean that’s a lot of carbon
every year that is going into these
ecosystems. OK, so let’s look and dissect
the table a little bit. Looking at tropical forests
like the rainforest in the Amazon that are
some of the most productive ecosystems in the world. You can see that over here
their net primary productivity per meter
squared is 2,000. And then you look down here at
the open ocean which on a per meter squared basis,
a very unproductive ecosystem, there are
tiny little phytoplankton, is 100. But looking further,
before I get to the but which is the punch
line. That’s the trouble with
animation. The biomass in the tropical
forest is enormous, obviously trees,
whereas the biomass in the open ocean is very tiny. But if we look
at the percent of the surface of the earth that is covered by
these two different ecosystems the open
ocean is huge compared to the tropical
forest. So on a global scale,
because of the aerial coverage here
these two ecosystems contribute equally, OK? So it’s a combination of net
primary productivity and the coverage of the
global ocean. OK. Let’s go back to that for a
minute. So let’s —
— talk about turnover times or mean
residence times. Just eyeballing it,
can you give me an estimate in terms of days,
months, years, decades, centuries,
order of magnitude for the mean
residence time of carbon in all the
phytoplankton in a marine ecosystem? Is it days, months,
years, centuries, decades? Well, don’t guess. Well, you can
guess but minutes is wrong. So having guessed minutes is
wrong now you use your analytical brain
and you look at this, mean residence
time, which is biomass divided by net primary productivity. Here’s the biomass. Round that off. And here’s the net
primary productivity. And the units here is years. Like a month did I hear? I didn’t hear. Right. It’s about one-tenth of year
which is about a month. About a month. Does everybody follow that? It’s just to round
this off, five over 50 is 0.1, biomass over NPP. How about for the
terrestrial ecosystem, what’s the average amount of
time a carbon atom spends in the average tree? Years, right? Decades. Many years. Maybe decades to centuries. This
is a way we think about these things. We don’t have
an exact number. But you want to get an order of
magnitude feeling. So carbon is turning over very,
very fast in the marine ecosystem but very
slowly in the terrestrial ecosystem. And the
simple way to think about that is that phytoplankton don’t have
trunks, but there’s a more complicated way to think about
it. OK, so now, all of this primary
productivity that we’ve made, all of
this photosynthesis, as we talked about,
is the base of the food webs in all ecosystems. And so we’re going to start to
dissect this. This is a marine food web
showing phytoplankton that are eaten by zooplankton. We used to talk about food
chains, but we well now that it’s not a chain. It’s really a very complex web
and very hard to put organisms and
assign them to particular sections. Phytoplankton are eaten by
zooplankton. Zooplankton are eaten by worms. You’ve got blue crabs,
barnacles, and the top predators shore
birds and sea bass. Now, an important
part of these food webs is also all of the carbon,
the primary productivity that is not eaten
while it’s alive. So some things just
die, right? You have dead carbon lying
around. And that dead carbon
falls into what’s called a different food web,
the detritus food web. And we’re going to talk about
that. And it comes from all of these
different components in the food web. So now we’re going to more
analytically look at the flow of carbon. And when we talk about
carbon we are also talking about energy,
right? Carbon and energy
are the same thing. I mean they’re
interconvertable. So we’re going to look at the
flow of carbon from the phytoplankton to
the zooplankton through that trophic level. And to do this we have to
talk about some definitions. And again this is in your
handout so I’m not going to write all this on
the board but we’ll just walk through
this. The flow of carbon or energy
through a trophic level, which is
one of these links, OK? This is one trophic level. This is
the next trophic level. Or you can also think of this
as an organism. This type of analysis applies
to both. And we start out with the
productivity at trophic level and minus one,
OK? And so the first
would be primary productivity coming into the system. Some of that
productivity, some of that carbon is not
ingested by the next trophic level. That’s lost as dead organic
matter, detritus, whatever we want to
call it. So that’s D sub n the portion
not consumed. Then some of it is ingested by
the organism, I sub n. And then some fraction of that
is assimilated by the organism. That
means it’s taken into its biochemistry and goes toward
building biomass. And some of it is lost as fecal
matter produced, that’s F sub n. And urine would also be a part
of this, waste products. And then some of it is then,
the rest of it is then available,
oh, some of it, this is important,
is lost as respiration. And then the rest is available
as productivity for the next trophic level. OK. So different types of
organisms. First, before we get to that,
using this analysis we can start to define
efficiencies of energy conversion through this system. And this is
because different types of organisms assimilate carbon with
different efficiencies. And that is important in the
flow of carbon through different types of ecosystems. So let’s look at the first
efficiency that would be I, ingested, reflecting the amount
that’s ingested relative to the amount that’s available. And this
is called the exploitation efficiency, OK? I sub n divided by
