Hi, I’m Ulf Riebesell. Did you know that
half the oxygen we breath comes from tiny
phytoplankton flourishing in the sunlit surface
layer of the ocean? These are all single celled
micro algae, tiny little machines that use
the sunlight and the available nutrients to
grow at enormous rates, a new generation almost
every day. Their total production rivals that
of all land plants although their total biomass
is 100 times less than that of land vegetation.
So let’s take a closer look at these amazing
creatures and the role they play in our planet’s
carbon cycle.
First how can it be that phytoplankton are
so much more productive per unit biomass than
land plants? Well, think of what land plants
have to do to harvest light, water and nutrients.
They need roots, stems, branches, leaves.
Each component containing lots of structural
material. Biomass, which is not active in
photosynthesis.
In contrast, phytoplankton don’t have to
overcome the same forces of gravity. They
drift almost neutrally buoyant in the surface
ocean so they don’t need all those structural
components. Also, they are surrounded by the
nutrients they need for growth. A single cell
combines all necessary parts to take up nutrients,
capture sunlight, photosynthesize, grow and
reproduce. They’re like miniature photosynthesis
machines.
To build their biomass, they need lots of carbon
dioxide. That’s shown in this graph by the
red arrows going into the ocean. In fact,
phytoplankton consume ten times more carbon
dioxide every year than humans release into
the atmosphere by burning fossil fuel. But
much of that is released again very quickly
by the algae themselves through resspiration,
by zooplankton eating the algae and by the
bacteria consuming the remains.
Some of the leftovers of this plankton feast
sink out of the surface ocean to greater depths,
like bread crumbs falling off the table. On
the way down they are slowly degraded by bacteria,
thereby releasing the carbon and nutrients
in deeper layers of the ocean. This process
pumps carbon from the surface to the deep
ocean, and that explains why it’s called
the biological carbon pump. A very small portion
of the carbon export reaches the sea floor
where it feeds the deep sea fauna or accumulates
as long term carbon storage in the sediments.
So it’s clear from this that the biological
pump is an important driver in our climate
system. Over the past decades, scientists
have therefore spent quite a bit of effort
trying to quantify the sinking flux of carbon
and other elements through the biological pump.
What they found is that the proportion of
carbon fixed by phytoplankton that makes it
into the deep ocean can vary from less than
1% in some unproductive regions of the open
ocean up to 50% in some highly productive
areas where massive phytoplankton blooms occur.
And the pump’s efficiency is very much dependent
on what phytoplankton are flourishing and
what food web develops in their wake.
Scientists believe that variations in the
strength and efficiency of the oceans biological
pump played a key role in climate changes
in earth history.
For example the recurring pattern from glacial
to interglacial times which characterized
our planet’s climate over the past couple
of million years from the warm interglacials
where the atmospheric CO2 partial pressure
was about 300 parts per million, about one-third
of the CO2 was gradually taken out of the
atmosphere and stored elsewhere on our planet.
Scientists believe that the most likely storehouse
for this CO2 is the deep ocean and that the
biological carbon pump is the vehicle responsible
for transporting the CO2 into the deep ocean
storehouse.
To switch from glacial maxima with their low
pCO2 values of 180 ppm back to the high CO2
interglacials, the gate of the ocean storehouse
must be opened again and the CO2 released
back into the atmosphere. What turns the switch
around, so what causes the shift from atmospheric
to deep ocean storage still puzzles scientists.
What is clear however is that ocean plankton
biology is a key driver in the earth’s climate
system and that the climate in turn is a primary
factor in controlling ocean productivity.
Since the beginning of the industrial revolution,
humans have dramatically perturbed the natural
carbon cycle. The atmospheric CO2 has already
increased by 100 PPM over the pre-industrial
maximum. An additional increase of the same
magnitude as glacial interglacial variations.
This increase will most likely double or triple
over the next few decades and further amplify
climate change. This will give rise to major
changes in the oceans environmental conditions.
With ocean warming and ocean acidification
as the two most prominent and probably most
influential changes.
In combination, these environmental changes
will trigger major alterations in plankton
composition and productivity. For example
warming and acidification are expected to
favor small phytoplankton species and to accelerate
the recycling of carbon in the surface ocean.
So how will the oceans biological carbon pump
respond to these changes? Will it continue
sequestering the same amount of carbon into
the deep ocean? As shown in this graph the
wheel of CO2 fixation via photosynthesis and
CO2 release via respiration and remineralization
may turn faster. But the net result would
be a weakening of the biological carbon pump.
Less carbon would be transported to depths,
more of it would remain in contact with the
atmosphere. Our understanding of the biological
consequences of ocean change is still insufficient
to make reliable predictions of these changes.
And there may be aspects in this complex interaction
between the climate system and ocean biology
which we haven’t spotted yet. But the science
on ocean change biology is in full swing,
so we are likely to soon hear about new discoveries
from this fascinating field of science.
What we’ve learned from all this? Well,
one key message from this is that the strength
and the efficiency of the biological carbon
pump very much depends on the productivity
and composition of the phytoplankton community.
Both plankton composition and productivity
are expected to change under future ocean conditions.
It doesn’t take much imagination to conclude
that the biological carbon pump is not going
to continue at the same rate in the future
ocean. What if the pump loses strength and
more carbon stays in the atmosphere? This
would amplify the human induced global warming
and this is what scientists refer to as a
positive feedback but positive in this context
doesn’t mean good. A positive feedback works
as an amplifier. It makes bad things worse.
It is these positive feedbacks that scientists
are so worried about.