P sub n minus one. The next one similarly would be
A sub n, the amount that’s assimilated
relative to the amount that’s ingested. And that is the assimilation
efficiency. And finally the amount
that goes to the next trophic level divided by the amount
that’s assimilated,
which is the production efficiency, the amount that
actually goes to productivity that is
assimilated. And these all
multiplied together give you the ecological efficiency. And that is
sometimes called the trophic transfer efficiency. That’s the amount of carbon
that is basically lost as you go through one
trophic level. And usually,
and we’ll talk about this in a minute,
this is 10% to 20% actually makes it through the system,
and the rest is lost to respiration or detritus
or fecal matter. OK, so let’s talk about now how
different organisms vary in terms of
efficiencies. We have, in terms of the
exploitation efficiency, if you’re
talking about, for example,
tree insects. So insects feeding on trees is
about 1% to 10%. They don’t take that
much of the tree. If it’s grass to animals it’s
more like 20%. And if
it’s phytoplankton to zooplankton it’s more like 20%
to 40%. In other words,
zooplankton harvests much of the primary productivity,
no, almost half of the productivity that the
phytoplankton have made. OK. What about assimilation
efficiency? How does this vary
between organisms? Well, this one,
you can think about it as if you eat
food that is, I was going to say not you,
but it is true, we are all
animals so it applies to us, too. If you eat food that is similar
in composition to your own bodily
composition you’re more efficient at
assimilating it because you don’t have to break it down as
much and reorganize it. So herbivores,
organisms that eat plant matter are
20% to 50% efficient in their assimilation. But carnivores is more like
80%. Because you are meat and if you
eat meat you don’t have as much
waste than if you eat a lot of fiber. So
there are big differences there. And then in terms of,
that is not a value judgment on whether you
should eat meat or not. I just want to make that very
clear. What? OK. But later on we’ll talk
about the difference between eating meat globally and being
vegetarians in terms of utilizing primary
productivity on the earth. But in
terms of production efficiency you have warm-blooded organisms
having a 2% production efficiency. Whereas cold-blooded have a 40%
production efficiency. Why would that be? Yes. If you’re warm-blooded,
does anybody know the technical term
for that? It is if you are homeotherm. Or what did you say? Endothermic. I’m not sure. I think that’s
chemistry. It sounded good,
though. So there are homeotherms and
heterotherms. It doesn’t matter. The point is that these have to
maintain their body temperature like we do. That takes a lot of energy. Whereas, these go with the flow
so to speak, that technical term. But that doesn’t take as much
energy. If it’s cold they just let
their body get cold. They don’t burn,
burn, burn to keep the temperature constant. OK, so that’s how organisms
differ. Now let’s
move on to the next chapter in which now we are going to look
at — We were looking at the flow
through one trophic level. Now we’re going
to connect a whole bunch of trophic levels. We’re going to look at the
flow of energy through this component of this food web and
do a more thorough analysis. OK, so this gets kind of messy
but let’s start here. I better use the powerful one. OK, so each one of these is
what we call a trophic level. And these are the primary
producers, the photosynthetic organisms. Here is our gross primary
production absorbing sunlight. Some of that is lost to heat. Some of that is lost
to respiration. Here is our little R sub A,
remember? Right here. And
some of it, the net primary productivity is available for
ingestion at the next trophic level which are the herbivores. So all we’re doing here is
ganging up a whole bunch of those individual
analyses. And then the next trophic
levels are the carnivores and then
the second carnivores. And the number of links you
have here is something that is obviously
determined by the efficiency of transfer from one to another
and the total amount of energy that comes
into the system. So here in our ecosystem we
have carbon being lost at each step to detritus. And you have feces at each
link. This all goes down and becomes
part of the detritus food web. And in
that you have two forms of carbon. You have particulate organic
carbon which is pieces of dead carbon
floating around, dead leaves,
dead phytoplankton,
whatever. And then you also have
dissolved organic carbon. When these plants die and the
phytoplankton die they burst open. And the glucose and amino acids
and all of that dissolve into the
water in the system. And that becomes
dissolved organic carbon which is available for this microbial
food web, an entirely different food
web that’s coupled to the system. So
you have the detritivore. This the grazing food web here. The waste
from that goes to the detritivores and the microbes. And then whatever
is left over after that is called refractory carbon. That means none of the
creatures are able to break it open and really get
energy out of it. And in the big picture what is
this? We talked
about it actually last time. Fossil fuel,
exactly. This is the carbon
that actually accumulates over time that when ecosystems are
finished processing all of the primary
production. OK, so I’m trying to time this
right so that we make sure we have time
for the movie. So let’s look at a couple of
ways we can think about this. Let’s just compare. If we look at the open ocean
ecosystem versus the tropical forest. And the average,
oh, I forgot one thing. OK. If we take now the gross
primary productivity. And I’m going to circle all of
the respirations. See the purple here? So this is all the carbon
that’s being fixed. This is that is lost
to respiration. And we can define a new
parameter which is net ecosystem production. And that is gross primary
production minus the respiration of all of the
autotrophies, R sub A, minus the
respiration of all of the heterotrophic components
of the system. So that would be the
herbivores, carnivores, detritivores,
microorganisms. That’s all the
carbon that’s respired and lost to CO2. So you can add this to this. So we’d have net ecosystem
production. Production equals GPP
minus RA minus the sum of all the heterotrophs. Now, in very mature
ecosystems this net ecosystem production is essentially
nothing. All right? Everything that’s produced is
consumed. For example,
in a tropical rainforest you don’t have a huge
buildup of organic carbon in a tropical rainforest. Everything
that’s produced is basically consumed and this is zero. But in a
young forest, say a plantation at the extreme
where you plant trees and they’re increasing in
biomass, then obviously this net ecosystem
production is a positive number. What if net ecosystem
production is a negative number? It won’t be there for long,
right? You’ve got to have things
photosynthesizing net at least enough to maintain the
ecosystem. I
mean you can have it for a transient for not any steady
state. OK. All
right. Talking about ecological
efficiencies again. If we talk
about the average — The average ecological
efficiency of the open ocean is about 25% and of
the tropical rain forest is about 5%. So the average number of
trophic levels, that’s basically the number of
links in the food web in these two systems is about 7.1
and 3.2. In other words,
when you have more efficiency of transfer from one link
to another you can have more links obviously. So getting back to
humans again, and this gets back to in terms
of what we might think about as a global human
society. If you go from wheat to man you
lose 90% of the energy in that
transfer. If you go from wheat to cows to
man you lose 90% here and you lose
90% here. So obviously in terms of
feeding the world it’s much better to go
directly from wheat to humans than to go from wheat to cows to
humans. You all know this but it’s
important to remember that. And unfortunately
the trend in the world is to go more from here to here instead
of the other way around. Just something to remember in
terms of the application of this
knowledge. OK. Finally very
quickly I’m going to skip over this and go to another way to
look at what we’ve been talking about. It’s just another diagram of
the same thing. You have the photosynthetic
organisms that the entire world depends on, this productivity
for food and fiber. And that most of that is lost
through respiration and the rest goes to the other organisms
including detritivores. So a big
question is how much of this global primary productivity,
this global photosynthesis have humans
taken over? It’s really hard to answer
that question, OK, but a lot of ecologists
have been working very hard at understanding the
fraction of global photosynthesis that has
been what’s called co-opted by humans. And the estimates range from
10% to 55%. The amount that we use
directly as food or fuel or fiber or timber is not that
great, but there’s a lot of productivity
that’s diverted as crop waste, burning, et
cetera. And land conversion obviously
uses up a lot of habitat and productivity. So the significant point here
is that as we co-opt this primary
productivity we change dramatically
all of the food webs that rely on it, and that is what is part
of the path to extinction of a lot of
species. And the big question is
how much can we co-opt? I mean we’re on the road to
taking over the primary productivity of the
earth, there’s no question, completely. Unless we set up reserves
that’s it, that’s where we’re marching. Now, when we’re in charge of
it, is it going to function the way we need
it to function to maintain our atmosphere and to provide us
with the food and fiber that we
need? That’s still an open question. OK,
so now I’m going to show you this really neat DVD because
these pictures of food webs are
deadly dull and don’t represent anything at
all of what the reality is like. So I’m going to show you three
weeks in the life of a real food web. And I want you to think about
two things when you’re watching it. Don’t just think you’re at home
in front of your TV watching a
nature show and going brain dead or
something. Think about what you’ve learned
in this class. So think
about the gigatons of carbon that are flowing through this
system. More importantly this entire
food web and everything that’s going on
in it is orchestrated by the information in the genes of the
organisms in the food web. That’s the information content
that structures this whole thing. And how it’s all orchestrated
and coordinated and happens the
same way more or less every year is
absolutely mind blowing as far as I’m concern and a major,
major challenge for ecology and for
molecular ecology. OK.

4 thoughts on “Lec 18 | MIT 7.014 Introductory Biology, Spring 2005

  1. …depends on what kind of single cells are you talking about. Bacterium and Archaea, for example, don't have them, but single celled organisms from Eukaria, like amoeba, do have them.

    Of course, as always, there are exceptions, like amitochondrial organisms who only have remnants of mitochondria in their cytoplasm… but I'm sure you weren't talking about them, heh.

Leave a Reply

Your email address will not be published. Required fields are marked *